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ANALYTICAL DEVELOPMENTS IN X- RAY EMISSION SPECTROMETRY. Item Type text; Dissertation-Reproduction (electronic) Authors PERRON, STEVEN JOSEPH. Publisher The University of Arizona. Rights Copyright © is held by the author. Digital access to this material is made possible by the University Libraries, University of Arizona. Further transmission, reproduction or presentation (such as public display or performance) of protected items is prohibited except with permission of the author. Download date 06/05/2018 01:04:35 Link to Item http://hdl.handle.net/10150/143053
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ANALYTICAL DEVELOPMENTS IN X-RAY EMISSION SPECTROMETRY.
Item Type text; Dissertation-Reproduction (electronic)
Authors PERRON, STEVEN JOSEPH.
Publisher The University of Arizona.
Rights Copyright © is held by the author. Digital access to this materialis made possible by the University Libraries, University of Arizona.Further transmission, reproduction or presentation (such aspublic display or performance) of protected items is prohibitedexcept with permission of the author.
Download date 06/05/2018 01:04:35
Link to Item http://hdl.handle.net/10150/143053
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Perron, Steven Joseph
ANALYTICAL DEVELOPMENTS IN X-RAY EMISSION SPECTROMETRY
The University of Arizona PH.D. 1982
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ANALYTICAL DEVELOPMENTS IN X-RAY EMISSION SPECTROMETRY
by
Steven Joseph Perron
A Dissertation Submitted to the Faculty of the
DEPARTMENT OF CHEMISTRY
In Partial Fulfillment of the Requirements for the Degree of
DOCTOR OF PHILOSOPHY
In the Graduate College
THE UNIVERSITY OF ARIZONA
198 2
THE UNIVERSITY OF ARIZONA GRADUATE COLLEGE
As members of the Final Examination Committee, we certify that we have read
the dissertation prepared by ____ S_t_e_v_e_n __ J_o_s_e~p_h __ P_e_r_r_o_n ______________________ _
entitled Analytical Developments in X-Ray Emission Spectrometry
and recommend that it be accepted as fulfilling the dissertation requirement
for the Degree of Doctor of Philosophy ------------------~~-----------------------------------
(C-'""L Date
II - 1-Date
Date
Date
Date
Final approval and acc~ptance of this dissertation is contingent upon the candidate's submission of the final copy of the dissertation to the Graduate College.
I hereby certify that I have read this dissertation prepared under my direction and recommend that it be accepted as fulfilling the dissertation requirement.
LJ~i;,=j Dissertation Director Date
STATEMENT BY AUTHOR
This dissertation has been submitted in partial fulfillment of requirements for an advanced degree at The University of Arizona and is deposited in the University Library to be made available to borrowers under rules of the Library.
Brief quotations from this dissertation are allowable without special permission, provided that accurate acknowledgment of source is made. Requests for permission for extended quotation from or reproduction of this manuscript in whole or in part may be granted by the head of the major department or the Dean of the Graduate College when in his judgment the proposed use of the material is in the interests of scholarship. In all other instances, however, permission must be obtained from the author.
To Celeste
"It's never too late to have a happy childhood"
Tom Robbins, 1980
ACKNOWLEDGMENTS
I want to express my sincere appreciation to the following
people for their help and guidance throughout my graduate career:
To Dr. Quintus Fernando, my research advisor; his advice,
support and direction have been invaluable during my years at the
University of Arizona.
To Hank and Hain Oona, whose help, advice and ahove all
friendship have been, and will always be, greatly valued and
appreciated.
To my family, and especially my parents, whose support and
understanding helped me through many trout led times.
And above all, to my wife Celeste. I thank her for her help
in preparing this manuscript and especially for her understanding,
patience and love throughout my years of schooling. This is truly ~
degree; I couldn't have done it without her.
I also want to sincerely thank Dr. John Leavit, Peter Stoss,
Dave Rollins and Steve Kirchner for their many contributions to my
research, and I acknowledge the financial support obtained from NSF
grant CHE78-l8576 and the University of Arizona.
TABLE OF CONTENTS
Page
LIST OF ILLUSTRATIONS • vii
LIST OF TABLES xi
ABSTRACT xiii
1. INTRODUCTION 1
Historical Development of X-Ray Spectrometry 4 Basic X-Ray Properties • • • • • • • • • • 5
Origin of X-Rays • • • • • • •• 5 X-Ray Nomenc lature •••• • • • • 9 Interaction of X-Rays with Matter • 14
Instrumentation • • • • • • • • • • • • • • •• 16 Excitation Sources •• • • • • • • • • • • • 18 Wavelength Dispersive Spectrometry 20 Energy Dispersive Spectrometry • • • • • • 23
2. RADIOISOTOPE INDUCED X-RAY FLUORESCENCE •
Instrumentation • • • • Analysis of Photographs • • • • •
Experimental ••• • Summary • • • • • • • • • • • •
Analysis of Dysprosium X-Ray Contrast Experimental • • • • Summary • • • • • • • •
3. PARTICLE INDUCED X-RAY EMISSION.
Media • •
Instrumentation • • • • • • • • • • •••• Qualitative and Semiquantitative Applications of
PIXE • . • • • • • • • • • • • Analysis of Backing Materials Analysis of Bullet Lead • Analysis of Bat Tissue • • • • Analysis of Metal Films • Summary • . • • • • • . • • • . •
Quantitative Applications of FIXE ••••• X-Ray Efficiency Curves • • • Analysis of Standard Reference Materials Analysis of Ferromanganese Nodules Summary • • . • • •• •••••••••
v
27
31 34 35 45 49 49 57
62
67
74 75 79 87 89 95 99
100 108 117 123
TABLE OF CONTENTS--Continued
4. HIGH RESOLUTION PIXE
Instrumentation • Van de Graaff Accelerator Sample Chamber • • • • Spectrometer • • • • • • • • • • • • Electronics • • • • • • • Operating Instructions • • • •
Experimental • • • • • • • Studies on Low-Z Elements • Molybdenum Extraction Products Cu(I)/Cu(II) Compounds
Summary • •••• • • • • • • • • •
5. CONCLUSIONS
APPENDIX A:
APPENDIX ~:
APPENDIX C:
REFERENCES
EXTRACTION OF MOLYBDENUM FROM FERROMANGANESE NODULES • • • •
NEUTRON ACTIVATION ANALYSIS OF BULLET LEAD
PROGRAM RATIO FOR PIXE QUANTITATION
Page
124
128 128 130 132 151 160 164 164 167 170 177
180
185
211
230
257
vi
LIST OF ILLUSTRATIONS
Figure Page
1.1. Fluorescence yield curves for K, Land M shell
1.2.
1.3.
vacancies
Principal electron transitions that give rise to characteristic X-ray line spectra • • • • • • • • • • •
Mass attenuation coefficients as a function of wavelength (for uranium) • • • •
1.4. A typical wavelength dispersive X-ray analysis
1.5.
2.1.
2.2.
2.3.
2.4.
2.5.
2.6.
2.7.
2.8.
2.9.
2.10.
system . . . . . . . . . .
A typical energy dispersive X-ray analysis system •
Geometric arrangements of radioisotope sources for X-ray fluorescence analysis • • • • • • • • • • ••••
The radioisotope X-ray fluorescence system used
Sample spectrum obtRined from the X-ray fluorescence analysis of a color Type-C print ••••••
X-Ray emission spectrum of a cellulose acetate film negative ••• • • • . • • • • • • • • • • •
X-Ray emission spectrum of an unidentified albumen print showing the presence of a gold chloride toner
X-Ray emission spectrum of a palladium emulsion black and white print by Tom Millea •••••••••••••
X-Ray emission spectrum of a platinum emulsion black and white print by Laura Gilpin • • • • • • • • • • • •
X-Ray emission spectrum of a "satista" print by Alfred Stieglitz •••.•.••••
X-Ray emission spectrum of the matting used to back some of the photographs analyzed • • • • •
X-Ray emission spectrum of the dry mount tissue used to mount photographs •••••••.• . • • • • • • •
vii
10
11
17
21
24
30
33
37
38
40
42
43
44
47
48
Figure
2.11.
2.12.
2.13 •
2.14.
2.15.
2.16.
3.1.
3.2.
3.3.
3.4.
3.5.
3.6.
3.7.
3.8.
3.9.
3.10.
3.11.
LIST OF ILLUSTRATIONS--Continued
1 MeV PIXE spectrum of Dy-DTPA X-ray contrast media
2 MeV PIXE spectrum of Dy-DTPA X-ray contrast media
2 MeV PIXE spectrum of rat feces to determine the elimination of Dy-contain1ng contrast media •
Am X-ray emission spectrum of rat urine containing the Dy-DTPA contrast media
Sample X-ray emission spectrum of dysprosium standard used to establish a calibration curve
Dysprosium calibration curve obtained from the Am X-ray emission spectra of DY203 in dilute aqua regia
K and L shell ionization cross-sections for 1 MeV (solid line) and 2 MeV (dotted line) protons •••••
The PIXE system • • • • • • • • •
Van de Graaff accelerator •
The sample chamber used in PIXE •
PIXE spectra of polyethylene with and without a carbon foil in the beam path
1 MeV PIXE spectrum of Kapton film showing impurities . . . . . . . . . . . . . . . 1 MeV PIXE spectrum of Nuc1eopore filter on Kapton backing . . . . . . . . . . . . . . 1.5 MeV PIXE spectrum of bullet lead showing the Pb La peak used to normalize counting times • • •
1.5 MeV PIXE spectrum of a Federal bullet fragment
1.5 MeV PIXE spectrum of a Winchester bullet fragment
PIXE spectrum of bat tissue to determine Cu/Zn ratios • • . • . • . • .••••••••
.
viii
Page
51
52
53
55
56
59
64
68
70
71
73
78
80
83
84
85
89
Figure
3.12.
3.13.
3.14.
3.15.
3.16.
3.17.
3.18.
3.19.
3.20.
3.21.
4.1.
4.2.
4.3.
4.4.
4.5.
LIST OF ILLUSTRATIONS--Continued
1 MeV PIXE spectrum of a thin molybdenum film on glass substrate · · · · · · · · · · · · 2 MeV PIXE spectrum of a thin molybdenum film on glass substrate · · · · . · · · · 2 MeV PIXE spectrum of indium film sputtered in argon . · . · · · · · · · · · · · · · · · 2 MeV PIXE spectrum of indium film sputtered in oxygen • • • • • •
Experimentally obtained K and L shell relative X-ray efficiency curves for 1 MeV protons • • •
Experimentally obtained K and L shell relative X-ray efficiency curves for 2 MeV protons with a 4 mil Al filter • • • • • • • • • • . •
1 MeV PIXE spectrum of NBS 1571 orchard leaves
2 MeV PIXE spectrum of NBS 1571 orchard leaves
1 MeV PIXE spectrum of ferromanganese nodule sample · . . . · · · · · · · . · · · · · 2 MeV PIXE spectrum of ferromanganese nodule sample · . . · · · · · · · · · · The high resolution PIXE system
Electron gun coverplate used to align the system
The variable curvature bent crystal spectrometer and PIXE sample chamber ••••
Variation of the radius of curvature of the Rowland circle with crystal position
Vacuum chamber used to stretch polypropylene for proportional counter windows ••••••
.
.
4.6. The experimental (solid line) and theoretical (broken line) amplification characteristics of our proportional counter using methane at atmospheric
·
·
·
pressure • • • • • • • • • • • • • • . • • • . • •
ix
Page
92
93
. . 96
97
107
108
110
. III
. 120
121
129
131
133
134
139
144
Figure
4.7.
4.8.
4.9.
4.10.
4.11.
4.12.
4.13.
4.14.
4.15.
4.16.
4.17.
4.18.
4.19.
4.20.
LIST OF ILLUSTRATIONS--Continued
The experimental (solid line) and theoretical (broken line) amplification characteristics of our proportional counter using 90% Ar/lO% CH4 at atmospheric pressure • • • • • • • • • • • • • • • • •
The electronics used for high resolution PIXE •
The preamplifier circuit incorporating an MC 1552 video amplifier chip •• • • • • • • • • • • • •
The preamplifier circuit incorporating an MC 1733 differential video amplifier chip • • • •
The Tennelec TC 214 single channel analyzer •
The lower level discriminator calibration for the Tennelec TC 214 single channel analyzer
The upper level discriminator calibration for the Tennelec TC 214 single channel analyzer . . . The experimentally determined calibration curve for
. .
the Philips-Norelco variable •• • • • • • • • •
High resolution PIXE spectrum of graphite showing 1 order diffracted off a PbSt crystal • • • • • •
High resolution PIXE spectrum of aluminum showing 7 ordes diffracted off a PbSt crystal • • • • • • •
High resolution PIXE spectrum of molybdenum showing 2 orders diffracted off mica •••••••
High resolution PIXE spectrum of coper showing 8 orders of K X-rays diffracted off mica and 4 orders of L X-rays diffracted off PbSt • • • • •
High resolution PIXE spectrum of Cuo , CuCl and CuC1 2 . . . . . . . . Variation of Cu L peak intensity for a variety of ion beams . . . . . . . .
x
Page
146
153
154
155
157
158
159
162
166
168
169
171
172
176
Table
1.1
2.1.
2.2.
3.1.
LIST OF TABLES
Relative intensities of principal X-ray spectral lines of analytical interest •
Commonly used radioisotope sources for X-ray fluorescence ••••••••••• • • • • •
Corrected peak areas for dysprosium standards in dilute aqua regia • • • • • • •
Properties of backing materials used in PIXE
3.2. PIXE analysis of bat tisue to determine Cu/Zn
3.3.
3.4.
3.5.
3.6.
3.7.
3.8
3.9.
3.10.
3.1l.
3.12.
3.13.
3.14.
ratios
Trace metal impurities in evaporated metal films
PIXE analysis of indium sputtered films • • • • •
Experimental thin target K-shell X-ray yield for 1 MeV protons • • • • • • • • • • • • • • • • • •
Experimental thin target L-shell X-ray yield for 1 MeV protons • • • • • • • • • • • • • • • • • •
Experimental thin target K-shell X-ray yield for 2 MeV protons with a 4 mil Al filter • • • • • •
Experimental thin target L-shell X-ray yield for 2 MeV protons with a 4 mil Al filter • • • •
Analysis of NBS SRM 1571 orchard leaves . Analysis of NBS SRM 1575 pine needles . . Analysis of NBS SRM 1577 bovine liver •
Analysis of NBS SRM l632a coal
Analysis of NBS SRM l633a fly ash . . Comparative PIXE and flame analysis of ferromanganese nodules • • • • • • • • •
xi
. .
.
Page
13
29
58
76
90
94
98
102
103
104
105
112
113
114
115
. . 116
122
Table
4.1.
4.2.
4.3.
4.4.
LIST OF TABLES--Continued
Properties of common crystals used in a crystal specstrometer • • • • • • • • • • • • • • . • •
Experimental gas amplification factors for a methan3 counter where ap 0.1378 and bla = 2 x 10 • • • • • • • • • • • • • • • • • • • • • • • •
Necessary memory contents of microprocessor to obtain desired spectriometer step size • • • • • • •
Observed counts and background in 2nd order Cu peak using various ion beams • • • • • • • • •
xii
Page
136
143
149
175
ABSTRACT
X-Ray emission techniques have been developed for the analysis
of unusual and difficult samples that cannot be analyzed by
conventional analytical techniques. Three X-ray techniques have been
investigated, including radioisotope-induced X-ray fluorescence
(RXRF), proton-induced X-ray emission (PIXE) and high resolution PIXE.
Low flux radioisotope X-ray sources have been used to non
destructive1 characterize the elements present in photographic papers
and emulsions. The information obtained has proven valuable for
cataloging and preserving photographic prints of historical
significance. Radioisotope X-ray sources have also been used in the
development of low-cost, portable instrumentation useful for
quantitating a variety of toxicological samples, including urine and
feces samples to determine the elimination rates of X-ray contrast
media containing dysprosium.
The PIXE technique has been applied to the analysis of
forensic samples, including bullet lead, tissue fragments and thin
metal coatings, and has been compared with other non-destructive
methods of analysis. Sample preparation techniques and analytical
procedures have been developed for general, thin target, quantitative
PIXE analysis. These procedures were applied to the analysis of five
NBS standard reference materials, and to the analysis of deep-sea
ferromanganese nodules.
A high resolution (2 eV) PIXE system has been developed to aid
in the deconvolution of overlapping X-ray peaks encountered in
conventional PIXE spectra. This system has been applied to the
measurement of chemical shifts in the X-ray emission spectra of
transition metal comounds.
CHAPTER 1
INTRODUCTION
Three areas of applications have grown out of X-ray technology
since the discovery of X-rays in 1895; these include medical and
industrial radiography, X-ray diffraction, and X-ray spectrometry. Of
these, X-ray spectrometry is especially interesting to chemists
because of the many potential applications to trace element analysis.
We first became interested in X-ray spectrometric techniques during a
search for analytical methods that could be used for a variety of
forensic, toxicological and environmental samples. In this search the
methods desired were to be sensitive, applicable to a wide variety of
elements, and non-destructive so that the results could be presented,
if necessary, as evidence in a courtroom. Flame methods, although
very sensitive and applicable to a wide variety of elements, were
rejected because almost without exception, they required dissolution
of the sample. Electrochemical methods also required the sample to be
in solution and were only applicable to certain elements of interest.
Neutron activation analysis was both sensitive and non-destructive,
but it could not be used at all for some elements of interest and the
operating conditions had to be varied to maximize the sensitivity for
each element. Also, the available counting facility and the computer
capabilities for automatic peak identification and quantitation were
unsatisfactory. As a consequence, the determination of a large number
1
of elements in a single sample became a formidable problem. X-Ray
emission methods seemed to meet all the requirements of our search;
they had the required sensitivity for most applications, all elements
of interest could be determined, and the method was non-destructive.
With most X-ray emission methods, all elements from Na through
U could be determined qualitatively in the original sample, and the
potential existed for quantitative analysis in the original form if
corrections were made for X-ray absorption and enhancement by the
sample matrix. If the sample were dissolved, accurate quantitation at
the parts-per-million level could b~ performed and the results
compared to those from flame or electrochemical analyses on the same
samples, since the sample is not altered or destroyed during
irradiation. There would be no residual radiation, in contrast to
NAA, and the start-to-finish analysis time would be less than that
required for most other analytical methods, considering the number of
elements determined. It appeared, therefore, that X-ray emission
methods would be ideally suited to the determination of trace elements
in the types of samples in which we were interested.
An X-ray facility was developed at the University of Arizona
for the analysis of unusual, one-of-a-kind and difficult types of
samples. This involved the development of new instrumentation, sample
preparation techniques and analytical procedures that could be used
for the qualitative and quantitative determination of trace elements
present in the samples. These developments are discussed throughout
the remainder of this dissertation.
2
This dissertation is divided into 5 chapters. The first
chapter serves as an introduction to the field of X-ray emission
spectrometry and includes a discussion of the historical development
of X-ray spectrometry, the basic physical properties of X-rays, and
the instrumentation used in X-ray emission analysis. The next three
chapters describe analytical developments made in three areas of x
ray spectrometry that were used in my research: Radioisotope Induced
X-Ray Fluorescence (RXRF) , Particle Induced X-Ray Emission (PIXE), and
High Resolution PIXE. In each chapter the history and background of
the technique are discussed, and the instrumentation used is described
along with representative analyses that demonstrate how the technique
has been developed for chemical analysis. The last chapter summarizes
my work and the current capabilities of the X-ray facility at the
University of Arizona. Future directions of the facility are
discussed, including possible applications and developments in
instrumentation.
Two appendices are also included which describe work done
using complementary techniques to more fully characterize some of the
samples analyzed by X-ray emission techniques. These include the
characterization of molybdenum compounds extracted from ferromanganese
nodules by infrared analysis (IR) and X-ray photoelectron spectroscopy
(XPS), and the elemental analysis of bullet lead by neutron activation
analysis (NAA). The final appendix includes the program RATIO that
was developed for use in PIXE analyses.
3
Historical Development of X-Ray Spectrometry
X-Ray emission spectrometric techniques have been well
established and widely used methods of chemical analysis for nearly
thirty years. Electron-induced X-ray emission spectra were first
applied specifically to chemical analysis by Hadding in 1922 (1). In
1925, von Hevessy suggested the use of primary X-rays rather than
electrons as an excitation source (2,3). Coster and Nishina in 1925
(4) and G10cker and Schreiber in 1928 (5) applied his idea of X-ray
secondary emission (X-ray fluorescence) to quantitative analysis, but
found the secondary emission technique to be less efficient by several
orders of magnitude, resulting in decreased sensitivity. Von Hevessy
noted the advantages of the technique, however, in the possibility of
analyzing liquid samples and in the almost complete absence of
continuous background. He suggested the construction of sealed X-ray
tubes fitted with beryllium windows to carry out excitation with soft
X-rays (6). By the 1930s concentrations in the range of 10-4 to
10-5 M could be measured, but the full development of X-ray
spectrometry had to wait for several technological advances; it was
not until the 1940s that high-powered, stable, sealed X-ray tubes,
large single crystals of synthetic rock salt and sensitive Geiger
counters were all brought together to permit rapid and reproducible x
ray fluorescence measurements (7). In 1948 Friedman and Birks built
the prototype of the first commercial X-ray fluorescence spectrometer
(8); this incorporated a sealed beryllium window X-ray tube, a
collimator system made of nickel tubes, a thin window Geiger counter
4
with associated electronic counting gear, and a sample introduction
device. After this spectrometer, vacuum systems soon followed, along
with the gas flow proportional counter and the lithium fluoride
crystal (3). By the late 50's detectors became commercially available
with dead times of less than 1 msec. and with output pulses
proportional to the energy of the incident photon (9); there was
little progress in energy dispersive techniques, however, until the
introduction of semiconductor detectors in the early 60's (10,11).
Bowman and co-workers first used semiconductors for X-ray analysis in
1966 (12), and since then energy resolution and count rate performance
have improved significantly due to progress in electronic design
(13,14). The introduction of guard-ring detectors in the early 1970s
(15) drastically reduced background resulting from incomplete charge
collection, so that by 1975 semiconductor detectors could be used for
routine trace multi-elemental analysis. Today most X-ray analysis
systems use an energy dispersive semiconductor detector, or a
semiconductor detector in conjunction with a crystal spectrometer, for
rapid, reproducible, routine chemical analysis.
Basic X-Ray Properties
Origin of X-Rays
When an element is bombarded with high energy particles or
photons, interactions occur between the bombarding particles and the
electrons makiug up the atoms of the element. These interactions
cause an energy change to occur within the atom, and the atom later
5
returns to its original energy state by giving up excess energy as
emitted radiation. The emitted radiation may include infrared,
visible, ultraviolet and x-radiation. X-Rays, considered to be photons o
of a wavelength between 0.1-100 A, may be emitted as either
noncharacteristic continuous spectra or as line spectra having
wavelengths characteristic of the emitting elements. X-Rays with a
continuum of energy are largely responsible for excitation of
characteristic secondary spectra and give rise to most of the
background in X-ray fluorescence; characteristic line spectra are
unique to any element and form the basis of all X-ray emission
techniques. These types of x-radiation will be discussed in more
detail.
Continuous x-radiation. Continuous X-ray spectra, also
known as bremsstrahlung, occurs when high speed electrons undergo
stepwise deceleration in matter. This happens, for example, in a
conventional X-ray tube where energetic electrons strike an anode
material to give characteristic anode lines superimposed on a broad
band continuous background. The intensity distribution of the
continuum is characterized by a short wavelength limit, Amin' and by
a peak maximum at approximately 2 Amin. The short wavelength limit
corresponds to the maximum energy of the exciting electrons, Vo, as
defined by Duane and Hunt (16):
A min hc Vo
(1.1)
6
where h is Planck's constant, and c is the speed of light. A is
expressed in Angstroms and Vo in kilovolts, and substitution of these
values gives:
A - 12.4 min -~ (1.2)
The intensity distribution of the continuum can be approximated by
Kramer's formula (17):
I(A) « i Z (1.3)
where leA) is the relative intensity at wavelength A, i is the X-ray
tube current and Z is the atomic number of the target material.
For practical purposes, continuous x-radiation occurs only
with electron excitation. A continuum excited by ions is weaker than
that from electrons by a factor Z2/m2, where Z is the charge of the
nucleus and m the nuclear mass. For example, the continuum intensity
for protons is (1/1800)2 that for electrons, due to the increased
mass, and it is successively less for deuterons, -particles and
heavier ions. In electron-excited X-ray spectrometry, the high
continuum background seriously restricts the minimum detection limit.
In X-ray fluorescence analysis, the primary continuum generated by the
X-ray tube provides the principal source of sample excitation and may
appear as background in the secondary spectra due to scattering by the
sample. The X-rays themselves do not generate continuous radiation
because photons do not undergo a stepwise loss of energy.
7
8
Characteristic X-Radiation. Characteristic X-ray spectra
consist of a series of discrete wavelengths characteristic of the
emitting element. These X-ray spectral lines arise from the
rearrangement of orbital electrons in the target element following
ejection of one or more electrons in the excitation process. For
example, if a K-shell electron is ejected during excitation, the atom
becomes unstable due to the presence of a "positive hole" in the K
shell; the atom can return to its original stable state by two
predominant processes. In the first process, the ground state is
regained by multiple electron transitions from outer orbitals. Since
the instability of an atom due to ionization decreases in the order K+ >
L + > M+ > N+, each time an electron is transferred the at iom moves to
a less energetic state and X-radiation is emitted at a wavelength
corresponding to the energy difference between the initial and final
states of the transferred electron. The process continues until the
energy of the atom is approximately that associated with normal outer
electron vibration (generally a few electron volts). The second
process is similar to the first, but in this case radiation is not
emitted following the transition of an outer electron to an inner
shell; instead, the atom gives up its energy by further ionization
within the atom in a process known as the Auger effect. Since
millions of atoms are involved in the excitation of a given sample,
both de-excitation routes will be taken and the intensity of emitted
X-radiation will depend on the relative efficiency of the opposing
processes. This relative efficiency is expressed in terms of the
fluorescence yield, w, and is defined as the ratio of the number of x
ray photons emitted within a given series to the number of vacancies
formed in the associated level. Figure 1.1 shows how the
distribution of the total de-excitation between the two processes
differs from element to element.
In practice, the processes involved in characteristic X-ray
emission are more complex than this description might suggest.
Electron transitions cannot occur from any higher orbital to any lower
one; only those transitions permitted by the following selection rules
are allowed: ~n ~ 0
~2 * 1
Aj ± 1 or 0
where n is the principal quantum number, 2 is the angular quantum
number and j is the vector sum of 2 and s, the spin quantum number.
Figure 1.2 illustrates the electron transitions which fit these
selection rules and give rise to the principal X-ray spectral lines.
The accepted nomenclature for these spectral lines is somewhat
unsystematic and will be described in the next section.
X-Ray Nomenclature
It is often confusing to find that four separate notations are
used in X-ray spectrometry to express a given electron transition.
For example, the two allowed L-shell to K-shell transitions can be
expressed as:
9
10
1.0-r---------------------,
0.8
"0 0.6 Qj :;
C III U Ul III
0 0.4 ::I u::
0.2
M Shell
~j 0.0-+----r--,----r----.-----.----.,.--.---.---.....----1
1
a 20 40 60 80 100
Atomic Number
Figure 1.1. Fluorescence yield curves for K. Land M shell vacancies.
I '~I
'M
' I WN-~;' .. I_V~;~v-1 -y~-V~~VV~
I I='
:':;il
l~~
"." II:
I' y
y Q
y~
... !?~
"~I
~ i:n
I
r-
----
-•
~'j:;::1Cf'T1c"&II.'lu"';'a:wl
......
......
.. _
... _ ..
....
....
N .. 9
...
...
t: .
. .,
_'"
_ ..
. :., ..
..... ~
.. _
N
• t
l I
.;;! ....
NO
' ... "l
Ofi
IIla
tOT
aTrJ
<I!
-I0§
~
__
...
. _C
Q_ ..
~ ...
......
......
.....
-: ....
......
......
.....
::;; ,,
_ ..
....
....
.. _
__
u -=~
'=
a.
..
... ,.J'
21
21
21
~:o ..
'';--
--.,-
-__
_ ."" .'"
.. ,'O
S"
"1"0
'" , ,
"'I" ,:
",,1'
I In
2
,1."
, o
Vl
2 "S
u.l
i 11
1191
.. , I'
ll' .
', J/I
S '12
."
sir
Z ""I
.'D
S',
Z
V
I ..
..
lOS
"
I 31
2 .Z
.,n
I
112
.. r
•• "
o II
I 1
.lS
llt
1'1~
. ""I"
I'!O,,,
2 1
Il
..
rov
: I
'12
..
,'.
""
I V
Z Z
,'
p"t
o 11
1 2
,1"._,
lI
eUI
I -
2 I
"21
2'."
, I
117
ZIp
",
o II
I Z
2
1"1
/1'
"I.'
o 11
11
1 I
"S
tll
Q'
I:
II i
"III
~"
\,
!;l
-I
OO
N
00
, ..r
..s'~
.:'~
;,
"!'. -1"
-=
--~
NUC'
(USI
.. /'
~.
/~~,/,
Fig
ure
1
.2.
Pri
ncip
al
ele
ctr
on
tr
an
sit
ion
s
that
giv
e ri
se
to ch
ara
cte
risti
c x
-ray
li
ne
spectr
a.
__
A
lon
g
the
top
are
re
lati
ve sp
ectr
al-
lin
e in
ten
sit
ies.
No
te th
at
the
L89
, L
8l0
, L
8l7
' L
s an
d
Lt
lin
es
show
n are
fo
rbid
den
tr
an
sit
ion
s.
I-'
I-'
K~l' KLIII
, 2p3/2 ~ 18, 2 1S or P3/2 ~ a
K~2' KLII
, 2p1/2 ~ 1 , 2 1S
or P1 / 2 ~ a 8
The first two methods of notation are most commonly found in textbooks
and tables. They are the most convenient to use, but they cannot
indicate satellite lines or forbidden transitions. The third method
is familiar to chemists and indicates exactly which electron states
are involved in a given transition. The last method is the correct
spectroscopic nomenclature for the third notation, but it is seldom
used by analytical spectroscopists (3).
The first method of notation, known as Siegbahn notation, will
be used throughout the remainder of this dissertation. In this system
the notation for an X-ray spectral line consists of: 1) the symbol of
the chemical element; 2) the symbol of the series (K,L,M, etc.), where
the series is determined by the final resting place of the transferred
electron; and 3) a lower case Greek letter, sometimes with a numerical
subscript, denoting the particular line in the series. Thus,
ionization in the K shell followed by filling of the K vacancy leads
to production of K-series radiation. The strongest line in the series
is called the a line and corresponds to a ~n = 1 transition. The S or
Y lines usually arise from ~n = I or 2 transitions. The symbol Ka, or
K~l 2' indicates the Kal~ pair and has a wavelength equal to the , average of the individual lines weighted for their relative
intesities: K~1:Ka2 2:1,
AK a
?AKN1 + AKa2 3
(1.4)
12
Table 1.1. Relative intensities of principal X-ray spectral lines of analytical interest
Kal
100 Lal 100 Mal 50
KaZ 50 LSI 75 MaZ 50
Kal Z 150 Lf,2 30 Mal •Z 100
KS I ZO LB3 5 MSI 100
LS4 3 MYI 100
LYI 10 M 1 5
Le 3
13
14
Similarly, the sym~ol KS indicates the KS l S3 pair. Table 1.1
indicates the typical relative intensities of the principal spectral
lines of analytical interest (18). Aside from these few conventions,
there is no systematic procedure for assigning Siegbahn notation;
however, an X-ray line resulting from a given transition in any
element will always be given the same symbol.
Interaction of X-Rays with Matter
When a beam of x-radiation interacts with matter, each
individual incident photon may undergo coherent or incoherent
scattering, photoelectric absorption, or transmission. Coherent
(Rayleigh) scattering occurs when an incident photon is scattered by a
loosely bound outer electron and no energy transfer occurs in the
process. Incoherent (Compton) scattering occurs when a fraction of the
energy of the incident photon is transferred to the bound electron,
causing the wavelength of the scattered X-ray to be slightly longer
than the incident wavelength. Photoelectric absorption occurs when
all of the energy of the incident photon is transferred to an electron
of the target atom causing complete removal from its initial site;
this is the process that results in emission of X-ray lines.
The fraction of X-rays that are transmitted by the sample
depends on the thickness and density of the absorbing material and on
the wavelength of the incident photons. The intensity of the
transmitted beam, I, can be expressed by the Beer-Lambert Law:
I I exp[-a(N/A)px] o
(1.5)
15
where 10 is the initial intensity, x is the thickness (cm) and P the
density (g/cm3) of the absorber. N is Avogadro's number (atoms/mole),
A is the atomic weight (g/mole) and cr represents the "cross section"
or imaginary target area (cm2/atom or barns) that each atom presents
to the photons. The cross section is related to the mass attenuation
coefficient, ~, by the relationship:
~ = cr(N/A)
so that:
I = 10 exp(-~px)
0.6)
0.7)
The mass attenuation coefficient is an atomic property of chemical
elements and is a measure of their X-ray stopping power.
The value of the mass attenuation coefficient, ~, is
determined by the amount of photoelectric absorption, T, and the
amount of scatter, cr, in a sample; the scatter is generally small
enough, however, that it can be neglected. Since photoelectric
absorption occurs at each energy level in an atom, the total
photoelectric absorption -- and therefore the mass attenuation
coefficient -- will be determined by the sum of the individual
absorptions:
~ = TTotal
0.8)
Since the photoelectric absorption varies greatly with wavelength and
atomic number:
T = K ~ • Z).. 3 A
0.9)
it follows that the value of the mass attenuation coefficient
increases with wavelength, and, at a specific wavelength, increases
with atomic number. Each time the wavelength increases to a value in
excess of a certain absorption edge ( ab), however, one of the
photoelectric absorption terms in Equation (1.8) drops out, resulting
16
in a sharp fall in the value of The absorption edge corresponds to
the binding energy of electrons in the appropriate levels and is the
longest wavelength (minimum photon energy) that can excite a specified
X-ray line series of a specified element. A plot of mass attenuation
coefficient versus wavelength shows sharp discontinuities at the
absorption edges in otherwise smoothly increasing lines (see figure
1.3). The rapid increase in as the absorption edge is approached
from the short wavelength side indicates a resonance effect in the
ionization process and it is expected that the most effective
wavelength to cause excitation will be just to the short wavelength
side of the absorption edges. This forms the basis of excitation in
secondary target X-ray fluorescence.
Instrumentation
X-Ray emission spectrometry covers a group of instrumental
techniques that are based on the measurement of wavelengths and
intensities of characteristic X-ray lines. An X-ray spectrometric
analysis involves four essential elements of instrumentation: 1)
excitation of the elements in a sample; 2) separation of the emitted
X-ray line spectra so that a line of any specified element can be
104~ ____________________________________________ ~
103
4
J E
102
~
C ~ Qj 4 0 to)
c .E iii :0 C
10 ~ < CI) CI)
'" 4 ::E
~1~------r---~--------r----+------~----~--~ 0.01 0.1 4.
Wavelength tAl 4 10
Figure 1.3. Mass attenuation coefficients as a fUnction of wavelength (for uranium).
17
18
measured individually; 3) detection and conversion of X-ray photons
into current pulses; and 4) readout and measurement of the detector
output data. Excitation can be any form of radiation capable of
producing vacancies in the inner shells of the atoms of interest in
the sample. Several types of excitation sources are available, and
they can be selected according to the sensitivity needed and the type
of information desired. The remainder of the instrumentation can be
divided into two types based on the methods used to separate
individual X-ray lines; these are commonly called wavelength
dispersive and energy dispersive methods. Each of these areas will be
described in more detail in the following sections.
Excitation Sources
Excitation of characteristic line spectra may be achieved by
means of X-rays, electrons, protons and other charged particles, or
radioisotope sources. Fluorescent excitation with an X-ray tube is the
most common method used for chemical analysis. As explained earlier,
X-ray tubes are primarily bremsstrahlung sources of photons so that
excitation is possible over a broad range of wavelengths. The
effective wavelength and intensity of the exciting radiation can be
varied by selection of the X-ray tube target, operating current and
voltage, and choice of filters and secondary targets, to give lower
limits of detection of about 1 ppm in the most favorable case. The
lower absolute limit of detection, however, is only about IO-8g , so
that rather large samples must be used.
19
Direct electron excitation was the first method used to excite
characteristic x-radiation and is the method aplied in X-ray tubes,
electron microprobe analyzers and electron gun spectrometers. Direct
electron excitation offers a 20,000 times increase in intensity over
fluorescent excitation for the same input power (19), but the total
useable power is less than that of a sealed X-ray tube because of
sample heating. In an X-ray tube most of the heat is carried away by
water cooling the anode, but in direct electron excitation the heat
must be dissipated by the sample; this limits the useable power of the
system and presents problems in the analysis of non-conducting
samples. The background interference from bremsstrahlung radiation
limits the lower limit of detection to about 100 ppm; because the
electron beam can be finely focused down to small areas on thin
samples, however, the lower absolute limit of detection is about
10-16g for direct electron excitation.
Proton sources offer several advantages over photon or
electron excitation, two of the most important being the very low
continuous background due to the increased projectile mass, and the
high ionization cross-section for low Z elements. These effects
contribute to a uniform lower limit of detection of about 1 ppm
throughout the periodic table, with a lower absolute detection limit
of about 10-14g using a focused 1 mm 2 beam. A1pha- and other charged
particles have also been investigated, but there is generally a large
Y-ray contribution to background and a severe target heating problem
due to the increased mass of the projectile. To alleviate these
problems, the beam intensity must be reduced, thus offsetting the
advantage of higher ionization cross section. At present, the
sensitivities are worse than for protons. Charged particle-induced
excitation will be discussed in more detail in Chapter 3.
20
Radioisotope sources can excite a sample by the direct
emission of a-, S- or Y-rays; they are small, inexpensive,
monoenergetic sources that are generally of low intensity. The total
photon yield is several powers of ten lower than that of a sealed x
ray tube, so this severely limits their sensitivity and applications.
Radioisotope excitation sources will be discussed in greater detail in
Chapter 2.
Wavelength Dispersive Spectrometry
In wavelength dispersive spectrometry the characteristic X-ray
lines of different elements are separated spatially using a crystal
spectrometer in much the same way as a diffraction grating separates
the lines in a visible spectrum. The essential features of a
wavelength dispersive crystal spectrometer are shown in Figure 1.4.
Generally, a sealed high-vacuum X-ray tube is used for wavelength
dispersive analysis. Primary X-rays excite characteristic X-ray
emission from elements in the sample by fluorescence; the secondary x
rays are emitted in all directions, but are limited by collimators to
a nearly parallel beam directed at the analyzing crystal. The crystal
diffracts each wavelength, A, at an angle, e, corresponding to Bragg's
law:
Wavelength Dispersive
Excitation Source
System
Spectrometer
DETECTOR TUBE POWER SUPPLY
T 2B
Figure 1.4. A typical wavelength dispersive X-ray analysis system.
21
nA 2 d sine (1. 10)
where n is the order of diffraction and d is the spacing between
planes of atoms in the crystal. When the crystal is rotated, the
angle e that it presents to the incident beam is varied and each
wavelength in turn satisfies the Bragg condition for diffraction.
This is an essential difference between optical emission
spectrography and wavelength dispersive X-ray spectrometry; in X-ray
spectrometry the spectral lines are diffracted individually in
sequence, while in optical spectrography the entire spectrum is
produced at once, either by a diffraction grating or a reflecting
prism.
22
As the crystal scans through a range e, the goniometer meter
drive rotates the detector at twice the rate of the crystal so that it
is at an angle 2e. In this way the incident sample beam and the
detector always present equal angles to the crystal so that the
detector is in the correct position to see any diffracted X-rays.
(The angle of incidence is equal to the angle of "reflection"). The
detector then converts each diffracted X-ray into a current pulse with
an amplitude proportional to the X-ray energy. In wavelength
dispersive spectrometry, detectors are usually either gas-proportional
counters or scintillation detectors. In a proportional counter a
current pulse results from ion pairs formed when the X-ray photon
interacts with a gas. The number of ion pairs formed is proportional
to the photon energy divided by the first ionization potential of the
23
gas. In this way different X-ray photon energies generate different
numbers of ion pairs which in turn cause current pulses of different
amplitudes. In a scintillation detector, photoelectrons generate a
number of visible light photons proportional to the X-ray energy. The
visible photons are detected by a photomultiplier tube, resulting 1n a
current pulse whose amplitude is again proportional to the X-ray
photon energy.
The current pulses generated by either detector are amplified
by a charge-sensitive preamplifier and an amplifier. A pulse height
selector sorts pulses according to amplitude and passes only those
having a preselected height. The discriminated pulses may then be
individually counted by a scaler-timer or integrated by a ratemeter.
The pulses are recorded as counts-per-second so that a strip chart
recording or multichannel scaler plot of count rate vs 28 will give
both the wavelength and intensity of elements in the sample.
Energy Dispersive Spectrometry
Although wavelength dispersive X-ray systems are capable of
very high energy resolution, their detection efficiency is quite poor.
Energy dispersive systems generally exhibit poorer energy resolution,
but they have inherently higher detection efficiencies and are capable
of simultaneous detection of a wide range of X-ray energies.
Essentially, energy dispersive spectrometery differs from wavelength
dispersive spectrometry only in the means used to separate spectral
lines emitted by a sample; in practice, however, this distinction
24
Energy Dispersive System
Cnorocl.ri.lic I A Elcitation
Anolyl. Source x.roy.~
;=: vvvv"I
I O,Ieclor 11 Po .. er Supply
H Oeteclor Pre-Amplirier
Pileup
I A ... mbly omplifier t---I t---I Rejlclor
Cooling Supporl Sy,ltm
Compul.r [ Multi-channel Dedicated ion
t--t Sloroge line) 01 off I,,,, Sy,lem
0010 Oulpul (plolter, prinler, '---1 Oisp;oy I lop., elc.)
Fip.ure 1.5. A typical energy dispersive X-ray analysis system.
25
leads to many differences in the instrumentation used in two methods.
The basic features of an energy dispersive X-ray system are
illustrated in Figure 1.5.
Most detectors used for X-ray analysis consist of a lithium
doped single crystal of silicon or germanium. The interaction of
ionizing radiation with the crystal creates an amount of free charge
proportional to the energy deposited by the incident photon. The
proportionality between charge and deposited energy is the key to
energy dispersive spectrometry; the high statistical precision of the
energy to charge conversion is the key to the high resolution of the
detector. To obtain the high resolution and low noise conditions
necessary for X-ray spectrometry, the detector and preamplifier stage
must be kept at liquid nitrogen temperature (77 0 K).
The free charge generated in the detector is swept out as a
charge pulse by an applied potential bias. The current pulse is
integrated by a charge sensitive preamplifier to produce a voltage
step proportional to the charge. The voltage pulse is then amplified
and shaped to maximize the signal-to-noise ratio. Pulse pileup
rejectors are used to eliminate overlapping peaks that give rise to
erroneous pulse amplitudes and a live time clock system is used to
correct for the rejection of the pulses. The shaped pulses are
processed in a multichannel pulse-height analyzer (PHA). Signals pass
into an analog to digital converter (ADe) and are stored in a
multichannel analyzer or small computer according to pulse height.
After calibrating the channel numbers according to energy, a resulting
26
spectrum is obtained of counts as a function of X-ray energy. This is
by far the most widely used method for X-ray analysis.
CHAPTER 2
RADIOISOTOPE INDUCED X-RAY FLUORESCENCE
Sealed radioisotope excitation sources have several appealing
characteristics compared to conventional X-ray tubes. They are small,
light, inexpensive, simple and highly reliable. They are inherently
low intensity sources; associated radiation problems impose an upper
limit of activity of about 100 mCi on radioisotope sources, whereas a
conventional X-ray tube can easily be operated at the equivalent of
10-1000 Ci (20). (The curie, Ci, is that mass of radioisotope in
which 3.7 x 1010 disintegrations occur each second.) Although the low
intensity is a disadvantage in many ways, it can be advantageous for
analyzing samples that may be damaged in the higher flux of an X-ray
tube. The low intensity of radioisotope sources can be compensated
for by longer counting times, since for all practical purposes the
excitation conditions will be absolutely stable during an analysis.
Occasional recalibration can compensate for any long-term decrease in
radioisotope intensity.
For X-ray applications, radioisotope sources can be
characterized by four basic properties: 1) the decay process and type
of emitted radiation (~-, S-, or y-emission or orbital electron
capture) which results in characteristic X-ray emission; 2) the energy
of the emitted radiation; 3) the half-life, or time required for half
of the atoms of the radioisotope to disintegrate; and 4) the activity
27
28
of the source. For X-ray spectrometric excitation, a suitable
radioisotope source should have a simple line spectrum at an
appropriate energy below 150 KeV (preferably a single intense y- or
X-ray line) with no accompanying high energy beta or gamma radiation,
a half-life of at least one year, and an activity high enough to yield
a source X-ray flux on the order of 107 photons/second this gives
rise to count rates of 10 3 to 105 cps for pure elements, assuming an
efficiency of 0.1 to 10% (18,21,22). Table 2.1 lists the properties
for several of the radioisotope sources commonly used for
radioisotope-induced X-ray fluorescence.
Four principal types of source-specimen-detector geometries
have been used for radioisotope-induced X-ray emission. These are
shown in Figure 2.1. The first type is the central or button source
in which the radioactive material lies in the middle of the detector
and excites the specimen directly; this geiometry is most efficient
for large detector windows and is most commonly used with proportional
counters or scintillation detectors (23). The second type is the
annular ring source; in this arrangement the source lies around the
detector and the radiation is directed upwards to excite the sample
directly; it is the preferred geometry for semiconductor detectors.
The last two types are commonly referred to as source-target or
secondary target sources, and may utilize either the central or
annular ring geometry as shown. In this arrangement the source
irradiates and excites a secondary target selected so that its
principal X-ray lin~ is at an energy just above the analyte absorption
Tab
le 2
.l.
Com
mon
ly
use
d r
ad
iois
oto
pe
sou
rces
fo
r X
-ray
fl
uo
resc
en
ce.
--(1
8).
Pri
ncip
al
Use
ful
rad
iati
on
T
yp
ical
H
igh
est
Rad
iois
oto
pe
rad
ioacti
ve
En
erg
y
sou
rce
at.
n
o.
sou
rce
dec
ay p
roce
ss
Half
-lif
e
Typ
e (K
ev)
acti
vit
y
ex
cit
ed
(C
i)
Tri
tiu
m-T
itan
ium
8
-l2
.3y
C
onti
nuum
3
-10
5
29
(Cu)
T
i K
-x-r
ays
4-5
Tri
tiu
m-Z
irco
niu
m
8-
12
.3y
C
onti
nuum
2
-12
5
30
(Zn)
Iro
n-5
5
DEC
2.7
y
Mn
K x
-ray
s 5
.9
0.0
2
24
(Cr)
Co
bal
t-5
7
DEC
270d
F
e K
x-r
ay
s 6
.4
0.5
98
(C
f)
y 14
Y
12
2 Y
13
6
Cad
miu
m-1
09
DEC
l.3
y
Ag
K x
-ray
s 22
0
.00
3
43
(Tc)
y
88
92
(U)
Iod
ine-
12
5
DEC
60d
Te
K x
-ray
s 27
54
(X
e)
y 35
Pro
met
hium
-147
8
-?.6
y
Con
tinu
um
12
-45
0
.5
60
(Nd)
A
lum
inum
Gad
oli
niu
m-1
53
DE
C 23
6d
Eu
K x
-ray
s 42
0
.01
88
(R
a)
y 97
Y
10
3
Lea
d-2
l0
8-22
y B
i L
x-r
ay
s 11
0
.01
62
(S
m)
y 47
Plu
ton
ium
-23
8
a. 8
9.6
y
U L
x-r
ay
s 1
5-1
7
0.0
3
82
(Pb)
Am
eric
ium
-241
a.
470y
N
p L
x-r
ay
s 1
1-2
2
0.0
1
69
(Tm
) y
26
N
59
.6
\0
Y
Geometric Arrangements of Radioisotope Sources
center-type arrangement
5 ,... .f. ';' 1 ! \! I , ~ •
::~~""" ''',' ,:',.:.; R !, ..,." F-4-' _\-
: I
J ~ ,of;. T
~ center-type
source - target arrangement
annular ring arrangement
5 =- ..
~"'--R-'1e,I.J,·' .;
T~' --:::~-' :
'" ' R , '" :~ T
I D \
~ annular ring
source- target arrangement
radioactive rodiation target X-rays sample X-rays
Figure 2.1. Geometric arrangements of radioisoto~e sources for X-ray fluorescence analysis. ~- (R) Radioisotope, (5) sample, (F) X-ray filter, (T) secondary target fluorescer,
30
(D) detector. The shaded area represents lead shielding.
31
edge. The X-rays from the secondary target then irradiate and excite
the specimen. Most commonly, y-emitting primary sources are used, and
high source activities are needed because of the inefficiency of the
secondary excitation process. Using this type of source-energy
selection, optimum excitation of specific elements can be achieved
since the secondary target arrangement gives extremely pure analyte
spectra, having high peak-to-background ratios, with detection limits
in the range of 50-200 ppm (24-29).
For most of our work, we do not find the secondary target
sources as useful because we are interested in a complete multi
element analysis. The broad energy range and added intensity of the
annular ring source offsets its higher background, making this the
preferred source for non-routine analysis; also, the high energy and
long half-life of the 24l-Am radioisotope make it especially suitable
for laboratory applications. We have used a 100 mCi 24l-Am annular
ring excitation source to obtain complete elemental profiles, from K
through Tm, in a single experiment. The instrumentation used and two
applications of this technique will be described later in this
chapter.
Instrumentation
All radioisotope-induced X-ray fluorescence analyses were
carried out using a 241-Am excitation source, an energy dispersive
lithium-drifted silicon, Si(Li), detector with associated
electronics, and a Nuclear Data 6620 X-ray analysis computer system
32
linked to the University's DEC-lO time-share computer. A schematic of
this system is shown in Figure 2.2.
The 24l-Am excitation sources used in these studies were
purchased from Isotope Products Laboratories (Burbank, California).
The sources measure only 2.50 in. overall diameter, with a central
hole diameter of 0.375 in. The sources are available in either the
direct or secondary target annular ring geometry.
The X-rays emitted from the sample were detected with either
an ORTEC vertical dipstick or KEVEX horizontal dipstick Si(Li)
detector. Both had beryllium windows to prevent back-scattered ions
and energetic electrons from reaching the detector. The detectors had
resolutions of 170-190 eV (measured at 5.898 KeV, the Mn Kline)
throughout the course of these experiments, and had a 100% efficiency
rating for X-rays from about 4-17 KeV. Charged pulses in the detector
resulting from X-ray ionization were swept out at a bias potential of
1000V. A cooled FET preamplifier together with a pulse pileup
rejector helped to minimize electronic noise and problems of
coincident X-rays. Detector count rates varied from a few counts per
second to several hundred, but were always held less than 1000 cps to
reduce pileup.
Analog signals from the detector electronics were fed to a 12-
bit ND 575 analog to digital converter (ADC). The digital signals
were sorted according to pulse amplitude and ultimately counted and
stored according to channel number. A calibration of energy versus
Figure 2.2.
Radioisotope X ray Fluorescence
System
I ( I I
sample~ 11,1 ~~ --------- . l~ \ Backing
2.11 ~m SOURCE It 'lD Pb Shield (annular ring)
r----L--- --,
Multi· Channel Analyzer
NUClear Data 6620
Computer
(LiqUid N: cooled)
RS·232 Control
Box
I I I I I I I
O;:C·10 Computer
Coumerl Timer
Display
The radioisotope X-ray fluorescence system used.
33
34
channel number was calculated by obtaining a spectrum containing six
peaks of known energy and determining the channel number of the
centroid of each peak. A least-squares fit of the energy-channel
pairs gave peak positions that were within 10eV of the expected values
in the calibrated region. In this way the energy spectrum of emitted
X-rays could be obtained so that the position of peaks in the spectrum
provided a qualitative identification of elements present in the
sample.
Spectra were acquired for a given preset live time in 1024
channels and were displayed while being collected on the CRT of the
ND 6620. The acquired spectra were stored on hard disk, as counts
versus energy, for future data reduction and peak identification and
could be transferred via phone link to the university's DEC-IO
computer for plotting.
Analysis of Photographs
One of our primary interests in radioisotope-induced XRF has
been for the qualitative analysis of various types of black and white
and color photographs that are a part of the collection in the Center
for Creative Photography (CCP) at the University of Arizona. No other
non-destructive analytical technique has been used to look at
elemental profiles of photographs. The only other techniques that
could be applicable to this type of analysis are the neutron
activation and autoradiography techniques that have been used for
whole-painting analysis (30-33) but these involve the use of a neutron
35
source and subsequent decay times and counting schemes. Radioisotope
induced XRF, on the other hand, is rapid and relatively simple, and it
leaves no residual radiation at the end of the analysis. We have
shown how elemental analysis by XRF can be used in the preservation,
conservation and identification of photographic materials.
Two major problems confronting museums housing large
collections of photographs are the identification of emulsion and
toners used in a print and the determination of a print's
authenticity. It is often impossible to visually determine whether an
emulsion contains platinum, palladium, silver or some other metal; it
is also difficult to determine whether a selenium or gold chloride
toner has been used to stabilize the emulsion. Since different
emulsions require different preservation procedures, it is critical to
the conservation of photographs that the exact chemical makeup of each
emulsion be determined. The same information needed for preservation
can also be used to determine authenticity and to further our
understanding of the medium and its practitioners. An elemental
profile may be used to characterize special emulsions, to identify
fakes and to date prints. We have shown that radioisotope-induced XRF
can be used for the routine elemental analysis of photographic papers
and emulsions (34).
Experimental
An initial feasibility study was conducted in the fall of
1979. The previously described instrumentation was used to analyze
several types of photographs from the CCP, including silver (Harry and
36
Weston) and platinum (Gilpin) black-and-white prints and Type C
(Shore), dye transfer (Harry) and carbro (Paul and Roche) color
prints, in addition to nitrocellulose and cellulose acetate negatives.
Representative spectra from 20 min runs are shown in Figures 2.3 and
2.4. The results of ~this study showed the capabilities of the method,
and in the summer of 1980 a more detailed study of the photographs was
carried out.
Initial experiments were conducted to determine the effect of
X-ray exposure on both black-and-white and color photographs, including
albumen, color and black-and-whate Polaroid, faded silver, Type C,
cibachrome and dye transfer prints. An extensive series of exposures
to direct X-rays from the 24l-Am source at various distances and for
periods of exposure ranging from several minutes to 24 hours showed no
noticeable effect on any of the prints studied. It was determined,
therefore, that a typical elemental analysis, taking from 1/2 to 3
hours, would have no visible or measurable effect on the photographic
materials studied.
When the non-destructiveness of this method was established,
the experimental set-up described previously was used to determine the
irradiation time needed to obtain useful spectra. A lucite holder was
designed to allow reproducible positioning of any desired portion of a
photograph over the source and detector at a set distance. The
spectra of various photographs were counted for 10 minutes, 30
minutes, 1 hour, 1 1/2 hours, 2 and 3 hours. It was determined that
the optimum irradiation time for black-and-white prints was about
6131
3 SH
ORE
C
-PR
INT
AM
SOU
RCE
Zn
200,,----------------------------~
4131
3 c
Ti
D
C
~ ~
100
~ ~
T
0 c
t Sr
U
A
m
N
T
S
2131
3~ I'~
II 0
25
30
10
15
20
D
IDIO
Y
(KE
Y)
Am
II
~~.-~
13
I I
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I I
I I
I I
II
I I
13
5 11
3 1
5
213
25
31
3 3
5
ENER
GY
(KEV
)
Fig
ure
2
.3.
Sam
ple
spec
tru
m o
bta
ined
fr
om
the
X-r
ay
flu
ore
scen
ce
an
aly
sis
of
a co
lor
Typ
e-C
p
rin
t.
W
-...J
1500
0 CE
LLU
LOSE
A
CETA
TE
NEG
ATI
VE
Ag
50
0
l 10
000
C
i ao
o-a
0 u U
• 1
N
I
T
S
50013~
10
0 0 5
10
1'5
20
/I
A
g IH
m (
I[Y
)
13 • '
.-.-
. .. ,
-,
. -.
-•
r-r
~. ~-'
)Hr-
r-r"
'-"'
-.,
13
5 31
3 3
5
ENER
GY
(KEV
)
Fig
ure
2
.4.
X-R
ay
em
issi
on
sp
ectr
um
of
a cell
ulo
se aceta
te fi
lm n
eg
ati
ve.
w
co
1 1/2 hours, while that for color prints was about 3 hours. With
longer counting times, the background problems became appreciable.
39
The next series of experiments was designed to qualitRtively
identify the elements present in specific prints selected by the CCP.
These were primarily early twentieth century prints made by Paul
Anderson, Imogen Cunningham, Laura Gilpin, Alfred Stieglitz and Edward
Weston, and contemporary prints by Tom Millea. The results of tests
conducted on colorphotographs from these collections proved
interesting and could lead to improved methods of identification and
preservation, but the data were insufficient to draw conclusions about
the colored emulsions examined. This discussion will therefore be
limited to only the black-and-white photographs analyzed, and four
photographs in particular that demonstrate the potential of this
method for the analysis of photographs.
The first photogaph is an anonymous nineteenth century albumen
print that was tested to verify whether a toner had been used on the
print to inhibit sulfide formation. The elemental spectrum of this
photograph (Figure 2.5) shows the presence of a small amount of gold
(Au) indicating that a gold chloride toner was used. The iron (Fe),
copper (Cu) and lead (Pb) peaks were found in almost all black-and
white photographs analyzed, and barium (Ba) was found in most
commercial papers, probably present as a barium sulfate filler.
The next three photographs demonstrate how XRF may be used to
identify the light sensitive element(s) used in a photographic
emulsion. The first of these is a 1979 print by Tom Millea entitled
Alb
um
en E
xhib
it I
12
50
~iTi--------------------------------------------------~
10
00
C
75
0
o U
N
T
S 5
00
25
0
fig
-....---
----
"-F
~
~ ~~-
o ~-
-;
;'"
o 10
2
0
ENER
GY
(KE
V)
30
4
0
Fig
ure
2
.5.
x--P
ay
em
issi
on
sp
ectr
um
o
f an
u
nid
en
tifi
ed
al
bu
men
p
rin
t sho~ving
the
pre
sen
ce o
f a
go
ld ch
lori
de
ton
er.
.p
o
41
Carmel Valley (CCP accession no. 79:205:001). This was recorded in
the curatorial files at the CCP as a platinum (Pt) print, but the test
results indicated a palladium (Pd) print as shown in Figure 2.6. In
contrast, the second photograph is verified as a 1932platinum print by
Laura Gilpin entitled Temple of Kukulean (CCP accession no.
77:023:022). The spectrum shown in Figure 2.7 shows strong platinum
lines and also weaker silver (Ag) lines, probably due to impurities of
silver in the emulsion. The absence of barium in both of these
photographs indicates that these were probably hand-coated papers, and
indeed both photographers are known to have coated their own paper.
The last photograph in this series is a 1917 Alfred Stieglitz print
entitled Portrait of Paul Strand (CCP accession No. 78:017:006),
identified on the back as a satista print. Standard literary sources
for photographic processes did not provide a definition of this
process; however, the spectrum shown in Figure 2.8 indicates a
combination platinum and silver print. Several months after this
analysis was performed, an advertisement was found in an early British
photographic journal that offered Satista paper for sale (35). This
paper was coated with a platinum or palladium emulsion and was
available commercially in the early 1900s.
In the last series of photographs analyzed we tried to
determine the feasibility of dating and authenticating vintage prints
by this method. Elemental profiles of original prints by Edward
Weston and Imogen Cunningham were compared with profiles of later
prints by Cole Weston and the Cunningham Trust, made from the same
Mill
ea
, To
m
79
:20
5:0
01
50
0-,-
--,-
--------------------
40
0
C
30
0
o U
N
T
S 2
00
10
0-
o o
10
P,I
20
ENER
GY
(KEV
)
Pa
llad
ium
30
4
0
Fig
ure
2
.6.
X-R
ay
emis
sio
n
spec
tru
m o
f a
pall
ad
ium
em
uls
ion
bla
ck
an
d w
hit
e p
rin
t by
To
m M
ille
a.
.l:
N
Gilp
in.
LalJ
r<1
77:0
23:0
;'?2
PI a
ti 11
11 rn
5e0-y--,---------------------------------------~
40
0
c 3
0e
o U
N
T
S 2
00
10
0
0-4~~~~~rT,-rT~~~_r~~~~rT~or~~~_r~~~
o 5
10
1
5
20
2
5
30
3
5
40
ENER
GY
(KEV
)
Fig
ure
2
.7.
';-R
ay
em
issi
on
sp
ectr
um
of
a p
lati
nu
m em
uls
ion
b
lack
an
d \v
hit
e p
rin
t b
y
Lau
ra G
ilp
in.
..,..
w
tmm
--
80
0-
C
60
0
o U
N
T
S 4
00
-
2131
3
~3Ii
pgli
lz.
/\1I
r!~d
if
3:0
17
:1I0
G
Sa
lisla
PI i
lll
13-1~-r'-~~-r'-~~-r'-~~~-r'-~-r-r~~-r-r'-~~~~
o 5
113
15
213
25
3
0
35
ENER
GY
CKEV
)
Fig
ure
2
.8.
Y-R
ay em
issi
on
sp
ectr
um
o
f a
"sati
sta
"
pri
nt
by
A
lfre
d S
tieg
litz
.
-J::'-
-J::'-
negatives, to see if an elemental analysis would unambiguously
distinguish between the prints. The results were largely
inconclusive, although it appears that either the barium-strontium
ratio or accurate quantitation of the silver content may help to
authenticate a print. More extensive studies, requiring the use of
carefully prepared standards and extensive analysis of authenticated
photographs by several artists, must be carried out before accurate
determinations can be made. The archives at the CCP should prove
invaluable for such a study.
Summary
45
The results of these studies demonstrate the capabilities of
XRF when emloyed for the analysis of photographs. The position of
each peak in the energy spectrum of the photograph provides a
qualitative identification of the elements present, and the area under
the peak provides a measure of the concentration of the element.
Since the conversion of peak areas to accurate concentrations is a
formidable problem and was not required in this initial study, no
quantitation was attempted.
The analyses of the first four photographs described
demonstrate the usefulness of qualitative analysis in cataloging and
preserving photographic materials. The light sensitive components of
the compounds used in an emulsion are readily discernible, as shown by
the platinum, palladium and silver peaks in the X-ray spectra
(Figures 2.6,2.7 and 2.8), and the presence of toners is readily
determined by the presence of gold or selenium peaks in the elemental
profile (Figure 2.5); the presence of barium provides a quick
indication of whether a paper was hand coated or obtained
commercially. This type of chemical information is critical to the
preservation or restoration of a print and may be of use in
identifying fakes or reproductions. The iron, copper and lead peaks
present were seen in all black-and-white photographs analyzed, as
indicated earlier, and arise, at least in part, from the mounting
tissue and matting as shown in Figures 2.9 and 2.10. No other peaks
were distinguishable above the background in the X-ray fluorescence
spectra obtained from these photographs; the presence of other
elemental peaks in future studies could help to identify a special
paper or emulsion.
46
It is apparent from these studies that the XRF technique is
ideally suited to the analysis of photographic material. It is a
rapid, multi-element technique that requires minimal sample
preparation and causes no damage to the sample. While a few XRF
systems exist for analyzing whole paintings (36), none had been
developed for the routine analysis of photographs. A simplified
system as described here can be used to solve many problems
encountered in the cataloging and preservation of photographs and
other printed paper media. In future studies this method will be
extended to the analysis of color photographs and to the quantitation
of elemental concentrations in both color and black-and-white
photographic prints.
C o U
N
T
S
t25
M
ATTI
NG
(BAC
KGRO
UND
SPEC
TRU
M)
s
t00
C
oCo
75
Fe Fe
50
25
V'\f'~~V'lr' rr
, o
,I
I i
I I
I I
I-rl
I i
I I
I I
I Iii Iii
o 5
10
1
5 EN
ERGY
(K
EV)
Fig
ure
2
.9.
X-R
ay
emis
sio
n s~ectrum
of
the
matt
ing
use
d
to b
ack
so
me
of
the
ph
oto
gra
ph
s an
aly
zed
.
~
.......
c a U
N
T
S
DRY
HaU
NT
TIS
SUE
600~1~1--------------------------------------------~
Sn
40
0
20
0 o
I ----
vy::""
P l
"rS
~.\.
i Y
I
I I
I I
I
o 1
0
20
ENER
GY
(KE
V)
30
4
0
Fig
ure
2
.10
. X
-Ray
e~
issi
on
spectr
um
o
f th
e
dry
m
ou
nt
tissu
e
use
d
to ~o
unt
ph
oto
gra
ph
s.
.p
ee
49
Analysis of Dysprosium X-Ray Contrast Media
New and beter X-ray contrast media are constantly being sought
for use in vasculature imaging. Heavy metal chelates have been
investigated, with emphasis on EDTA and related polyaminocarboxylic
acid ligands that have high metal-ion affinities and form extremely
stable water soluble complexes. In an on-going project in the
Toxicology Program at the University of Arizona, salts of a dysprosium
diethylenetriaminepentaacetic acid complex (Dy-DTPA) are being
investigated for use as contrast media.
Of critical concern in the investigation of potentially useful
contrast media is the rate at which the compound is eliminated from
the body. Previous excretion studies of DTPA complexes utilizing 99Tc ,.
l69yb , ll3 In , Cu and Fe have shown that greater than 90% of the
complex is excreted in the urine during the first day following
administration (30-32). Preliminary experiments were carried out to
determine the rate of elimination of the Dy-DTPA complex. Rats were
administered the complex intravenously and urine and feces were
separately collected in metabolism cages for analysis by 24lAm X-ray
fluorescence and PIXE (see Chapter 3).
Experimental
Initial qualitative studies of the rat feces were carried out
using the PIXE technique. This is inherently more sensitive than
radioisotope XRF and is discussed in detail in chapter 3. Standard
200 mg samples of the Dy-DTPA compound were weighed out and pressed
into pellets at 20,000 psi using a 13 mm KBr hydraulic press. The
50
pellets were then mounted on KaPton® (Dupont) covered aluminum
planchets for PIXE analysis. samples of the rat feces were prepared
similarly and loaded together with the standard for analysis.
The Dy-DTPA pellets were analyzed with a 200 nA proton beam at
both 1 and 2 MeV (using a 4 mil aluminum filter at the higher energy
to eliminate low energy X-rays) to obtain a set of standard spectra.
These are shown in Figure 2.11 and 2.12. The peaks between 5.5 and
8.5 keV arising from L-shell X-rays are cleaner peaks, with lower
background than the M-shell peaks between 1 and 3 KeV, and therefore,
these were the peaks of choice for the analysis. The rat feces were
then run under identical conditions at 2 MeV, but no appreciable
amount of dysprosium was detected in the samples analyzed (see figure
2.13). It was concluded, therefore, that very little dysprosium, if
any, is eliminated in the feces and that therefore it is probably
excreted in the urine. This follows from the earlier studies cited on
DTPA complex elimination. For further analyses we therefore decided
to use the 241Am source for the determinations since the urine samples
could be run as either solids or liquids and sample changes could be
effected more easily.
Since the metabolism cage is designed in such a way that food
particles and unknown amounts of drinking water may combine with the
urine, the urine samples collected were filtered and freeze-dried to
remove water; this allowed a determination to be made based on % dry
weight. A sample of the freeze-dried urine was prepared as above and
. . . .. 241A . h'l d an 1n1t1al analys1s uS1ng a m source W1t a S1 ver secon ary
20
00
0
15
00
0
c o U1
00
00
N
T
S
50
00
Dy
Dy
spro
sIu
m
DT
PA
A2
72
I H
eV
N
o F
It
M·
lin
es
Dy
l li
nes
o r
i 'r-=
---='-
"..,
I \
{ 'V
\
C>
iii
o 2
4 6
8
ENER
GY
(K
EV
)
Fig
ure
2
.11
. 1
MeV
PI
XE
sp
ectr
um
of
Dy-
DT
PA
X-r
ay co
ntr
ast
med
ia.
10
VI
i-'
c o U
N
T
S
3mH:l~J-
20
01
30
-
10
01
30
-
Dy U
Dy
l~I,2
Py,-
:pr·
c>~
i "m
01
'1 II
lI
?l'l
2 fI
..,V
Dy
l ~
1,3
Dy
l~2
4 m
Il fll
Dy
Ltl
I ---.J
o I
r I~Tr·l T
l-r-
...-
rrr-
T-Y
--'
'1
4 5
6 7
8
ENER
GY
(KEV
) F
igu
re
2.1
2.
2 M
eV
PIX
E
spec
tru
m o
f D
y-D
TPA
X
-ray
co
ntr
ast
med
ia.
Ie
VI
N
15ee
e-
Fe
leee
e~
II c
j 0 U
N
T
S
seee
-l
Fe
Mn
e-t
-i'~
4 6
Dy
spro
siu
m
Feces
Sam
ple
A
27
5
2 M
eV
4 m
il
fll
40
0J ~
30
0
Sr
" tJ ~ 2
00
-T
C
100
1 A
, Br
Zn
0
A
10
12 ~ (
kE
Y)
14
Cu
Ie
ENER
GY
(KEV
)
12
14
1
6
Fig
ure
2
.13
. 2
MeV
PI
XE
sp
ectr
um
of
rat
feces
to
det
erm
ine
the eli
min
ati
on
of
Dy
-co
nta
inin
g co
ntr
ast
med
ia.
16
V1
W
54
target showed that the dysprosium compound was indeed excreted in the
urine (Figure 2.14). Several attempts were made to reconstitute the
freeze-dried samples, the most successful being with dilute aqua
regia. In this way the samples could be run either as solids or
liquids by radioisotope XRF, and the results could be checked with
flame methods requiring a liquid sample.
A calibration curve was made of varying concentrations of
dysprosium in dilute aqua regia. Since the amount of Dy-DTPA compound
was limited, standards were made from dysprosium oxide, DY203. (The
compounds yielded identical dysprosium X-ray spectra.) Standards were
prepared from 1000 ppm to 10% Dy, and three 100 ul aliquots were
successively spotted and dried on 13 mm Nucleopore filter disks placed
on thick Kapton backings. Alternatively, the liquid samples could be
run directly with an appropriate thin-window sample cup, or solid
standards could be prepared using freeze-dried rat urine from
controls. We could not obtain control samples so no solid sample
calibration curves were prepared.
The samples were analyzed using the direct 60 KeV 241Am source
for maximum excitation. Duplicate samples of each concentration were
run for a preset live-time of 10.0 min and X-ray spectra were
obtained. The background corrected area under the peak at 6.47 KeV
was measured as indicated in the representative 4% dysprosium spectra
shown in Figure 2.15. The measured peak areas for the standards are
shown in Table 2.2 and a plot of peak area (at 6.47 KeV) versus
concentration of dysprosium is shown in Figure 2.16.
5013
13
4130
13
C
3131
313
o U
N
T
S 21
3131
3
1131
313-
; I>
v
I>v
RAT
UR
INE
CO
NC
fNTR
ATF
AM
SOU
RCE
AG
SECO
ND
ARY
I'
v I>
v
~
13 -t-
I -J
-.-r--r-l--·J~-T--r-'-·l
5 11
3 1
5
213
ENER
GY
(K
EV
)
Ag
line
s
r-T
-I 25
Fig
ure
2
.14
. Am
X
-ray
em
issi
on
sp
ectr
um
of
rat
uri
ne co
nta
inin
g
the D
y-D
TPA
co
ntr
ast
med
ia.
Ln
Ln
2513
130-
2131
3'~1
0~
L
r0""-~
N
T
SI
1301
313
5131
30-1
1\
Jill
r:>
I V
I
o 5
4"
DY
SPR
OSI
UM
ST
AN
DA
RD
AM
SOU
RCF.
ZS
AO
OT
-----------
I]
Dy
2-1
~ \'
c IIi
lIIIQ
0 U
N
T
S
I-
5011
3 " ,-
G.B
B
.B
20
ENER
GY
CKEV
)
La(
1.2
Pea
k a
rea
=
16
49
07
7.B
B
.B
9.B
1
9.B
£N£R
OY
(K
EY
)
'TT
II'I
3
0
35
4
0
Fig
ure
2
.15
. S
ampl
e X
-ray
em
issi
on
sp
ectr
um
of
dy
spro
siu
m
stan
dard
u
sed
to
esta
bli
sh
a
cali
bra
tio
n c
urv
e.
--
The
sh
aded
pea
k a
rea w
as
use
d
for
cali
bra
tio
n.
V1 '"
"'
57
Once the calibration curve was established, experimental
problems developed with this project in the Toxicology Program and no
samples were available for elimination rate studies. Progress has
temporarily been suspended until new compounds can be made and better
methods of sample collection can be developed. It appears, however,
that either radioisotope-induced XRF or PIXE can be used for the
routine monitoring of dysprosium elimination studies.
Summary
The results of these studies demonstrate the quantitative
capabilities of XRF for monitoring dysprosium elimination rates in x
ray contrast media studies. Our experiments resulted in a dysprosium
calibration curve as shown in Figure 2.16. As seen, non-linear
behavior is observed above about 4%, deviating significantly above 6%
Dy. This is due primarily to self absorption in the sample. The
calibration curve is linear in the range of 0.1 to 4% Dy, and the
lower limit can probably be pushed below 1000 ppm using longer
counting times -- although background problems were already becoming
significant. It may be possible to lower the detection limit using
the 241Am source with a copper secondary target, since this would give
a low background and the principal emission line of eu at 8.04 KeV
would be just greater than the 7.85 KeV absorption edge of dysprosium;
much longer counting times would be required, however. Of course, one
could achieve lower detection limits using PIXE also; if such low
limits are not required by the study, however,the simplicity of the
I..~
58
Table 2.2. Corrected peak areas for dysprosium standards in dilute aqua regia.
% Dy Peak area (cts)
Run 1 Run 2
0.1 5,,604 5,572
0.5 12,624 19,137
1.0 47,195 38,595
2.0 87,313 78,328
3.0 125,863 121,387
4.0 164,907 171.128
5.0 181,6113 181,422
6.0 210,203 196,041
7.0 218,735 214,692
8.0 245.133 238,128
9.0 248,915 248,726
10.0 255,125 249,642
r\
(f)
t- Z
::J
0 U
v 4: w
fr:
-1:
~
4:
W
a.. X
I05 3 21 11 0
-f
0
, Dy
spro
siu
m C
ali
bra
llo
n
Cu
rve
CD
Y20
3 In
d
il
aq
. reg
ia)
(i ~
X
:~
· · 0
· • · )(
CJ
~
/ /
;'
/ ~/
/ /
f{
2 4
6 8
10
PERC
ENT
DYSP
ROSI
UM
Fig
ure
2
.16
. D
ysp
rosi
um
cali
bra
tio
n
curv
e o
bta
ined
fr
om
the
Am
X-r
ay e
mis
sio
n sp
ectr
a o
f D
Y20
3 in
d
ilu
te
aqu
a re
gia
.
V1
1.0
radioisotope XRF method makes it preferable. The instrumentation
needed for radioisotope XRF could be set up in the animal lab itself
for routine studies.
60
One of the principal problems in this study was that of sample
preparation. We were given freeze-dried urine samples with no
explanation of the collection and handling processes and no control
blanks. The blanks could have been crucial in this case for
background corrections and peak stripping if we had found significant
iron levels in the urine, since iron at 6.3 KeV interferes with the
dysprosium line at 6.4 KeV. As we have so often found ~n our analyses,
it is critical for the analyst to be involved with, or at least
informed about, sample collection and preparation from the very outset
of the research work. Ideally, in future studies, it would be
preferable to collect a small amount of urine directly to avoid many
of the problems associated with freeze-drying; this should not involve
catheterizing the rats since only a few drops of sample are needed.
If this is not possible, then filtered freeze-dried samples should be
used directly, along with a solid sample calibration curve, to
eliminate the process of reconstituting the urine.
We have shown that radioisotope-induced XRF can be used
successfully to monitor dysprosium elimination in X-ray contrast media
studies. The method is rapid, simple and can be extended to compounds
incorporating metal ions other than dysprosium. A set-up such as the
one used in this study could easily be incorporated in a toxicology
lab for routinely monitoring either liquid or solid samples with
minimal sample preparation. In future studies alternative excitation
sources may be investigated to lower the detection limits, and
improved sample collection techniques should be investigated.
61
CHAPTER 3
PARTICLE INDUCED X-RAY EMISSION
Until the mid 1970s virtually all analytical applications of
X-ray spectrometry involved the use of either energetic electrons or
high-powered X-ray tubes for excitation. In recent years, however,
the use of high energy particle beams for excitation has grown
tremendously. The combination of particle excitation and detection of
resultant X-ray emission forms the basis of what is generally referred
to as particle-induced X-ray emission (PIXE) spectroscopy. As
indicated earlier, the majority of PIXE analyses are conducted using
1-3 MeV proton beams for excitation, and because of this one will
often see PIXE referred to in the literature as proton-induced X-ray
emission.
As early as 1912 it was recognized that X-rays were produced
by proton bombardment (40), but proton excitation was not applied to
actual chemical analysis until 1964 (41). In the same year, Birks
compared X-ray-, e1ectron- and proton-induced X-ray emission and
suggested that the high X-ray production cross sections for proton
bombardment might give increased analytical sensitivity over the other
methods (42). In 1970 Johansson showed experimentally that a
combination of proton excitation and semiconductor X-ray detection
could provide analytical sensitivities down to 10-11g (43). During
the next few years many papers were published that verified these
early results and discussed some possible applications of the
62
\
63
technique (44-55). In 1976, Johansson and Johansson published an
extensive review of PIXE which summarized the instrumentaion, accuracy
and precision and many applications of the technique (56). In recent
years there have been many advances in PIXE instrumentation and PIXE
has been applied to the analysis of a variety of sample types,
including aerosal particulates (57-60) biological and clinical
samples (61-67) and geological materials (68-70). Two international
conference~ have been held on PIXE and its analytical applications,
the proceedings of which have been published in Nuclear Instruments
and Methods (71,72). There are now about 100 laboratories engaged in
PIXE work of some kind, and the number of publications dealing with
PIXE is on the order of two hundred papers per year. While PIXE is
still in a phase of rapid growth, it is now established as a standard
method of elemental analysis.
Interest in PIXE stems primarily from an increased sensitivity
over other X-ray emission techniques. This increased sensitivity
arises both from a low continuous background inherent in ion
excitation, as discussed earlier, and from a high cross section for
inner shell ionization. Values for ionization cross sections have
been determined theoretically and experimentally in a number of
different ways; Kirchner reviewed some of these in 1981 (73). Figure
3.1 shows the K and L shell ionization cross sections for 1 and 2 MeV
protons as a function of atomic number. If compared to Figure 1.3,
one sees that the cross section for protons is much greater than for
X-rays, and that the proton ionization cross section is a smoothly
10--
4 I
I I
n 2
0
'w
60
8
0
lCl(3
ATO
MIC
N
UM
BER
Fig
ure
3
.1.
K a
nd
L sh
ell
io
niz
ati
on
cro
ss-s
ecti
on
s fo
r 1 ~eV
(so
lid
li
ne)
and
2
MeV
(d
ott
ed
li
ne)
pro
ton
s ..
0\
.J:-.
65
varying function of atomic number, or wavelength, whereas the X-ray
cross section exhibits characteristic dips and peaks corresponding to
the absorption edges. The ionization cross section is also a function
of the incident particle energy; there can be an increase in cross
section by a factor of almost 200 in going from 1 to 2 MeV protons. A
further increase is achieved by going above 2 MeV, but there is an
accompanying increase in background resulting from increased
bremsstrahlung and Compton scattering in the detector due to Y
radiation from proton-induced nuclear reactions. It is for these
reasons that the highest sensitivity is obtained using 1-3 MeV
protons.
The absolute sensitivity of PIXE depends on many factors,
including the beam conditions, irradiation time, experimental
configuration, type of detector, background conditions and sample
matrix. For a thin target, defined as a target through which the
proton beam will pass completely without losing more than 10% of its
energy. the sensitivity can be roughly estimated by the formula:
where:
N = A n cr prod (gJ 4rr ) rt
N = the number of counts in a peak
(3.1)
A the number of atoms of an element being irradiated
n = the number of protons hitting the target per cm2
crprod the cross section for X-ray production crwk
where cr "" ionization cross section
w = fluorescence yield
k = relative transition probability
n/4n = the solid angle subtended by the detector
E = the efficiency of the detector
t the transmission through the filter
For example, assuming that: (a) 100 counts are needed in the KUl,2
peak for a quantitative analysis of zinc; (b) a 1 mm2 proton beam at
200 nA is used for a 30 minute run at 2 MeV using a 4 mil Al filter
for attenuation; and (c) the solid angle of detection is .003 with
66
100% efficiency at this energy, the sensitivity can be determined as:
n = 200 x 10-9cou1/sec x 1 proton x ~1~0~0~rnm~2~2 x 1800 sec
Imm2 1.6x10-19cou1 1cm (3.2)
n = 2.25 x 1017 protons/cm2
(J prod 27.7 barns = 27.7 x 10-24cm2
n .003 x 4n
E = 100%
t 32.3%
A 100 atoms/cm2 (2.25x1017 )(27.7x10-24 )(.003)(1)(.323)
(3.3)
A 1.66 x 1010 atoms of zinc or
1.66 x 1010 atoms Zn x 65.37 g Zn 6.02 x 1023 atoms Zn
8 10-12 f· 2 1. x grams 0 Z1ne per em
67
For a typical thin target of 1 mg/cm2, this converts to approximately
2 ppb sensitivity.
This calculation demonstrates some of the considerations
involved in quantitating PIXE results. For thick targets the
calculations become more cumbersome, since the cross section for X-ray
production must be corrected for the change in projectile energy as it
slows down in the sample. Also one must consider the attenuation of
primary emitted X-rays and the enhancement of secondary X-rays
resulting from photoelectric excitation as the primary X-rays travel
through the sample. Ahlberg (74) and Van der Kemp (75) provide
excellent examples of the calculations involved in thick target PIXE.
These calculations do not prove very useful for PIXE quantitation,
even in thin samples, however, since the values of cross section and
fluorescence yield are good to only about 30%. The actual sensitivity
for a given element under a given set of conditions must be evaluated
from experiment.
Instrumentation
All PIXE analyses were carried out at the University of
Arizona Regional Accelerator facility using a 2 MeV Van de Graaff
accelerator, a Spectromagnetic analyzer magnet, a laboratory-built
vacuum sample chamber, a Kevex Si(Li) detector with associated
electronics and a Nuclear Data computer system as described
previously. A schematic of this system is shown in Figure 3.2.
One to 2 MeV proton beams at beam currents of 10 nA-l mA were
generally used for PIXE analyses, although ion beams of He+, N2+, O2+
Pa
rtic
le A
cce
lera
tor
(Van
de
Gra
aff
)
L _
__
__
__
__
--',
Po
rtic
le
Bea
m
Fi~ure
3.2
. T
he
PIX
E
syst
em
.
Pile
-up
R
eje
cto
r
Pre
am
pl i
f ie
r
Sam
ple
Cha
mbe
r M
oooe
nerQ
et ic
B
eam
Am
plifi
er
--,
Co
mp
ute
r '-
-_
--II sy
stem
Mu
lt ic
ha
nn
el
An
aly
zer
Ma
gn
etic
Top
e st
orag
e
Q'\
(X
l
69
and Kr+ were also available. Ion beams were generated with a High
Voltage Engineering Model AN 2000 Van de Graaff accelerator, depicted
schematically in Figure 3.3. High positive potentials are built up at
one end of the accelerator by means of a rotating belt. Positive
ions, generated by a radiofrequency field, are repelled by the
positive potential and are accelerated down an evacuated column. The
beam is focussed electrostatically and the ion of interest is
magnetically separated according to its mass/charge ratio and
deflected using a Spectromagnetic analyzer magnet. The beam is then
stopped down to a diameter of .025 inches before it enters the sample
chamber. The entire system is maintained at approximately 10-6 torr
by means of two Sargent Welch turbmolecular pumps.
The PIXE sample compartment was a laboratory-built modular
chamber consisting of a 21 cm x 14 cm x 13 cm aluminum box with a
quartz view port and a gate valve to isolate the chamber from the beam
port. A schematic of the chamber set up for a Si(Li) detector and
solid sample cover plate is shown in Figure 3.4. Other cover plates
could be interchanged for gas targets or electron excitation of solid
samples, and a crystal spectrometer could be substituted for the
detector for high resolution work. The solid sample cover plate, used
most often, incorporated a rotating sample wheel that could hold up to
20 samples mounted at a 450 angle to the incident proton beam. The
wheel was geared to allow rotation or translation in the direction of
the beam from outside the evacuated chamber. The sample wheel itself
was made of aluminum and was electrically isolated from the cover
(;o
ntl
U1
Jo
us
U
t=lt
+
FO
l:u
s.ln
g E
leclr
od
e
-6
10
to
rr
+
u+
+
pre~wur i
~t:.L1
+ 2
H
eV
3UO
p
si
Fig
ure
3
.3.
Van
d
e G
raaff
accele
rato
r.
IllC
lI!i
hc
!io
I
gIU
UI1
L1
(Iu
t L
"nl
toll
112
~
1&
8 ;;; .,
-...I o
Mag
net
-1a
-6to
rr
Car
bo
n
Foil aper
ture
Fig
ure
3
.4.
The
sa
mpl
e ch
ambe
r u
sed
in
PIX
E.
Liq
uid
NZ c
oole
d
Fil
ler
chon
qer
Be wil'lOON~
II" n
oy
:C'=
=i ...
. 0"
."
/ /
I K
apla
n fo
il I
'l :
" !
l"l' "
'n'
I I
/
Sam
ple
whe
el
Sam
ple
fara
day
cup
curr
ent
chm
ger
-30
0 V
'-l
I-'
72
plate and the rest of the sample chamber so that it could be used as a
Faraday cup for monitoring the beam current. A suppressor ring was
located in front of the sample wheel and kept at a potential of -300
volts. Rapton film was used to shield metal parts of the sample wheel
from the proton beam to prevent extraneous X-ray emission.
A 15-25 pg/cm2 carbon foil was placed at the end of the beam
aperture, in front of the sample wheel, to reduce the buildup of
positive charge on non-conducting samples. High positive charges
cause energetic photoelectrons to be accelerated back toward the
sample target, resulting in a bremsstrahlung continuum that can
obscure features of the emission spectrum at high energies. Low
energy carbon electrons knocked out of the foil help to neutralize the
positive charge buildup. The effectiveness of the carbon foil in
reducing background bremsstrahlung is dramatically shown in the two
spectra of polyethylene shown in Figure 3.5 (76). Each spectrum was
obtained during a 5 minute irradiation at 1 MeV, one with the carbon
foil in the beam path and the other without. The increase in
sensitivity and resolution is evident.
A wheel containing filters of various types and thicknesses of
materials was located between the sample wheel and the detector to
allow selective absorption of unwanted X-rays. Generally no filter
was used for the low energy (1 MeV) analyses, and a .004 inch Al
filter ws used for the higher energy (2 MeV) analyses. This enabled
us to reduce the low energy background and high count rates from
characteristic X-rays from low Z major element components (Z < 26).
Fig
ure
3
.5.
c 0 u N
T
S
20
00
-
15
e0
10
0e
50
0-
50 ~' I ! I I i I 1 : 1
CI
I •
I ..
I "
I "
I "
: :
~ w
ith
cor
bon
:
: :
foil
I I
I ,
• I
I •
I ,
I I
I I
I
t,
5: ~
I
' ...
, I
\,/ ~
.: !
' .. ,
.' "
" ~
Co.
k
A ..
c M
o U
M
T
. .. M
n
'-.M
"
.e-l
,0~V~
'·'Vft,w
-\. c.
r.,
C.
....
....
..-
T
-r-I~
wit
hout
car
bo
n
foil
4 Ii
•
7 •
_<U
Y,
\ I
I ~",
(,
Ab
, f.
Mn
o -r--f-:
"-r
I "
I o
2 4
6 8
ENER
GY
CKEV
)
PIX
E sp
ectr
a o
f p
oly
eth
yle
ne
\l1
ith
an
d \
l1it
ho
ut
a carb
on
fo
il
in
the
bea
m p
ath
.
-...J
W
74
The characteristic X-rays emitted by elements in the sample
were detected with a Kevex Model 2010 Si(Li) detector. The detector
and its associated electronics were discussed earlier, and more
complete details on Si(Li) detectors are given by Woldseth (77),
Gedcke (78) and Russ(79). The measured resolution of the detector
used was 185 eV at 5.898 KeV, and the efficiency was given by Kevex as
100% for the Ka X-rays of atomic numbers 21 (Sc) through 42 (Mo).
The remainder of the electronics and the computer system are identical
to those discussed in Chapter 2; detector pulses were fed into a
Nuclear Data 6620 X-ray analyzer system, and an energy vs. channel
number calibration curve was obtained for each beam energy to allow
output in terms of X-ray energy. The low energy (0-8 Kev) calibration
was set at 8.4607 eV/ch. and the high energy (5-35 KeV) calibration
was set at 33.662 eV/ch. The ND hardware and software involved with
data reduction will be discussed in subsequent sections.
Qualitative and Semiquantitative Applications of PIXE
The PIXE facility at the University of Arizona was first
developed for multielement qualitative analysis. At the time, this
was the only multielement technique available to the department of
chemistry. The technique was especially appealing because of the
minimal sample preparation needed for a complete elemental
determination. Qualitative analyses could be run simply by
irradiating a sample with protons at one or more energies and
analyzing the resultant X-ray emission spectra.
75
The ND program PEAK could be used to search out and identify
peaks according to their energy, but more often the programs IDENT and
KLM were used to manually establish X-ray peak identifications for
qualitative determinations. IDENT listed all possible X-ray line
identifications for a given peak position selected by a system marker
on the display terminal, and KLM listed all X-ray line energies for a
given element and displayed markers at the appropriate energy
positions on the collected spectrum. Both programs used an X-ray
library that had been previously established by running ultra-pure
standards under various beam conditions. The programs could be run
during the actual data collection so that the down time between
samples for data analysis was minimal. The combination of PIXE and
the Nuclear Data X-ray analysis system allowed rapid, non-destructive
qualitative analysis of a wide variety of sample types.
Analysis of Backing Materials
For qualitative PIXE analysis, samples were generally mounted
in their original form on "thick" backing materials. Thick in this
case means that the proton beam loses a significant fraction of its
energy in passing through, or is stopped completely within, the
backing material. A number of backing materials were investigated and
compared in terms of impurities, performance in the beam and
hydrophobicity. The results of the investigation are summarized in
Table 3.1. Of the materials tested, Kapton (Dupont) was
unquestionably the best both with respect to impurities and beam
stability. It showed minimal deterioration even when subjected to
76
Table 3.1. Properties of backing materials used in PIXE.
Performance ImEurities in Beam Hydrouhogicity
Mylar P*,Ca*,Fe,Cu Good Hydrophobic
Millipore Cl,Ca,Fe,Cu,Br* Poor Hydrophilic
Nucleopore S,Cl,K,Cr,Fe,Ni,Cu Fair Hydrophilic
Kapton Cl,Ca,Cr,Fe Excellent Hydrophobic
* Indicates greater than trace amounts.
77
several microamps of beam current at 2 MeV. The PIXE spectra of thin
Kapton foil showed only traces of Cl and Ca at 1 MeV (see figure 3.6)
and no impurities at 2 MeV. A thicker Kapton foil obtained from
Dupont showed traces of cr and Fe at the higher energy.
Kapton was the backing of choice for all solid samples. Many
samples were run with no preparation other than mounting the target on
a 1/2" x 3/4" Kapton backing using a quick drying epoxy (Hardman).
When it was desired that the irradiated area be representative of an
entire sample, and the destruction or alteration of the sample was not
important, the sample was dried, pulverized in a freezer mill (Spex)
at liquid nitrogen temperature, and homogenized using a Wig-L-Bug
amalgamator to insure complete mixing. A known amount of the powdered
sample (100-200 mg) was then weighed into a 13 mm die and pressed into
a pellet at 20,000 lbs using an 11 ton hydraulic press (Carver). The
pellets, which were 2-3 mm thick, were then epoxied onto Kapton
backings as above. PIXE spectra of geological samples prepared in
this manner were reproducible to better than 1%.
Kapton could not be used for liquid samples because the
extreme hydrophobicity of the material prevented a homogeneous
deposition of liquid residues upon evaporation. As solutions dried
on the Kapton, differential crystallization occurred which gave rise
to particles of significant thickness relative to the penetration
depth of the proton beam. To enhance homogeneous deposition, we tried
modifying the Kapton surface by acid etching, physical abrasion
and coating the surface with a thin layer of evaporated carbon, but
1000
] TH
IN
KAP
TON
FILM
1 M
eV
No
F
II
80
00
11
"" .. ....
~ 600B
l I \
c ,.
. ..
0 U
H
T
N
I;
call
T
S 4
00
0 1
1 \
.lr~'lllilllll
llll
llll
,
4 5
e 7
•
DII
!RII
T
OlD
>
20
00
o ~
o 2
4 6
8 10
ENER
GY
<KEV
)
Fig
ure
3
.6.
1 H
eV
PIX
E
snectr
um
of
Kan
ton
fi
lm
shm
.lin
g im
pu
riti
es.
'-l
co
localized concentrations of residue still occurred, giving
irreproducible results with errors of up to 50%.
79
Although Nucleopore and Millipore filters contained
considerably higher levels of contaminants and were more prone to beam
damage than Kapton, the filters were completely wetted by liquid
samples, resulting in a fairly uniform layer of residue upon drying.
A combination of Kapton and Nucleopore proved acceptable for analyzing
most liquid samples. The combination used consisted of a 1/4"
diameter circle of Nucleopore filter sandwiched between two layers of
Kapton (1/2" x 3/4") with a slightly less then 1/4" diameter hole cut
out of the top layer. The low energy spectrum of this backing is
shown in Figure 4.7. In most cases the levels of contamination in the
Nucleopore were considerably lower than the corresponding sample
concentrations. Known volumes of solution were pipet ted directly onto
the circular filters and dried under an infrared lamp. When dried,
the sandwiched sample was mounted on the sample wheel using double
stick tape and were positioned so that the proton beam would cover the
circular filters upon irradiation. Once in the sample chamber, the
final positioning of the sample was aided by the visible fluorescence
of Nucleopore in the beam. PIXE spectra of samples prepared in this
manner were reproducible within about 5%.
Analysis of Bullet Lead
A criminal investigator can often link an evidence bullet to a
suspect by comparing striations on the evidence bullet to those on
test bullets fired from the suspect's gun. Bullets cannot be directly
c o
:-~0000
15
00
0
U 1
00
00
N
T
S
50
00
o o
NUCL
EOpO
I<E
FIL
TFk
I
H ...
V
No
F
II
~oo
""""
c ~
0 U
N
T
II
20
0
IWl-
, a-
A
r.
1/1
B-t
-r'-
'-'-
r-'-
"' "
1"1
j~I"
1 .,"-,~~~,F'j
3 4
51
1
7 d
I!:l
lfk
gy
(I<
I!V
)
"I
10
---I
--,-
4 2
6 8
ENER
GY
(KE
V)
Fig
ure
3
.7.
1 M
eV
PIX
E
spec
tru
m o
f N
ucl
eop
ore
fil
ter
on K
apto
n b
ack
ing
.
CXl
o
81
compared, however, when the evidence bullet is badly mutilated or when
the suspect's gun is not available for examination. An alternative
method of comparison uses a "fingerprint" of the trace elements found
in the lead of each bullet. Several techniques have been used to
detect the trace elements present, including atomic absorption
(80,81), neutron activation analysis (82-84), spark source mass
spectrometry (85,86) and lead isotope analysis (87,88), but to date
each technique has suffered either from a lack of sensitivity or
because it alters the sample and is therefore not desirable for legal
applications. The PIXE technique is essentially non-destructive and
has the capability of simultaneously determining all elements with an
atomic number greater than eleven that are present at the parts-per
million level. We have applied the PIXE technique to bullet analysis
for qualitatively identifying the trace elements present. In future
studies, quantitation of the trace elements may lead to the
identification of the caliber, manufacturer and possibly even batch
number of a suspect bullet.
Boxes of .22 and .38 caliber bullets from several
manufacturers were supplied by the state crime lab (Phoenix) for
analysis. Bullets were randomly selected from each box, and samples
were prepared by slicing off thin (0.1-0.5 mm) cross sections from the
nose, middle and tail portions of the bullet using a Teflon coated
utility knife blade. A plexiglass holder was made for holding and
cutting the bullets to ensure per~endicular uniform cuts so that the
sample geometry would be constant. The cross sections \lere then
mounted on a Kapton backing and positioned on the sample wheel for
analysis.
82
The bullet samples were run at I, 1.5 and 2 MeV at beam
currents of approximately 0.1 pA. The most useful single beam energy
for comparison was 1.5 MeV. The total counting time was normalized on
the lead La line to 50,000 counts peak height as seen in Figure 3.8.
When the collected spectra are expanded, several trace elements are
seen. Figure 3.9 and 3.10 show two spectra of middle sections of .22
caliber Winchester and Federal bullets run at 1.5 MeV. In general,
the same elements are present but in widely varying concnetrations.
In addition to the eleven elements shown (Sn, Sb, Cr, Fe, Co,Ni,Cu,
Zn, Au and Bi), four additional elements could be detected at 1 MeV
(Na, Cl, K,Ca) and two at 2 MeV (Ag, Te), giving a total of 18
elements detected in the bullet samples analyzed; this can be compared
to 4 elements detected by atomic absorption, 6 by neutron activation
and 26 by spark source mass spectrometry. Several of the elements
detected by SSMS should be present at levels determinable by PIXE,
including Si, P, S, As, Ti and Se, but the huge lead lines overlap and
interfere with the smaller trace element lines at those energies. If
the lead lines could be eliminated, then conceivably more trace
elements could be detected by PIXE. One possibility is to take
advantage of the selective absorption close to the K-edge of filter
materials. For example, the use of a Ga filter may improve
sensitivity since the La l ,2 X-rays of Pb (10.52 KeV) are absorbed more
C
0 U
N
T
S
XI0
5 CC
2 BU
LLET
SA
MPL
E I
. 5-.
,---
~---
----
----
-__
____
___
. 1.
5 M
eV
----
----
----
----
-Pb
,Mn
1.0
l II
0.5
Sn,
Sb
Pb
Ll
Pb
Lo(
Pb
Lp
Pb
L.-
o . 0
-I
i I
I "P
1 I
f'-f
't (~
I "r
o 5
10
IS
2
0
ENER
GY
(KEV
)
25
30
35
Fig
ure
3
.8.
1. 5
H
eV
PIX
E
spectr
um
o
f b
ull
et
lead
sh
mv
ing
th
e
Pb
Let
peak
u
sed
to
n
orm
ali
ze
co
un
tin
g
tim
es.
:::
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F
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ET
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PLE
1.5
MeV
3eee-r
T:I ;,-
-------
--.. ..-
.. -----~.---.------------
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b P
h l{
loC
o l~
Ph u'
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III ~ I
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, \ S
n
T
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leee~
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Fe
IIA
u\
IBI \
II BI
e -I
I (
I "'
---T
'-o
5 1
0
15
ENER
GY
(KEV
)
Fig
ure
3
.9.
1.S
MeV
PI
XE
sn
ectr
um
of
a F
ed
era
l b
ull
et
frag
men
t.
213
00
.p
.
c o U
N
T
S
3131
313
2131
313
Ieee
13
- - - - - - - - - - - - - - 13
\J BU
LLET
SA
MPL
E
Pb
M
el.
Pb
M~ Sn
~Sb Sb
Fe
Co
C
u
Cr
Fe N
j
I I
I I
I I
5
1.5
MeV
Pb
L'"
Pb
Lt
Bj ~
Zn
V\:
V ~
I I
I I
I
Ie
ENER
GY
(KEV
)
Pb
LO Bj
I
Pb
L¥
Bj ~ L.
I
I I
I I
I
15
Fig
ure
3
.10
. 1
.5 M
eV
PIX
E
spec
tru
m o
f a
Win
ches
ter
bu
llet
frag
men
t. 2
0
0:>
Ln
86
effectively than either higher or lower energy X-rays due to the Ga
absorption edge at lO.37KeV. The systematic use of filter materials
between the sample and detector could decrease the size of many of the
lead lines, although this may involve several irradiations with
different filters and beam conditions.
It has been shown that the PIXE technique can be used to non
destructively identify many elements present at trace levels in bullet
lead. This is as far as this project was taken. The next step should
involve the determination of relative ratios of the trace elements to
accurately compare bullet samples. As described earlier, quantitation
of thick targets is an extremely difficult problem~ and this is
further complicated by the high concentration of lead in bullets. In
initial quantitation studies it may be desirable to begin with
dissolved bullet samples from which thin targets can be made. This
nullifies one of the main advantages of the PIXE technique for
forensic work, its non-destructiveness, but it may be useful to try
various methods to eliminate the lead (i.e., precipitation or
extraction), leaving the trace elementes for quantitation. For thick
target work, NBS standard lead (SRM 1132) or Kirk lead standards
(Morris P. Kirk & Sons) may be helpful for generating efficiency
curves, although in each only a few elements are of certified
concentration. The scope of this prnject is tremendous. In order to
determine the caliber, manufacturer and batch number of a bullet, one
shou ld be able to estimate the probability of a "match" occurring,and
such a study would have to be based upon a statistically random
sampling of the bullet population. Such studies have been done on
window glass and paints previously (89,90) and involve accurate
quantitation of many elements in a large number of samples. The FIXE
method of bullet analysis should be optimized and automated before
such a project is undertaken.
Analysis of Bat Tissue
87
As an example of the semiquantitative analyses that can be
quickly performed on biological samples, an early FIXE project will be
described that concerns the determination of copper/zinc ratios in
bat reproductive organs. This was a joint project with the Department
of Anatomy at the University Health Sciences Center. The project was
unique from the standpoint of sample size, since the bats were kept
alive for continuous study and only minute sections of the bat gonads
were dissected from the living organs. Ten different milligram-sized
samples were obtained for analysis by FIXE.
Since only rough copper/zinc ratios, and not absolute
concentrations, were required, the samples were run in their original
form as thick targets. The samples were already desiccated when
delivered and targets were prepared by sandwiching the dried tissue
sample between two layers of Kapton (1/2" x 3/4") in which a small
hole (approx. .5 mm diameter) had been cut in the top layer using a
stainless steel syringe needle. The Kapton sandwiches were then
mounted on the sample wheel using doublestick tape.
88
A 0.1 pA proton beam at 2 MeV was used with a 1 mil Al filter
for the analyes. A typical spectrum obtained is shown in Figure 3.11.
A ND peak search program was used which fitted gaussian peaks and gave
background corrected peak areas. The fraction of counts due to the
nickel K6 peak and the fraction of counts due to the copper Ka peak
were subtracted from the copper Kal 2 and zinc Kal 2 peaks, , , , respectively, to give peak areas at 8.04 KeV and 8.63 KeV
corresponding to the amounts of copper and zinc present. Since the
relative X-ray yield was nearly the same for pure Cu and Zn at 2 MeV,
no further attempt was made to correct for X-ray efficiency. The Ka l ,2
peak areas for Cu and Zn and the Cu/Zn ratios obtained for the ten
samples are summarized in Table 3.2.
The main problem encountered with these samples was in
mounting the samples, since they were so small. Normal mounting
procedures could not be used because the tape or epoxy would
inevitably be exposed to the proton beam. The method of sandwiching
the sample worked well and virtually eliminated contamination due to
sample preparation. These were the first of many biological samples
that were analyzed by PIXE. In general, this type of sample lends
itself well to dissolution and thin target quantitation, but for quick
elemental ratios this thick target procedure works well.
Analysis of Metal Films
Another early PIXE project involved the analysis of evaporated
and sputtered metal films. The first set of samples were run for the
Department of Physics at the University of Arizona. This involved a
BAT
TISS
UE
(4B
) M
V.2
68
C
4000
,,
-,-
-------------
K
K
30
00
~ ~
Zn
F.
I I
C
0 ~ 20
00
T
S
~ \11
11 \
Cu
1000
0,1
;:
:::
o 2
4 6
8 10
ENER
GY
(KEV
)
Fig
ure
3
.11
. PI
XE
sp
ectr
um
of
bat
tissu
e
to
dete
rmin
e
Cu/
Zn
rati
os.
12
00
1.
0
90
Table 3.2. PIXE analysis of bat tissue to determine Cu/Zn ratios.
Peak Area (cts. ) Cu/Zn
Cu Zn Ratio
MV. 265 (lA) 1078 7425 0.15
MV. 266 (2A) 442 2915 0.15
MV. 267A (3A) 2802 19R7 4.93
MV. 267B (4A) 1156 2085 0.55
MV. 267C (SA) 306 657 0.47
MV. 267D (lB) 9992.5 4774 2.09
MV. 268A (2B) 19886 trace trace Zn
MV. 268B (3B) 2847 2151 1. 32
MV. 268C (4B) 6085 19131 0.36 !'
MV. 268D (5B) 663 923 0.72
qualitative analysis of evaporated metal films on glass microscope
slides to determine metal impurities. Three films were analyzed,
including Ni, Cr and Mo. Each sample was mounted directly on the
sample wheel and analyzed at I MeV with no filter and at 2 MeV with a
4 mil Al filter for a preset live time of 1000 sec. Representative
spectra of the Mo sample are included as Figures 3.12 and 3.13.
91
The impurities in the metal films, after subtracting out the
X-ray peaks due to the glass slide, are summarized in Table 3.3. The
high tungsten peaks shown in the Mo spectrum in Figure 3.13 are due to
the tungsten boat used. All other lines are probably due to
impurities in the metal used. Nothing else can be inferred about
these samples. No information was given as to the method of
preparation, sample thickness, etc. This points out a major problem
in carrying out a PIXE analysis, the analyst must be given as much
information about the sample as possible, or should be involved with
sample preparation from the beginning. This point cannot be
overemphasized. We encountered this problem many times when running
samples for outsiders, and as with this analysis, we could tell what
elements were present, but we could deduce nothing else about the
sample, often to the extreme displeasure of the sample owner.
Although quantitation of the trace metals in these samples was
desirable, we could not obtain good numbers at the time they were run.
As with the bullets, quantitation of trace impurities in a metal
matrix is a very difficult problem. With the techniques to be
XI0
5
1.0
13
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C
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ENER
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(K[V
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8
Fig
ure
3
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. 1
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PI
XE
sp
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n g
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ure
3
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.
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PIX
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spec
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m o
f a
thin
mol
ybde
num
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lass
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bst
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, ,rl.
Table 3.3. Trace metal impurities in evaporated metal films.
*
Metal film
Nj
Cr
Mo
Impurities
Cr*, Fe*, Y. Zr, Mc*
Ti, Fe*, Ni, Rb, Zr*
Cr*, Fe*, Ni, W*
indicates greater than trace amount.
94
described in the next sesction we could now obtain reasonable numbers
in this type of sample prepared as a thin target.
95
A similar type of sample was analyzed for the Catalyst
Research Corporation (Baltimore, MD). This consisted of two glass
slides with a sputtered film containing indium as the major component
and antimony and nickel as minor components. One slide had been
sputtered in an Ar atmosphere and the other in 20% O2, Each slide was
analyzed directly using a 0.1 pA proton beam at 2 MeV with a 4mil Al
filter. The spectra obtained are included as Figures 3.14 and 3.15.
An efficiency curve was made using ultrapure In, Ni and Sb standards
(Spex) and this was used to obtain elemental ratios from the Kal,2
peak areas of Ni, Sb and In. From the elemental ratios, weight ratios
with respect to In were obtained and from these weight percents were
determined. The results are summarized in Table 3.4. These results
are semiquantitative at best, since a thin target efficiency curve was
used for a thick target, and no corrections were made for X-ray
enhancement or absorption due to the film thickness, which was
unknown. For simply comparing the amounts of minor components under
various sputtering atmospheres, however, this procedure is adequate.
Summary
From these examples of qualitative and semiquantitative
applications of FIXE, it is evident that the FIXE technique is ideally
suited for this type of analysis. Virtually any type of sample can be
analyzed, usually in its original form, with little or no sample
preparation necessary. From two or more analyses using different
50
00
40
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C
30
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N
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20
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13
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TT
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IIJ
AR
GO
N
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In
In
40
0
<II 'E 5
---
l I
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o Ii" ,--
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20
22
24
2
6
28
3
0
En
erg
y
~.u
HI
In In
S
It
jLJlil
JJY~
~ 1
-."
r--r
-r--
r-l
"-r
rT
-l
"-T
-r-,
-r-r
rr-
r-r-
,-.-
r-r-
l 1
0
15
2
0
25
31
3
ENER
GY
(K
EV
)
Fig
ure
3
.14
. 2
MeV
PI
XE
sp
ectr
um
of
ind
ium
fi
lm sn
utt
ere
d in
arg
on
.
',,0
0'
1
c o
40
00
30
00
-
U
20
00
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S
1130
0
111D
.11.11
1 F
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.
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1-"
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'--"
.-'
20
22
24
26
28
30
En
erg
y
In
", .
. j..
HI
HI
/I
5r.
Zr
,,-
Sr
Zr
1 __
--_J\~--.I~~~ ~
-0
----T
"'-
,-T
--r
-1 -
-r-
r-T
1 n
T--T
-.-
-r-I-T
I--'-
1 I
I ,--
.=...-,
S
I 0
I 5
213
25
31
3
Etl
ER
GY
C
KE
V)
Fig
ure
3
.15
. 2
HeV
P
IXE
sn
ectr
um
o
f in
diu
m
film
sn
utt
ere
d
in
ox
yg
en
.
-0
......
98
Table 3.4. PIXE analysis of indium sputtered films.
Element Elemental ratio Wt. Percent
Sputtered in °2 Ni 0.026 4.805
In 1.000 94.80
Sputtered in Ar
Ni 0.084 14.135
In 1.000 85,923
Sb 0.020 0.016
beam conditions, a complete determination of all elements from Na
through U can be made in a rapid nondestructive manner. The technique
is therefore ideally suited for the qualitative analysis of forensic
and toxicological samples, or virtually any solid, gaseous or liquid
residue sample.
99
For many samples, a quantitative determination of the elements
present was desired, and it was therefore obvious that our PIXE
facility had to be developed for routine quantitative analysis. As
have most other facilities, we first approached this problem by
developing methods for quantitating thin PIXE targets. This will be
described in the next section.
Quantitative Applications of PIXE
Quantitation by the PIXE technique must be based on a seri~s
of experimentally determined curves relating relative X-ray yield and
atoEic number, since the literature values for ionization cross
section and fluorescence yield are not accurate enough to permit
absolute quantitative analysis. Such a series of curves have been
developed at our PIXE facility for general thin target PIXE analysis
at 1 and 2 MeV. The development and use of these efficiency curves
will be briefly discussed in the following section. Kirchner (73)
gives a more detailed explanation of the origin of these curves and
compares them with theoretical efficiency curves calculated from
literature values of X-ray production cross section, detector
efficiency and filter mass attenuation coefficients. In the last two
100
sections, the efficiency curves are used to analyze NBS standard
reference materials and ferromanganese nodules to establish the limits
of sensitivity, accuracy and precision of the PIXE technique for thin
target quantitative analysis.
X-Ray Efficiency Curves
Standard X-ray spectra of all elements from Na to U were
collected for use in generating thin target X-ray efficiency curves.
High purity 1000 ppm standards were purchased from Spex Industries,
Inc. These were found to contain less than 5 ppm of anyone element
as a contaminant. Thin targets of the pure elements were prepared for
PIXE analysis by depositing 20 ul of the solution onto Kapton-backed
Nucleopore, as described previously. The targets were dried under an
infrared lamp and mounted on the PIXE sample wheel using doublestick
tape. X-ray emission spectra were collected at 1 and 2 MeV, using a
4 mil Al filter at the higher energy, for a preset live time of 15
minutes. The pure element X-ray spectra were stored on hard disk for
use in determining relative line intensities and in deconvoluting
complex spectra.
Replicate analyses of several hundred mixtures of pure
elements in various combinations were performed to determine the
relative Ka and La X-ray yields for all elements under several
operating conditions. Thin targets were prepared as before using
20 pI each of from 5 to 15 standard solutions. Spectra were obtained
at 1 and 2 MeV using a 50 nA proton beam. Irradiation times were
normalized by stopping data accumulation after a preset number of
101
counts was obtained in the chosen peak area of a standard spiked
element. For this purpose, a hardware feature of the ND 6620 system
was used which allowed magnification of a chosen peak area with
automatic background subtraction and peak integration. This feature
was also used to determine peak areas for quantitation. for samples
containing only a few elements and no spectral interferences, the
region of interest was manipulated to directly obtain the net number
of counts in a peak. For spectra containing overlapping peaks, this
feature was used to obtain individual peak areas by the stepwise
stripping of the contribution of individual peaks to the total
spectrum. The stored standard spectra of pure elements were
normalized and background subtracted on an interference-free
channel, and the standard elements were then sequentially stripped
until the spectra were deconvoluted. Although this method was time
consuming for complex spectra where many subtraction steps were
required, it proved to be the most reliable and allowed us to observe
each step in the data reduction so that changes could be implemented
as needed.
Through this procedure individual peak areas were obtained,
and from a knowledge of the relative number of atoms of each element
irradiated, the relative number of counts obtained from an equal
number of atoms could be determined. Tables 3.5-3.8 show the
experimentally determined K and L shell X-ray yields for I and 2 MeV
protons. From these values, relative response factors were determined
and X-ray efficiency curves were generated for 1 and 2 MeV protons as
Table 3.5. Experimental thin target K-she11 X-ray yield for 1 MeV Protons.
Atomic Number Element Relative Yield
11 Na 43.42
12 Ma 14,063
13 Al 19,375
14 Si 21,868
15 P 20,560
16 S* 17,962
17 C1 14,459
18 Ar* 11,470
19 K 8,026
20 Ca 6,570
21 Sc 5,096
22 Ti 3,744
23 V 2,799
24 Cr 2,096
25 Mn 1,543
26 Fe 1,151
27 Co 861. 7
28 Ni 645.1
29 Cu 486.0
30 Zn 363.1
* Indicates interpolated values.
102
Table 3.6. Experimental thin target L-she11 X-ray yield for 1 MeV protons.
Atomic Number Element Relative Yield
35 Br 37,895 36 Kr* 7,025 37 Rb 36,478 38 Sr 35,389 39 Y 34,300 40 Zr 32,913 41 Nb 31,327 42 Mo 30,050 43 Tc 28,607 44 Ru 26,320 45 Rh 24,641 46 Pd 23,725 47 Ag 22,035 48 Cd* 20,110 49 In 18,111 50 Sn 15,080 51 Sb* 14,317 52 Tc 13,304 53 1* 11,842 54 Xd* 10,900 55 Cs 9,043 56 Ba 8,530 57 La* 7,721 58 Ce 5,512 59 Pr* 6,233 60 Nd 5,653 61 Pm* 4,313 62 Sm 4,255 63 Eu 3,428 64 Gd 3,180 65 Tb* 2,804 66 Dy 2,652 67 Ho 2,214 68 Er* 2,185 69 Tm 2,069 70 Yb 1,977 71 Lu* 1,328 72 Hf* 1,150 73 Ta 1,043 74 H' 989 75 Re* 924
* Indicates interpolated values.
103
Table 3.7. Experimental thin target K-she11 X-ray yield for 2 MeV protons with a 4 mil A1 filter.
Atomic Number Element Relative Yield
20 Ca .066 21 Sc 1.39 22 Ti 8.07 23 V 52.83 24 Cr 131.8 25 Mn 307.9 26 Fe 479.0 27 Co 678.9 28 Ni 794.5 29 Cu 902.5 30 Zn 896.4 31 Ga 858.1 32 Ge 785.3 33 As 681.8 34 Sc 584.1 35 Br 502.9 36 Kr 413.5 37 Rb 337.7 38 Sr 277.0 39 Y 228.2 40 Zr 184.0 41 Nb 148.1 42 Mo 119.6 43 Tc 94.13 44 Ru 74.19 45 Rh 58.41 46 Pd 46.53 47 Ag 29.09 48 Cd 25.24 49 In 21.38 50 Sn 16.39 51 Sb 10.52 52 Te 5.36 53 I 4.15 54 Xe 3.51 55 Cs 2.88 56 Ba 2.54
104
Table 3.8. Experimental thin target L-shell X-ray yield for 2 MeV protons with a 4 mil Al filter.
Atomic Number Element Relative Yield
56 Ba 10.2 57 La 15.3 58 Ce 21.2 59 Pr 32.4 60 Nd 40.1 61 Pm 43.4 62 Sm 52.5 63 Eu 57.2 64 Gd 68.2 65 Tb 120 66 Dy 390 67 Ho 598 68 ER 735 69 Tm 932 70 Yb 1080 71 Lu 1262 72 Hf 1425 73 Ta 1490 74 l.J 1585 75 Re 1520 76 Os 1485 78 Ir 1403 79 Au 1385 80 Hg 1273 81 Te 1185 82 Pb 1120 83 Bi 1110 84 Po 1070 85 At 954 86 Rn 935 87 Fr 863 88 Ra 797 89 Ac 689 90 Th 665 91 Pa 605 92 U 534
105
106
shown in Figures 3.16 and 3.17. Any X-ray yield values that could not
be determined experimentally were interpolated from the curves.
The program RATIO was written by Hain Oona (Department of
Physics) to convert peak areas to concentrations. This program is
included as Appendix C. The values listed in Tables 3.5-3.8 were used
in the program as the standard relative response factors for the
analysis of all thin target PIXE samples. RATIO converted peak areas
to relative atomic ratios by dividing the peak areas by the relative
response factors, and concentrations were then directly calculated
using the known concentration of one element in the sample. This was
often determined by adding a known spike of some element, quantitating
one element by another technique, or by the use of a standard that had
a composition similar to that of the sample. This method of
quantitation was applied to a number of samples of widely varying type
and composition. The analysis of two groups of samples will be
discussed in the remainder of this chapter.
Analysis of Standard Reference Materials
Five NBS standard reference materials were analyzed to
evaluate the accuracy of the experimental thin target X-ray efficiency
curves. These reference standards included NBS orchard leaves (SRM
571), pine needles (SRM 1575), bovine liver (SRM 1577), coal (SRM
l632a) and coal fly ash (SRM l633a). Aproximately 250 mg of each
material was digested in a Teflon-lined stainless steel bomb (Parr)
with a 3:3:1 mixture of Ultrex (Baker) HN03' HF and HCl acids at 1100
4130
00
I M
eV
31
30
00
-
(f)
I- Z
::J
U u
2130
130-
ItJ >
H
I- <{
-1
llJ a:
1013
130
I e-r
,or
r .. ~
10
313
40
513
60
713
80
91
3 le
e
AT
OM
IC NU~1BER
Fig
ure
3
.16
. E
xp
eri
rnen
tall
v o
bta
ined
K
an
d
L sh
ell
re
lati
ve
X-r
av eff
icie
ncy
cu
rves
for
1 M
eV
pro
ton
s.
I-'
o -..J
20
00
-,.------
2 M
rN
15
00
--
(f)
1-- :J
u U
ILl
lmlO
->
H
1-
- « ...J
ld
a::
500~
1 I
\ K
She
ll
0-t
1'"
~'"
'1 2.
1
--' r ..
..... ""',
.;J-rl-
-r--.-
--.l
3
0
60
9
0
70
8
0
40
so
H10
AT
OM
IC NU~IBER
Fig
ure
3
.17
. E
xp
eri
men
tall
y o
bta
ined
K a
nd
L s
hell
re
lati
ve
X-r
av eff
icie
ncy
cu
rves
fo
r 2
MeV
p
roto
ns
wit
h
a 4
mil
A
l fi
lter.
I-'
o ex:>
109
for 2.5 hours. The acids had been analyzed previously and contained
only a few ppb of contaminants. The cooled solutions were diluted
with high purity saturated boric acid (Baker) and brought to volume
with distilled deionized water. After thorough mixing, 100 pI
a1iquots of the solution were spotted onto Kapton-Nucleopore
sandwiches, as described previously, and dried under an infrared
lamp. These targets were then mounted on the PIXE sample wheel for
analysis.
The samples were irradiated with a 50 nA 1 MeV proton beam
for 1/2 hr using no filter and a 2 MeV proton beam for 1 hr using a 4
mil Al filter. High and low energy X-ray spectra were collected as
shown in the representative orchard leaves spectra in Figures 3.18 and
3.19. Analytical concentrations were calculated from the background
subtracted deconvoluted peak areas at both energies and the
experimentally determined X-ray efficiency curves. One certified value
from each standard was used to calculate ratios in the determination.
The average values obtained from four independent determinations are
given in Tables 3.9-3.13 along with the NBS certified values for each
standard.
The concentrations of elements for which no PIXE values are
given were below the detection limits of this method. In many cases,
the volatility of the elements may contribute to the difficulty in
detecting these elements (eg. Se, Cd, Sb, Bi, As). The low values
obtained for As, Br and Pb, even though they are present at detectable
concentrations, are undoubtedly due to the volatility of the elements
1)0
01
10
- I K
~ ~C"
Ro
Mg
8
00
--1
00
00
-
~ S; c
1/1
e D u
0 N
lJ !4
0o
-N
T
II
/I
P ,... ->
20
m)g
-'~
""i"
"'i T
j ~i"i-
'i"""i
...-. r
t,rt
8 10
"r
JlO
Y 0
''''"
0
Cn
---I
----
r o-I-'-r~
4 o
I G
l-'-I-~~
8 1
0
EI'Er~GY
Cr.
[V)
Fig
ure
3
.18
. 1
MeV
PI
XE
sp
ectr
um
of
NBS
15
71
orc
har
d
leav
es.
f-'
f-'
o
r: II
11 rr 1 " ,.
" I (
r'"
Fe,
Mn
11
1,1
_
Mnl
l Fe
1\11
_ I~ Zn
C
u Ph
,As
Sr
Mj\JiM
A'b~
) V\~
"V''VW
1f'~'l
/A'
10
2,.-
1 n-:
I -\'
--r
-r-
'-,--r-I-r-T
' 1-
-'--
-1-1
--1-
'-,-
-1--
1-1
-'-'
---'
'I
5 1
0
15
2
0
2S
Etl
mr,
y
(KE
V)
Fig
ure
3
.19
. 2
NeV
PI
XE
sp
ectr
um
of
NBS
15
71
orc
hard
le
av
es
(usin~
a 4
mil
A
l fil
ter).
.... .... ....
112
Table 3.9. Analysis of NBS SRM 1571 orchard leaves.
PIXE Analysis Atomic Number Element Thin Thick NBS Value
11 Na 82 llg/g 12 Mg .607- .41% .62% 13 Si 480 Vg/g 540 llg/ g 15 P .20% .30% .21% 16 S 2200 llg/g 1560 llg/g (1900) wg./g 17 C1 700 llg/g (690) llg/g 19 K 1.51% 1.92% 1.47% 20 Ca 1.83% 1.80% 2.09% 24 Cr 2 llg/g 1 llg/g 2.6 llg/g 25 Mn 95 llg/g 110 llg/g 91 llg/g 26>~ Fe 300 llg/g 300 llg/g 300 llg/g 27 Co (.2) llg/g 28 Ni 1. 3 llg/g 29 Cu 13 llg/g 10 llg/g 12 l1g/g 30 Zn 22 llg/g 21 llg/ g 25 llg/ g 31 Ga (.08) llg/g 32 Ge 33 As 3 llg/g 4 llg/g 10 l1g/g 34 Se .08 llg/g 35 Br 7 llg/g 5 llg/g (10) llg/g 37 Rb 10 llg/g 11 llg/g 12 llf!/g 38 Sr 31 llg/g 33 llg/g 37 llg/g 48 Cd .11 l1g/g 51 Sb 2.9 llg/g 52 Te (.01) llg/g 53 I (.17) -}lg/g 55 Cs (.04) llg/g 56 Ba 38 llg/g 44 llg/g 82 Pb 15 11g/g 45 llg/g 83 Bi .1 llg/g
113
Table 3.10. Analysis of NBS SRM 1575 pine needles.
PIXE Analysis Atomic Nubmer Element Thin Thick NBS Value
12 Mg .22% .12% 13 Al 582 l1g/g 400 l1g/g 545 l1g/g 14 Si 248 l1g/g 320 l1g/g 15 P .10% .05% .12% 16 S .15% .12% 17 C1 300 l1g/g 19 K .35% .52% .37% 20 Ca .37% .28% .41% 21 Sc (.03)l1g/g 24 Ti 25* Mn 675 llg/g 675 llg/g 675 l1g/g 26 Fe 188 l1g/g 155 l1g/g 200 l1g/g 27 Co (.1) llg/g 28 Ni 2 l1g/g 3 llg/g 3.5 llg/g 29 Cu 3 l1g/g 3 llg/g 3.0 l1g/g 30 Zn 110 llg/g 120 llg/g 33 As 2.1 l1g/g 35 Br 5 l1g/g (9) l1g/g 37 Rb 13 l1g/g 12 l1g/g 11.7 l1g/g 38 Sr 5 l1g/g 4 118/g 4.8 l1g/g 48 Cd (.5) llg/g 51 Sb (.2) l1g/g 63 Eu (.006) llg/g 80 Hg .15 l1g/g 81 Te .05 l1g/g 82 Pb 7 l1g/g 5 llg/g 10.8 llg/g 90 Th .037 l1g/g 92 u .02 l1g/g
114
Table 3.11. Analysis of NBS SRM 1577 bovine liver.
PIXE Analysis Atomic Number Element Thin Thick NBS Values
11 Na .20% .90% .243% 12 Mg 580 llg/g 420 llg/g 604 llg/g 14 Si (17) llg/g 15 P 1.3% .41% (1.1)% 16 S .95% .72% 17 C1 .19% ( .27)% 19 K .95% .83% .97% 20 Ca 131 llg/ g 120 llg/ g 124 llg/g 24 Cr .088 llg/g 25 Mn 8 llg/g 11 llg/g 10.3 llg/g 26* Fe 268 llg/g 268 llg/g 268 llg/g 27 Co (.18) llg/g 29 Cu 189 llg/g 175 llg/g 193 llg/g 30 Zn 128 llg/g 125 llg/g 130 llg/g 33 As .055 llg/g 34 Sc 1.1 llg/g 35 Br 11 llg/g 8 llg/g 37 Rb 20 llg/g 18 llg/g 18.3 llg/g 38 Sr (.14) lJg/g 42 Mo 2 llg/g 3 llg/g (3.4) llg/g 47 Ag (0.6) }lg/g 48 Cd .27 llg/g 49 In (0.5) }lg/g 51 Sb (.005) llg/ g 53 I (.18) lJg/g 80 Hg (.016) llg/g 81 T1 (.05) -}lg/g 82 Pb 34 }lg/ g 92 U (.0008)
115
Table 3.12. Analysis of NBS SRM 1632a coal.
PIXE Analysis Atomic Number Element Thin Thick NBS Value
13 Al 2.8% 1.3% (3.07)% 14 Si 3.1% 5.2% 16 S 1.6% 1.1% (1. 64) % 19 K 20 Ca 21 Sc 5 llg/g (6.3) llg/g 22 Ti .17% .13% (.175)% 23 V 46 llg/g 28 llg/g 44 llf!./g 24 Cr 36 llg/g 15 llg/g 34.4 llg/g 25 Mn 20 llg/g 37 llg/g 2R llg/f!. 26* Fe 1.11% 1.11% 1.11% 27 Co 28 Ni 18 llg/g 16 llg/g 19.4 llg/g 29 Cu 16 llg/g 15 llg/g 16.5 llg/g 30 Zn 28 llg/g 26 pg/g 28 llg/ g 31 Ga 7 llg/g 8 llg/f! (8.49) llg/g 33 As 9.3 llg/g 34 Se 1 llg/g 2.6 llg/g 37 Rb 34 llg/g 28 llg/g (31) llg/g 48 Cd .17 llg/g 55 Cs (2~4) llg/p, 58 Ce (30) llg/g 72 HF (1.6) llg/g 80 Hg .13 llg/g 82 Pb 12.4 llg/g 90 Th 4.5 llg/g 92 u 1.28 llglf!.
116
Table 3.13. Analysis of NBS SRM 1633a coal fly ash.
PIXE Analysis Atomic Number Element Thin Thick NBS Value
11 Na .22% .08% .17% 12 Mg .38% .24% .455% 13 A1 15% 11% (14)% 14 Si 18% 24% 22.8% 15 P 16 S 17 C1 19 K 1.8% 2.3% 1.88% 20 Ca 1.2% 1.4% 1.11% 21 Sc 34 lJg/g 30 lJg/g (40) lJg/g 22 Ti .8% .6% (.8) % 23 V 280 lJg/g 245 lJg/g 300 lJg/g 24 Cr 200 l1g/g 135 lJg/g 196 llg/g 25 Mn 195 lJg/g 222 llg/g 190 lJg/g 26* Fe 9.4% 9.4% 9.4% 27 Co (46) lJg/g 28 Ni 112 llg/g 115 lJg/g 127 lJg/g 29 Cu 120 lJg/g 118 lJg/g 118 lJg/g 30 Zn 218 lJg/g 200 llg/g 220 llg/g 31 Ga 55 lJf'./g 60 llg/g 58 lJg/g 33 As 38 llg/g 43 llg/g 145 lJg/g 34 Se 8 lJg/g 5 lJg/g 10.3 llg/g 37 Rb 150 llg/g 135 llg/g 131 lJg/g 38 Sr 825 llg/g 828 llg/g 830 lJg/g 42 Mo 30 llg/g 27 lJg/g 20 llg/g 48 Cd 1.0 llg/g 51 Sb (7) lJg/g 55 Cs (11) llg/g 56 Ba .15% .10% (.15)% 58 Ce 230 lJg/g 240 llg/g (180) lJg/g 63 Eu (4) lJg/g 72 HF (7.6) llg/g 80 Hg .16 lJg/g 81 Te 4 lJg/g 3 )lg/g 5.7 lJg/g 82 Pb 65 llg/g 28 llg/g 72.4 llg/g 90 Th 28 llg/g 22 lJg/g 24.7 llg/g 92 U 11 lJg/g 8 )lg/g 10.2 )lg/g
117
and the heat generated by the proton beam. Difficulties also occurred
due to peak overlap. In the spectra of orchard leaves (Figures 3.18
and 3.19), the intense Mg Ka1,2 peak almost completely obscures the
weaker Na K 1,2 peak. Also, difficulties in the detection of Co and
the poor results that were obtained for Ni arise from spectral
overlaps with the Fe K-1ines since Fe is present at several times the
concentration. Greater detector resolution to separate overlapping
peaks would greatly improve the results of these analyses.
Generally, reasonable agreement was made for the non-volatile
elements, Na-U, that were present at greater than 10 ppm
concentration. The inter-sample precision of the method was better
than 10% in most instances, although for mUltiple analyses of a single
sample the precision was usually better than 2%. In general, an
accuracy of 10-20% was obtained for elemental concentrations in these
samples in the absence of losses due to volatility" and problems caused
by high backgrounds and overlapping emission lines.
Analysis of Ferromanganese Nodules
A major consideration of the PIXE facility at the University
of Arizona has been the quantitative determination of elements in deep
sea ferrmanganese nodules. These are heterogeneous mixtures
containing manganese oxides (15-30%), iron oxyhydroxides (8-26%), and
various amounts of calcite and silicates as major components, along
with associated minor components of Cu, Ni and Co which are of
strategic or economic importance. In addition, some 30-40 other
118
elements have been found in varying concentrations in the nodules,
some of which are highly toxic. In the near future large scale
recovery of metals from deep sea nodules will be undertaken, and an
analytical method is needed that can simultaneously monitor the
concentrations of a large number of elements throughout the processing
scheme. The PIXE technique is ideally suited to this type of
analysis. It can play an important role not only in the analysis of
complex ferromanganese nodules, but also in the development of
environmentally compatible process schemes for the separation and
extraction of valuable metals from the nodules.
Seven samples of ferromanganese nodules were analyzed by PIXE
and compared to flame technique results obtained by Steve Kirchner
(University of Arizona) and John Jong-Hae Lee (University of Hawaii).
Four samples from the Pacific Ocean (labeled MOH, VAL, KK and CRU)
were provided by the Hawaii Institute of Geophysics, and three samples
from the Atlantic Ocean (labeled D-S, D-20 and D-28) were provided by
the Lamont Doherty Geological Observatory. A more complete
description of these nodules is supplied by Kirchner (73). The
samples were air dried and ground to less than 100 mesh size. Thin
targets were prepared from the samples as described for the NBS
standards. 200-300 mg of each nodule sample was digested in a Parr
bomb with a 3:3:1 mixture of Ultrex HN03' HF and HCl at 1100C for
several hours. Three 100 ~l a1iquots were pipetted onto Kapton backed
Nucleopore filters, as before, and dried under an infrared lamp. Five
119
replicate targets were prepared for each nodule. These targets were
then mounted with doublestick tape for PIXE analysis.
A 50 nA proton beam was used to irradiate the samples for
1/2 hr at 1 MeV and for 1 hr at 2 MeV using a 4 mil Al filter.
examples of the nodule spectra obtained are included as Figures 3.20
and 3.21. Elemental concentrations were calculated as described
previously from the peak areas and the experimental X-ray efficiency
curves. The comparative results of 5 replicate analyses using both
PIXE and flame methods are shown in Table 3.14. Details of the flame
analyses may be obtained from Kirchner et al. (68). In addition to
the 22 elements for which concentrations are reported, P, S, Cl, Cr,
Se, Br, Y, Zr and Ge were detected by PIXE but their concentrations
were not determined.
The standard deviations for the PIXE determinations were of
the order of 10%, compared to 5% or less for the flame methods. Most
of this is probably due to inhomogeneity in the prepared targets. As
with the NBS standards, consistently low results were obtained for the
volatile elements As and Pb. The determination of Co in the presence
of high Fe concentrations was again unreliable, as was the
determination of Tl in the presence of relatively high concentrations
of Pb and As. The rest of the PIXE data shown in Table 3.14 are in
good agreement with the results obtained by flame methods. It is
therefore apparent that PIXE can be a valuable analytical tool for the
routine analysis of ferromanganese nodules.
50
00
0-T
----
----
----
----
----
.40
00
0-
C3
00
00
o U
N
T
5
20
00
0
10
00
0
0-I
-="1
(3
: .......
,
-'11(
',1
2
C
D
U " T • I .
....
~------
Mn
1- .....
CI
2 3
5 •
CH
lJU
n
(KC
V)
MIl
k"
4 6
ENER
GY
(KEV
)
Mn,
Fe
Fig
ure
3
.20
1
MeV
PI
XE
sp
ectr
um
of
ferr
om
ang
anes
e n
od
ule
sa
mp
le.
8 H
I
I-'
N o
50
00
40
00
C
30
00
o U
N
T
S
20
00
-
10
00
-
0-f
--r-
rL
o
MI.
KIi
'
h-I
\ "",
CU
K"
CU
t\u
llik
u
:~IIII ,
""1<..
1111
11 I
I
1'1"
11
k. ~I
I
...... ,.
.---
-
.....
c ..
...
o U
M
T .... ,-
Cu
t··
·· I···· I···".···".···· I'
'.G
7 .•
U
t._
1
2.5
I' .
• 17
."
fHfa
W
(IC
[V)
~)I
t\
ll~1
~. "l II
ttl~
n'I\'1
M''
'ri T
Ill
iT
30
ENER
GY
CKEV
)
2 •.•
3S
Fig
ure
3
.21
. 2
MeV
PI
XE
sp
ectr
um
of
ferr
om
ang
anes
e n
od
ule
sa
mp
le
(usi
ng
a
4 m
il A
1 fil
ter).
I-'
N
I-'
Tab
le
3.1
4.
Co
mp
arat
ive
PIX
E
and
fl
ame
an
aly
sis
of
ferr
om
ang
anes
e n
od
ule
s.
trol
l VA
K
K
0-5
D
-20
C
ru
rUC
E F
lom
e P
IXE
F
lam
e P
IXE
F
lam
e P
IXE
F
lam
r PU
CE
Fla
me
rlX
E
Fla
me
Na
1.7
0
1.91
%
1.29
%
2.15
%
!.05
%
2.1
1:
2.03
%
1.1,
5%
1. 7
8%
1.57
%
1.05
%
1.88
%
HI:
1.5
2:
1. 7
0%
1.41
%
1.28
7.
1.64
%
1.42
%
1.43
%
.90%
1.
68%
1.
40%
1.
57%
1.
76%
Al
1,.0
1%
3.62
%
1.70
%
1.72
%
1.00
%
1.11
%
1.35
%
1.11
%
2.97
%
3.56
%
2.01
%
1.88
%
51
13.2
4%
12.7
8%
5.36
%
6.06
%
9.47
%
11.1
1,%
12
.80%
11
.13%
8.
38%
6.
11%
11
.34%
9.
72%
K
.61%
.6
8%
.83%
.7
7%
.58%
.6
2%
.42%
.3
5%
.60%
.6
3%
.79%
.8
8%
Ca
4.83
%
4.20
%
.97%
.9
3%
.63%
.5
9%
1.73
%
1.52
%
1.31
,%
.92%
1.
32%
1.
25%
T1
2.51
%
2.42
%
1.02
%
1.10
%
1.58
%
1.72
%
1.05
%
.93%
.6
9:1;
.7
4%
.73%
.6
8%
V
395\
1g/g
I,
80ug
/1I
270I
lg/g
35
0ug/
g 35
1lg/
g -
685I
Jg/R
67
0\lg
/g
4101
lg/g
460
\lS
/g
2951
1g/g
46
01lg
/8
Hn
6.29
%
6.29
%
19.8
1%
19.8
1%
10.3
4%
10.3
4%
13.0
0%
13.0
0%
13.7
0%
13.7
0%
21.0
0%
21.0
0%
Fe
11
. 94%
12
.25%
6.
82%
6.
54%
10
.98%
10
.58%
16
.89%
17
.10%
13
.05%
. 1
2.56
%
7.35
%
8.37
%
Co
.20%
.1
8%
.05%
.1
0%
.10%
.1
5%
.08%
.1
1%
.09%
.1
5%
.12%
.1
8%
Ni
.16%
.1
5%
.99%
.9
8%
.31%
.3
5%
.22%
.1
9%
.27%
.3
4%
.93%
1.
04%
Cu
.02%
.0
3%
.68%
.7
0%
.30%
.2
9%
5891
1g/g
n
Oll
g/g
.1
2%
.12%
.6
3%
.67%
Zn
3761
1g/8
39
0ug/
g 10
0l,l
lg/g
102
01lg
/g
848u
C/g
8(
'O\.l
g/g
637\
1g/g
6I
Ol1
g/g
4131
1g/g
42
0llg
/g
687\
1g/8
71
,O\lg
/g
A8
351l
g/g
801l
g/g
55\1
g/g
62\1
g/C
53
ug/g
78
\1g/
g 88
\1g/
g 15
3\1g
/g
33\1
g/&
70
1lg/
g 33
\1g/
g 90
\lg/
8
Rb
8\1g
/,
10\l1
l/g
10\l
S/8
14
ug/ p
, l1
\1g/
g 14
11g/
g 12
11g/
g 12
\1g/
g 18
\1g/
g 21
\18/
g 14
\1g/
g 16
\1g/
8
5r
4811
lg/g
48
01lg
/g
"989
Ilg/
g 11
301l
8/&
16
20\l
8/g
1590
1lg/
g 82
811g
/g
730I
lg/g
47
6\1g
/8 4
50\l
g/g
528\
1g/g
540
\lg/
g
tIo
3011
l8/g
29
01l8
/g
697\
1g/g
n
Oll
g/g
29
5118
/g
280 l
lg/g
6
83
ug
/,
700l
lg/g
30
511g
/g
320U
g/g
6441
1g/g
63
0llg
/8
Cd
-27
Ug/
g 80
1lg/
g 12
01lg
/g
--
-25
\1g/
g -
40ll
S/g
-
271l
8/g
B.
.14%
.1
1%
.32%
.2
6%
.37%
.3
4%
.26%
.2
5%
.17%
.1
6%
.41%
.3
4%
Tl
3811
8/8
60\.l
1l/g
16
511g
/g
1501
lg/g
5l
1g/g
28
11g/
g 96
1lg/
g 10
1\1g
/g
97\1
g/g
U8
uS
/g
138u
g/g
129\
1g/8
Pb
5261
1g/g
70
0\lg
/8
43D
1lgf
g 51
01lg
/g
9381
1g/g
11
4011
8/g
1146
\18/
g 13
20\l
g/g
4931
18/g
62
01lg
/g
4271
lg/g
830
uS/g
D-2
8
PIX
E
Fla
llle
1.43
%
1.35
%
1.62
%
1.1,
1%
2.59
%
2.76
%
8.37
%
7.27
%
.52%
.4
8%
.79%
.8
3%
1.0
U
.82%
589\
1g/g
610
\lg/
g
12.9
3%
12.9
3%
13.8
3%
14.2
%
.22%
.1
7%
.• 29
%
.35%
.14%
.1
2%
495u
8/&
I,
70Il
S/g
4811
8/g
92\1
g/g
l2\1
g/g
16\1
g/g
474\
1g/g
450
1lg/
g
280\
lg/8
320
\lg/
g
-34
11g/
g
.13%
.1
8%
8711
g/g
98\1
g/S
637\
1g/g
73
0Ilg
/g
I-'
N
N
Summary
Thin target preparation procedures applicable to a wide
variety of sample types have been described, and X-ray efficiency
curves that can be used with any thin target PIXE sample have been
determined. Two examples of quantitative analyses included in this
section demonstrate how the PIXE technique was developed for the
123
rapid quantitation of complex samples. The NBS standard reference
materials served to establish the precision and accuracy of the
technique, and the ferromanganese nodules served to demonstrate how a
complete elemental analysis can be performed on an extremely difficult
"real-world" sample; virtually every possible interference is
presented by the more than thirty elements present at widely varying
concentrations in the nodule.
This work has shown that thin sample targets of complex
materials can be successfully analyzed by PIXE. Reasonable numbers
can be obtained for all elements Z > 11 in two PIXE runs at 1 and
2 Mev, even for samples as complex as ferromanganese nodules. An
accuracy of 10-20% can be obtained for all elements present at from
1 ppm to several percent, in the absence of problems due to volatility
and overlapping emission lines. The precision and accuracy of the
technique can be improved if inhomogeneities in the thin target
samples can be removed and if sample heating is minimized. Greater
detector resolution would also improve the accuracy of the method and
ease the automation of data reduction and quantitation.
CHAPTER 4
HIGH RESOLUTION PIXE
As indicated in the previous chapter, accurate quantitation of
PIXE spectra is often limited by the resolution of the detector.
Situations are frequently encountered whereby the X-rays emitted from
one element interfere with those emitted from other elements. This
occurs when quantitating a minor element in the presence of large
concentrations of neighboring elements in the X-ray spectral series;
when quantitating light elements, where the Ka peaks overlap or the KS
line of one element overlaps with the Ka line of the next; or when
quantitating samples containing a mixture of light and heavy elements,
where the M and L X-rays of the heavy components overlap with the K X
rays of the light components. In these cases the X-ray peaks cannot
be sufficiently separated by means of conventional energy-dispersive
detection techniques.
At the present time, little interest has been directed toward
the study of PIXE spectra using wavelength-dispersive X-ray detection
systems. Watson (91) has successfully used a wavelength dispersive
system for the analysis of Si and AI, Willis (92) has applied such a
system to the analysis of environmental samples, and Oona (93) has
used the technique to look at sulfur X-ray emission. From these
studies it is apparent that the high resolution capabilities of
wavelength-dispersive X-ray spectrometry can be successfully exploited
124
125
to analyze samples that contain elements which cannot be determined by
conventional energy-dispersive PIXE techniques.
As mentioned previously, in wavelength dispersive X-ray
spectrometry the several X-rays emitted by a specimen are displaced
spatially by crystal diffraction on the basis of their wavelengths.
The Bragg law shows that each characteristic wavelength, A, is
diffracted at a specific crystal angle, e, such that:
n A = 2 d sin e (4.1)
where n is the order of diffraction and d is the spacing between
planes of atoms in the crystal. The Bragg condition is satisfied as
long as the relative angular rotation of the detector is kept at twice
that of the analyzing crystal; if a crystal is rotated through an
angle eo, the detector must be rotated at twice the speed so that it
is always at angle 2eo and in position to intercept the diffracted X
rays. In principle, the detector receives only one wavelength at a
time, so that the counting rate of X-rays emitted by a given element
is not limited by the counting rates of X-rays emitted by other
elements in the sample, as is the case with an energy dispersive
detector. The fundamental disadvantage of this system is that
intensity losses are high; the efficiency of a high resolution
wavelength dispersive system can easily be two orders of magnitude
worse than that for a Si(Li) detector. On the other hand, the
resolution of a wavelength dispersive system can be nearly two orders
126
of magnitude greater than that attainable with a Si(Li) detector. For
example, in the work reported by Willis et al., the observed FWHM of a
spectral peak was about 2 eV for the Ka line of Si, compared with a
typical value of 170 eV for a peak in a Si(Li) derived spectrum (92).
The results of the early high resolution PIXE studies showed
that resolutions of better than 1.5 eV could be obtained using a
wavelength dispersive crystal spectrometer with a proportional
counter detector (94). We felt that this resolution was sufficient
not only to separate interfering peaks in conventional X-ray spectra,
but also to enable us to look at chemical shifts, or small changes in
X-ray energy due to the chemical environment of the atom. In this way
both elemental and molecular information could be obtained using the
same technique. Urch (95) has shown that X-ray emission spectra
provide a powerful tool for determining the electronic structure of
materials. X-ray emission resulting from electron transitions that
originate in the valence orbitals can reflect changes that occur in
the vicinity of an atom upon bond formation. In molecular orbital
concepts, the valence orbitals become directly involved in bond
formation, producing a new set of energy levels from which
transitions are possible. This effect can be manifest through shifts
in X-ray peak positions, changes in peak shapes and peak splitting.
According to Meyers (96), X-ray emission spectroscopy
possesses an inherent advantage over other methods for obtaining
information about chemical bonding. Due to the essentially atomic
nature of the initially ionized state and the large emission energies,
127
X-ray spectroscopy is capable of analyzing solids at high resolution,
unlike UV spectroscopy which necessarily involves broad bands that
are poorly resolved. Also, X-ray spectroscopy has an advantage over
photoelectron spectroscopy in its ability to distinguish electrons of
different symmetry since the K spectra arise from p--s transitions, L
spectra from d--p or s--p, etc. (97). Furthermore, X-ray emission
spectroscopy provides information about the bulk sample, whereas X-ray
photoelectron spectroscopy provides information about several surface
layers only.
The effects of chemical bonding upon X-ray spectra were first
noted over fifty years ago (98), but were uninterpreted primarily due
to the lack of rigorous bonding theory. Much of the initial work (99-
104) concentrated on transition metal complexes due to their large
transition energies and easier measurement in the hard X-ray region
(O.l-lOAO). Most work centered on K~, chemical shifts, for which a
large body of data has been accumulated (105). Work was later
extended to compounds of the second periodic row elements (106-111),
using both K and L emission spectra.
We have tried to extend work done on chemical shifts to
include Land M emission spectra of the transition metals. The higher
X-ray fluxes and low background characteristics of the PIXE technique
should allow observation of low energy and low intensity lines that
could not previously be detected using wavelength dispersive X-ray
fluorescence techniques. A vacuum PIXE system combined with a high
resolution crystal spectrometer, such as the system described by Oona
128
(94), should allow determinations of elements down to boron and
chemical shifts as small as 1 eV. Such a system has been developed at
the University of Arizona and used to investigate a number of
transition metal complexes, including those of Mn, eu and Mo. The
results of these investigations will be presented in this chapter.
Instrumentation
All experiments discussed in this section were done using a
6 MeV Van de Graaff accelerator, the PIXE sample chamber described
earlier, adapted to use a Philips Norelco single crystal
spectrometer, a stepping motor with digital controller for scanning, a
modified flow through proportional counter detector with a homemade
preamplifier and associated electronics, and a Tracor Northern 1700
multichannel analyzer. A schematic of the instrumentation used is
shown in Figure 4.1. Each of these will be discussed in detail.
Van de Graaff Accelerator
As described previously, a beam of singly charged ions of
destined energy is obtained from the Van de Graaff accelerator, mass
analyzed by a magnet and directed into a sample chamber where the ions
collide with the sample target. A 6 MeV accelerator was used to allow
greater variability in projectile ions and beam energies and to allow
higher beam currents than could normally be obtained with the 2 MeV
accelerator. For these experiments, either 2 MeV protons or 4 MeV H2+
projectiles were generally used at beam currents of approximately
1 )lA.
Fig
ure
4.1
.
6 M
eV
Van
de
Gra
alf
Acc
ele
rato
r
De
tect
or
Cry
sta
l-L
----
, T. i
,---
=-:
....
J1
1--
-t -j
--M
on
oe
ne
rge
tic
Be
am
S
am
ple
C
ha
mb
er
The
h
igh
re
solu
tio
n P
IXE
sy
stem
.
Sp
ect
rom
ete
r
Ch
Ara
cle
rist
ic
X-r
ays
f-'
N
\0
130
Sample Chamber
A sample chamber that was identical to that described in the
previous chapter was used for these experiments. For normal use, the
solid sample cover plate described previously was used for proton
excitation so that several samples could be loaded at once to minimize
pumping time. Again, the sample wheel itself was used as a Faraday
cup to monitor the beam current.
For alignment purposes, an electron gun arrangement was
interchangeably used with the solid sample cover plate. The gun
consisted of a heated thoriated tungsten wire, a grounded shield with
a small hole to direct the electron beam, and a single copper sample
holder that could be kept at positive potentials up to 3 KeV. A
schematic of this cover plate is included as Figure 4.2.
Two quartz viewing plates were built into the sample chamber,
one on top to allow correct positioning of the sample in the beam
path, and one on the back of the chamber to facilitate aligning the
beam. A hole was drilled through the sample wheel at position #14 so
that when in the proper position, the proton beam would pass
completely through the sample wheel to project an image of the beam on
the quartz plate. This image should take the shape of a 1/8 in
diameter hole that was drilled in a metal plug inserted into the end
of the beam aperature. No carbon foil was used in these experiments.
A 2-inch diffusion pump was used to evacuate the sample
chamber. Two other diffusion pumps were used on the crystal
spectrometer and on the beam line entering the sample chamber. When
r Nay
'
I .. gel~ _
eo.
.,."
.,- r
thori~
ted tu
ng
sten
--
---~.~~
.....
----'-
-n
icke
l su
pp
ort
s
-=-~~~
. ~
shie
ld
_,. , .
. ___
.. , _
_ .11 ..
... _~
"
..... '
""'"
'
'~:~ :
2~:~r-
~---~~
~~-·
insu
lato
r
[, ··
,4, ~I
". "
, \.'
, ...
:.-
'.
gro
un
d
". "..
. '< .
'.< .. :~
>I
... -~-;...-.~~-'~.
'. .
'-,
to
"+"
po
ten
tial
to
tr
ansf
orm
er
Fig
ure 4
.2.
Ele
ctr
on
gu
n
co
verp
late
u
sed
to
a
lig
n't
he
syst
em
.
I-'
(...
l
I-'
isolated from the crystal spectrometer, pressures of 10-6 torr could
be maintained in the sample chamber, but when the entire system was
opened up, 10-5 torr was the best that could be obtained. This was
primarily due to leakage through the detector window in the crystal
spectrometer.
Spectrometer
The crystal spectrometer used was scavenged off an electron
microprobe and was of the variable curvature type (112). A schematic
of the spectrometer showing all critical dimensions is included as
Figure 4.3. As with any crystal spectrometer, the angle of incidence
must be equal to the angle of reflection to satisfy the Bragg
condition; this is accomplished by a series of gears which rotate the
detector at twice the rate of the crystal. In addition, with a
variable curvature spectrometer, accurate focusing is obtained by
varying the radius of the Rowland (focusing) circle, as the
spectrometer is scanned through 28 0• This is accomplished by
continuously bending the crystal by a cam which puts pressure on the
center of the crystal. This bends the crystal into a cylindrical
surface such that the radius of curvature, R, varies with the Bragg
angle, 8, as R = L/(sin 8), where L is the distance from the crystal
axis to the source and detector. In this way the source can be held
in a fixed position and the X-ray image kept focused in the
proportional counter as it is scanned through 28. Figure 4.4 shows
132
Fig
ure
4.3
.
~-
fro
m
6 M
eV
Van
de
Gra
aff
t
,--__ -
Ij, __
! 11+
~"'
6.5'~
";
h
"'l
uart7
. w
ind
ow
Sa
mp
le C
ha
mb
er
(a)
bea
m al'
era
t u
r(',
(b)
('II
t ra
nc!
' sl It
.
(c)
mas
kll
lg sIlt
.
(d)
ex
it fl
ll t,
focu
s
Va
ria
ble
C
urv
atu
re
Be
nt
Cry
sta
l S
pe
ctro
me
ter
0.1
25
"
0.0
28
"
0.O
J5"
fl.0
2R
"
In
The
v
ari
ab
le c
urv
atu
re b
en
t cry
sta
l sp
ectr
om
ete
r an
d
PIX
E
sam
ple
ch
amg
er.
f-'
W
W
P.\TH OF PROPORTIONAL COUNTER
0-- PROPORTIONAL COU:-<TER
SOURCE·
jA \ /
V "'---// RCII"U:\D
CIRCLE
CRYSTAL IN TI;'Q P~S ITIONS
Figure 4.4. Variation of the radius of curvature of the Rowland circle with crystal position. -- L is the distance from source to crystal and crystal to detector, and R is the radius of the Rowland circle.
134
135
the relative positions of the source, crystal and detector, and shows
the how the Roland circle varies as a function of detector position.
Crystals. Flexible "pseudo crystals" or "soap film crystals"
were used exclusively with this spectrometer. These are crystals
built up from successive mono-layers of metal stearates formed when an
organic solution of stearic acid is added to water containing metal
ions (113). Molecules of stearate salt are produced at the surface of
the water, oriented so that the hydrophylic ends of the molecules dip
into the water while the hydrophobic tails are on top. When a
hydrophobic plate (usually mica rubbed with ferric stearate) is
dipped into the solution, individual molecules attach themselves to
the plate with the hydrophilic ends out; then, as the plate is
withdrawn, a second set of molecules attach themselves to the
monolayer already deposited, but in the reverse direction. Successive
depositions like this form a "pseudo crystal" having a d spacing equal
to twice the length of the stearate molecules; since the stearate o
chain is roughly 25 A long, this gives a pseudo cyrstal with a 2d
spacing of approximately 100 A.
Other methods of pseudo crystal formation involve building up
multilayer sandwiches of two different metals by vacuum deposition, or
by forming organometallic crystals from salts of phthalic acid. The
most useful of these has been potassium hydrogen phthalate (KAP),
which has a 2d spacing of 26.4 A, allowing dispersion of Ka radiation
down to oxygen. Table 4.1 lists the 2d values and useful wavelength
ranges for sime of the more popular pseudo crystals used.
Tab
le 4
.1.
Pro
pert
ies
of
com
mon
cry
stals
use
d in
,a cry
stal
spectr
ow
ete
r.
2d
Use
ful
reg
ion
C
ryst
sl
Fo
rmu
la
CAO)
0
.)
Com
men
ts
LiF
(2
00)
LiF
4
.02
8
0.3
51
-3.8
4
Goo
d fo
r KK
to
L~L·
lith
ium
flu
ori
de
Hig
h in
ten
sity
an
d d
isp
ers
ion
.
PET
C(C
H2O
H)4
8
.74
2
0.7
62
-8.3
4
Low
b
ack
gro
un
d,
good
fo
r p
en
taery
thri
tol
Al-
Sc
K.
ADP
NH4H
2P04
10
.64
0
0.9
82
-10
.15
G
ood
for
Mg
KCl.
amm
oniu
m
dih
yd
rog
en
Goo
d re
flecti
on
eff
icie
ncy
. p
ho
sph
ate
oCOOK
K
HP,
KA
P I~
26
.63
2
2.3
2-2
5.4
1
Goo
d fo
r lo
w Z
ele
men
ts.
po
tass
ium
hy
dro
gen
A
vera
ge re
flecti
on
eff
icie
ncy
. p
hth
ala
te
COOH
Pb
St,
LO
D [C
H3
(CH
3) l6
COO
] 2Pb
1
00
.68
8
8.7
5-9
5.7
5
Goo
d fo
r lo
ng
wav
elen
gth
s,
dow
n le
ad
ste
ara
te,
to 1
1K.
Ave
rage
re
flecti
on
le
ad o
cto
dec
ano
ate
eff
icie
ncy
LTC
[CH
3(C
H2)
22C
OO
]2Pb
1
56
.0
11
. 35-
124
Goo
d fo
r u
ltra
lon
g w
avel
eng
ths.
le
ad
te
traco
san
oate
Lo
w
refl
ecti
on
eff
icie
ncy
.
~
W
0\
137
In any whole crystal only small sections are actually perfect
crystals, and these sections are oriented at slightly different angles
and determine the characteristic "rocking curve" of the crystal. The
rocking curve gives the actual width of the diffraction band due to
crystal imperfections. The curve is generally measured with a double
crystal spectrometer which illuminates the second crystal with
monochromatic X-rays. When traced out in this manner, the rocking
curve determines the practical resolution possible. The absolute
resolving power of a crystal, as for an optical grating, is given by
A/~A = N, where N is the number of layers that is useful for
diffraction. Generally about 100 monolayers are suffcient for the
wavelength range in which we are interested. The dispersion of a
crystal is given by:
1 cos (4.1)
For a specified wavelength, the best dispersion occurs with 2d values
slightly larger than the wavelength and at large e angles. Since the
angular width (2e) of a line is the same for all wavelengths, the
resolution is poorest at small e angles. The choice of crystal for a
spectrometer should depend on the desired dispersion and narrowest
rocking curve, as well as the wavelength range and reflection
efficiency of the crystal material. Often one must sacrifice a good
rocking curve for high reflection efficiency.
Background may arise from fluorescent photons from the crystal
itself, caused by excitation from X-rays and electrons originating in
138
the sample chamber. the elecron problem may be eliminated by placing
deflecting magnets in the target chamber, near the entrance slit, and
in the spectrometer near the masking alit. The fluorescence can
usually be eliminated by pulse height selection.
Detector. For all analyses a gas flow proportional counter was
used to detect the diffracted radiation. The counter was made from a
I-inch stainless steel cylinder with a .0005 inch center wire, and
with a side entrance window 1 mm wide by 4 cm long that was fitted
with an adjustable slit system. Since we were interested in the low
energy X-ray region, extremely thin windows had to be used on the
proportional counter. We used stretched polyproylene for the windows
because of its excellent transmission properties and meclanical
strength.
A technique suggested by C. Y. Fan (114) was used to make the
X-ray windows. Commercially available 1.5 mil polypropylene was
clamped to the top of a circular vacuum chamber having a 7-inch
opening (see Figure 4.5). The chamber was connected to the pumping
port of a leak detector with a needle valve attachment to accurately
control the pumping speed. As the chamber was slowly pumped, the
polypropylene stretched until stress rays were seen to develop non
uniformly in the film. At that point the pumping speed was decreased
and continued until these areas became thin enough so that
interference patterns were seen on the stretched foil. Stretching
continued until the red colors turned to blue, or when extremely
1.5 mil polypropylene
9"
I I
L Vacuum chamber
inlared lamp
"O'~rin9
seal
needle valve
to vacuum
Figure 4.5. Vacuum chamber used to stretch polypropylene for proportional counter \oJindmoJs.
139
140
careful, the blue colors turned to violet. At that point the foil was o
about 3000 A thick and extremely fragile. The vacuum was quickly shut
down and the chamber slowly brought back up to atmospheric pressure.
This procedure resulted in a usable piece of polypropylene, about
1 1/4 x 2 1/2 inches, that could successfully transmit low energy X-
. db· . 0-5/ rays em1tte by car on and could ma1nta1n at least a I atmosphere
pressure differential. After experimenting with several different
stretching conditions, it was found that a usable stretched area about
3 times larger could be obtained by heating the clamped polypropylene
before and during stretching using an infrared lamp. Enough material
for 3 or 4 windows could be obtained from a single stretching.
As do most thin windows on proportional counters, these thin
polypropylene windows leak slightly, and therefore the counter gas
must be continually replenished. For this reason a flow proportional
counter was used. The principle of operation is identical to that of
a sealed proportional counter.
In a proportional counter, an X-ray interacts with the active
gas volume of the detector to undergo a photoelectric absorption,
whereby the energy of the X-ray photon is imparted to a photoelectron.
This energetic electron, in turn, interacts with the gas to produce
primary ion-electron pairs along its path until its energy is spent.
when 3 potential is applied across the counter, the electrons migrate
toward the positive anode wire and the positive ions migrate to the
counter walls. If the applied potential is high enough, the electrons
may accelerate to such a velocity that they collide with gas atoms
141
along their path to produce secondary ion-electron pairs. The
secondary electrons so formed may produce still more ion pairs, the
electrons from which produce more, and so. This avalanching process
forms the basis of internal electron multiplication in gas
proportional counters. In general, each electron produced by an X-ray
photon initiates only one avalanche, and the avalanches are largely
free of any interaction; the number of avalanches is therefore
substantially the same as the number of primary ionizations, and since
all electrons are collected, the total collected charge is
proportional to the X-ray photon energy.
The gas amplification factor, A, is the number of electrons
collected on the anode wire for each primary electron produced by the
initial photoelectron. If no is the number of primary ion pairs
produced, and n is the number of electrons that arrive at the wire,
A = nino' The actual amplification is a function of counter geometry,
bias voltage and the type and pressure of counting gas. Rose and
Korff (115) have derived an expression for the amplification factor,
simplified by Staub (116) to give:
1n A = k IVo ap r /-Vo - 1 ] log b/a L Vt
where p is the gas pressure in cm mercury, k is a constant
(4.1)
charcteristic of the gas used, b is the inner radius of the counter
and a the radius of the wire anode in inches, and Vt is the threshold
voltage at which multiplication occurs. Vt is a function of the gas
pressure and is determined by:
Vt =Y P (a log b/a)
Equation 4.1 can therefore be rewritten as:
In A
or:
In A
klVO ap I Vo log b/a yap log b/a - 1
k
Y (10gV~/a )- k rapA:-~-g-b"""';"/-a
(4.2)
(4.3)
(4.4)
For most of our applications, pure methane at atmospheric
pressure was used as the counting gas. The amplification
characteristics of our counter are well summarized in an article by
Gold and Bennett (117). Their counter IV consisted of a cylindrical
142
counter body with a = .0005 in. and b = 1.065 in., and used methane at
38 cm pressure. Our counter had a = .00025 in. and b = 0.500 in.,
with methane at 76 cm pressure. Comparing these parameters with
equation 4.5, one sees that the two counters should behave
identically, since the parameters ~p and bla are the same. Table
4.2 summarizes Gold and Bennett's experimentally determined
amplification factor for this counter over a range of bias voltages.
These data are plotted in Figure 4.6; the resulting curve should
closely approximate the amplification characteristics of our
proportional counter.
143
Table 4.2. Experimental gas amplification factors for a methane counter whe~e ;-ap = 0.1378 and b/a = 2x103.
In V 103 In A V A V
6.6 1.20 735 2.42
6.7 1.40 812 3.12
6.8 1.66 898 4.44
6.9 1.92 992 6.72
7.0 2.22 1097 11.4
7.1 2.51 1212 29.9
7.2 2.82 1339 43.7
7.3 3.10 1480 98.4
7.4 3.40 1635 260
7.5 3.68 1808 776
7.6 3.98 1998 2.84x10 3
7.7 4.28 2208 1.27x10 4
7.8 4.56 2441 6.81x10 4
7.9 4.90 2697 5.49x10 5
7.95 5.10 2836 1. 91x10 6
8.0 5.07 2981 3.66x10 6
8.05 4.85 3134 3.98x10 6
108
Met
ha
ne
Pro
po
rtI
on
al
Co
un
ter
/
137i
/ /
/ ~
106
105
z 0 H
104
r- <t
~/
u H
lL.
103
H
..J a..
.t?
l:
<t
102
~
/ 1
0-1
/'
./
I J
J I
J
50
0
10
00
1
50
0
20
00
2
50
0
30
00
3
50
0
BIA
S VO
LTAG
E (V
)
0
Fig
ure
4
.6.
The
ex
peri
men
tal
(so
lid
li
ne)
and
th
eo
reti
cal
(bro
ken
li
ne)
am
pli
ficati
on
ch
ara
cte
risti
cs o
f o
ur
pro
po
rtio
nal
co
un
ter
usi
ng
met
han
e at
atm
osp
heri
c
pre
ssu
re.
i-' ~ ~
145
The values of and k in equation 4.4 can be determined from
this graph. Taking two points at the extremes of the linear region,
we have:
(a) Vo
(b) Vo
1485, A
2480, A
102 , and
105, where
log bla = 3.301, /log bla = 1.817, and lap
Substituting into equation 4.5, we have:
and
(b)
k (1485 ) Ii485 In A = Y 3.301 - k (0.1378) I~
4.605 = 449.86 1 - 2.923(k) y
In A k = -y
11.513
2480 _ k(0.1378) /2480 3.301 3.301
751.29 1 - 3.777(k) y
0.1378
or
or
Solving these two equations simultaneously, we obtain:
(k/y)
k
-2 3.27 x 10 and
3.46
so that according to the Rose and Korff theory,
(4.5)
for our counter. This curve is plotted as a dotted line next to the
experimental gas amplification curve in Figure 4.7.
In early experiments a mixture of 10% methane/90% argon was
tried as a counting gas. The gas amplification curve for our counter,
108
107
10
6 -
z 1
05 0 H
t- ~
10
4 u H
\J.
. H
...
J 1
03 0
. ~ ~
102 1
0 1- 60
0
Arg
on
-Melh
an
e
(90
/10
) P
rop
orl
lon
al
Co
un
ler
./ -::
? P
' -:
?
80
0
10
00
1
20
0
14
00
1
60
0
18
00
BIA
S V
OLT
AG
E (V
) o
2013
0
Fig
ure
4
.7.
The
ex
peri
men
tal
(so
lid
li
ne)
and
theo
reti
cal
(bro
ken
li
ne)
am
pli
ficati
on
ch
ara
cte
risti
cs o
f o
ur
pro
po
rtio
nal
co
un
ter
usi
ng
90
%
Ar/
lO%
CH
4 at
atm
osp
her
ic
pre
ssu
re.
f-'
.j:>
. (j
'\
147
using this mixture at atmospheric pressure, is shown in Figure 4.7
along with the Rose and Korff calculated amplification curve whereby:
In A 13.83 x 10-3Vo - 0.2992 Vo (4.6)
As seen in these curves, this gas mixture was not useful for our
purposes because at bias voltages as low as 1500-1800 V the counter
was acting in the geiger mode. Although this is useful for
determining how many X-rays are detected, the signal is no longer
proportional to the X-ray photon energy, and it is not possible to
pulse height discriminate the signal. We therefore determined that
pure methane was the best counting gas to use for our experiments.
Using methane, bias voltages up to about 3000 v could be used so that
very low energy X-rays could be distinguished above the noise.
Stepping Motor. A Syn-Rapid stepping motor was used to drive
the spectrometer goniometer and gear mechanism which determined the
detector (and crystal) angular position. This was controlled by a
Heathkit ET-3400 single board microcomputer interfaced through a
MC6820 peripheral interface adapter chip. A complete description of
this stepping motor controller, with program, is given by Peter
Rathman (118). The stepping motor was advanced a preset number of
steps whenever a pulse from a TC 555P counter/timer (used to monitor
the beam current) was received at its "interrupt request" (IRQ) input.
The number of steps advanced was determined by the product of the
numbers stored in memory locations OOOD and 0010. The most common
148
step sizes used and the contents of OOOD and 0010 are summarized in
Table 4.3. The IRQ signal was also fed to the external channel
advance input of the TN1700 multiscaler so that channel advance and
scanning position were synchronized. The '~old" potentiometer on the
TC555P (which controlled the length of the master output pulse) was
adjusted so that data collection was inhibited until the new position
was reached and the stepping motor was no longer turning; this was
accomplished by feeding the amplified data signal from the detector
through a linear gate, and inhibiting the output of the gate to the
MCS for the duration of the delay time. This is explained in more
detail in the next section on electronics.
An on/off switch was used in series with the power supply
lines to the stepping motor to shut it off for manually setting the
goniometer position. Without this, as long as voltage is supplied by
the controller to the motor, the setting of the goniometer cannot be
adjusted., Microswitches were also used at each limit of the
goniometer travel to protect the stepping motor. When either switch
was tripped, ihe voltage supply line was '~pen" and no power was
supplied to the motor; as soon as the direction was reversed, the
switch closed and power was again supplied. For these experiments,
scanning was only done in one direction. The spectrometer was ~
manually set to 0.00 (26), or any other desired starting angle, and
the stepping motor advanced the scanning to the limit of the
goniometer travel (about 820 ). With minor modifications in the
control program the spectrometer could be continuously scanned in both
Table 4.3. Necessary memory contents of microprocessor to obtain desired spectrometer step size.
Step size (28) Memory Contents OOOD 0010
.01 04 01
.02 04 02
.04 04 04
.08 08 04
.16 08 08
149
150
directions, but the MeA used did not have these capabilities.
Instead, each spectrum should be scanned a number of times, comparing
ea~h scan with previous scans before it is added to the multichannel
analyzer memory and the spectrometer manually set to zero.
Tuning the Spectrometer. Exact alignment of the spectrometer
is necessary to obtain the best spectral resolution. The following
steps, although tedious, must be used to properly align the system:
1) With the spectrometer open, the detector was set at 00 and the
entrance slit and masking slit were rotated until they were parallel
with the crystal and detector slit.
2) Initially, a flexible mica crystal was placed in the crystal
holder, and the detector was moved to a midway position. A
bright, diffuse visible light source was used to uniformly illuminate
the entrance slit of the spectrometer. This produced a line image on
the crystal and a reflected image on the detector. Adjustments were
made so that the line image formed at the detector was parallel to and
in the center of the proportional counter window.
3) It is important that the line image remain centered and in
focus over the entire 28 range. Checks were made to keep the focused
image in the proportional counter as it was scanned, and adjustments
were made alternately to the crystal flexing cam (using the focus
knob) and the crystal 8 angle (using the rocking adjustment) until the
best image was obtained over the limits of the detector travel.
151
4) When the above seemed satisfactory, the mica crystal was
replaced with a lead stearate crystal and the electron gun coverplate
was fastened to the sample chamber. Generally a carbon target was
used with the electron gun to maximize the resolution for low energy
X-rays. The spectrometer was closed and the entire system evacuated.
5) The detector was activated and the carbon ~ X-rays were
observed. Minor adjustments were made to maximize the signal,
alternately using the focusing and rocking controls. Careful
adjustment of these controls produced the strongest signal and optimum
resolution.
6) At times, when interested in analyzing compounds emitting X
rays of higher energy, either a copper or aluminum target was used in
place of carbon. The aluminum was especially useful, since one could
tune-up on several of the 7 orders visibly diffracted by the crystal.
7) When the strongest signal was obtained using this procedure,
the electron gun was replaced with the solid target coverplate and the
system was evacuated overnight. A final check was made using the
proton beam to assure optimum alignment of the spectrometer.
Electronics
The output signal from the proportional counter was fed into a
low noise preamplifier, a pulse shaping amplifier and a single channel
analyzer before it was input to a linear gate whose output went to a
multichannel analyzer. The current monitoring signal from the sample
wheel Faraday cup was fed into a current digitizer whose digitized
output went to a dual counter/timer. The master output signal from
the counter/timer was fed to the stepping motor controller, the
channel advance input of the MCA and the DC inhibit input of the
linear gate. A schematic of the electronics is included as Figure
4.8.
152
The preamplifier used was an in-lab design that utilized a
wide band video amplifier IC chip. In early work, an MC l552G chip
was used in the circuit shown in Figure 4.9. This chip had an
internal gain of 50-100, although the overall gain of the preamp was
about 2. These chips are increasingly hard to find, so in later work
a new preamp was designed around an MC 1733C differential video
amplifi~r. This is a low noise wide band amplifier with differential
input and output that offers fixed internal gains of 10, 100 or 400.
A schematic of this preamp is included as Figure 4.10. A more
complicated version of this circuit, which gives lower noise, was
designed by Sheehan and Lamoreaux in 1974 (119), but no schematics for
the circuit were available. Any of these three preamplifier designs
could be used successfully with the system. In each case, the
preamplifier was built into an insulated box using a 9v battery power
supply to eliminate noise pickup from power lines and outside sources.
The 9 volts was stepped down to the voltage required by the IC chip
using a series of Zener diodes.
The preamplifier signal was fed into an Ortec 452 spectroscopy
amplifier for pulse shaping and further amplification. This amplifier
was used in the negative input mode, with a unipolar output in the
+lOv range. It was experimentally determined that a 1 ~sec time
Fig
ure
4.8
.
X-r
ay
)----
--
_~~~~l
~i>---
0.
QI u II)
Tp
nll
Pll
'c
Co
un
tpr/
T
imer
.... f
l j
~ Ii
<1l
>
-51l
The
ele
ctr
on
ics
use
d
for
hig
h r
eso
luti
on
PIX
E.
<-
---
TN
17
00
fIC
A
seA
U"eo
, 1
Gat
!' HC
S
p It
her
or
Am
p ),
PIIA
......
U1
W
I'RE
MU
'
OU
T
IIV
IN
<"" l
kV
) IIV
OIl
T
O,l
mf.
l o
---ll-------·
lK
f,rt
'un
d
r;Om
rd
SUI:;\
Y --[
.... ~---
---,--
, 1
0
0
--1 ""
2 7K
I r-W
W,
0-
I L
. .!W,,_
I 11
10
.OO
lmf.
1 I
(,kV
r-----I .O
Olm
fd
c.kV
-----ill---
4.7
mf,
\
15V
ti
c
IIC
I55
2G
IIp
en
..,.
g:l
ln o
f 5
0
gild
=
g
ain
o
f lO
O
Fig
ure
4
.9.
The
p
reaM
pli
fier
cir
cu
it
inco
rpo
rati
ng
an
Me
1552
v
ideo
am
pli
fier
ch
ip.
f-'
V1
.J:-
----
----
..
-hV
5
t1p
plv
O.l
mrd
so
v lK
I
7 r.
s
R
'J 11
1
50
. f-
-
=-g
n1
uIl
d
I'rp
nn1p
-f
(,V
OU
T q
UII
I'Iy
_L
) l'
-.L
n----
---rl-
l!lf
lLL
21
K
F'"''
.0
----"T
"----O
IIV
.·
ut
7.
'1 II
1'
,
1111
:
11-
.OU
hn
rd
22H
11\'
----
-0
In
1="
J H
t:I7
1 It
:
I!n
ln o
f 4
00
wh(
'n
jum
l"'r
ed
(.,.
)kV
)
Fig
ure
4.1
0.
The
p
ream
pli
fier
cir
cu
it
inco
rpo
rati
ng
an
Me
17
33
dif
fere
nti
al
vid
eo
am
pli
fier
ch
ip.
f-'
lJ1
lJ1
156
constant gave the best results. The amplilfier coarse gain and fine
gain potentiometers could be varied depending on the energy region of
interest, but were generally set at 10 and 1.0, respectively, with a
bias voltage of about 3000 on the proportional counter.
The signal from the spectroscopy amplifier was fed to a
Tennelec TC2l4 single channel analyzer where the low and high energy
ends could be discriminated to look only at the energy region of
interest. To set the upper and lower level discriminators, the main
amplifier output of the TC2l4 (see figure 4.11) was fed directly to
the TN 1700 set for pulse height analysis, and internal amplifier and
discriminator. The bias voltage setting on the proportional counter
and the amplifier settings on the TC 214 were adjusted for a
particular element by obtaining a good peak shape in the energy region
of interest on the TN 1700. The lower and upper level discriminators
(LLD and ULD) of the TN 1700 were then set to eliminate low energy
electronic noise and high energy interference and clipping. The TN
1700 discriminator settings were then converted to TC 214 settings
using the experimentally determined graphs shown in Figures 4.12 and
4.13. The discriminated output of the TC 214 was then fed to the input
of an ORTEC 426 linear gate, and the output of the linear gate went to
the multichannel scaling input (MCS) of the TN 1700. Both the
discriminated output and the main amplifier output of the TC 214 were
terminated with son terminators.
At the same time, the current from the sample wheel Faraday
cup was fed to an Ortec 439 current digitizer, which gives a digitized
TE
NN
EI.
EC
'11
.: 21
1,
SIN
I:J.
E
Cll
hNN
EI.
hN
hl.V
ZE
fI
-"
-_
~Ilir.
I//~
,-,
lnv
.
\...
) '-
h
IliI
'U\'
Rh
,'p
ing
: tn
t Ip
nl.1
.f"
po
l:u
lty
: In
vp
rt
lim
ing
' I.
E
mil
lie:
6
F.
6 E
m:1
x:
IOV
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f"\ .
OB
rl m.D
1-
---0 b "
'-'~-BJ ~ ..
o E
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(fr(
lm U
r-te
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tlh
lN
MIT
'
OU
TPU
T
IllS
CR
IH1
Nh
TE
D
OU
11'U
'I"
(I n
1'
11/1
) (t
II
I f I
IP'1
r r.
ate
)
Fig
ure
4.1
1.
The
T
enn
elec
TC
214
sin
gle
ch
ann
el a
naly
zer.
J-I
V1
-..
.J
'<t"
N
u l- t.- o a -I
6 5 4 3
-I
2
o
Iwt,
.:
min
llTlIu
n ~t'tlll1'~s
of
l.I.lJ'~
'IN
171l
tl:
n.l
llv
n:
214
: U
.Il'
"
2 3
----------------
r,o (
l T
f'rm
inat
io
n
4 5
6 7
LLD
o
f TN
1
70
0
(vo
lls)
Fi~ure
4.1
2.
The
lo
wer
le
vel
dis
cri
min
ato
r cali
bra
tio
n fo
r th
e T
enn
e1ec
TC
214
sin
gle
ch
ann
el a
naly
zer.
I-'
U1
0:
>
6-~---------------------------------------------------------------~
5 11
1.11
of
n:
2 If
. "
F I
I E
4-
.".. N
0 3
t- '+- 0 Q
2 -l
:J
c)o n
Tf'
rmll
1:lt
Io
n
10
2 3
4 5
6
ULD
o
f TN
17
1010
(v
olt
s)
Fig
ure
4.1
3.
The
u
pp
er le
vel
dis
cri
min
ato
r cali
bra
tio
n
for
the
Ten
ne1
ec
TC
214
sin
gle
ch
ann
el an
aly
zer.
I-'
U1
'-0
160
count rate proportional to the current. This was set for positive
polarity at 10-7 coulombs/pulse. The digitized output signal was fed
to a Tenne1ec TC555P counter/timer. When a preset number of counts
was reached, a pulse was output to the external channel advance input
of the mu1tisca1er and to the IRQ input of the stepping motor
controller. In this way the crystal and proportional counter were
moved to the next 26 position and the MCS advanced to the next
channel. The output pulse length was variable (0.1-10 sec), using the
hold time potentiometer on the TC 555P, and was set just longer than
the time needed for the stepping motor to advance to the next
position. The pulse was fed to the DC inhibit input of the linear
gate, so that during the time of the pulse the signal going to the MCS
was gated off to inhibit data collection while the stepping motor was
advancing.
Operating Instructions
The following procedure should be followed when operating the
high resolution PIXE system:
1) Load the samples and pump down; it is generally advisable to
include a sample of carbon or aluminum for tuning and calibrating the
system. The vacuum should be on the order of 10-5 or better. Note:
the system should always be evacuated and let up to air slowly so as
not to break the detector window.
2) Turn on the counter gas supply to the preamp and adjust the
needle valve on the rotameter so that the float is barely above zero
161
(this gives a pressure of just above one atmosphere). Turn on the
preamp and turn on the bias voltage power supply. The bias voltage
supplied to the preamp should be increased slowly to the operating
voltage; sudden voltage surges can "blow" the preamp.
3) With the stepping motor switch in the 'off' position, manually
crank the goniometer to the ~ang1e of the X-ray line of interest.
The position can be calculated from:
e = arc sin (nA) 2d
or, assuming a PbSt crystal with 2d = 100.7,
28 = 2 arc sin (0.123 x n ) Kev
and referring to the experimentally determined spectrometer
calibration chart in Figure 4.14.
(4.7)
(4.8)
4) With a proton beam hitting the sample, place the TN 1700 in
the PHA mode with input from the main amplifier output of the TC 214.
As described in the previous section, the bias voltage, amplifier
settings and discriminator settings should be set to get a clean,
well-shaped peak for the element(s) of interest in the PHA mode. The
TC 214 main amplifier signal can also be fed to an oscilloscope for
continuous monitoring of the signal pulse height during scanning.
5) Switch the TN 1700 back to the multisca1ing mode with input
from the Ortec 426 linear gate. The memory should be cleared and set
80
70
~
W
t-6
0
w
L:
0 ~
t- U
111
0..
(f)
Z
0 111
30
.J
(9
Z
<{ <P
(\J
o
CALIBRATlor~
CU
RV
E
FOR
P
-N
VA
RIA
RL
E
CU
RV
ATU
RE
S
PE
CT
Rot
1ET
ER
10
2
0
TflT
Imm
rrrrm
mTT
nnTT
TTTr
nmnn
pnrrm
l1
30
40
50
6
0
70
80
CA
LCU
LA T
ED
?
e A
NG
LE
Fig
ure
4
.14
. T
he
ex
peri
men
tall
y d
eter
min
ed cali
bra
tio
n
curv
e fo
r th
e P
hil
ips-
No
relc
o
vari
ab
le
cu
rvatu
re b
en
t cry
sta
l sp
ectr
om
ete
r.
t-' '" N
up for the number of channels needed to collect the spectrum of
interest.
163
6) The cycle switch on the Tennelec counterltimer should be in
the "single cycle" position. The stepping motor can then be plugged
into the controller power supply and the step size input into memory
locations OOOD and 0010 as explained earlier. The stepping motor scan
is then initiated by entering D800E.
7) Manually crank the goniometer to the angular starting position
and turn the stepping motor switch to lion". Set the dwell time at
each position by adjusting the preset counts on the Tennelec
counter/timer; this actually sets the number of ~coul/channel or
~coul/step.
8) Initiate the scan by pressing the '~cquire" button on the
TN 1700 and then moving the cycle switch on the counter/timer to the
"recycle" position. As soon as the preset number of counts are
reached, the data collection is gated off, the stepping motor and MeS
are advanced to the next position, and data collection begins again in
the next channel.
9) Usually the scanning was stopped manually by changing the
cycle switch on the counter/timer back to the "single cycle." Data
acquisition was stopped on the TN 1700 and the stepping motor switch
was turned to off. The goniometer could then be reset for another
scan.
10) The acquired spectrum could be punched onto paper tape or
dumped to' the laboratory NOVA computer for plotting. Alternatively,
the spectrum could be added to a previously stored spectrum in the
TN 1700 memory.
Experimental
164
The instrumentation described previously was used to provide
high resolution X-ray emission spectra of a number of chemical
compounds. Initially, our intent was to use this system to aid in the
identification of the molybdenum sublimates that result from the
sulfation of ferromanganese nodules (see Appendix A). Information on
chemical shifts from this technique, in conjunction with data from
X-ray photoelectron spectroscopy (XPS), infrared and X-ray powder
diffraction studies, should provide conclusive identification of the
Mo reaction products. We also wanted to use the system to extend the
lower range of the PIXE spectra to enable us to look at the low-Z
elements, and to look at a series of Cu(I), Cu(II) and mixed valence
copper compounds that we have been interested in for many years. The
information obtained from these studies is summarized in the following
sections.
Studies on Low-Z Elements
Initially, a great amount of effort was dedicated to obtaining
a good high resolution spectrum of carbon. The Ka emission of carbon
(at 0.277 KeV) is nearly in the same region as the ~ emission of
molybdenum (at 0.31 KeV), and it was reasonable, therefore, to try to
get a good spectrum of the more intense K-lines at this energy before
attempting to see the weaker M-lines. Also, since carbon was
inevitably present in all samples due to surface contamination, pump
oil, etc., this made a good calibration line.
Carbon was first used as the target in the electron gun
165
arrangement described earlier to align the system. No spectrum could
be observed until the proportional counter gas was changed from Ar-CH4
to pure methane, allowing us to operate at bias potentials above
2000V. Under these conditions, a carbon peak could be seen at about
520 • (The expected angle, according to the Bragg equation, would be
52.70 using a PbSt crystal.) The solid sample cover plate was then
substituted for the electron gun arrangement, and a H2+ ion beam was
used to excite a spectrographic grade graphite target. Problems were
initially encountered when high levels of radioactivity were noted
near the sample chamber. Apparently a nuclear reaction was initiated
that had a measured half-life of about 10 minutes. When a H+ beam was
used instead, no such reaction occurred. The problem was subsequently
solved when the Pd leak on the hydrogen source bottle was found to be
contaminated with deuterium. The D2 apparently caused a conversion of
C12 to N13 , giving off neutrons, and the N13 decayed by positron
emission to 013 with a half-life of 10.0 minutes. When the Pd leak
was changed, no further problems were encountered using the higher
mass beams, although we generally continued using 2 MeV H+ ion beams
for most analyses. A 2 MeV high resolution spectrum of carbon is
included as Figure 4.15. The high background at low 26 values was due
to an intense straight-through X-ray peak that was at a maximum at 00,
and slowly decreased to background levels at about 200•
~ K
a
1st
ord
er
52
.58
0
CAR
BON
S
PE
CTR
UH
2HeV
H
+ b
eam
329
4402
cts
Fig
ure
4
.15
. H
igh
reso
luti
on
PI
XE
sp
ectr
um
of
gra
nh
ite
shm
.;rin
g 1
ord
er
dif
fracte
d o
ff
a P
bS
t crv
sta
l.
I-'
0\
0\
167
The carbon peak behaved erratically when tuning-up on the
single peak, and an Al target was therefore substituted both on the
electron gun and on the PIXE sample wheel. A good Al spectrum was
obtained showing 7 orders of diffraction off the PbSt crystal. A
representative PIXE spectrum is included as Figure 4.16, showing a
scan from 40 to 800 using a 1 pA 2 MeV proton beam and a proportional
counter bias voltage of 2850 volts. The 4th order Al line (at 38.00 )
was used to tune up the spectrometer by systematically varying the
rocking and focusing controls to obtain maximum intensity and
resolution as described earlier. When optimum focusing was achieved,
the FWHM of the 4th order Al peak was 0.240 26, or about 3 eVe
Molybdenum Extraction Products
Before analyzing the more complicated air-sensitive molybdenum
compounds that related to the Mo extraction of ferromanganese nodules,
pure Mo metal and Mo03 were analyzed using this system. Pellets of
each were prepared as described earlier, using a Carver hydraulic
press and a 13 mm KBr die. The samples were mounted and analyzed
using a lpA H+ beam at 2 MeV. With no discrimination, only the Mo L
lines off the mica backing were seen for both compounds as shown in
Figure 4.17. When the higher energy L-lines were discriminated out to
see the low energy M-lines off the PbSt, no lines were visible in the
spectrum. Apparently carbon, present on the sample surface and in the
crystal and proportional counterwindow, was absorbing the molybdenum
M-X-rays, since the absorption edge of carbon lies just below the M-X
ray emission energy of molybdenum. If such is the case, the
l I \ 1. v
1st
o
rder
8.4
4A
O
8.8
0
1 tL ttl 2n
d o
rder
18
.40
I r,
~~ I.
I r~\
I III
'\~l'V
IVV \
...v.
-J'\.
., 56
U
5
5th
o
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175
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ure
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igh
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molybdenum M-lines would never be seen, since it is virtually
impossible to eliminate carbon from the system. The L-X-ray spectra
of the two compounds were virtually identical, showing no shift in
peak energies, and therefore, these could not be used in the
identification of the sublimate products from the Mo extraction of
ferromanganese nodules. Consequently, it was decided to look at the
higher energy L-emission of some of the first row transition metals.
Cu(I)/Cu(II) Compounds
170
A series of copper compounds were prepared as before,
including Cu metal, Cu(C2H302)2' Cuel and CuCI2• It was hoped that
chemical shifts could be seen in the L-X-ray emission spectrum going
from Cuo to Cu+ to Cu++. The samples were initially analyzed using 2
MeV protons with no discriminatiton, and a spectrum was obtained
similar to that shown in Figure 4.18. Both the K-lines off the mica
backing and the L-lines off the PbSt were observed. By selective
discrimination, either the 8 orders of K-lines or the 4 orders of L
lines could be eliminated from the spectrum. For our purposes, the K
lines were eliminated using a LLD setting of about 1.6 with a bias
voltage of 3000V on the proportional counter. The line widths of both
the 2nd and 4th orders were measured as 0.360 FWHM, and the higher
order lines were used for comparisons because of the greater peak
separation at higher 28 angles.
The emission spectra for the compounds did not seem to vary,
as shown in the copper chloride series in Figure 4.19, but problems
'Ii I r.a
\
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~I 8
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ure
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uCl
and
C
uC1
2.
I-'
"-I
N
173
were encountered with the samples when we tried to compere the
energies of the emission lines observed. Within minutes of starting
the analysis, the samples were reduced in the beam to Cu metal so that
any indication of a chemical shift was impossible to detect. This
phenomenon was first noticed for Cu(C2H302) when the vacuum was lost
as soon as the proton beam hit the target. Upon opening the sample
chamber, the smell of acetic acid was evident, and wherever the proton
beam had contacted the target, the sample had changed to copper metal.
The same effect was later noticed for CuCI and CuCI2 , but to a lesser
extent. It appears that this problem is related to the high
temperatures generated by the proton beam. If the current was reduced
to much below I ~A to decrease the amount of sample heating, the
signal intensity decreased prortionally and counting times became
prohibitively long. Even at I~A, a typical scan often took up to
3 hrs.
We tried to make the copper compounds more heat conductive by
mixing in ottCL compounds. Since the eventual aim of this research
was to characterize compounds in ferromanganese nodules, Fe203 and Mn02
were first considered as mixing agents. Neither was satisfactory,
since the Fe203 would not hold together in the beam, and the Mn02
glowed red hot and showed signs of tunneling where the beam hit.
Powdered molybdenum metal was next used, since it held up extremely
well in a I ~A beam in the earlier molybdenum studies. This did help
to eliminate the outgassing problems and reduction of the copper
sample, but only at mixing ratios higher then 1:3 (Mo:Cu). At these
ratios the beam only intercepted a fraction of the copper it would
normally, and the CU signals were markedly lower in intensity.
174
Also, the Ka:KS ratios appeared altered, GO it was felt this was not a
useful method for the reduction of sample heating. It appears that a
new sample chamber must be designed that incorporates sample cooling
and/or secondary X-ray emission from beam stable primary targets to
eliminate this problem. These alternatives will be discussed more
fully in the final section.
The last series of experiments dealt with maximizing the Cu
emission using various types and energies of ion beams. He+ was first
investigated at beam energies of 2, 3 and 4 MeV. + The lower energy He
beams were considerably worse than 2 MeV protons, but the 4 MeV He+
ions gave nearly the same intensity as 2 MeV protons, with a lower
background. At the time we could not go above 4 MeV with the Van de
Graaff, but higher energy He+ beams may well prove to be superior to
2 MeV proton beams. A 2 MeV Ne+ beam was tried, but no peaks at all
were observed. Next, a series of H+, H2+ and H3+ beams at different
energies were examined. The results are summarized in Table 4.4. If
the count rates and backgrounds are normalized to the number of
equivalent energy protons and plotted as in Figure 4.20, it can be
seen that 2 MeV H+ and 4 MeV H2+ beams give nearly the same high peak
to-background signal, although the absolute height of the peak will be
higher using 4 MeV He+ beams. In future studies, this may be the ion
beam of choice, although similar tests should be made with several
elements of higher and lower z.
Table 4.4. Observed counts and background in 2nd order eu peak using various ion beams.
Ion Beam Max. cts bkg.
2 MeV H+ 300 45 +
2.5 MeV H 360 120
3 MeV H+ 600 300
2 MeV H + 225 35 2+
3 MeV H2 390 35
4 MeV H + 600 100 2
2 MeV H+ 185 25 3+
3 MeV H3 360 25
175
EX
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177
Summary
For most analyses, energy dispersive PIXE is the preferred
technique because of the increased sensitivity over wavelength
dispersive methods. At times, however, a higher resolution system is
desired to aid in deconvoluting overlapping peaks or to observe X-ray
fine structure, and so a high resolution wavelength dispersive PIXE
system has been developed. This system has a resolution of about
2 eV, which is more than sufficient to separate interfering X-ray
lines from any elements present at observable concentrations.
Although the resolution of this system should also be
sufficient to observe chemical shifts in a number of transition metal
compounds, several problems have arisen which prevent the system from
being used for that purpose. Foremost of these has been sample
decomposition due to the high energy of the ion beams used for
analysis; the samples either are reduced or are burned up in a beam of
sufficient intensity to observe the emitted X-ray lines. Before
further work can be done in this area of high resolution PIXE, this
problem must be corrected, and it appears that any solutions to the
problem will inevitably involve major design changes in the PIXE
system.
Cahill (120) has claimed that the use of on demand beam
pulsing techniques, developed by Thilbea (121), can greatly reduce the
amount of heat generated in a sample. This would involve major
modifications to the electronics in our PIXE system, but is one
possible solution to the heating problem. A liquid nitrogen cooled
178
sample wheel may also be of help in reducing sample decomposition and
lopses due to heating. This technique was used by Oona (122) to cool
thin layers of powdered sulfur compounds on a thin backing, but it is
not known whether it would work for thick non-conducting targets,
since the heating appears to be localized in the vicinity of the beam
spot. Another technique which has not been investigated to date, is
secondary target PIXE. A series of beam stable metals could be used
as primary targets, and selected so that the energy of the emitted X
rey lies just above the absorption edge of the element of interest.
This same technique is used in modern X-ray fluorescence systems. In
the PIXE system very high beam currents could be used to give high,
monochromatic primary X-ray fluxes that would cause little sample
damage. The monochromatic primary X-rays should prove ideal for
chemical shift studies. This system would also be attractive for XPS
studies because of the low bremmstrahlung background emitted from this
type of source. The high backgrounds in conventional ESCA have been a
major deterrent to doing quantitstive XPS. With proper sample chamber
design, it should be possible to have a Si(Li) detector, crystal
spectrometer and electron analyzer operating together to allow
conventional PIXE, high resolution PIXE and XPS studies on one system.
The preliminary work done on high resolution PIXE has
presented a number of interesting and challenging problems and opened
up many avenueS for future research. The following list summarizes
some of the problems which should be addressed in the near future:
1) Beam stable compounds, such as metal alloys, should be
analyzed with the present system to determine chemical shifts.
2) The two detection systems should be combined on the same
system to allow simultaneous energy dispersive and wavelength
dispersive analysis.
179
3) A new sample chamber should be designed which permits water or
liquid nitrogen cooling of the sample during analysis.
4) Secondary target PIXE should be investigated, for the purpose
of carrying out conventional and high resolution PIXE and possibly
ESCA studies.
CHAPTER 5
CONCLUSIONS
This dissertation reports the development and use of X-ray
emission techniques for the analysis of unusual and difficult samples
that cannot be analyzed by conventional analytical techniques. Low
flux radioisotope X-ray sources have been used to non-destructively
characterize the elements present in photographic papers and emulsions
from the collections of the Center for Creative Photography; the
information obtained has proven valuable for cataloging and preserving
photographic prints of historical significance and has led to the
inclusion of an X-ray facility in the new CCP building for the routine
elemental analysis of photographs. Radioisotope sources have also
been used in the development of low-cost, portable instrumentation
useful for quantitating a variety of toxicological samples, including
urine and feces samples to determine the elimination rates of X-ray
contrast media containing dysprosium.
The PIXE technique has been applied to the analysis of
forensic samples, including bullet lead, tissue fragments and thin
metal coatings, and compared to other non-destructive methods of
analysis. Sample preparation techniques and analytical procedures
have been developed for general, thin target, quantitative, PIXE
analysis. These procedures were applied to the analysis of deep-sea
ferromanganese nodules and of NBS standard referen~e materials,
including orchard leaves, pine needles, bovine liver, coal and fly
180
ash. We have shown that the PIXE technique can be a valuable
analytical tool for rapid, non-destructive multie1ementa1
determinations of extremely complex samples.
181
A high resolution PIXE system has been developed to aid in the
deconvolution of overlapping X-ray peaks encountered in conventional
PIXE, and has been applied to the measurement of chemical shifts in
the X-ray emission spectra of first row transition metals. This
technique introduces some exciting new areas of research which should
prove useful for developing a more complete bonding picture for many
compounds of analytical interest.
Several areas of future research using the present X-ray
facilities have already been outlined at the conclusion to each
chapter. The present facilities must be continually updated and
improved, however, to keep X-ray emission techniques competitive with
other analytical methods. Assuming unlimited support and manpower,
the following suggestions are made to extend the work presented here
and to update the facilities to utilize state of the art technology:
1) The computer capabilities of the present X-ray facility should
be upgraded to facilitate the automation of the equipment and of the
data reduction, and to integrate the data obtainable from
complementary simultaneous detection systems.
2) Since this PIXE work was completed, several advances have been
made in thick target quantitation. Many of these involve an iterative
process based on thin target approximations with adjustments for X-ray
absorption using absorption filters between the sample and detector.
These advances should be integrated into the present FIXE facility,
and the sample and filter changing systems should be automated to
require less direct interaction of the operator.
3) The feasibility of a proton microprobe has been demonstrated
by other workers. A PIXE system capable of micron size spatial
resolution would prove extremely useful for forensic applications.
Such a system should definitely be considereo at the University of
Arizona.
182
4) The possibility of using the 6 MeV accelerator at the
University of Arizona for Rutherford backscattering is presently being
considered. These studies can well be coupled with FIXE experiments
to allow depth profiling studies and analysis of low Z-elements.
5) Ultimately, one would like to obtain not only elemental but
chemical information from X-ray emission studies. It may be possible
to use high resolution X-ray emission techniques for obtaining
detailed information about the electronic structures of compounds.
This will require the use of high resolution detectors with higher
efficiencies than are now obtainable with a crystal spectrometer. The
combination of X-ray emission spectroscopy, X-ray photoelectron
spectroscopy and ultraviolet photoelectron spectroscopy should provide
a more detailed description of bonding in molecules.
6) To incorporate these new areas of XES, and to couple XES with
other techniques, a new, more versatile sample chamber must be
designed for the present FIXE system at the University of Arizona.
183
This should include provisions for calibrated, computer controlled
sample movement during analysis, sample cooling, automated sample and
filter selection, and analysis by secondary target emission.
Secondary target emission should prove extremely useful for trace
level determinations of specific elements and for chemical shift
determinations using a high resolution detector. The new sample
chamber should be designed to allow simultaneous detection by a
variety of systems, including: a Si(Li) detector for conventional
analysis; a Ge(Li) detector for the determination of light elements
by -emission; a high resolution crystal spectrometer detector; and an
electron analyzer.
7) The possibility of using the monochromatic X-ray emission from
beam-stable PIXE targets as a source for ESCA studies should be
considered. The very low bremsstrahlung background generated by this
system should prove attractive for quantitative ESCA determinations.
Also, the combination of an XPS analyzer and Si(Li) detector on the
same system, using monochromatic X-rays from a proton-excited metal
target, may lead to improved experimental values of cross-sections and
fluorescent yields.
8) Finally, all possible attempts should be made to make a
smaller, more inexpensive PIXE system. A recently developed
accelerator can deliver protons at an energy of SO MeV per meter
length; it is being scaled up to deliver BeV and GeV protons for other
purposes, but if it were scaled down instead, a foot-long accelerator
would result that would be ideal for PIXE. With appropriate magnets,
184
pumping system and detector, a bench top FIXE system could be made and
incorporated into any analytical laboratory.
APPENDIX A
EXTRACTION OF MOLYBDENUM FROM FERROMANGANESE NODULES
For several years our research group has been interested in
the extraction of molybdenum from ferromanganese nodules. John Jong
Hae Lee (University of Hawaii) has used the experimental set-up shown
in Figure A.l to break down the iron-manganese nodule matrices and
selectively extract molybdenum from ferromanganese nodules by
sublimation (123). Using a 4:1 volume ratio of S02:02 at 4000 C and a
flow rate of 50 ml/min (adjusted with a He carrier gas), Lee obtained
colored condensates at the outlet of the sample tube which were
identified by PIXE to be essentially pure molybdenum (see Figure A.2).
He further determined that the yield increased and the reaction time
decreased when the nodule was impregnated with 2-5% (w/w) of halides,
such as NaCl, NaBr and CaC12• The sublimates, consisting of deep
green (over 10%), brown (25%) and yellow (remainder) material, formed
in bands and could be separated by color. By using X-ray powder
diffraction and halide analysis, Lee determined that both the green
and brown materials were molybdenum tetrachloride (MoC14), and the
yellow material was molybdenum dioxydichloride (Mo02C1 2). He has
suggested the following reaction steps in this extraction:
1) disruption of the major Mn and Fe metal oxide matrices with
S02 and S03 present in the gas mixture.
2) reduction of Mo03 in the nodule to Mo02 by S02'
185
+-
(f)~ '.
f"
Con
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Fig
ure
A
.I.
Sch
em
ati
c re
pre
sen
tati
on
O
F L
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up
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35
Fig
ure
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I'IX
E
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um
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I-'
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3) reaction of the Mo02 and remaining Mo03 with the halides in
the solid matrices to produce volatile metal halides according to the
reactions:
188
Evidence for this reaction scheme was given by reacting a mixture of
Mo03 and NaCl alone under the same experimental conditions. The
product obtained was essentially all Mo02C12 with only traces of MoC14
present, indicating that the second reaction above predominated. Lee
suggests that this results because the Mo03 is in direct contact with
the chloride ions, whereas in ferromanganese nodules spiked with NaCl,
there is a time lag during the disruption of the nodule matrices
during which the Mo03 is reduced to Mo02•
We have investigated the possibility that the major product
formed in the nodule extractions is actually not MoC14 , but rather
some other product that may have a Mo:Cl ratio of 1:4, possibly
MoOC14• In order to verify the products obtained by this technique, we
attempted to characterize them more fully using a variety of X-ray and
non-X-ray analytical methods; these included high resolution PIXE, X
ray photoelectron spectroscopy (XPS), X-ray powder diffraction and
infrared spectroscopy. Standards of MoC14, MoOC14 and Mo02C12 were
obtained from Alfa for analysis and comparison to the reaction
products. The results of these analyses are summarized in the
following sections.
Characterization of Molybdenum Standards
189
During early sample preparation studies using the Alfa
preparations of MoC14 and MoOC14 , it was noted that both compounds
underwent an interesting series of color changes when exposed to air.
The MoOCl4 instantly turned dark brown and emitted a white pungent gas
as it rapidly changed to dark blue; the dark blue is probably
indicative of the mixture of oxy-hydroxides known as "molybdenum blue"
that often form when Mo-bearing compounds are hydrolyzed (124,125).
The MoCl4 changed rather slowly to an oily dark brown (which, upon
dilution, was actually a very dark, concentrated yellow solution), and
after standing for a week it also turned to dark blue. Neither of the
colored residues from MoOC14 or MoC1 4 dissolved in CC14 , but both
dissolved, at least partially, in acetone and water. The yellow
(MoC14) residue seemed to dissolve more readily, giving a dark yellow
liquid, but it left an insoluble precipitate that remained even after
several days. The blue (MoOC14) residue dissolved more slowly, but
after a day it had dissolved completely, leaving no precipitate, to
give an intense blue solution. IR spectra were obtained from the oily
residues, as films on AgCl plates, and UV-VIS spectra were obtained
from the aqueous solutions, but both gave broad bands which were of
little use.
According to Larson and Moore (126), MoOC14 decomposes to form
MoOC13 and C12. It appears from my observations, however, that the
white gas evolved was instead HCl: the decomposition only occurred
when water was present; the evolved gas did not color KI paper; and
white NH4Cl fumes were seen when the gas contacted NH3 vapor.
Apparently, though, the brown intermediate formed from MoOC14 was
MoOC13• It appears that both MoOC14 and MoC14 may undergo the same
reaction to form molybdenum blue (at very different rates), so the
brown intermediate formed from MoC1 4 may also be MoOC13• To check
this, standards of MoOC13 and its starting material, MoOC14 , were
prepared using the following synthesis schemes:
reflux)
and
190
Both reactions must be carried out under dry nitrogen because of the
extremely hygroscopic nature of the compounds. After repeated
attempts I was able to obtain both products in high yield and purity.
After filtering and repeated washings of the MoOC13 with benzene under
anaerobic conditions, it was noticed that the benzene apparently
dissolved a small amount of the product, giving it a yellow color.
After standing for a week in the absence of air, the yellow benzene
solution slowly changed to a blue color.
The dried preparations of MoOC14 and MoOC13, along with the
Alfa standards, were sealed in glass vials under a dry N2 atmosphere
and stored in a desiccator for analysis by the techniques mentioned.
191
All sample preparations discussed in the remainder of this section
were carried out in a glove box under a dry N2 atmosphere using these
standards.
Infrared Analysis
The molybdenum compounds used in these studies were difficult
to work with because of their extreme sensitivity to moisture. It was
desired that a technique be developed which could quickly and easily
identify these Mo compounds without breakdown or hydrolysis to
molybdenum blue. Infrared specstroscopy was, therefore, the first
method investigated.
Initial attempts to obtain infrared spectra of these comounds
as solids met with little success. First, KBr pellets were prepared
using a Wig-L-Bug homogenizer and Carver hydraulic press. Although
the pellets were pressed in a N2 filled glove bag, they seemed to pick
up water, forming the characteristic molybdenum blue color. Next
polyethylene pellets were prepared by mixing the Mo compounds with
polyethylene powder in a Wig-L-Bug homogenizer and pressing in a
heated Al die under vacuum. This technique has been used by Huffman
(127) to look at compounds in the far IR. Again, the compounds
discolored,forming a dark green-brown for the MoC14 and dark blue for
the MoOCI4• We then tried to obtain infrared spectra of the
compounds as liquids.
Larson and Moore (126) show that MoOC14 is soluble in CC14 ,
giving a strong absorption peak at 1009 em-I, but that MoOC13 is not
192
soluble in CC14 (nor chloroform, methylene chloride nor benzene).
Bader and Huang (128) show that MoClS is soluble in CC14, so it was
likely that MoC14 would be also. Dried CC14 was tra~sferred
anaerobically to evacuated test tubes containing my preparations of
MoOC14 and MoOC13 and the Alfa preparations of MoC1 4 , MoOC14 and
Mo0 2C12• Both preparations of MoOC14 dissolved readily to give dark
red solutions, but the other three compounds dissolved only slightly
giving fine colloidal dispersions. When infrared spectra of the
solutions were obtained, both MoOC14 solutions gave peaks at 1011 cm- l
(the M=O stretch); no peaks were seen for the other solutions, whereas
the M=O peak was expected for MoOC13 and Mo02C1 2• According to the
CRC handbook (129), dilute alcohol and ether should dissolve Mo0 2C12
and MoC1 4• USP ethanol was transferred anaerobically into evacuated
test tubes of the above standards, and this time all the comounds
dissolved as follows: both MoOC13 and MoC14 gave dark brown
solutions; Mo02C1 2 gave a very light blue-green solution; and MoOC14
gave an emerald green solution with a white gas given off. IR spectra
were obtained for each solution with the results summarized in Table
A.l. Upon exposure to air, the MoOC13 and Mo02C12 solutions turned to
blue, while the MoC14 solution remained dark brown. The emerald green
MoOC14 solution changed to a brown intermediate (shown in Table A.I as
MoOC14(br») and then to dark blue. Both the emerald green MoOC14 and
the brown intermediate solutions gave identical IR spectra. From
these results, it appeared that an alcohol solution of the molybdenum
compounds would give unique, useable spectra that could be used to
Table A.l. Summary of IR peaks seen fer Mo compounds dissolved in ethanol.
Peak Compound
-1 (em ) MoOCl3 MoC14 Mo02Cl2 MoOCl4 MoOCl4
(br) (red br) (It,bl) (gr) (br)
1011 m '17 w w
987 vs vs s s
956 s s s
919 s vs vs
778 s
vs very strong; s strong; m medium; w weak.
193
194
fingerprint the sublimates from the Mo extraction of ferromanganese
nodules. The solutions formed were the result of reactions between the
molybdenum compounds and the ethanol, however, and as such were not
representative of the actual Mo compounds. We therefore investigated
the possibility of carrying out gas phase IR analysis on these
compounds using a heatable cell to sublime the solid Mo compounds.
There are several advantages to obtaining the gas phase
infrared spectra of these Mo compounds: the sample can be completely
isolated from air or moisture during sample preparation and analysis;
the obtained IR spectrum is of the pure compound; and higher
resolution may be obtained in the gas phase than in the liquid or
solid phase (130,131). Also, the high volatility of the molybdenum
sublimates makes gas phase studies particularly appropriate for these
compounds. Reber Brown (132) has analyzed the Mo standards using a
Perkin Elmer heatable gas IR cell and a Perkin Elmer 398IR
spectrophotometer. For all but the Mo02C1 2 sample, NaCl windows were
used on the gas cell, effectively limiting the scanning rangee of the
spectrophotometer to 4000 cm- l to 600 em-I. In each case, the solid
samples were loaded into the cell in a glove box under dry N2, and the
cell was then heated while in position in the PE 398 untilthe sample
sublimed (up to a maximum of 2000 C). Samples were run for a scan time
of 15 minutes at both medium and narrow slit widths. The resulting IR
spectra are summarized in Table A.2. The cell was cleaned between
samples by alternately purging the cell with dry N2 and evacuating it
while heating; this process was repeated until an IR scan showed a
195
flat baseline. The cell was then cooled, opened and rinsed with
absolute ethanol.
A unique spp.ctrum was obtained for each compound, as indicated
in Table A.2. From these results, it appears that MoOC14 can be
distinguished from the others by a strong peak at 995-965 cm- l ; MoC1 4
by a strong peak at 795 cm- l , MoOC13 by a strong peak at 1025 cm- l ,
and Mo02C1 2 by a weak doublet at 1600 and 1500 cm- l • In future
studies, it would be useful to extend the scanning range down to
200 cm-1 so that the Mo-Cl stretches could also be observed. It
appears that this technique will work well for fingerprinting the Mo
sublimates from the extraction of ferromanganese nodules. The solid
products can be sublimed in the gas cell, as was done for the
standards, or the gas cell could be connected directly to the quartz
reaction tube by means of the 18/9 stainless steel balljoint on the
gas cell, thereby minimizing sample preparation and the risk of
contamination. The cell can also be evacuated prior to heating to
cause the samples to sublime at a lower temperature, reducing the risk
of thermal decomposition. This technique should prove very useful for
the identification of the Mo compounds obtained from the sulfation of
manganese nodules.
X-Ray Powder Diffraction
A modified 1000 watt Phillips Electronic Instruments X-ray
generator with a CU excitationt tube and a 114.6 mm diameter Debye-
Scherrer camera were used to obtain X-ray powder diffraction patterns
for the standard Mo compounds. The system was initially aligned to
Table A.2. Summary of peaks found for gas phase IR analysis of Mo compounds. -- (129).
Peaks Compound
MoOC13
MoC14
Mo02C12
MoOC14
3640 w
2950 s
3040-2890 fw fw f
2885-2660 fw fw f
1600 v]
1500 w
1260 s s s s
1180 w
1085-1070 s s s s
1025 s
995-965 s
810 s s s
785 s
685 s
s = strong; w = weak; f fine structure
196
197
produce good powder patterns for NaCl. After alignment was complete,
1 and 2 mm diffraction capillaries were prepared in a dry box for each
of the standards and sealed with wax to prevent exposure to the
atmosphere during analysis. The samples were then run at 42 KeV,
40 mA, using CU Kal ,2 X-rays for diffraction. Repeated attemts were
made to obtain good powder patterns for the four standard compounds
(MoC1 4, MoOC14' MoOC13 and Mo0 2C1 2). A good powder pattern of Mo02C1 2
was obtained after a 3.5 hr run using developing and fixing times of 3
min and 10 min, respectively. The d-spacings observed are summarized
in Table A.3. The values obtained correspond well with those obtained
by Lee (123) for Mo02C1 2 •
Several exposures were made, varying from 2 hrs to 24 hrs
(using two pieces of film for the longer runs), for MoOC14 , MoC14 , and
MoOC13 , but no useable powder patterns were obtained for any of the
compounds. The problem was one of signal-to-noise; looking at
slightly darker lines on a dark background. It appeared that the
standards were not very crystalline and that X-ray absorption and
scattering were major problems. Longer exposure times therefore would
not help, but acceptable powder patterns should result if the
background could be eliminated. Arrangements were made with the
Department of Mining and Metallurgy at the University of Arizona to
use their X-ray diffraction system, which incor?orates a scanning
proportional counter detector in place of a Debye-Scherrer camera. In
this way the backgrounds could be eliminated through discrimination to
give much cleaner powder patterns. Dave Rollins (Department of
198
Table A.3. Characteristic d-spacings observed for Mo02C12.
d-spacing Intensity
7.012 vs
3.791 vs
3.431 w
2.773 m
2.290 vs
2.250 vs
1. 964 m
1. 757 s
1.500 w
1.180 w
vs very strong: s strong; m medium; w weak
199
Chemistry, University of Arizona) was going to run these samples as a
special project for a class on X-rays, but to date they have not been
run (133). This is soimething that should definitely be investigated
in future work on this molybdenum project. If the backgrounds can be
eliminated to give clear powder patterns for these compounds, then
this would provide an easy unambiguous method for identifying the
sublimation products resulting from the sulfation of ferromanganese
nodules.
High Resolution PIXE
As described in Chapter 4, a high resolution PIXE system was
constructed to look at chemical shifts in the M-level X-ray spectra
resulting from PIXE analysis of these comounds. These experiments
were unsuccessful, however, largely due to absorption of the Mo X-rays
by carbon on the sample surface and in the crystal and proportional
counter window. The K-absorption edge of carbon (at 0.283 KeV) lies
just below the energy of the most likely molybdenum M-line (at
0.331 KeV), so it is doubtful the molybdenum M-lines could ever be
seen with this system. The K- and L-level chemical shifts are too
narrow to be seen (they are on the orderof 0.1 eV) and as such the
chemical state of the molybdenum standards could not be determined from
shifts in the X-ray energy spectra using this system.
X-Ray Photoelectron Spectroscopy (XPS)
Since the Mo standards were so suited to gas phase IR studies,
gas phase XPS was also used to obtain ionization energies for the
200
compounds. The precision of the gas phase technique permits the
detection of relatively small shifts in ionization energies from one
compound to another, and therefore is the preferred technique when
applicable. A standard GCA McPherson ESCA-367 instrument was used
with a modified sample chamber that incorporated a heatable gas
ionization ~e11 as shown in Figure A.3 (134). Samples were placed in
a screw-on reservoir so they could easily be loaded without removing
the entire cell. The X-ray source was protected from sample
contamination by a thin A1 window which contained the sample vapors in
the gas ce11,and by an isolated diffusion pump for the source region.
To correct for energy drift during data collection due to changes in
pressure or instrument conditions, argon was introduced into the cell
through a Granville-Phillips variable leak valve and run along with
the sample vapors as an internal standard. Any drift detected in the
Ar 2p (3/2) line (at 248.62 eV) could be used to correct for drift in
the observed sample ionization energies between signal averaging
scans. Data collections were repeated as many times as sample
volatility allowed, alternating between argon reference scans and data
collection scans. Each data collection was then referenced to the
average of the argon positions just before and after the data
collection.
Representative spectra showing the Mo 3d 3/2 and 5/2 and the
C1 2d 1/2 and 3/2 peaks for MoC14 and MoOC14 are included as Figures
A.4-A.7. The calculated shift in ionization energy was +0.30 eV for
Mo and +0.663 eV for C1 going from MoC14 to MoOC14• This represents
ANALYZER OPENING
..,..-GASKET
SAMPLE mll""ll����___ RESERVOIR
-V TEMFERATUR E PROBE
Figure A.3. XPS heatable gas cell.
201
(f)
-4-'
( J o U
ion
izali
on
en
erg
y
(eV
)
24
6
24
4
24
2
24
0
23
8
23
6
Mo
3d
5'2
H
oCI 4
Mo
3d
3'2
, _
1-_
_
~/
----
----
' " '
-
1008
10
10
1012
10
14
1016
10
18
ele
clr
on
k
ineli
c
en
erg
y
(eV
)
23
4
III
Fig
ure
A
.4.
Gas
p
has
e X
PS
spec
tru
m o~
mol
ybde
num
3
d(3
/2)
and
3
d(5
/2)
pea
ks
for
MoC
14·
N o N
(I)
..aJ C
J o U
ion
iza
lio
n
en
erg
y
(eV
)
246
244
242
240
238
236
234
Mo
3d
S '2
HoD
el"
I ,
Mo
3d 3
'2
__
__
__
__
_ -
.,1
"
III
I I
I _
__
__
__
__
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~
I I
,I
I
, "
,I
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I II
1008
10
10
1012
10
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1016
10
18
ele
clr
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k
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(eV
)
Fig
ure
A.s
. C
,RS
p
has
e X
PS
spec
tru
m o
f m
olyb
denu
m
3d
(1/2
) an
d 1
d(5
/2)
pea
ks
for ~oOCI4.
N o w
(/) .., C
J o o
212
1042
ion
izati
on
en
erg
y
(eV
)
21
0
20
8
206
20
4
20
2
20
0
CI
2p
3/2
It I
\ ,
I /
'\
Irr----....
"-I I
HoC
I 4
I II
II
1111
II
I II
1
'1'
I ,
I II
I"
I I
I I
I
1044
10
46
1048
10
50
1052
ele
ctr
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k
ineti
c
en
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(eV
)
Fig
ure
A.6
. G
as
ph
ase
XPS
sp
ectr
um
of
ch
lori
ne
2p
(1/2
) an
d 2
p(1
/2)
pea
ks
Fo
r ~foC14.
N o -I>-
(J)
...j
.)
C
J o o
212 , ",
1042
ion
izali
on
en
erg
y
(eV
)
210
208
206
204
202
CI
2p
3/2
CI
2p1/
2
,y.'
,; ~--
-7,
/ ,
. '" /
/ ,
/ , ,
"
HoO
CI4
"
1044
10
46
1048
10
50
1052
ele
clr
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en
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(eV
)
200
Fi~ure
A.7
. n
as
ph
ase
XPS
sp
ectr
um
of
ch
lori
ne
2p
(1/2
) an
d 2
p(3
/2)
pea
ks
for
MoO
CI 4
.
N o U1
206
the average of many runs after correction for shifts in the Ar
calibration peaks as explained earlier. No shifts were determined for
the Mo02C1 2 since it had obviously decomposed during the analysis.
In future work it may be beneficial to introduce the sample
vapors into the gas cell from outside the sample chamber using a glass
sample cell equipped with a needle valve and vacuum fittings. If the
samples are not volatile enough to sublime into the gas cell at rOOm
temperature, the glass sample cell could be heated from outside using
an oil bath. Since the samples can be loaded into the glass cell and
sealed in a glove box under dry N2, and leaked into the heatable gas
cell through the needle valve after the chamber is evacuated, the
possibility of sample exposure to air during loading would be
elimlinated. Problems of sample decomposition due to contact with a
metal surface would also be eliminated.
The gas phase XPS technique looks very promising for
establishing the identity of molybdenum halides and oxyhalides when
combined with the IR and X-ray powder diffraction techniques discussed
previusly. In future studies the UPS spectra should also be obtained.
The combination of these techniques should provide a great deal of
information about the reaction mechanism involved during the sulfation
of ferromanganese nodules.
Molybdenum Extraction from Ferromanganese Nodules
To date, we have been unable to reproduce Lee's extraction
results except when using pure Mo03• Except for one instance when
water vapor was introduced into the reaction tube by accident,
207
separate colored bands of product have never been obtained. A slightly
modified version of Lee's experimentval set-up was used to keep out
any moisture and to simplify the process of removing the product from
the sample tube under anaerobic conditions. A schematic of the set-up
used is included as Figure A.B. Higher flow rates of the reaction
gases were required by this system, and generally the flow rates were
simply increased until bubbles could be seen to form in the gas
scrubber. Also, UHP nitrogen was substituted for the He carrier gas
because of cost.
A mixture of Mo03 and NaCl wns reacted according to Lee's
instructions,and a yellow sublimate was obtained as predicted. When
the same reaction was run using NaCl spiked ferromanganese nodules,
however, the same yellow sublimate was obtained instead of the
expected green, borwn and yellow bands. Both of the reaction poroducts
were analyzed by IR and X-ray powder diffraction, and they matched
identically the spectra and powder patterns obtained above for
Mo0 2C1 2• The extraction was tried repeatedly, and each time only the
yellow Mo02C1 2 sublimate resulted. When exposed to the atmosphere,
the yellow Mo0 2C1 2 changed slowly to blue (or rapidly when in contact
with a metal surface). It is possible that in Lee's set-up water
vapor could enter the reaction tube to hydrolyze the Mo02Cl 2 product.
Lee never indicated the order in which his bands formed, but Fernando
(135) believed the order was: nodule--yellow-green-brown. If such is
the case, water vapor may enter fom the scrubber to cause the
formation of the green and brown products, which may be MoOC14 and
~
02
N2
(A)
A.
LIn
de
" g
as
flo
w m
eIe
r
B.
Dry
ing
lu
be
C.
LIn
db
erg
-lu
be
fu
rna
ce
D. T
em
pe
ratu
re
con
sole
E.
Sa
mp
le l
ub
e
(C)
sam
ple
(H)
F.
Ho
I a
ir
blo
we
r
G.
Co
lle
cllo
n t
ub
e
H.
Co
nd
en
ser
I. W
ater
va
po
r tr
ap
J. G
as t
rap
Fig
ur2
A.B
. S
et-u
p
use
d
for
the ex
tracti
on
of
mol
ybde
num
fr
om
ferr
om
ang
anes
e n
od
ule
s.
N o 00
209
MoOC13 , respectively; these two compounds, having Mo in the +6 and +5
oxidation states, could rapidly form the molybdenum blue complex which
is a series of oxyhydroxides with a mean Mo oxidation state between 5
and 6 (124). At this point, this is purely speculation, and the role
of water in the extraction scheme must be investigated more fully.
Lee's proposed reaction scheme was examined using pure Mo02
(obtained from Alfa). According to his predictions, when Mo02 is
mixed with NaCl and reacted with S02 and °2, essentially all of the
product should be Mo02C1 2• When this was tested, again the only
product obtained was a yellow sublimate which proved to be Mo0 2C1 2•
It appears, therefore, that under identical conditions both Mo02 and
Mo03 react to give Mo02Cl2, which is in complete disagreement with
Lee's reaction scheme. These experiments should be rerun under more
closely controlled conditions, but if verified, a new reaction scheme
must be proposed.
Summary
The ground work has been laid for a more complete study of the
reaction products obtained from the Mo extraction of ferromanganese
nodules. Three techniques have been developed to characterize the
reaction products, including: gas phase infrared; gas phase XPS; and
X-ray powder diffraction. Several discrepancies have been found with
the work reported by Lee, and it appears that both his identification
of the major reaction product and his reaction scheme may be wrong.
Work must now be done to duplicate Lee's results and to develop a new
reaction scheme consistent with the results obtained from both
studies. The following suggestions are made for future
investigations:
210
1) To further develop the analytical techniques to identify the
reaction products: X-ray powder patterns should be obtained using a
proportional counter detector so that backgrounds can be eliminated;
the IR scans should be extended down to 200 em-I; and a new sample
cell should be investigated for the XPS work to eliminate sample
contact with the metal walls of the cell.
2) Since it appears that water vapor may play an important role
in the reaction scheme, this should be carefully investigated using
drying tubes. placed before and after the reaction tube. to control
moisture. The effect of sample drying time should also be
investigated in this light.
3) The effect of the S02:02 ratio and over-all flow rate should
be investigated to see if this affects the reaction products.
4) The experiments using Mo02 should be repeated to verify that
Mo02Cl 2 is the only resulting product.
S) Since Lee says that NaCI. KBr or CaCl 2 may be added to the
nodules. the use of KBr may help establish the reaction mechanism. If
Lee's reaction scheme is correct. MoBr4 and Mo02Br2 should be formed
as end-products.
APPENDIX B
NEUTRON ACTIVATION ANALYSIS OF BULLET LEAD
In order to compare PIXE and NAA for the non-destructive
analysis of bullet fragments, it was decided that previous NAA results
should be verified and all operational parameters should be optimized
using a computer simulation to determine the most advantageous
irradiation and counting schemes for bullets. The program BULLET was
devised for this purpose and is included at the end off this section.
To determine which elements should be included in the program
library, the result of Haney's spark source mass spectroscopy (SSMS)
bullet experiments (85,86) were used, since SSMS has been the most
sensitive method found to date for looking at the trace elements in
bullet lead. His results showed that a total of 26 elements were
present at measurable levels. From these 26 elements, it was
determined which had isotopes suitable for NAA. This was done by
correlating the relative abundances, half-lives and cross-sections of
each isotope which could undergo a (n,Y) type reaction. The results
are shown in Table B.l. The decay scheme for each isotope was then
obtained from the Table of the Isotopes, and from the absolute
intensities and the energy of the gamma rays, it was determined which
gamma rays would be suitable for analysis. The resulting set of
isotopes and gamma rays was included in the program BULLET as the
working library and is shown in Table B.2.
211
212
Table B.1. NAA parameters of elements present in bullet lead.
Element Isotope Tl/2 % C1 ppm
Boron none (. 02s) .01 - 0.3
Sodium }Ta-24 900 m 100 0.53 l.0- 10
Aluminum AJ-28 2.31 m 100 0.23 0.1 - 10
Silicon Si-31 157.2 m 3.09 0.11 0.1 - 3
Phosphorous none (no .01 - 0.1
Sulfur S-37 5.06 m 0.014 0.14 1.0 - 300
Chlorine Cl-38 37.3 m 24.47 0.431 1.0 - 10
Potassium K-42 744 m 6.88 1.10 l.0 - 10
Calcium Ca-47 6523.2 m 0.0033 0.3 1.0- 10
Ca-49 8.8 m 0.18 1.10
Titanium Ti-51 5.8 m 5.34 0.14 0.1 - 3.0
Chromium Cr-51 40032 m 4.31 17.0 0.1 - 1.0
Manganese }In-56 154.8 m 100 13.3 0.1 - 1.0
Iron Fe-59 64800 m 0.33 1.20 1.0 - 100
Nickel Ni-65 153.6 m l.08 1.50 0.1 - 5.0
Copper Cu-64 768 m 69.09 4.50 10 - 1000
Cu-66 5.1 m 30.91 2.30
Zinc Zn-69 828 m 18.57 1.10 0.1 - 10
Zn-71 2.4 m 0.62 0.11
2400 m 0.62 0.11
Arsenic As-76 1590 m 100 4.S0 1.0- 1000
Selenium Se-81 61 m 49.82 0.5 0.1 - 10
18.6 m 49.82 0.6
Silver Ag-108 2.42 m 51.82 ? + 35 5.0 -75
Cadmium Cd-l07 390 m 1. 22 1.0 0.1 - 10
Cd-lIS 3210 m 28.86 l. 24
Tin Sn-123 40 m 4.72 0.2001 l.0 - 6000
Sn-125 9.7 m 5.94 0.1004
Sn-125 13536 m 5.94 0.1004
213
Table B.1. NAA parameters--Continued.
Element Isotope T1/2 % cr ppm
Antimony Sb-122 4032 m 57.25 ? + 6.06 0.4 - 5%
Sb-122 4.2 m 57.25 ? + 6.06
Sb-124 21m 42.75 3.345
Tellurium Te-129 67 m 31. 79 0.157 1.0- 50
Mercury Hb-197 1440 m 0.146 905 0.1 - 50
Hg-197 3900 m 0.146 905
Thallium none 0.1 - 10
Bismuth none 5.0 - 500
Lead none 95 - 99%
214
Table B.2. Suitable isotopes of elements present in bullet lead.
Isotope y (MeV) A.I.
Na-24 1.368 1.0 2.754 0.99
Al-28 1. 78 1.0
S-37 3.09 0.90
Cl-38 1.64 0.38 2.17 0.034
K-42 1.52 0.18
Ca-47 0.49 0.057 0.81 0.057 1. 33 0.763
Ca-49 4.11 0.10 3.10 0.89
Ti-51 0.94 0.046 0.61 0.014 0.32 0.954
Cr-51 0.32 0.09
1m-56 0.845 0.979 1. 81 0.294 2.12 0.155
Fe-59 0.191 0.023 1.10 0.553 1. 29 0.435
Ni-65 0.37 0.044 1.114 0.16 1.48 0.247
Cu-66 1.04 0.093
Cu-64 0.511 0.62
Zn-69 0.44 1.0
215
Table B.2. Suitable isotopes--continued.
Isotope y (MeV) A.1.
Zn-71 0.385 0.80 0.495 0.83 0.609 0.93
As-76 0.559 0.34 0.657 0.054 1.22 0.038
Se-81 0.103 1.0
Ag-108 0.511 0.02 0.633 0.018
Cd-107 0.511 0.99
Cd-115 0.335 0.99 0.490 0.10 0.523 0.265
Sn-123 0.16 1.0
Sn-125 0.325 0.996
Sn-125 0.815 0.015 0.91 0.0l3 1.068 0.038
Sb-122 0.511 0.023 0.561 0.663
Sb-122 0.026 1.0 0.061 1.0 0.075 1.0
Sb-124 0.025 1.0 0.01 1.0
Hg-197 0.077 1.0 0.l35 1.0 0.191 0.019
216
As shown in the printout for the BULLET program, for a given
set of irradiation and decay conditions the expected activity for each
isotope in the library is calculated, as well as the over-all activity
and dose rate. Since the concentration of each element in bullets was
reported as a range, both maximum and minimum expected activities are
calculated. The program also gives the option of printing out the
program library. By eliminating the printout of isotope activities, a
rapid printout of the total activity and dose rate for a given set of
reactor conditions is possible. This is useful for determining
irradiation conditions and for filing the necessary forms to obtain
reactor time. Also, if one generated a detector efficiency vs. energy
curve for the detector used and incorporated the results into the
program, the program could easily be revised to give an output of only
those gamma rays which could be expected to be seen under a given set
of operating conditions. The selection of irradiation and delay times
could then be continuously varied for a given set of reactor
conditions to predict the optimum irradiation and counting scheme
that would allow the determination of the greatest number of elements.
This was not done for our initial study.
Experimental
Using the University TRIGA reactor at full power, irradiation
times of one minute (in the RABBIT) and two hours (in the SUSAN) were
selected from the results of the BULLET program. The available
11 2 12 I 2 ) neutron fluxes were 7x10 nlcm -sec) and 2.1xlO n (cm -sec for the
SUSAN and RABBIT, respectively.
217
1.00 g samples of bullet lead were prepared by taking thin
cross-sections of Remington .22 caliber bullets and removing the outer
circumference to eliminate surface contamination. Three such samples
were encapsulated for irradiation in the SUSAN and one for irradiation
in the RABBIT. A reference standard was also prepared from 1000 ppm
stock solutions of Sn, As, Mn, Ti, AI, Zn, Cr, Cd, Cu, Ni, Fe and Ca.
Because of its much larger concentration, Sb was purposely left out to
determine which elements could be seen if Sb were not present to
obscure the spectra. 200 ~l samples of the appropriate metals were
added together and diluted to a volume of 50 ml with a 1000 ppm lead
solution. A one ml aliquot of this standard was then pipet ted onto a
piece of Whatman ~4l filter paper and dried overnight. This resulted
in a standard that contained 680 ~g of lead and 20 ~g of each of the
elements: Sn, As, Mn, Ti, AI, Zn, Cr, Cd, Cu, Ni, Fe and Ca. The
standard was encapsulated for irradiation in the RABBIT. It should be
noted that this standard was intended only for qualitative purposes,
and no attempt was made to quantitate the elements present in bullets.
Count rates were determined by an Ortec Ge(Li) detector
coupled to a NOVA microcomputer in the Nuclear Engineering Department
at the University. Counts were taken randomly for the RABBIT samples
throughout a period of 8 days, the first taken seven minutes after
leaving the reactor. Counting was begun on the SUSAN samples 9 days
after irradiation to allow a decrease in the counting dead time and
in the activity due to Sb, which has decay times of 2.8 days and 60.2
days for its two most activated isotopes (Sb-122 and Sb-124).
218
Results
The results of this study are inconclusive at best, and at
worst verify that this technique is not suitable for the routine non
destructive analysis of bullet fragments to identify their origin. At
the time these experiments were completed, no good counting lab
existed at the University. There was no available method of storing
the acquired spectra, nor was there an available peak identification
program on the Nuclear Engineering system. Peak identification was
done manually by comparing the energies of observed peaks with tables
and cross-checking to match several gamma rays to a particular isotope
or element when possible. From the results of nine counts taken for
the one minute RABBIT samples, the only element that is undeniably
present is Sb, with possible traces of Te, Cd, Cu and K present as
well. The reference standard showed only As, Sn, K and Al present,
but counts were made on this sample only during the first day. The
longer irradiations indicated the presence of only Sb and Te. There
was a problem due to the long half-lived isotopes of Sb; the Sb
present overshadows anything else in the spectrum, which is expected
since it is present at percent levels, whereas the other elements are
present at ppm levels.
Conclusions
From these experiments, it appears that Sb and Te are
definitely present in the bullet lead, and probably Cd, Cu and K as
well. These do not coincide with the elements reported previously by
219
Lukens and Pate (82-84), but different irradiation and counting times
were used. It is apparent from both sets of NAA data that this method
does not give useful information on a sufficient number of trace
elements to allow conclusive bullet comparisons. Although NAA is an
extremely effective technique for the determination of trace amounts
of a few elements, it is generally not suitable for the determination
of a large number of elements unless radiochemical separation
techniques are used. At this time, therefore, it appears that the
PIXE technique holds much more promise for matching and identifying
the origins of bullets.
A good counting laboratory does now exist in the Space
Sciences Center at the University of arizona. with appropriate
programs for peak search, deconvolution, and identification, and for
elemental quantitation. These experiments could now be repeated with
more reliable results. If this work is to be repeated. the following
items should be completed in reference to the NAA work:
1) An efficiency curve should be constructed for the Ge(Li)
detector used for counting.
2) The BULLET program should be altered to generate irradiation
and delay times systematically so that predictions can be made as to
which elements may be seen after a certain delay time.
3) Several sets of bullets should be irradiated and counted as
dictated by the BULLET program.
4) Lead standards should be obtained so that accurate
quantitation of the trace elements can be accomplished.
When these steps have been accomplished, NAA of bullets in its
optimum capacity can be compared with PIXE.
BULLET Program
220
The following pages include the program BULLET, written in
BASIC. This program is designed to calculate the activities one can
expect from the elements in bullet lead under various irradiation and
delay conditions. The program library, based on SSMS analyses of
bullet lead, includes thirty isotopes as indicated in Table B.2. The
library has been stored as DATA statements, beginning at line 1160,
using a 3-line format for each isotope as follows:
line 1: Isotope name; half-life in minutes; % abundance; cross
section in barns; minimum concentration in ppm and
maximum concentration in ppm.
line 2: Gram atomic weight of the element.
line 3: Number of gamma rays; gamma energy and absolute intensity
for each gamma ray.
A printout of the library may be obtained by entering "YES" to
'~sotope listing?" The program is now dimensioned for up to 50
isotopes. If more are desired, the dimension statement in line 20
must be altered.
The program is self explanatory, giving prompts where data
input is required. The printout of individual isotope activities may
be suppressed by deleting lines 600,610,'760 and 900.
BULL
ET
11 :
40
25
-0C
T-8
2
100
REM
***
** PR
OGRA
M
BULL
ET
FOR
NEUT
RON
ACT
IVA
TIO
N
ANAL
YSIS
***
** 11
0 M
ARGI
N 13
2 12
0 DI
M
T(5
0),P
(50)
,C(S
O),
L(5
0),H
(50)
,Z(5
0),N
(50)
13
0 RE
M .
140
REM
PROG
RAM
G
IVES
TH
E O
PTIO
N
OF
PRIN
TIN
G
OUT
THE
PROG
RAM
LI
BRA
RY.
150
REM
A
YES
OR
NO
<Y
OR
N
) AN
SWER
IS
RE
QU
IRED
. IF
NO
T W
ANTE
D,
FLOW
SK
IPS
TO
160
REM
LIN
E 25
0 17
0 RE
M 18
0 PR
INT
"ISO
TOPE
L
IST
ING
";
190
INPU
T 0$
20
0 IF
O
$="N
" TH
EN
540
210
PRIN
T 22
0 PR
INT
230
REM
240
REM
PRIN
T HE
ADIN
G FO
R LI
BRAR
Y LI
STIN
G
250
REM
26
0 PR
INT
"ISOTOPE",·HALF-LIFE","~
AB
UN
D."
,"C
.S."
,"C
ON
C
(PPM
)"
270
PRIN
T "--
----
-","
----
----
-","
----
----
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----
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----
----
--"
28
0
REM
·290
RE
M PR
OGRA
M
READ
S AN
D PR
INTS
TH
E IN
FORM
ATIO
N IN
TH
E BU
LLET
LI
BRA
RY;
300
REM
T
= H
ALF
-LIF
E 31
0 RE
M P
= %
ABU
NDAN
CE
320
REM
C =
CRO
SS
SECT
ION
33
0 RE
M L
= M
INIM
UM
CONC
ENTR
ATIO
N 34
0 RE
M
H =
MAX
IMUM
CO
NCEN
TRAT
ION
350
REM
Z =
ATO
MIC
W
EIGH
T
N
N
I--'
.' \'
360
REM
G =
GAM
MA
ENER
GY
370
REM
A =
ABS
. IN
TEN
SITY
38
0 RE
M 39
0 FO
R 1=
1 TO
30
40
0 RE
AD
E$
,T(I
),P
(I),
C(I
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(I),
H()
41
0 PR
INT
420
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T 43
0 PR
INT
ES
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CI)
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0 PR
INT
450
READ
Z
46
0 RE
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N C
I )
470
FOR
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N
CI)
480
READ
G
CW),A
(W)
490
PRIN
T"
GAM
MA:
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50
0 NE
XT
IrJ
510
NEXT
I
520
PRIN
T 53
0 RE
STOR
E
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540
PRIN
T "A
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INT
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T)"
560
REM
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PERI
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TAL
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DIT
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S AR
E IN
PUT
BY
THE
OPE
RATO
R.
THES
E IN
CLU
DE:
57
0 RE
M
(1)
SAM
PLE
SIZE
IN
GR
AMS
580
REM
(2
) TR
IGA
FAC
ILIT
Y
USE
D,
AND
590
REM
(3)
POW
ER
OF
THE
REAC
TOR.
60
0 RE
M FO
R TH
E FA
CIL
ITY
, "S
" SH
OULD
BE
IN
PUT
FOR
SUSA
N,
"R"
FOR
RABB
IT
610
REM
AN
D "G
H"
FOR
THE
GLOR
Y H
OLE
. TH
E A
PPRO
PRIA
TE
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RON
FLUX
IS
TH
EN
620
REM
SU
BSTI
TUTE
D
FOR
<F>.
63
0 RE
M 64
0 PR
INT
"SAM
PLE
SIZE
(G
M)"
; 65
0 IN
PUT
H7
660
PRIN
T 67
0 PR
INT
"FA
CIL
ITY
";
680
INPU
T F$
N
N
N
690
PRIN
T 70
0 PR
INT
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ER
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710
INPU
T P
720
IF F~~"5"
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0 73
0 IF
F$
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EN
770
740
IF
F$="
GH
" TH
EN
820
750
PRIN
T "E
NTER
S,
R
OR
GH"
760
LET
F~='SUSAN"
770
LET
F=7.
0000
0Et0
9*P
780
GO
TO
840
790
LET
F$="
RA
BB
IT"
800
LET
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1000
0E+I
0*P
BID
GO
TO
B4
0 82
0 LE
T F$
="G
LORY
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LE"
830
LET
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0*P
840
PRIN
T 85
0 RE
M
860
REM
TH
E IR
RAD
IATI
ON
TI
ME
AND
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Y TI
ME
ARE
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T BY
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E OP
ERAT
OR
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INT
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ME
(MIN
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890
INPU
T T1
90
0 PR
INT
910
PRIN
T 92
0 PR
INT
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AY
TIM
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IN)"
; 93
0 IN
PUT
T2
940
PRIN
T 95
0 PR
INT
960
PRIN
T 97
0 RE
M
980
REM
EX
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INTE
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1000
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INT
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ON
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CIL
ITY
: "F
S 10
10 P
RIN
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1040
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INT
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1060
RE
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70
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0 10
80
FOR
1=1
TO
30
1090
RE
AD
E$
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(I),
C(I
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(I),
H(I
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(I)
1100
RE
M
1110
RE
M
MAX
AND
MIN
NO
. OF
AT
OMS
ARE
CALC
ULAT
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AS:
11
20
REM
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M)
X 6
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23
X Z
ABUN
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M)
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11
30
REM
11
40
LET
Nl=
L(I
)*1.
0000
0E-0
6*6.
0200
0Et2
3*P
(I)/
Z(I
) 11
50
LET
Nl=
Nl*
H7
1160
LE
T N
2=H
(I)*
1.00
000E
-06*
6.02
000E
t23*
P(I
)/Z
(I)
1170
LE
T N
2=N
2*H
7 11
80
REM
11
90
riEM
12
00
REM
12
10
REM
12
20
REM
12
30
REM
IRRA
DIA
TIO
N
(5)
AND
DECA
Y (D
) PA
RAM
ETER
S AR
E CA
LCUL
ATED
A
S:
S =
1
-E
XP
(-.6
93
X T
(IRRA
D)/L
AM
BDA
) AN
D D
= E
XP
(-.6
93
X T
(DEC
AY
)/LA
MBD
A)
1240
LE
T S
=(1
-(E
XP
(-.6
93*T
I/T
(I»»
12
50
LET
D=
EX
P(-.
693*
T2/
T(I
» 12
60
REM
12
70
REM
UP
PER
AND
LOW
ER
AC
TIV
ITIE
S IN
DP
S AN
D CU
RIES
AR
E CA
LCUL
ATED
A
S:
1280
RE
M
A =
N
X N
EUTR
ON
FLUX
X
CRO
SS
SECT
ION
(BA
RNS)
X
S X
D
1290
RE
M
AND
1300
RE
M
R =
A/3
.7E
I0
1310
RE
M
1320
LE
T A
l=N
l*F
*C(I
)*1.
0000
0E-2
4*S
*n
1330
LE
T A
3=A
1/3.
7000
0Etl
0 13
40
LET
A2=
N2*
F*C
(!)*
1.00
000E
-24*
S*n
N
N
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",
1350
LE
T A
4=A
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7000
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0 13
60
PRIN
T E
$,A
l,A3,
A2,
A4
1370
RE
M
1380
RE
M TH
E TO
TAL
ACT
IVIT
Y
IS
SUM
MED
FO
R AL
L IS
OTO
PES
1390
RE
M 14
00
LET
Vl=
VltA
I 14
10
LET
V3=
V3+
A3
1420
LE
T V
2=V
2tA
2 14
30
LET
V4=
V4f
A4
1440
RE
M
1450
RE
M
THE
DOSE
RA
TE
(R)
FOR
EACH
GA
MMA
RAY
IS
CALC
ULAT
ED
AS:
14
60
REM
R
= (
AC
TIV
ITY
X
6
X 1
000
X S
UM
(G(W
)*A
(W»)
/3.7
EI0
14
70
REM
14
80
REA[
t N
14
90
LET
Bl=
O
1500
FO
R W
=1
TO
N
1510
RE
AD
G(W
),A(W
) 15
20
LET
D(W
)=G
(W)*
A(W
) 15
30
LET
Bl=
BltD
(W)
1540
NE
XT
W
1550
LE
T R
2=A
3t6*
Bl*
1000
15
60
LET
R4=
A4*
6*B
l*10
00
1570
P
RIN
T'
DOSE
RA
TE
(MRE
M/H
R)=
"R2"
, M
IN",
R4'
, M
AX'
1580
PR
INT
1590
PR
INT
1600
RE
M 16
10
REM
TO
TAL
DOSE
RA
TE
IS
SUM
MED
16
20
REM
1630
LE
T X
l=X
ltR2
1640
LE
T X2=X2~R4
1650
NE
XT
I 16
60
PRIN
T 16
70
PRIN
T 'T
OT
AL
:',V
l,V3,
V2,
V4
N
N
l.11
1680
PR
INT
1690
PR
INT
·TO
TAL
DOSE
R
AT
E:·X
l,X2
1700
PR
INT
1710
PR
INT
1720
PR
INT
·ANO
THER
DE
LAY
TIM
E·;
1730
IN
PUT
R$
1740
IF
R
$=·N
· TH
EN
1760
17
50
GO
TO
92
0
1760
PR
INT
"ANO
THER
IR
RAD
. T
IME
·; 17
70
INPU
T R$
17
80
IF
R$=
·N·
THEN
18
00
1790
GO
TO
88
0 18
00
PRIN
T ·A
NOTH
ER
FAC
ILIT
Y·;
1810
IN
PUT
R$
1820
IF
R
$=·N
· TH
EN
1840
18
30
GO
TO
670
1840
PR
INT
1850
GO
TO
28
30
1860
RE
H 10
70
REM
"TH
E D
Aln
IS
ST
ORED
FO
R EA
CH
ISO
TOPE
IN
A
3-L
INE
FO
RMAT
A
S:
1880
RE
M
ISO
TO
PE,H
AL
F-L
IFE
,Z
AB
UN
D.,C
.S.,L
OW
ER
CO
NC
.,HIG
HER
CO
Ne.
1890
RE
M
ATOM
IC
WEI
GHT
1900
RE
M
NO
. OF
GA
MMA
RAYS
,GAM
MA
ENER
GY
, A
BS.
INTE
NSI
TY
OF
EACH
19
10
REM
19
20
REM
19
30
DA1A
N
A-2
4,9
00
,l,.
53
,1,1
0
"194
0 DA
TA
22.9
9 19
50
DATA
2
,I.J
68
,1,2
.75
4,.
99
19
60
DATA
A
L-2
8,2
.31
,l,O
.23
,1,1
0
1970
DA
TA
26.9
8 19
80
DATA
1
,1.7
8,1
19
70
DATA
S
-37
,5.0
6,1
.40
00
0E
-04
,.1
4,1
,30
0
2000
DA
TA
32.0
6
GAi1M
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Y.
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N '"
2010
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TA
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20
20
DATA
C
L-3
8,3
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44
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43
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20
30
DATA
3
5.4
5
2040
DA
TA
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34
20
50
DATA
K
-42
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4,.
06
B3
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0
2060
DA
TA
39
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2070
DA
TA 1,1.~2,.18
2080
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TA
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2140
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57
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2370
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N
-0
APPENDIX C
Program RATIO for PIXE Quantitation
230
*****
HELP
FI
LE
FOR
USE
WIT
H TH
E PR
OGRA
M
RATI
O ***
** (T
O U
SE,
TYPE
: "T
YPE
HEL
P,"
)
TO
RUN
THE
RATI
O PR
OGRA
M,
TYPE
: RU
N RA
TIO
(CR)
TH
E PR
OGRA
M
THEN
RE
SPON
DS
WIT
H AN
"*
". TH
IS
TELL
S YO
U TH
AT
THE
PROG
RAM
IS
W
AITI
NG
FOR
INPU
T.
AT
THIS
PO
INT
THER
E AR
E SE
VERA
L PO
SSIB
LE
OPT
ION
S.
THES
E AR
E SU
MM
ARIZ
ED
BY
THE
FOLL
OWIN
G.
C,F
ILE1
,FIL
E2<C
R>
WHE
RE
C I
S A
SIN
GLE
CH
ARAC
TER
COMM
AND
AND
IS
ONE
OF
THE
FOLL
OWIN
G.
C:
CREA
TES
DATA
FIL
ES
FOR
USE
BY
THE
RATI
O PR
OGRA
M
L:
LIST
S DA
TA
FILE
S TH
AT
WER
E CR
EATE
D BY
"R
ATY
IO'
H:
MO
DIF
IES
EXIS
TIN
G
DATA
FI
LE
S.
R:
CALC
ULAT
ES
THE
ATOM
IC
RATI
OS
S:
TERM
INAT
ES
EXEC
UTI
ON
(STO
PS
THE
PROG
RAM
)
THE
SECO
ND
ARGU
MEN
T "F
ILE1
" IS
TH
E FI
LE
NAME
UP
ON
WHI
CH
ALL
BUT
ONE
OF
THE
ABOV
E CO
MM
ANDS
OP
ERAT
E.
THIS
M
UST
BE
A F
IVE
(S)
CHAR
ACTE
R FI
LE
NAM
E.
FOR
EXAM
PLE,
TO
CR
EATE
FI
LE
NAME
D 'G
LOR
P'
THE
REOU
IRED
TY
PE
IN
WOU
LD
BE
C.G
LORP
<CR)
N
VJ .....
\. )
THE
PROG
RAM
TI
IEN
PRO
CfED
S TO
AS
K Q
UES
TIO
NS,
W
HICH
SH
OULD
GU
IDE
THRO
UGH
THE
REST
OF
TH
IS
MOD
E.
TO
GET
OUT
OF
THIS
M
ODE,
TY
PE
"0'
(ZER
O)
FOR
AN
ATOM
IC
NUM
BER.
TO
LIST
'GLORP~,
YOU
WOU
LD
TYPE
L,G
LORP
AFTE
R W
HICH
IT
TY
PES
OUT
WHA
TEVE
R IS
IN
IG
LO
RP'
, AN
D GO
ES
BACK
TO
W
AITI
NG
FOR
INPU
T BY
TY
PIN
G
OUT
A 1*
••
MOD
IFYI
NG
FILE
S IS
PE
RFOR
MED
THE
SA
ME
WAY
. ON
CE
M,G
LORP
HA
S BE
EN
TYPE
D IN
Q
UES
TIO
NS
WIL
L BE
AS
KED.
AL
SO
THE
MOD
IFY
SUBR
OUTI
NE
PROM
PTS
YOU
WIT
H '**
" TO
GE
T DA
TA
INPU
T OT
HER
THAN
Q
UES
TIO
NS.
TH
E ON
E CO
MMAN
D W
HICH
M
OST
OF
THE
TIM
E RE
QU
IRES
A
MO
DIF
ICA
TIO
N
IS
THE
R C
OMM
AND.
YO
U W
OULD
TY
PE
R,F
ILE
l,FIL
E2
IN
THIS
CA
SE,
SIN
CE
NORM
ALIZ
ATIO
NS
ARE
TO
BE
DONE
W
ITH
RESP
ECT
TO
A S
TAND
ARD,
TW
O FI
LES
HUST
BE
RE
AD
IN.
THE
PROG
RAM
IS
W
RITT
EN
SO
THAT
'F
ILE
1"
CORR
ESPO
NDS
TO
AN
UNKN
OWN
SAM
PLE
WHI
LE
'FIL
E2'
CO
RRES
POND
S TO
TH
E ST
ANDA
RD
DATA
D
ESIR
ED
FOR
AN
ALY
SIS.
R,X
l1tH
A,S
TAN
D
WOU
LD
READ
DA
TA
FROM
BO
TH
"XDA
TA"
AND
ISTA
NDI •
TH
E PR
OGRA
M
WOU
LD
THEN
AS
K FO
R A
NOR
MAL
IZAT
ION
ELEH
ENT(
ATO
HIC
NU
HBER
) TO
TE
RMIN
ATE
BOTH
TH
E CR
EATE
AN
D M
ODIF
Y PR
OGRA
M
TYPI
NG
A '
0'
N
W
N
IN
THE
APP
RORP
IATE
PO
INT
SHOU
LD
DO
THE
JOB
.
ONE
FIN
AL
NOTE
: FI
LE1
OR
FILE
2
NEED
NO
T AL
WAY
S BE
SP
ECIF
IED
ON
CE
THE
FILE
HA
S BE
EN
READ
, IT
IS
IN
CO
RE A~D
IS
USEA
BLE
TO
THE
OTHE
R CO
MM
ANDS
. ON
CE
L,F
ILE
I IS
TY
PED
ONE
COUL
D M
ODIF
Y IT
BY
M
EREL
Y TY
PIN
G
"L"
ALSO
ON
CE R,F
ILE
1,FI
LE
2 HA
S BE
EN
TYPE
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D YO
U W
ANT
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IMM
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TELY
US
E FI
LE3
AS
A
STAN
DARD
ON
E CO
ULD
TYPE
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THER
(1)
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OR
(2)
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3
N
W
W
F'a~
e 1
0001
0 00
020
0003
0 00
040
0005
0 00
060
0007
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080
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0 00
100
0011
0 00
120
0013
0 0
01
40
00
150
0016
0 00
170
0018
0 00
190
0020
0 00
210
0022
0 00
230
0024
0 00
250
0026
0 00
270
0028
0 00
290
0030
0
C **
****
****
** P
ROGR
AM
RATI
O
FOR
PIX
E AN
ALYS
IS
****
****
****
****
C
C
TH
IS
PROG
RAM
IS
US
ED
IN
THE
OU
AN
TITA
TIV
E A
NA
LYSI
S OF
PI
XE
SAM
PLES
C
TO
CALC
ULAT
E EL
EMEN
TAL
RATI
OS
BY
DIV
IDIN
G
THE
X-RA
Y PE
AK
AREA
S OF
AN
C
AN
UNKN
OWN
SAM
PLE
BY
THE
PEAK
AR
EAS
OF
A S
TAND
ARD
X-RA
Y FI
LE
AND
C N
ORM
ALIZ
ING
THE
HIGH
AN
D LO
W
ENER
GY
SPEC
TRA
TO
A S
ING
LE
ELEM
ENT.
C
C
THE
PR
OGRA
M
MAY
BE
USED
TO
CR
EATE
UN
KNOW
N SA
MPL
E FI
LES
AND
C S
TAND
ARD
REFE
RENC
E FI
LE
S,
TO
LIST
AN
D M
ODIF
Y EX
ISTI
NG
DA
TA
FIL
ES,
AN
D C
TO
CALC
ULAT
E EL
EMEN
TAL
RATI
OS
OR
WEI
GHT
PERC
ENTS
FO
R AN
UN
KNOW
N C
SAM
PLE
USI
NG
TH
E EN
TERE
D ST
ANDA
RD
REFE
RENC
E FI
LE
AND
A K
NOW
N CO
NCEN
TC
RAT
ION
OF
ONE
ELEM
ENT
FROM
AN
AL
TERN
ATE
TECH
NIO
UE.
C
C
THE
FI
LES
ARE
DIM
ENSI
ONED
FO
R 10
0 EL
EMEN
TS
IN
THE
FOLL
OWIN
G LI
NES
. C
DAT
Al
IS
AN
ARRA
Y FO
R TH
E UN
KNOW
N SA
MPL
E,
CON
TliIN
ING
TH
E AT
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C
NUM
BER
AND
EITH
ER
A "
l"
IND
ICA
TIN
G
A H
IGH
ENER
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SPEC
TRUM
, OR
A
C
"2"
IN
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ATI
NG
A
LOW
EN
ERGY
SP
ECTR
UM,
AND
THE
PEAK
AR
EA
FOR
THE
C E
LEM
ENT;
DA
TA2
IS
A S
IMIL
AR
ARRA
Y FO
R TH
E RE
FERE
NCE
STAN
DARD
, AN
D C
CON
TAIN
S TH
E SA
ME
INFO
RMA
TIO
N.
TITL
EI
AND
TITL
E2
ARE
THE
NAM
ES
C G
IVEN
TO
TH
E SA
MPL
E AN
D RE
FERE
NCE
SPEC
TRA
, RE
SPEC
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, DU
RING
C
FIL
E CR
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ON
. EL
EMEN
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EIGH
T AR
E TH
E DA
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FILE
S IN
TH
E PR
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M
C L
IBRA
RY
THAT
CO
NTAI
N TH
E TW
O LE
TTER
AB
BREV
IATI
ON
FOR
THE
ELEM
ENTS
AN
D C
TH
EIR
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IC
WEI
GH
TS.
THE
TTY
IS
DEF
INED
AS
OU
TPUT
DE
VICE
5
. C
DIM
ENSI
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DA
TA
1(10
0,2)
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15)
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MEN
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/
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c 00
310
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0 00
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0034
0 00
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0 00
370
0038
0
C T
HE
PROG
RAM
LI
BRA
RY
IS
INPU
T IN
TH
E FO
I.LOW
ING
DATA
ST
ATE
MEN
TS;
C '
ELEM
ENT"
CO
NTA
INS
A T
WO
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TER
SYM
BOL
FOR
EACH
EL
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T (H
Tl
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C
AND
CA
N CO
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IN
UP
TO
100
ELEM
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'WEI
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CON
TAIN
S TH
E AT
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C
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CH
OF
THE
ELEM
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ALY
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600
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c C T
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IS
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AT
THE
OPER
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CA
N IN
PUT
C T
HE
SIN
GLE
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IN
DIC
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IS
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DONE
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AHD
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DATA
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S AN
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610
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C
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S DE
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INED
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R TH
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R TH
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ALL
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ATIO
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300
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0069
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720
0073
0 00
740
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760
0077
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780
0079
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800
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890
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0 00
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0094
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ENE
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ENE
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LE
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F
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IT
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AREA
15
ZE
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E VA
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E FI
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IF(R
AT
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HE
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T BE
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OVE
OR
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L TO
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E NO
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IT
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AREA
IS
ZE
RO,
AND
IF
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TH
E C
VA
LUE
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OH
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MES
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E FI
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L V
ALU
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FIN
AL
(I)=
RA
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) GO
TO
400
500
IF(A
RA
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(I).E
D.O
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0 C
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F
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ZE
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IS
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R FI
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AT
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EATE
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NTAI
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E C
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MAL
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FOR
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EN
ERGY
RU
NS.
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HE
PROG
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NO
W PR
INTS
OU
T A
MES
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TO
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E TE
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C W
HICH
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L RA
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: 70
0 W
RIT
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TY,2
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-i:' N
0097
0 00
980
0099
0 01
000
0101
0 01
020
0103
0 01
040
0105
0 01
060
0107
0 01
080
0109
0 01
100
0111
0 01
120
0113
0 01
140
0115
0 01
160
0117
0 01
180
0119
0 01
200
0121
0 01
220
0123
0 01
240
0125
0 01
260
0127
0 01
280
0129
0
C T
HE
ATOM
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NO.
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THE
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FOR
ELEM
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L RA
TIO
S IS
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AD
AND
C S
TORE
D AS
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IF
A Z
ERO
IS
INPU
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THE
FLOW
RE
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S TO
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MAI
N PR
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REA
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OR
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