B . A . R X > - 5 4 7 . . . : • " • • • ' "-~?::Wi±

GOVERNMENT OF INDIAATOMIC ENERGY COMMISSION

STUDIES OF K 5tRAY EMISSION IN THE THERMALFISSION OF U« s AND SPONTANEOUS FISSION OF Ci'«s*

V . : ' ' . - ; ; • • • > • - • - ; - ' • • - ' - ; ' ' - . : - .

I. S, Kapoor, S. E . Kataria,^ S. R. S. Morthy, D. M. Nadkarni,

Nuclear Physics Division '

| ^

:T( FofiJiciB

n . A . R . C . -547

GOVERNMENT OF INDIAATOMIC ENERGY COMMISSION

ini

U

STUDIES OF K X-RAY EMISSION IN THE THERMALFISSION OF U2 3 5 AND SPONTANEOUS FISSION OF

byS. S. Kapoor, S. K. Kataria, S.R.S. Murthy, D. M. Nadkarni,

V.S. Ramamurthy, P. N. Rama Rao and R. RamannaNuclear PhyBics Division

BHABHA ATOMIC RESEARCH CENTREBOMBAY, INDIA

1971

*Report on work performed under research contract No. 535/RB withInternational Atomic Energy Agency, Vienna, on "FundamentalStudies of the fission process in heavy nuclei"

CONTENTS

SECTION I

SECTION II

a.

General Outline of the Project

x Experiments

Experimental

i. Experimental Arrangementii. Electronics

iii. Counting Rates

b .

c .

SECTION in.

a.

b .

c .

d.

e .

Analysis of the data

Results

K X-ray Half Liv 38 Versus FraAtomic Number

Introduction

Principle of the method

Experimental

i. Layoutii. Electronics and data taking

Alpha-X-ray data

Analysis of Results

i. Background correction

ii. Determination of observed K X-rayintensities from specificed Z fragments

iii. Determination of X-ray half livesVersus Z

Page

1

5

5

579

10

15

20

20

21

23

23

24

26

27

23

30

31

f. Discussion of the Results

SECTION IV

a.

b.

c.

d.

Page

: Studies of K X-ray Multiplicity from 41Cf252 fission Fragments

Introduction 41

Experimental Set-up 42

Electronic Arrangement 42

Data Analysis 46

i. Heavy-Heavy or Light-Light X-ray 51Coincidence Data

iL Light-Heavy X-ray Coincidence Data 53

Results and Discussion 54

SECTION V : Summary 63

REFERENCES 67

I. GENERAL OUTLINE OF THE PROJECT

It is known from earlier investigations'1' of the fragment

deexcitation process that soon after scission, the fission fragments

each having about 16 MeV excitation energy are accelerated to large

kinetic energies owing to Coulomb repulsion between them. Promptly

thereafter, a greater part of the excitation energy is dissipated by

neutron emission. The residual excitation-energy is then emitted in

the form of gamma rays. Approximately 8-10 MeV of energy is

carried away by about 8-LO gamma rays. About 85 fo of the total gamma

rays are emitted with a half life of about 10 sec, and the remaining

15% have half-life in the region of 10 sec. . A small fraction of

about 6% of these gamma transitions undergo internal conversion

primarily in the K and L shells giving rise to conversion electrons

and characteristic X-rays. In recent years, the studies of the radia-

tions emitted in the nuclear fission process have been a subject of con-

siderable interest. It has been so mainly due to the recognition of the

fact that the fragment nuclei emitting these radiations span the neutron

rich region far away from the line of {^-stability, and cover a large

region of nuclear periodic table as shown in Fig. 1. This region is

not easy to reach by other conventional means and fission process

provides a natural means of studying the nuclear spectroscopy of

these nuclei. Detailed investigations of the radiations originating from

fragment nuclei of known mass and nuclear charge have been made

-2-

possible only recently, with the availability of high resolution lithium-

drifted Silicon and Germanium detectors, the multiparameter data acqui-

sition system and the on line computers. With the availability of these

facilities, recent efforts* " ' are aimed at studying the fission gamma

rays, conversion electrons and K X-rays as a function of the mass and

nuclear charge of the emitting fragment.

In this project, detailed investigations of the K X-rays emitted

in the thermal neutron induced fission of U and in the spontaneous

fission of Cr" were undertaken by carrying out three different types

of experiments. The first experiment involved a three parameter study

of the kinetic energies of the pair fragments and the coincident K X-rays

for the case of thermal fission of U235 . We will refer this as EjEgE^

experiment, where the fragment kinetic energies Ei, E? of the two

fragments and the X-ray energy E are recorded event by event using

the multiparameter data acquisition system. These data have provided

information on the K X-ray yields as a function of average fragment

inass and total kinetic energy. Details of this experiment are given in

section II. The second series of experiments described in Section III

were aimed to obtain information on the K X-ray emission times as a

function of the nuclear charge of the emitting fragment for the thermal

fission of U"5# The experiments on the correlated emission of the K

X-rays for the case of spontaneous fission of Cf252 described in

Section IV, provided information regarding the average multiplicity of

-3-

K X-rftys and light-heavy correlation in the X-ray emiseion process.

All these investigations were carried out using lithium drifted silicon

detector system for X-ray energy measurements and with the four

parameter data acquisition system obtained under contract from IAEA.

The results obtained from all these studies are summarised in Section V.

Cf2M FragmentsU*36 Fragments

N

M 54 56 58

F I G 1

The region of the periodic chart spanned by

Of252 and U256 f ission fragmants.

-5-

II. E.E E EXPERIMENTS

a) EXPERIMENTAL

(i) Experimental Arrangement

A schematic diagram of the experimental arrangement is

shown in Fig. 2. The Bource foil holder assembly was specially designed

to mount the fragment detectors also in the same holder at a small dis-

tance of 0. 5 mm and 3 mm from the source foil. The source foil con-

sisted of about 100 Ugm/cm of U 3^ electrosprayed onto an area of 1 sq.

cm of a thin VYNS foil coated with a very thin film of gold, which was

first adhered to the foil-detector holder assembly. Since in this system,

the fragments are stopped within 0. 5 mm on one side, this arrangement

has the following advantages: (i) it eliminates the uncertainties asso-

ciated with the determination of solid angle of detection of X-rays

emitted from flying fragments, (ii) it improves the fission counting

rate enabling the experiments to be carried out with a low flux and

consequently with a low background. The X-ray energy was measured

with 0. 6 cm2 x 3 nun Si(L,i) detector cooled to liquid nitrogen tempe-

rature and coupled to a cryogenic FET amplifier. The energy resolu-

tion of the X-ray detector system in terms of FWHM of 26. 25 Kev

line of Am2 4 1 was 0. 88 Kev for low count rates for the system used in

this work. During the experiment the actual energy resolution attain-

able was about 1 Kev due to a slight deterioration of the system reso-

lution caused by highly saturated background pulses.

-6-

FiG-2

SOURCE ANDDETECTORS,,ASSEMBLY^

-1 MIL AL $&£ rBERRYLIUM WINDOWS

- ! MIL ALUMINUM

CRYOSTAT WITH COOLEDSi (L i ) DETECTOR ANDFET ASSEMBLY

STAINLESSSTEEL

SPRING WIRECONTACT —

SOURCE U"235 —

TOP PLATEOF CHAMBER

FRAGMENT .•*DETECTORS AJ*

A

-VYNS BACKING

-PERSPEX

e e

DETAILS OF ASSEMBLY OFSOURCE AND DETECTORS

Schematic diagrani of the e_: erirfE •••tal arrangeTS:n'tfor the E..B-E oxperiraent.

-7-

A colllmated neutron beam from thp C1RUS reactor was used

for this work. To reduce the fast neutron and / -ray content of the

beam, the beam from the reactor core passed through 15 cms of

quartz and 25 cms of BiBmuth and finally through a Steel collimator

which reduced the beam size to 1.25 cms. The thermal neutron flux

at the foil was about 5 x 10"n/cm /sec. The vacuum chamber housing

the foil-detector assembly was made to have very thin entrance and

exit windows of aluminium for the incident neutrons, to minimise the

beam scattering and therefore the background field in the region of X-

ray detector. The source foil detector assembly was placed in line with

and making an angle of 45° with the collimated neutron beam, and the

x-ray detector was placed at right angles to the beam direction at a

distance of about 3. 0 cmB from the source foil. The x-rays were

viewed through two 10 mil Be windows, one in vacuum chamber and

the other in the cryostat. Since in this arrangement the X-rays were

viewed through the fragment detector, thin wafers (thickness 350 microns)

of diffused junction silicon detectors were employed to have small atte-

nuation for the fragment X-rays,

(ii) Electronics

The block diagram of the electronic arrangement is shown in

Fig. 3. The pulses from the fragment detector system after suitable

amplification were fed to a single channel analyser to cut off the alpha

pulses. The at c. analyser was externally strobed by the zero cross-

over output of the bipolar pulses from the amplifier of the second frag-

X-RAYDETECTOR

A PARAMETER! RECORDINGSYSTEM

DELAYEDnCOINC.

FRAGMENTDETECTOR

FRAGMENTDETECTOR

oo

BLOCK DIAGRAM OF ELECTRONIC ARRANGEMENT FOR E, E 2 E x EXPER;M=MT

FIG. a

-9 -

ment detector system. This arrangement provided timing pulses corre-

sponding to fragment-fragment coincidences. The timing pulses from

the X-ray detector system corresponding to zero cross-over and the fis-

sion timing pulses were fed to a double coincidence unit of resolution

time iUsec. The EjE£ double coincidences were scaled down by a

factor of 256 and these sealer output pulses and the triple coincidence

EjE2Ex pulses were fed to an OR gate, the output of which gated the 4

parameter system in the external delayed coincidence mode. The pulses

from the respective linear amplifiers containing the information on the

energies Ej, K^ and Ex were fed to the three ADC's (1, 3 and 4) of the

multiparameter system and a flag signal from sealer output was fed to

ADC 2. In this way, the pulse heights corresponding to either Ej E£

coincidences or EiE^Ex coincidences were recorded event by event on

a paper tape. A high precision pulser fed at the input of the x-ray

detector was first calibrated into energies using the e. m. radiations

from a A m ^ ' source. A careful channel versus energy calibration

was obtained at the start of each run and further checked at the end. In

the present set of experiments about Z. 5 x 105 events containing roughly

equal number of EiE2Ex and EjE, events were recorded. The record-

ing of E,E2 data was required to obtain an accurate mass calibration.

(iii) Counting Rates

The fission counting rates in the individual detectors D^ and D2

and the coincident EjE2 counting rates were about 1Z0, 30 and 30 per

sec respectively. The triple coincidence EJE2EJJ. counting rate was

-10-

about 5 per minute. The singles background rate in the X-ray detector

was about 200 per sec in the fragment X-ray energy region and about

1500 per sec in all the energy range. The chance coincidence rate in

the X-ray energy region with a resolution time (2 T ) of 1 Usec was

therefore only 10% of the total triple coincidence rate. In this senee,

background effects were rather small since the major source of back-

ground in the triple coincidence data originates from the unavoidable

true coincidences with the compton scattered fission gamma rayB. How-

ever, the background counting rate of about 1500 per second mainly of

the saturated pulses in the x-ray detector had the effect of somewhat

deteriorating the actual x-ray energy resolution obtainable during the

experiment to a value of about 1. 1 keV.

The radiation damage of the fission detectors was continuously

monitored by recording their leakage currents and fragment kinetic

energy distributions. The time for which continuous recording of data

could be carried out was limited to 15 days during which the detector

oD, was exposed to about 1. 5 x 10 fission fragments since after this

the effects of radiation damage became significant. During a period of

15 days ahout 1. 2 x 10^ triple coincidence events and roughly equal

number of double coincidence events were recorded on the paper tape of

the 4 parameter system.

(b) ANALYSIS OF THE DATA

The data on the punched paper tapes were transferred on to a

magnetic tape with the 160 A satellite computer. The data were then

-11-

properly decoded, and written on another tape in CDC 3600 computer

compatible format. This tape was used for further analysis of the data.

Fragment masses were obtained from the double kinetic energy

data through the energy-momentum conservation relations. In order

to take into account any small shifts in the fragment kinetic energy dis-

tributions during each run the double coincidence data of nearly each

day were first separately sorted out to obtain the kinetic energy distri-

butions. The first moments P^ and PJJ of the pulse height distributions

for each set of data were used for the calibration of pulse heights into

fragment kinetic energies. The energy to mass conversion was carried

out by means of an iterative process which incorporated mass dependent

(11)energy calibration proposed by Schmitt et al and neutron corrections

based on the results of Maslin et al* ' . Using this procedure the eva-

luation of prompt fragment masses for each event was individually done

and transcribed onto a magnetic tape. The pulse heights for the x-ray

channel in each event was also converted into x-ray energy and recorded.

After these transformations of the data, the unbiased mass distributions

for the cases when no x-rays were recorded was sorted out. The

observed mass distribution was found to be wider because of mass dis-

persions introduced due to neutron emission and experimental effects.

The experimental effects are attributable mainly to significant experimental

dispersion in the fragment kinetic energies introduced by the source

thickness, the proximity of the source foil, with the fragment-detector

and the detector resolution. The observed mass distribution was

-12-

corrected for mass dispersion effects by the method of Terrel .

The removal of mass dispersion corresponding to

3000

in5 2000

CD(X<

Q

1000

SCHMITT ET AL

PRESENT DATA

160

I

140 130

PROMPT FRAGMENT MASS120

FIG. 4

The frvL, icv I -if, c i r i t r i b u t i o r . •;.:. .i ..-."'exper iment c c a ^ a r e d '.vi!;h t h e r eoa l ' . o of

.i,- t :

-14-

the heavy and the light fragments respectively are moving towards the

fragment detector which is placed very close to the source foil and

towards the X-ray detector. In this case the fragment motion is limited

to only about 0. 1 cm and hence the change of the solid-angle due to

varying emission times of X-rays is not appreciable. The following

results have been obtained from the analysis of only such events.

The unbiased mass distribution Y(M) and the distribution

Y(MT JJ) in coincidence with light or heavy fragment K X-rays are re-

lated by the following relation.

YX(ML. H> / Y = Px T (*>

where p (MT JJ) *8 ^ e average number of K X-rays emitted from

the masses M^ or MJJ. 7) (M), T(M) are the average detection effi-

ciency and the transition coefficients for the K X-rays characteristic

of fragment mass M, and -TL is the solid angle of X-ray detection.

The solid angle _Q_ of X-ray detection was determined by

comparing the number of L-X-rays detected per alpha decay of U

in the given experimental setup with the calculated number'14' of L>X

rays emitted per alpha decay. In the present arrangement the X-ray

intensities specially those from light group, were attenuated in passing

through the fragment detector. The transmission T of the X-rays of

different energies was calculated using the known values of the frag-

ment detector thickness and the absorption coefficients' ' for silicon.

The transmission was also measured for the experimental arrange-

ment by measuring the L X-rays in coincidence with natural Cephas

of U^3* present in the foil, with and without the fragment detector

-15-

being present and was found to be in agreement with the calculated

values. T̂ was calculated from the known photoelectric and total absor-

ption cross sections in silicon.

(c) RESULTS

The mass distributions Y (M) and Y^(M) in coincidence

with the photon energies in the region of 10-21 keV and 21 to 45 keV

corresponding to the light fragment X-ray and heavy fragment X-ray

regions respectively were first sorted out. The mass distributions

YC(M) in coincidence with compton scattered gamma rays were sorted

out by fixing the energy window in the region 50 to 60 keV. Since a known

fraction of events under the light and heavy X-ray peaks corresponds to

compton scattered gamma background, the observed mass distributions

Y;r(M), Y (M) were appropriately corrected using Yc(M) to obtain the

mass distributions Yx (M) and Y (M) in coincidence with K X-rays

alone.

The K X-ray yield per fragment PxtMj^ pj) was then obtained

using Eqn. (1). The analysis was carried out separately for the two

fragment kinetic energy groups of 150-170 MeV, 170-190 MeV and for

all fragment kinetic energies respectively. The values of PX(M) versus

final fragment mass (after neutron emission) obtained for the three

cases are shown in Fig. 5 for the heavy fragment group. In calculating

the final fragment mass from initial masses the neutron number 2) (M)

for different K. E. intervals obtained by Maslin et a r were used. The

total K X-ray yield per fission for the light and heavy groups obtained

for different total kinetic energies are shown in Fig. 6. For the average

1.0

AVERAGE K.E.

K.E. INTERVAL 170-190 MeV-

K.E. INTERVAL 150-170 MeV.

130 WOFRAGMENT MASS ( M f )

150FIG.5

160

K X-rcv yields per frf\/;ment versus ?v •".after noj."rc%: emission for the thret. 'rr.•

UJ

U_ 0-4

a»!. as01

DCX

0-2

0.1

0-1/J SacHEAVY FRAGMENTSLIGHT FRAGMENTS

-aI

100 130 150 170 190 210 230

TOTAL KINETIC ENERGY (MeV)

K X-ray yieldsfragrant groupsenergy.

FIG.f

fmrr-ent for t^e ll^'.nta function of the tot.sl

-18-

fragment K. E., the K X-ray yields for the light and heavy fragment

groups are found to be 0.11 + 0. 02 and 0. 30 \_ 0. 01 respectively.

The observed increase in the total K X-ray yield from the

heavy group as the kinetic energy decreases is consistent with lie

(16)results of Wyman et al for 0-1 neec interval . This behaviour can

•

be ascribed primarily to the change in the fragment maBB distribution

with kinetic energy. For example, as the kinetic energy decreases the

mass distribution becomes broader with the consequent increase of

the fractional heavy fragment yields in those regions which are away

from the closed shells (Z = 50, N = 82), and where the X-ray yields

are higher.

The present results on the variation of K X-ray yield per

fragment versus fragment mass for the total kinetic energy show the

(A- ft ft*already known feature ' of low yields in the region of closed

shells Z = 50 and N = 82 and increasing yield as one moves away

from the closed shells into the regions of permanently deformed nuclei

at N = 88. Furthermore' although the X-ray yield variation with mass

for the two kinetic energy groups are found to follow essentially the

same behaviour, there is an indication that for the high kinetic energy

group, the yields are somewhat larger than for the low kinetic energy

group in the deformed region. On the other hand for fragment masses

less than 136 the X-ray yields for low kinetic energy case are con-

sistently greater than that for high kinetic energy group. The increase

in the yields in the deformed region for.uigh kinetic energy case may be

- 1 9 -

ascribed to the presence of a larger initial angular momentum in this

case leading to a higher population of states undergoing rotational

transitions with a larger probability of internal conversion. In the case

of nuclei in the spherical region where most of the internal conversion

originates not from the transitions between rotational states but from

the single particle transitions in odd A or odd-odd nuclei, the dependence

on initial fragment spin is not expected to be significant. The higher

X-ray yield for low kinetic energy group (high initial excitation energy)

can be the reBult of a larger number of transitions expected as the re-

sidual excitation energy after neutron emission is also expected to be

greater in this case.

-20-

III. K X-RAY HALF LIVES VERSUS FRAGMENT ATOMIC NUMBER

(a) INTRODUCTION

It has been pointed out earlier ' that the electron vacancies

that give rise to X-rays from fission fragments arise primarily from

internal conversion of nuclear transitions during the deexcitation of frag-

ment nuclei. Once such a vacancy is created, the electron transitions

giving rise to X-rays take place very quickly ( & JO sec). Thus the

time of emission of X-rays following fission-is determined by the life-

times of the nuclear transitions being converted. Consequently a study

of K X-ray emission times from individual fragment nuclei can provide

useful information needed to characterize these transitions. A few

studies'1*' °) have been carried out in the past to determine average time

of emission as a function of fragment masses. Due to the effects of

mass resolution, these times of emission represent only a suitably

weighted average over a number of neighbouring nuclei. With a high

resolution Si(Li) detector X-ray spectrometer it is now possible to

determine the average X-ray yields originating from fragments of dif-

ferent nuclear charges and therefore to infer the X-ray emission times

emitted from fragment nuclei of specified nuclear charges.

In this work the spectra of the K x-rays emitted from U236

fragment nuclei were measured with a high resolution Li drifted silicon

detector for the time intervals of 0-110 nsec and 110 nsec - 1000 nsec

after fission. In each time interval the x-ray spectra were measured

for the two cases of the emitting fragment moving towards the x-ray

-21-

detector and away from it. From the analysis of these spectra, the

observed intensities of the K x-rays from different Z fragments were

obtained for the above cases. From the observed ratios of the x-ray

intensities in the time, interval 0-110 nsec for the cases of emitting

fragment moving towards and away from the x-ray detector, the average

x-ray emission times for different fragment nuclei were determined. A

comparison of the spectra in the two different time intervals gave infor-

mation regarding the presence of any relatively long half life components

for X-ray emission originating from any fragment nucleus.

(b) PRINCIPLE OF THE .METHOD

The Fission detector D, the source foil S and the X-ray detec-

tor Dx are placed in line with each other as shown in the schematic

diagram of Fig. 7 where d, dj, do are the distances of the x-ray detector,

fission detector and the Berylliuin window from the Bource foil respec-

tively. If Nj and N2 are the number of X-rays detected in the cases

of emitting fragment moving away from D and towards D respectively,

the ratio Nj /N2 is equal to -fl- j / XLg where -TL j- and -O- 2 are the

effective solid angles of x-ray detection in the two cases respectively.

The solid angles Si-l and - ^ - 2 will depend on d, dJt d2, area of x-ray

detector aad the average x-ray emission times, and to a lesser degree

also on the area of source foil and the fission detector. It is apparent

that as the x-ray emission timen increase, the average point of emis-

sion moves away from the foil. Consequently -H- j / _Q-2 increases

SCHEMATIC EXPERIMENTAL SETUP FOR X-RAY HALFLIFE MEASUREMENT

Be WINDOW

X-RAY OETECTOR

FISSION OETECTOR

i

rv

FIG.7

t ic diagran of z':.\- experimentfl sctjo forlial+'-lil'e .iQfisur- :er. , .

-23-

with the increase in the emission timee. Therefore from the observed

values of Nj /N2 the average X-ray emission times can be determined

aB a function of the decay constant A and fragment velocity ft . The

exact calculations of -TL j, / XL^ as a function of the decay constant X

and the fragment velocity ft were carried out for the present geometry

with a Monte Carlo programme using computer CDC-3600. The average

life times T for x-ray emission from fragment nuclei of specified

nuclear charges were then determined by a comparison of experimental

values of Nj /N2 and the calculated values of -TL . / SX^,

(c) EXPERIMENTAL

(i) Layout

235 2

A source of U of thickness 200 u.g/cm was coated on a

VYNS film by the electrospraying technique. The source foil and a

surface barrier fission detector were mounted inside a vacuum chamber

having a 10 mil Beryllium window, such that the foil, fragment detector

and the Be window were in line and parallel to each other (Fig. 7). The

fragment detector and the beryllium window were at distances of 1 cm

and 1.5 cms respectively frozn the foil. A collimated neutron beam

from the CIRUS reactor was usnd for this work. To reduce the fast

neutron and gamma ray content of the beam, the beam from the reactor

core passed through 15 cms of quartz and 25 cms cf Bismuth and finally

through a steel collimator which reduced the beam size to 1. 25 cm.

The thermal neutron flux at the foil was about 5 x 10 n/cm2/sec. The

vacuum chamber housing the foil detector assembly was made to have

-24-

very thin entrance and exit windows for the incident neutrons to mini-

mise the beam scattering and therefore the background field in the

region of the X-ray detector. The x-ray detector was a 1. 1 citfi x

0. 3 cm Si(Li) detector cooled to liquid nitrogen temperature and coupled

to cryogenic FET preamplifier. The energy resolution of the x-ray

detector system in terms of FWHM of 26. 25 keV line of Am2 4 1 was 0. 8

keV for low count rates. During the experimental runs, the actual

energy resolution attainable was about 1 keV due to a slight deterio-

ration of the system resolution caused by the highly saturated background

pulses and any long drifts. The X-ray detector was placed at right

angles to the beam direction at a distance of 3. 0 cms from the source

foil. The X-rays were viewed through two 10 mil Be windows, one of

the vacuum chamber and the other of the cryostat and the x-ray detec-

tor was optimally shielded to minimize background.

(U) Electronics and data taking

A block diagram of the electronic arrangement is shown in

Fig. 8. The bipolar outputs of the fragment detector amplifier and the

X-ray detector amplifiers (both in delay line shaping mode) were fed

to two zero cross over (ZCO) units to provide timing pulses. The gain

of the fission detector amplifier was suitably adjusted to avoid trig-

gering of the zero cross over unit by the natural alpha pulses. The

ZCO outputs were fed to a fast coincidence unit of resolution time

(2 t ) equal to 110 nsec and also to a slow coincidence unit of 2 t equal

to 1 u sec. For the purpose of x-ray pulse analysis with optimum .

DDL SHAPING

FRAGMENTOETECTOR

"V. BIPOLARL \ . 0 U T w

DDL SHAPING

X-RAYDETECTOR

""NsBIPOLAR

V . DELAYEDL^*\OUTAMP/* '

ACTIVE FILTER

z.coUNIT t

2.CO

UNIT

COINC2T«110 ns

BASE LINERESTORER

i

m> t

I

COINC

h

GATE

APf> Anr? Ann AIVA1 i

i

0

BLOCK DIAGRAM OF ELECTRONIC ARRANGEMENT FOR X-RAY EMISSION TIME EXPERIMENT

Flft.f

- 2 6 -

resolution, the output of the x-ray preamplifier was fed to an active

filter amplifier with a Gaussian pulse shaping network followed by a

base line restorer, which provided a delayed unipolar output for pulse

analysis. The unipolar outputs of the X-ray detector amplifier and the

fragment detector amplifier were fed to the two ADCs of the 4 parameter

system, which was gated with the slow coincidence output. The output

of the fast coincidence unit was fed to the third ADC to flag the fast

coincidence events. In this way, the pulse heights corresponding to

the kinetic energy of the fragment and the energy of the x-ray and the

timing label pulse were recorded event by event. A high precision

pulser fed at the input of the X-ray detector was first calibrated into

energies using the e.rn. radiations from a Am source. A careful

channel versus energy calibration was^obtained at the start of each run

and further checked at the end, the energy calibration and the system

stability was further monitored during each run by simultaneously

recording a fixed pulser output.

The singles fission count rate was about 27 per sec. The frag-

ment X-ray coincidence rate was about 8 and 11 per minute in the reso-

lving time of 110 nsec and 1 lAsec respectively. In the continuous

running of the experiment for about a month, about 2. 5 x 10 events of

the fragment-X-ray coincidences were recorded.

(d) ALPHA-X-RAY DATA

The present method of determination of emission times re-

quires that the geometry of the set up and in particular the distance

-27-

between the foil and the X-ray detector be known accurately. Since

the measured distances could be subject to slight uncertainties, the

•olid angle of X-ray detection was also determined experimentally by

counting the number of L X-rays per alpha decay of U2 3 4 present in

the source foil wherein 54 keV transition to the ground state of Th2 °

is almost'totally converted in the L subshells. The spectrum of the L.

x-rays in coincidence with alphas was recorded on a pulse height ana-

lyser with the same electronic arrangement by gating the analyser with

the coincidence pulse. From the alpha-x-ray coincidence data, the

experimental solid angle of x-ray detection was determined in the fol-

lowing manners: The total number N. of L x-rays emitted per alpha,

decay was first calculated from the known branching ratios of alpha de-

cay and the measured valued ' of 0. 48 for the average L fluorescence

yield. The observed spectrum of L x-rays was corrected for the detector

efficiency and the total cumber NTJ of L x-rays reaching the detector

per alpha decay was calculated. The solid angle -*"*— of x-ray detection

was then obtained from the relation Si- = NA/NB . This solid angle

corresponds to the X-ray emission at the source foil itself, and was

experimentally determined to be equal to (£0068 £ . 0001).

(e) ANALYSIS OF RESULTS

The peak to valley ratio in the singleifragment pulse height dis-

tribution was observed to be about 10 to 1. On the basis of the recorded

fragment pulse heights, the identification of the fragments into the

light and the heavy group could therefore be carried out without any

significant intermixing.

-28 .

The sorting out of the recorded data on the magnetic tape was

carried out with a CDC 3600 computer. Fig. 9 shows the observed

spectra of the K x-rays in the time interval of 0

-29-

.' • LIGHT FRAGMENTS MOVING TOWARDS• * . X-RAY DETECTOR

1OOO|- * '. • (a)

• (0-HOnsec Data)

• (110-1000 nsec Data)

500

\

0

u

k HEAVY FRAGMENTS MOVING* TOWARDS X-RAY DETECTOR

1C00• (0-110 nsec Data)

, , . • (110-1000 nsec Data)

»•• •500

200_ I I

100 150 200 250CHANNEL NUMBER

FIG.S

The spectra of K X-rays emitted in the two timeregions of 0-110 nsecs. and 110-1000 nseca. forthe cases of (a) l i^ht fragment moving towardsthe X-ray detector and (b) heavy fraf^ent movingtowards the X-ray detector.

-30-

coincidences being leBS than 2%.

(ii) Determination of observed K X-ray intensities from specified

Z fragments

The observed number N(E) of K X-rays per unit energy

interval per fission can in general be expressed a»

where

N (Z) = Number of K X-rays per fission reaching the detectorfrom element Z,

R • = Ratios of the areas of the {, th X-ray component to thesum of all components for element Z,

= Standard deviation of the energy resolution function.

£ z >^ = Energy of the (/th component belonging to eleixient Z.

T| (E) = X-ray detection efficiency.

In order to determine NfZ) the spectrum for the light and

heavy X-ray groups were separately analysed. The background cor-

rected spectrum of each group was fitted to Eq. (1) with a least square

fitting code using the CDC-3600 computer and with the energies and

relative intensities of K

-31-

source served as a counter check for the calculation of T\(E). In the

fitting procedure the variance

oUi

ui2

35

(b)

IN=50

Io .

5a:

atLUQ. 8aUJ

I 6ID

a(A 4

2 -

Q EMITTING FRAGMENT KOVING TOWARDS X-RAV DETECTOR

§ EMITTING FRAGMENT MOVING AWAY FROM X-RAV DETECTOR(a)

_ n» I ft rx. -is-w

33 41 43 45 49 51FRAGMENT ATOMIC NUMBER

53 57 59 61

(a)- The observed intensities ffx(z) of the K X-rays per fission versus f rag asn« unumber in the tine interval of 0-110 naecs. for the cases of the eraittin..; fratner.tmoving towards and a'vay from the X-ray detector, (b )-The nven^e X-w.y e:niscicn

-33-

Monte Carlo method. In these calculations the X-rays were assumed

to be emitted exponentially with a single decay constant'X and -H--,

S\. 2 were determined for a range of values of X and fragment velo-

cities B . These calculations took into account the finite size of the

source foil and the stopping of the fragments in the beryllium window

and the fragment detector. The Doppler change in the solid angle re-

sulting from an anisotropic emissioti in the laboratory system due to

the motion of the emitting fragments was also incorporated in the cal-

culations using the relation XL = -TLO (1 + ji cosO)2 where Q ie the

angle of x-ray emission with respect to fragment direction. Fig. 11

shows the calculated values of -TL^/S^., versus d where d is related

to life- time "T by d = C &T.

The calculated solid angle for T = 0 was found to be equal to

0. 0067 in excellent agreement with the value obtained from the analysis

of alpha-X-ray data. Also the calculated ratio - ^ - 2 / -O-j for f = °o

is euqL to 0. 27, which is in agreement with the value 0. 26 for the ratio

Nv,/N where Nv, and NYO are the intensities for the two c isesJtj XT XJ •*•£

from Z = 52 in the time range of 110-1000 nsec. These agreements

provide an experimental check on the accuracy of the geometry used

for solid angle calculations.

From a comparison of the experimental results of Fig. 10(a)

and the calculated ratios (Fig. 11) for appropriate j3 , the average X-

ray emission times T* versus Z were obtained and theBe are shown

in Fig. 10(b) and Table I. From the data of Fig. 10(a) and (b) and the

GM5

UJ

<UJ

xiO10

oUJ

ill

aifl

8

6

4

2

(b)

N=50

\

p o -tf-1-I

COI

Q EMITTING FRAGMENT MOVING TOWARDS X-RAY OETECTOR

B EMITTING FRAGMENT MOVING AWAY FROM X-RAY DETECTOR

33 35 37II > I

ta)

41 43 45 " 49 51FRAGMENT ATOMIC NUMBER

53 59 61

( a ) - The observed in tens i t ies K -̂fz) of the K X-rays per fission versus, fraf-.aanljnumber in the time interval of 0-110 nsecs. for the cases of the emittin:., frairrtetmoving towards and away from the X-ray detector . (b)-The a v e r s e X-xtiy einissicn•times T vers'js the fragment atomic nunber.

-33-

Monte Carlo method. In these calculations the X-raye were assumed

to be emitted exponentially with a single decay constant 'X and - ^ - I .

-Tl_ 2 were determined for a range of values of X and fragment velo-

cities B . These calculations took into account the finite size of the

source foil and the stopping of the fragments in the beryllium window

and the fragment detector. The Doppler change in the solid angle re-

sulting from an anisotropic emission in the laboratory system due to

the motion of the emitting fragments was also incorporated in the cal-

culations using the relation _TL- = -TLO (1 + B cosO)2 where 0 is the

angle of x-ray emission with respect to fragment direction. Fig. 11

show8 the calculated values of -TL^/S^., versus d where d is related

to life time ¥ by d = C &T.

The calculated solid angle for T = 0 was found to be equal to

0. 0067 in excellent agreement with the value obtained from the analysis

of alpha-X-ray data. Also the calculated ratio -^-z' -^1 ioT "^ = °°

is euqL to 0. 27, which is in agreement with the value 0. 26 for the ratio

Nv,/N whsre Nv, and N,,., are the intensities for the two cases*1 Xj X I fa

from Z = 52 in the time range of 110-1000 nsec. These agreements

provide an experimental check on the accuracy of the geometry used

for solid angle calculations.

From a comparison of the experimental results of Fig. 10(a)

and the calculated ratios (Fig. 11) for appropriate p , the average X-

ray emission times T versus Z were obtained and these are shown

in Fig. 10(b) and Tible I. From the data of Fig. 10(a) and (b) and the

£=0.22

CALCULATED RATIO OF SOLID ANGLES VERSUS

d (CMS)

The calculated ratios _/2-;2/l.j versus ci

•ftI

dsXX>0Cms

calculated value of JTL for different T~ and B the' K-X ray yield

per fission were derived and these are shown in Fig. 12(a). For the

sake of comparison, we have al6o shown in the same figure the K X-ray

yield per fission determined in an independent earlier experiment^ ',

where the X-rays from the fragments stopped in a time less than 5x10 "^sec

were measured. The observed good agreexnent between the two indepen-

dent measurements shows the validity of the present method of deter-

mining the solid angle for x-ray detection from flying fragments and

consequently the X-ray emission times. The fragment charge yield curve

estimated by Wahl et al'1 8 ' is also shown in Fig. 12(a) . The K X-ray

yields per fragment obtained from 'iie present data on K-X rays yield

per fission and the above charge yield curve are shown in Fig.22(a). The

K X-ray yields per fragment obtained from the present data on K-X rays

yield per fission and the above charge yield curve are shown in Fig. 12(b).

(f) DISCUSSION OF THE RESULTS

The observed features about X-ray emission times are broadly

speaking, as follows:

(i) Small X-ray emission times of the order of 0.1 nsec in the

region of N = 50 in the light fragment group. The very small X-ray emis-

sion times and the X-ray yields in the region can be attributed to the

presence of wide level spacings in this region giving rise to faster decay

and low interval conversion, (ii) Comparatively long emission times

for 43TC, 52Te and 5gCe. In the present setup, from the ratio of

one could get only the lower limit of the emission times for these

z111

IU.oc 0.5UJQ.

U>

O .081-

.06UJ

a.

JL

(b)

111FRAGMENT CHARGE YIELD

EXPTL.K X-RAY VIELO

ffl33 41 43 45 49 51 53

FRAGMENT ATOMIC NUMBER

• 2

15 55

o:.10 CL

oUJ

.05 UJ

©

o55 57 61

( a ) - F X^ray yield per f leal on versus fragment atomic number The dotted bars referto the resul ts of Hef.17» .Also shown in the figure i s the charge yield curveestimated by Wahl et a l v " " ^ (b)« K X-ray yield per fragment versus atomic number.

-37-

nuclei as the sensitivity of the technique is limited for emission

times after 3 nBec. For the case of 52Te, a significantly large fraction

of K X-ray intensity is observed in the time interval 110-1000 nsec

showing the presence of predominantly delayed component and these

observed fractions are given in Table n. The half life for this delayed

component has been estimated to be about 200 nsec, if only a single de-

layed component is assumed. For the case of fragment nuclei 43TC,

5gCe no significant yie'd in the 110-1000 nsec time interval is observed

above the background thereby implying that the long life component is

either of very small intensity or the average emission time is less than

about 100 nsec. Isomeric half life for all known isotopes in the region

N = 50-58 are found to vary from days to seconds. It may be therefore

inferred that the isomeric transition half lives have significantly de-

creased in the case of neutron rich fragment nuclei (Z = 43, N ^ 62) due

to the onset of permanent deformation with the addition of extra neutrons.

For the case of 55CS, a noticeable yield above the background is

observed in the time region 110-1000 nsec data, the presence of an

intense fast component and a less intense slow component with emission

time of the order of 100 nsec is concluded in the case of X-ray emission

from 55CS. (iii) For the remaining fragment nuclei, average X-ray

emission times are around 1 nsec.

The broad features of the K X-ray yield per fragment charge

plotted in Fig. 12(b) are found to be similar to those for emission from

Cf252 fragr f̂~ --=

-38-

for U^3° fission'1'' . These features are the observed increase in the

X-ray yield as one moves, away from the closed shell region of N = 50,

Z = 50 and N = 82, a significantly lower yield for 5^Xe as compared to

the neighbouring odd Z nuclei and an increasing yield for N £> 88.

These observations on the relative probability of internal conversion

process in fragment nuclei for different Z have been earlier' ' '

qualitatively correlated with the expected properties of the low lying

states in these neutron rich nuclei.

-39-

TABLE I

Results from 0-110 nsec data

K X Ray YieldAtomic number per fission Mean Emission Time

Z (0-110 nsec) (nsec)

35

36

37

38

39

40

41

42

.43

44

.003

.009

.015

.016

.020

.018

i 026

.017

.006

.001

j_ .001

+_ .002

+_ .002

+ .002

+_ .002

+ .001

+_ .002

+ .002

+ i 001

+ .001

0.2 +_

0. 2 +

2. 2 +

0. 8 +

0. 2 +

0.4 +

0.8 +

1.4 +

7. 3 +_

0. 4 +

. 1

.1

.7

.5

.1

.1

.2

.3

.2

.1

Light Group Yield 0. 13

50

51

52

53

54

55

56

57

58

59

Heavy Group Yield 0.32

004 +

008 +_

015 +_

063 +_

034+_

076 +_

054+_

034+_

022 £

008 +

.002

.002

.002

.003

.003

.002

.003

.002

.002

.001

0. 1 +

0. 5 +

4.3

0.4 +

1.0:+

2.0J;

1.2+_

0. 8+_

4.8

0. 1 +

. 1

. 1

.1

.2

.7, 5.3.2.2

. 1

-40.

TABLE II

Results of 110 - 1000 nsec data

Fragment Observed K X Ray Yield (110-1000 nsec)Atomic Number Intensity per 104 Yield (0 - 1000 nsec)

Z fissions

50 5 .12

51 5 .06

52 110 .79

53 20 .03

54 10 .03

55 42 .06

56 10 .02

57 10 .03

58 6 .03

59 4 .05

IV. STUDIES OF K X RAY MULTIPLICITY FROM Cf252

FISSION FRAGMENTS

(a) INTRODUCTION

It is known that the prompt K x-rays emitted in fission result

from the internal conversion process during the / -deexcitation of

fission fragments. Several workers' ^' have carried out experi-

ments to determine the average yield of K x-rays per fragment as a

function of the fragment charge Z, (or mass M) for the case of sponta-

neous fission of Or and also for the case of thermal neutron induced

fission of various nuclei. The results of these experiments contain

information only on the average number of transitions per fragment

which are internally converted. For a proper interpretation of the

data on the K x-ray yields, it is further necessary to inquire about the

shape of the x-ray emission distribution function f^n), where f^(n) re-

presents the fraction of events in which n x-rays are emitted in a

cascade ( £ fz(n)= 1)-In the present experiments we have determined

both the first moment (IT ) and the second moment (n 2) of the x-ray

emission distribution function fz(n) for fragments of specified nuclear

charges to learn about the cascade emission of K x-rays from

nuclear charges.

The experimental arrangement consists of two Independent

x-ray detectors operated in triple coincidence with each other aad

with fission to record the energies of the coincident x-rays. In additioa,

the independent spectrum of x-raye in each detector in coincidence

-42-

with fission are also recorded. From the analysis of this data infor-

mation about n(Z), n^(Z) and about the simultaneous x-ray emis-

sion probability from complementary fragments are obtained.

(b) EXPERIMENTAL SET UP

A schematic diagram of the experimental assembly is shown

In Fig. 13. The fission fragments from the Cf source were detected

in 2TT geometry by a parallel plate mini-ionization chamber filled

with pure Argon gas. A Cf " source of strength about 5 x 10' fission

per minute, coated on a nickel backing formed the cathode of the ioni-

cation chamber. The cathode-anode separation was kept about 0.15 cm

and the chamber was operated with a voltage of 100 volts. The walls of

the fission chamber were made of perspex and the perspex windows

through which the x-rays were viewed were reduced to a thickness of

0. 5 mxn to minimize the attenuation of the x-rays. The energies of the

x-rays were measured by two cooled Si(Li) detectors A and B each of

size 1. 0 cm x 0. 3 cm placed on either side of the ionization chamber

assembly at distances cf about 2. 0 cm from the source foil. Each of

the Si(Li) detectors was housed in a cryostat and coupled to a cooled

FET preamplifier. The energy resolutions of X-ray spectrometers

A and B invterms of the full width at half maximum of 26. 25 keV line

of Am2 4 1 were 0. 8 keV and 1.0 keV respectively.

(c) ELECTRONIC ARRANGEMENT

A block diagram of the electronic arrangement is shown in

Fig. 14. The pulses from the ion chamber and the x-ray detectors were

DETBIAS.ION CHAMBER OUTPUT

DETBIAS.

' s * ^ J \ / ' y * t* ' f ' '4 J

FET OUTPU1

COLD FINGER Si (Li) Si (Li) COLD FINGER

SCHEMATIC OF THE EXPERIMENTAL ASSEMBLY

FIG. 13

-.s.-iati'.1 diacrr.;n of the experimental r.etuj)the X-ray ruultiplicity experiment.

X-RAY DETECTORA

AMP

FISSIONDETECTOR

LAMP

BX-RAY DETECTOR

DISC

DISC

CSINC.

DISC

SCAI+ 128

•0 COlNCc

COINCSCAi£R-M2B

O-

I DELAYED• COMCCENCE

ADC

Q

IADC ADC

2 | 3O I

15MSacDELAY

Ex.

ADC

O

GATINGPULSE

^"•"^• MOfwFfSSJOW

ITORSCALER

BLOCK DIAGRAM OF ELECTRONIC ARRANGEMENT F O R E X A E X B F EXPERIMENT

-45-

amplified and fed to discriminators to cut off the natural alpha pulses

and noise pulses respectively. The out-put of the three discriminators

were fed to two double coincidence and a triple coincidence units as

shown in Fig. 14. The double as well as the triple coincidence resolu-

tion time was 1 p.sec. The double coincidence pulses (EXF, EXF )

after being scaled down by a factor of 128, and the triple coincidence

(Ex Ex2

F) P"*868 were fed to an OR gate, the output of which gated the

4 parameter system in the delayed external coincidence mode. The

amplifier outputs from the fission fragment detector and the two x-ray

detectors representing Ff E,,. and Ev were fed to the three ADCs of the

4-parameter system. Thus the triple coincidence events of the type

E x . E F (x-rays detected in both the x-ray detectors A and B and in

coincidence with fission) together with the double coincidence events

of the type Ex F and E F (K x-rays detected in either A or B X-ray

detector in coincidence with fission) were recorded event by event on

to the 4 parameter data acquisition system coupled to a paper punch.

The pulses from the fission chamber,although not containing the infor-

mation about the fragment kinetic energy, were used to discri-

minate against natural alpha pile-up pulses from the Cf̂ source.

During the experiment, the fission events were monitored continuously.

The two x-ray detection systems A and B were previously energy

calibrated using the x-rays from Am241 source and in between the runs

of the experiment this was checked by means of an energy calibrated

precision mercury pulser. For an on-line check on the energy cali-

bration of the x-ray system, the pulser was set at 50 keV which gave

-46-

a peak due to chance coincidences with fission. The K x-ray peak

from the Nickel backing of the Cf " source also helped to keep a

check on the stability of '.he system and energy calibration during the

experiment.

(d) DATA ANALYSIS

The recorded data were analysed to obtain the following spectra

for the total number of fission events: (1) Independent energy spectrum

Of K x-vays detected in A and B systems, We will refer to these

independent spectra as NA(E) and N;B(E) respectively. (Z) Energy

spectra of K x-rays detected in system A for the triple coincidence

events of the type Xj X£-F for those cases in which the photons detected

in system B belong to the energy regions of (a) light fragment K x-rays

(10-24 keV), (b) heavy fragment K x-rays (25-50 keV) and (c) compton

scattered gamma rays (50-60 keV). We will refer to these spectra

as NA (E), N̂ ~ (E) and N^ (E) respectively. The observed spectra

NA(E), NA(E) and N^(E) are shown in Figs. 15-17.

These measured K x-ray spectra were converted into K x-

ray yield from specified fragment nuclear charges Z using the least

squares fitting code described earlier in section in e(ii). The code

tock into account the efficiency of detection of detector A and the cor-

rections for the background counts arising from the true coincidences

between the fission and the compton scattered fission gamma-rays.

Using the numbers Ng , Ng and NR of double coincidences per

fission between fission and x-rays detected in system B in the energy

:oo

3000

20 00

uoo

10 0Q

i r I I I I

INDEPENDENT SPECTRUM-OF

K X-RAYS DETECTED IN SYSTEM A

NA(E)

putice

I ' ' ' I ' I I 1 1 1 1 1 1 1 1 L 1 L50 100 150

CHANNEL NUMBERFIG.15

Inde pent! e-it spectr.rn of the K X-rpyadetected i.n oysfcem A.

200 250

400

300

200

en

CO

UN

1

100

;

•

-

- i*- I- 5

: i•••• •**• ••

•XV•

j 1 •

•

•

•

•

•

• 4

•

• i i t i i

NAL(E)

m

m

*

* * • - .

• •

i . . . • i i i 1 f 1 1 k 1 I 1

03I

50 100 150

CHANNEL NUMBER200 250

FI6-16

Spectravi of K X-rayc detected i n sysSc . A fort : icse GKSotj in which a l i y h t fracrna'it ••' T - r y^.-.K 'aeon detected in s y s t e i "D.

ou

300

200

100

• i

ii

i i

i .

KX

RA

Y

- "5

:• i* •

"V

m m

I I

1 |.

1 " 1

t

•t • / *V* •

i I 1

•

#

•

*

i i i

1 1 ' . '

•

•

<

* •

*•

•••

i 1 i i

i i | i i

KlIN

•V.

1 1 1 1 1

1 1 1 1 1

1 1 1 1 1

1 , 1 1

1 1

\ 1

1

•

i i i "

so 100 150CHANNEL NUMBER

200 250FIO.I7

i J p a c t . r u - " , .}i" K X - n ; - t . J o ' . " ! ' " • l e i ' j r . eyat•;••' / . C O J - f i e ; . f

c u o c i n v/hic-i o nonvy frr.;-; i^i-L K " - r n . y h'r..i beo:T -1" tor; tncl

i n s;\ f-i t r M B.

-50-

region 10-24 keV, 25-50 keV, and 50-60 keV respectively and the

above spectra, we obtained the distributions YX(Z), Yx and

YX(Z) of K X-ray yields for a specified change Z per fission for those

selected fissions in which another photon falling in the energy region

of light fragment x-rays, heavy fragment x-rays and comptun scattered

i rays respectively has also been detected in system B. The un-

biased K x-ray yields v x ( ^ ) Pe r fission detected in detector A, were

i

also obtained using the number of fission monitors. The triple coin-

cidence spectra YZ'(Z) and YV(Z) this obtained contain two compo-

nents: one representing events in coincidence with the genuine K x-rays

in the energy regions of 10-24 keV and 25 to 50 keV detected in system

B and the second representing events in coincidence with the compton

scattered / -rays detected in system B In these same energy regions.

Corrections to the triple coincidence spectra for the events of the

second type were made as follows: The spectral shape of the component

which Is in coincidence with the compton scattered j -rays was taken

to be the same as that of the experimentally observed Y (Z). Let

fB and fg denote the fraction of the compton background counts

under the light and the heavy fragment K x-ray peaks respectively in

the spectrum NB(E). Then the spectra Y j * (Z) and Y ^ Z ) of the

K x-rays detected in system A in coincidence with the light fragment

group and the heayy fragment group of K x-rays detected in system

B were obtained from the relations:

-51-

YXC(Z)]

Thus we get the spectrum of K x-rays detected in system A for the

following three cases; (I) the spectrum of K sprays per fissica, v?ith=

out regard to any x-ray detection by system B (ii) the spectrum

Y^OC(Z) of K x-ray per fission when a light fragment x-ray is detected

in system B and (iii) the spectrum Y X(Z) of K x-rays per fission

when a heavy fragment K x-ray is detected in system B.

Let -H- j and - ^ - ^ be the solid angles of X-ray detections

for systems A And B respectively. The solid angle il_j was deter-

mined from the measured total x-ray yield from the light and heavy

fragments per fission, YX(Z) and the known value of average total

K x-ray yield per fission (viz. 0. *1 per fission). If Y(Z) be the

fragment charge yield per fission .stnd f̂ O*) be the fraction of

fragments of charge Z that emit n number of K x-rays in a cascade,

it follows that

Yx(Z) = Y(Z) ( £ n fz(n)) J L x (1)

W h e " fz(n) = 1

The average yield of x-rays per fragment, n(Z) is given by

^(Z) = £ n fz{n) (2)

(i) Heavy-Heavy or Light-Light X-ray Coincidence Data

The triple coincidence X-ray yields Y^X(ZH) corresponding

-52-

to the cases when both systems A and B detect x-rays from the same

fragment charge, say heavy fragment charge ZJJ, is given by

N B

where 7) (ZJJ) is the efficiency of detection in system B of the x-rays

emitted from charge ZJJ and

Y(ZR) T^(ZH) n (ZH) _Q_2 (4)

from eqns. (1), (3) and (4) it follows that

RH* (ZH) = Y " X ( ZH } = ^ n ( n " 1 ) f z H l n ) < r l ( Z H ) ( 5 )

YX(2H)

where2 / - v v/"7 \ ~n l

-53-

From the measured values of R and ~5, the second moment n2 of the

distribution function £7(n) can therefore be determined from Eqs. (6 & (7).

Since corresponding to a specified nuclear charge Z we have a

number of fragment masses contributing to x-ray emission, the distri-

bution function. ftnj represents a suitably weighted average of the

responding quantity for the various isotopes

fz(»)= £z,A >A

andOJ(Z.A) = 1

where (JO(Z, A) represents the relative yields of different isotopes of

fragment charge Z. Fig. 18 shows n(Z), the unbiased K x-ray yield

per fragment, and also n*(Zjj). n*(Zj,). In Fig. 19 is shown

n (Z), the second moment of the x-ray emission distribution function fQ.

(ii) Light-Heavy X-ray coincidence Data

Now, n(Z) is the normal (unbiased) average yield of K x-ray

emission per fragment from fragment charge Z. It is of interest to

find out if this yield is altered if one selects only those fission events in

which the complementary fragment charge also emits a K x-ray. If

P (ZJJ, ZL) denotes the probability of simultaneous emission of K x-

rays per complementary fragment (charge) pair (ZH, ZjJ one can

write

-54-

The triple coincidence x-ray yield YH x(ZL) or ^ ^ j

corresponding to the case when the detector A detects K x-rays from

a light fragment charge Z^ (heavy fragment charge Zu) and the detec-

tor B detects K x-ray from the complementary heavy fragment

charge Z-_ (light fragment charge ZjJ is given by

PX(ZH, ZL) 7 ] ^ ) Sl2 - O - 7 | / N H (9)

where NB = V^ SL2

also Yx(ZXi) = Y(ZL) ^ ) Si-X (10)

From Eqs. (8), (9) & (10), we get

) = J ^ =L n(ZH) n(ZL) Yx (ZjJ n(ZH).

and similarlyK L ( Z H ) =

Y * ( Z H ) ^ (il)

The value of K (ZL) and K (ZH) thus determined are shown in

Fig. 20.

(e) RESULTS AND DISCUSSION

The normal K x-ray yield per fragment is plotted in Fig, 18

as a function of fragment atomic number Z. It is observed that in the

heavy fragment region (Z = 51 to 57) the yield of K x-ray; from odd Z

fragments is significantly higher than those from even Z nuclei, whereas

a similar dependence was not observed in the light fragment group.

This effect of odd-even nature of fragment charge on the K x-ray emis-

sion probability from heavy fragments was observed earlier by

-55-

ft r^ff

OUI

2»

O0

8*2aUiV)

at3-.

J _ JL...J, » I.

-56-

(9)Watson et al . Since the x-ray emission is known to be mainly due

to the internal conversion during the V -deexcitation of the fragments,

the yield of K x-ray from a fragment charge Z is dependent on the

relative predominance of low-energy / transitions for which the

internal conversion coefficient is large. Thus the observed large K

x-ray yield per fragment in the region of N = 88 corresponds to the

well known deformed region where a predominance of low energy

transitions is expected. Also shown in Fig. 18 is the K x-ray yield per

fragment from fragment change Z when it is known to have already

emitted one K x-ray. The most striking feature of this plot Is that

for almost all fragment charges the second x-ray emission probability

is significantly large which shows that x-ray emission is in general

a cascade process. Another noteworthy feature of this plot is that

the yield of K x-rays per fragment from a fragment charge (Z) which

is known to have already emitted one K x-ray is, in general, higher

than the average unbiased K x-ray yield per fragment from the same

charge. In view of the fact that the average K x-ray yield per frag-

ment, n(Z), is less than unity and there exists a significantly large

probability for multiple (or cascade) x-ray emission, one can con-

clude that a very sizable fraction of fission events do not emit any x-

rays. Furthermore, the multiple x-ray emission probability is con-

siderably larger for the heavy fragments than for the light fragments.

There is evidence which indicates that multiple K x-ray emission

probability depends on the odd-even nature of the emitting fragment

-57-

charge. Further, it is seen that for some fragments (Z = 53, 55, 57,

59, 60, 61), the second x-ray emission probability is greater than

unity indicating a fairly large probability of emission of more than 2 K

x-rays per event in these cases.

The second moment n^ of x-ray emission distribution function

fz(n), defined in Eqn. 6, has been plotted in Fig. 19 as a function of frag-

ment charge. The average second moment n2 of the x-ray emission

distribution function is nearly constant for all lig'it fragment charges

except Z = 43 which has a larger n2 value. This larger value of She

width n^ of the distribution function f(n) for Z = 43 indicates a larger

probability of multiple x-ray emission from this fragment charge com-

pared to its neighbours. In the heavy fragment region the width n^ of

the x-ray emission distribution function shows a strong dependence on

the oddreven nature of the fragment charge and also there is a large

increase in n2 for fragment nuclei in the deformed region (N "%> 88)

indicating an increased cascade x-ray emission in these cases.

Fig. (20} shows a plot of the coefficients K1" (ZR) and K H ( Z L )

as a function of the fragment charge. In general the value of the co-

efficient lies close to unity within the experimental errors but in a few

cases the value is greater than unity. There is a tendency for K

and KH (ZjJ, corresponding to a complementary fragment charge pair,

to be unequal for some charge pairs. Since the coefficient K, defined

in Eqn. (8), represents the degree of enhancement (or decrease) of the

"observed" average K x-ray emission probability from one of the frag-

• t i l i i t i

00

45 50FRAGMENT ATOMIC NUMBER

RG.1t

55 60

The second noraori I of the X-ray, emission d ia t r iba t ionfunction versus the fragrne-'Tt atonic 'nu:.ibc;r.

K

5

4

3

2

1 -

e KL(ZH)

vOI

3959

4058

4157

4256

4355

4454

45 ZL53-ZH

X X-rny correlat ion coefficientr-; K vernuothe atomic numbers of the pair fragments.

-60-

ments when the other fragment has emitted an x-ray, a deviation from

unity of the K coefficient can arise as a result of the following reasons:

(i) There could exist genuine correlations in the x-ray emission from

the complementary fragments due to the common conditions existing at

the scission point. For example, the x-ray emission probability may

sensitively depend on the spins of the fragment pairs which are expected

to be highly correlated.' (ii) the measured correlation could be purely

"instrumental". By "instrumental" we mean the following: When x-ray

emission from a particular fragment charge is being studied, there al-

ways exists, as was pointed out earlier, the unavoidable mixing of the

various isotopes of the same charge contributing to x-ray emission.

If the K x-ray emission probability is independent of the isotopic com-

position of the element under study, the coefficient K will be unity. If

however, the x-ray emission probability is a function of the isotopic

composition of the emitting fragment, the measured value of K will

deviate from unity even in the absence of a physical correlation between

the x-ray emission process of the two fragments. We discuss below

two such possibilities: (a) It is seen from Fig. 18 that the average K

x-ray emission probability is dependent on the odd-even nature of the

fragment charge. If, on this basis, one assumes that for the same frag-

ment charge, nuclei with odd neutron numbers emit more x-rays than

those with even N, then in the study of simultaneous x-ray emission

from fragment pairs, the isotopic distribution of the complementary

fragments under study will be different from the normal case and this

-61-

could lead to a value of K different from unity, (b) A similar biasing

will also be introduced if the x-ray emission probability is a function

of the mass of the emitting fragment (either increasing or decreasing).

Let us assume, for illustration, that the x-ray emission probability

increases with increasing mass for the same charge in both the light

and the heavy fragment groups, when x-rays from one of the fragments,

say the light fragments, have been detected in detector B, and the x-

rays from the other fragments being studied using detector A, we are

selectively looking at those events where the light fragments have a

higher mass to charge ratio. Consequently, the heavyfragments will

have a lower mass to charge and hence a lower average K x-ray emiss-

ion probability for the same charge, that is K will be less than unity.

One can show by similar arguments that if the emission probability de-

L H

creases with increasing mass in both fragment groups, K and K will

continue to be less than unity. On the other hand if the emission proba-

bility has an opposite dependence on mass in the light and the heavy

fragment groups, a value of K greater than unity will be obtained.

It is seen from Fig. 20 that the measured values of K for differ-

ent fragment pairs are greater than unity in most cases. On this basis

one can derive the following general conclusions: If the measured

correlation arises as a result of the mass dependence of the x-ray

emission probability, then it follows that the x-ray emission probability

has an opposite dependence on mass for complementary fragment charges.

Such a dependence can be expected as a result or the N = 82 shell.

-62-

Looking at the systematics of the energy of the first 2+ states of even-

even nuclei, a general increase in the level spacing of the low lying

levels is expected as one approaches fromceither side the N = 82 shell

for the same Z. Consequently, for fragment pairs of specified nuclear

charges, the interval conversion probability may be decreasing with

increasing neutron number for higher fragments and vice versa for

heavy fragments. The observed values of K different from unity may

also be taken to indicate the presence of an odd-even effect with respect

to neutron number. Alternately, there could also exist a genuine corr-

elation in the x-ray emission from the fragment pairs, due to common

condition existing at scission.

-63 -

V. SUMMARY

In this project, detailed investigations of the K x-rays emitted in235

the thermal neutron fission of U and in the spontaneous fission of252

Cf were undertaken by carrying out three different types of experiments

described in Section II, in & IV. Several new and interesting results con-

cerning K x-ray emission from fragment nuclei have been obtained, which

are summarised below:

(i) From a three parameter study of the kinetic energies of pair235

fragments and K x-rays in the thermal fission of U the K x-ray yield

per fragment versus final fragment mass was determined for two kinetic

energy intervals. The broad features of the observed heavy fragment x-

ray yield curve included low x-ray yields in the region of closed shells

Z = 50, N = 82, and increasing yields as one moves away from closed

shells into the region of permanently deformed nuclei It is found that

for the high kinetic energy group as compared to the low kinetic energy

group, the x-ray yields are somewhat larger in the region of deformed

fragment nuclei (N^r88) and are consistently smaller in the region of spher-

ical nuclei (2~50, N-^82). The increase in the yields in the deformed

region for high kinetic energy case can be ascribed to a larger initial angu-

lar momentum in this case leading to a higher population of states undergo-

ing rotational transitions with a larger probability of internal conversions.

For the spherical region, the effect of initial fragment spin is not significant

and the larger initial excitation energy is found to result in larger x-ray

yield. This result implies two different types of cascades in the two regions

of spherical and deformed nuclei. It is found that the total K x-ray yields

from the heavy fragment group increases, while from the light fragment

-64-

group remains essentially constant with dec rea se in the fragment total

kinetic energy.

(ii) F r o m the analysis of the data of the experiments to study x-

ray emission t imes it is found that the x - r ay emission t imes a r e of the

order ofO. 1 nsec in the region of N = 50 in the light fragment group. K

x-ray emiss ion from nuclei Tc, Te and Ce is found to have an aver-43 52 58

age longer half life. For the case of ,-7Te, a significantly l a rge r fraction

of K x - r a y intensity is observed in the t ime interval 110-1000 nsec showing

the presence of a predominantly delayed component. The half life for this

delayed component is es t imated to be about 200 nsec. if only a single de-

layed component is assumed. For fragment nuclei Tc, Ce no signifi-43 58

cant yield in the 110-1000 nsec time inverval is observed above the back-

ground implying that the long life component i s either of v e r y small inten-

sity or the average emission t ime is less than 100 nsec. F o r the case ofCs, a noticeable yield above the background is observed in the time range

55

110-1000 nsec . Considering the average emission time of 2 nsec inferred

from the r e su l t s of 0-110 nsec data, the presence of an intense fast com-

ponent and a l e s s intense slow component with emission t imes of the order

of 100 nsec i s concluded in the case of x - r a y emission from Cs. For55

the remaining fragment nuclei, the average x - r a y emission t imes a re found

to be about 1 nsec. The K x - r a y yield per fragment ve r sus fragment atomic

number shows increasing yields as one moves away from the spherical r e -

gion near N = 50, Z = 50 and N = 82 and a significantly lower yield fromXe as compared to the neighbouring odd Z nuclei and an increasing yield

54

mve-

-65-

for N >,. 88.

(iii) The experiments described in Section IV are aimed to

8 tig ate an entirely new aspect of the K x-ray emission in fission namely

the K x-ray multiplicity and a possible correlated emission of K x-rays

from fragment pairs. These investigations were carried out for K x-ray252

emission from Cf fragments. We have determined the correlation coe-

fficient K for simultaneous emission of K x-rays from the fragment pairs

and have discussed in Section IV the implications of this coefficient being

different from unity.

The average K x-ray yield per fragment versus Z shows an odd-

even structure, and an increasing yield with the onset of deformed region

(N »̂ 88). The present investigations have shown a new feature that al-

though the average K x-ray yield per fragment is considerably less than

unity ( ~ 0. 2 for fragments in the light group and ~ 0. 4 for those in the

heavy group), those fission events that do lead to x-ray emission give

rise to a cascade of x-rays rather than a single x-ray. This further imp-

lies that a large fraction of events do not lead to x-ray emission. These

result's should be taken into account in any theoretical investigation of K

x-ray emission in fission. In particular the following pointc are worth

emphasizing:

(i) The cascade emission of x-rays from fragments is a general

rule rather than an exception. Only fragment nuclei with charge Z = 40,

42 and 52 show appreciable deviations from this general trend. Such a

-66-

behaviour of the Tellurium (Z = 52) fragments is understandable since

this element is known to have a long half life x-ray component (refer

Section III), which most probably arises from a well defined angualr

momentum isomeric state.

(ii) Ambng fragments that do lead to a cascade emission of

x-rays, the multiple x-ray emission is found" to be more striking for

fragments in the heavy group. In particular, for the fragments in the

deformed region, (N ^88) the present results show that in those cases

where one x-ray is already emitted, the average yield of the additional

K x-ray is already emitted, the average yield of the additional K x-rays

is even more than one per fragment implying an appreciable probability

for the emission of three or more x-rays during the cascade deexcitation

of a single fragment nucleus. It may be pointed out that in the region

of deformed heavy fragments a higher second x-ray emission probabi-

lity may be associated with a preferential selection of events with larger

initial spin and in the spherical region with the preferential selection of

particular odd N isotopes for which x-ray yields may be larger.

ACKNOWLEDGEMENTS

The assistance provided by The International Atomic Energy

Agency for the purchase of the four parameter data acquisition system

is gratefully acknowledged. We wish to express our thanks to

Mr. B. R. Balial for assistance in the data acquisition on the four para-

meter unit and also in the maintanance of the electronic instruments.

-67-

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2. S. A. E. Johansson; 'Gamma rays in fission1 in spectroscopyedited by K. Siegbahn

3. L. E. Glendenin and H. C. Griffin, Phys. Letters 15 (1965) 153.

4. S. S. Kapoor, H, R. Bowman and S. G. Thompson, Phys. Rev. 140(1965) B 1310.

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10. L. E. Glendenin, H. C. Griffin, W. Reisdorf and J. P. Unik,'Proceedings of Second IAEA Symposium on the Physics & Chemistryof fission', Vienna (1969) 781.

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16. E. M. Bohn, B. W. Wehring and M. E. Wyman, Phys.' Rev. 188(1969) 1909

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