Level 3 Laboratory Project Report - Cameron Anderson

Cameron Anderson - The Set-Up and Application of a Germanium Gamma Ray Detector

I. Introduction

Since its discovery in 1896 [1], the use of nuclear radia-

tion to determine the structure and composition of the

world around us has been standard practice. Easily ma-

nipulated and detected, radiation allows small and large

objects to be probed to gain information about their

composition or their fundamental building blocks, using

‘invisible radiation to make a visible image of an invisi-

ble object’ [2]. Early investigations capitalised on the

charge of alpha and beta radiation to peer deep into nu-

clear structure whereas the observability, negligible ab-

sorption and scattering in air, contrary to alpha and beta

particles [3], makes gamma radiation the superior choice

when it comes to imaging larger objects.

I.I Gamma Radiation and Detection

Gamma radiation consists of photons with energies typi-

cally in the region of the electromagnetic spectrum be-

tween 0.1Mev and 10Mev [3]. Although weakly ionis-

ing, their high energies ensure they are highly penetra-

tive. The creation of a gamma ray photon occurs in an

unstable radioactive nucleus. Most alpha and beta decays

leave the nucleus in a final excited state. These excited

states decay to the ground state energy though emission

of one or more gamma rays [3]. The activity of a radio-

active source will decrease exponentially as a function of

time,

𝐴 = 𝐴0𝑒−𝜆𝑡. (1)

Where A is the activity at time t, A0 is the source’s activi-

ty at t = 0, and λ the decay constant of the radioactive

source, related to the half-life of the source, τ1/2 by

𝜆 =𝑙𝑛2

𝜏1/2. (2)

Variable attenuation of gamma radiation makes it ideal

for imaging. Interactions with matter occur in a variety

of ways, with over 99% of interactions dominated by the

Photoelectric effect, pair production and Compton Scat-

tering. The Photoelectric effect is prominent with low-

energy gamma rays and a high Z (atomic) number of

attenuating matter (important in heavy atoms like lead

[4]), with the inverse being true for pair production.

Compton scattering dominates in intermediate energies,

particularly for low Z values. [5]

As mentioned previously, the observability of gamma

rays makes them perfect for imaging. High resolution

semiconductor detectors are readily available, with a

coaxial germanium detector being used for the duration

of this investigation. Spectrometry using germanium

detectors is preferable for investigating radionuclides

due to the sharply defined characteristic energies of

gamma rays produced by different radioactive nuclei. [6]

The excellent resolution of a germanium detector allows

these characteristic energies to be resolved.

Germanium gamma ray detectors contain a germanium

crystal which, when bombarded with gamma ray pho-

tons, excite electrons within the crystal structure to cre-

ate an electron-hole pair, allowing the electrons to trans-

verse the energy band gap of germanium. These elec-

trons are subsequently swept away by an applied electric

field, created by applying a large voltage across the ger-

manium crystal. The charge collected from liberated

electrons, which is proportional to the energy deposited

into the detector by the gamma ray photons [7], is con-

verted to a voltage pulse which is subsequently ampli-

fied for measurement. One drawback of the germanium

detector arrangement is that a band gap of germanium

The Set-Up and Application of a Germanium Gamma Ray Detector for Imaging using

Gamma Rays and Fluorescent X-rays

Cameron Anderson (and Paul Gape)

Level 3 Laboratory Project, Final Report

Michaelmas Term 2014 Investigations into the set-up of an Ortec® germanium gamma ray detector are conducted, leading eventually to 2D

imaging of simple lead based objects using gamma rays from a radioactive Co60 source. Simple, well defined objects

are successfully imaged, with difficulties in determining any irregular defects in objects.

The success of imaging with gamma rays allows the use of imaging with X-rays to be studied, with the detection set-

up altered to apprehend hard X-ray photons, imaging using the principle of X-ray fluorescence to identify a known

element is conducted. Although trivial scans were successful, many areas are opened up for further study to fully re-

fine the process, making it applicable to industrial and academic areas of study.

Material Atomic

Symbol

Atomic

Number

Operating

Temperature

Band Gap

(eV)

Electron-hole

creation energy ε

(eV)

Density (gcm-3

)

Germanium Ge 32 Liquid N2 (77K) 0.67 2.96 5.32

Table 1. Properties of semiconducting germanium within the detector implemented for the duration of the investigation. [7] Properties

displayed are at the operating temperature of 77K


large enough to ensure thermally excited electrons do

not become charge carriers must be established. Cooling

the crystal to liquid nitrogen temperatures safeguards

against this. Some operating parameters of germanium

can be seen in table 1.

The practical applications of gamma ray imaging are

extensive, with the prominent areas of research being

medical physics and astronomy. In medical physics, con-

trary to x-rays, the defining feature of gamma ray imag-

ing is that the source of energy is usually located inside

the body, introduced by a radioactive pharmaceutical. [8]

An image of emission can then be produced using gam-

ma ray detection techniques. With regards to astronomy,

the primary purpose of high energy gamma ray detection

is to observe obscure phenomena in extreme astrophysi-

cal environments. [9] Gamma ray’s advantageous prop-

erties make them applicable to many areas of physical

research.

I.II X-rays and X-ray Fluorescence

One step below gamma rays in the electromagnetic spec-

trum we find X-rays, though this is not a definite bound-

ary. The regions of hard, high energy X-rays overlap the

regions of soft, low energy gamma rays. [10] This being

the case, it is not unreasonable to adapt gamma ray im-

aging techniques to image hard X-rays, i.e. photons with

wavelengths in the region 0.1-0.01nm. [10]

The generation of X-rays is predominately completed

using an X-ray tube assembly. [11] A beam of cathode

electrons run through the tube and interact with the ma-

terial of the anode, slow down and stop. Some energy

loss in this process forms X-rays known as ‘bremsstrah-

lung’ or braking radiation. [12] This will not be the con-

cern of this investigation, which will focus on the pro-

duction of X-rays thorough X-ray fluorescence.

Atomic X-ray fluorescence occurs when an incoming

photon excites electrons at a low electron orbital (K

shell). The stripping of electrons from this shell creates a

vacancy in the orbital. This vacancy is hence filled by an

electron in a higher energy level (L or M shell) to stabi-

lise the atom. This de-excitation of an electron from a

higher energy level will emit a characteristic X-ray

unique to this element. [13]

Fig. 1 The process of X-ray fluorescence, the secondary release

of a photon from an atom after absorption of an incident

photon.

The specificity of the X-ray photon emitted by the atom

in the fluorescence process allows the atom to be identi-

fied. If the energy of this characteristic X-ray can be

measured and compared to a previously measured value,

one can confidently say the element is present in the

item being studied (Although the specific isotope of the

element cannot be identified [14]).

The applications of analytical methods using X-ray fluo-

rescence are considerable. These include non-destructive

testing, medical research, trace-element analysis and

analysis of samples in situ for geological exploration.

[15] The non-destructive property of this type of imag-

ing makes it preferable for a range of fields, for example,

in the analysis of pigments and inks in historical manu-

scripts. [16] In medical physics, the presence of heavy

elements in biologically important molecules can be

investigated. [17] Solid state detectors with high energy

resolution, such as the germanium detector implemented

in this investigation, have made it possible to study X-

fluorescence for in-vivo investigations. [18]

As far as this investigation is concerned, detailed study

into the set-up of the Germanium gamma ray detector

will be completed. Determination of the optimum exper-

imental parameters to detect firstly gamma rays from a

specified source, and subsequently X-rays leading to the

detection of X-ray fluorescence will be the main aim of

the experiment. The arrangement of the detector will be

used to take scans of known objects to investigate the

limitations of the experimental arrangement as a gamma

ray/X-ray fluorescence imaging set up.

II. Methods

II.I Gamma Ray Source

For the duration of the germanium detector configura-

tion and imaging, the radioactive source used was a

Co60 (Cobalt 60 isotope) gamma ray emitter. With an

initial activity of 458kBq and a half-life of 1925 days,

the current activity of the source was determined to be

66.7kBq from equation 1, due to the fact the source was

obtained with its original activity on 23/02/2000 (i.e.

time since initial activity ≈ 5350 days). It was assumed

due to the large half-life of the source that any drop in

activity for the duration of gamma ray scans was negli-

gible.

As with many gamma emitters, Co60 decays via beta

radiation, which in turn leaves the nucleus in an excited

state. Gamma photons will then be released as the

nucleus returns to the ground state. Two characteristic

gamma photons are realsed in different quantities, as

seen in figure 2, with energies of 1.17Mev and 1.33Mev

respectively.


Fig. 2 Decay scheme of Co60, two beta decay processes lead to

two separate gamma ray emissions. [19]

II.II Germanium Detector Configuration

The detector implemented in the investigation contained

a germanium crystal of diameter 49.6mm and length

35.3mm, with a 3mm space between the end cap of the

detector and the crystal. Absorbing layers in the detector

included a 1.27mm thickness of aluminum to protect the

crystal, and 0.7mm of inactive germanium.

To set up the germanium gamma ray detector to eventu-

ally scan items, two main electronic settings required

optimum determination. These were the amplification of

signals received from the detector and the bias voltage

across the germanium crystal within the detector. A sim-

ple flow diagram of the electronic set up of the system

can be seen in figure 3.

Fig. 3 Electronic set up to configure detector, a high voltage

supply allows the detector to create pulses, which are in turn

amplified and passed to a PC with suitable analysis software.

The software used for the primary stages in the investi-

gation was Ortec®’s Maestro® software, allowing full

energy spectra to be easily analysed. It aided in the con-

figuration of the detector by allowing trivial calculations

of spectral properties to be completed. Signals from the

amplifier were transferred to the software via a multi-

channel buffer.

Firstly, the gain setting of the amplifier in the electronic

system was investigated in an attempt to locate the opti-

mum amplification for signals from the detector to be

read in Maestro®. Maestro® itself operates by placing a

voltage pulse detected in a certain channel for visualisa-

tion. As the gain of the amplifier is increased, the range

over which these voltage pulses are placed widens. For

example, a small amplification will lead to all pulses

from the detector being in a very small range of voltag-

es; hence a very tight spectrum will be observed in

Maestro®, and vice versa for a high amplification. If

amplification is too high, a characteristic gamma energy

peak will be spread widely between channels, and hence

may be difficult to observe. This being the case a wide

range of amplification gains were investigated and a

spectrum taken of Co60 for each one. An amplification

that placed the two Co60 peaks centrally within the

spectrum was considered to be adequate. Gains investi-

gated included 5,8,10,12,15,17.5,20,25 and 30, with a

counting time in Maestro® of 120s to ensure a complete

spectrum was observed. A gain of 20 was eventually

selected (see part III: Results/Discussion).

The high voltage supply was the next electrical compo-

nent to be configured. This voltage over the germanium

crystal within the detector was required to pick up volt-

age signals when a photon event occurred in the crystal.

A voltage that was not substantial enough would mean

photon events would be been missed, hence not being

amplified and subsequently displayed in Maestro®. To

determine the optimum voltage a range of voltages were

selected and an energy spectrum of Co60 taken over 60s.

To assess the change in photon events recorded at each

voltage the net-area of both characteristic peaks of Co60

were measured by Maestro®. [20] A bias voltage of

3.5kV was selected (see part III Results/Discussion).

Although Maestro displays a measured spectrum, it re-

quires calibration to display a spectrum in terms of ener-

gy, and not as previously mentioned, by channel number.

This is a trivial calculation done within Maestro but does

require the user to manually determine the energy of two

points on the spectrum from which the remaining chan-

nel energies can be determined. This was completed

initially with the Co60 source, with known peaks at en-

ergies of 1.17Mev and 1.33Mev [21].

Due to the fact only changes in counts were compared

for the duration of the experiment, and not the change

relative changes in energies of photons hitting the

detecor, an investiagion into the energy efficiency of the

dector was not required, it was assumed that the

efficiency was constant for the duration of all

experimental procedures.

II.III Scanning Movement Set Up

After configuration of the system to detect gamma rays,

a method of moving an object to be scanned was re-

quired. The method implemented consisted of an X-Y

stage to move in two dimensions. Within the stage a

small area was removed to allow a source to be placed

directly behind the object for scanning, creating an emp-

ty scanning area of 40mm x 35mm. The stage also con-

tained a small opening on its reverse side to allow a ra-


dioactive source to be easily added to the system. The

small opening also acted as an attempt to collimate the

beam of gamma rays from the source, although the pro-

ficiency of this was not tested. The X-Y stage itself can

be seen in figure 4.

To move the X-Y stage in a desired direction, two step-

per motors were required, one for movement in the hori-

zontal direction (X) and one for movement in the verti-

cal direction (Y). Each stepper motor had three voltage

inputs; drive, which moved the motor one ‘step’ of a

known distance; direction, which consisted of an either

high or low voltage input to determine the direction the

motor would turn; and finally, MS4, which also required

a high/low pulse, but determined the size of the step tak-

en by the motor (not applied in this investigation). Each

motor was then subsequently ground.

Fig. 4 X-Y stage complete with two stepper motors to move in

2 dimensions.

II.IV The Complete Imaging Process

To complete the imaging system the scanning and detec-

tion process needed to be combined into one system.

This was completed through a National Instruments®

BNC 2121 connector box, containing a mixture of ana-

logue and digital outputs for driving the motors on the

X-Y stage, and a PFI input to take count readings from

the germanium detector, the complete system can be

seen in figure 5.

To compute the movement and count measurement pro-

cesses LabVIEW™ code was written to complete the

scanning, completing certain tasks as follows:

1. Move the X-Y stage to a desired starting coordinate.

2. Take a count reading for a previously determined

time period.

3. Move to the next desired X-Y scanning coordinate.

4. Repeat steps 2 and 3 until scan is complete.

This process was completed over a square array shape

determined by the user, due to the constant physical size

of the scanning area, an increase in scanning array size

would lower X and Y motor ‘step’ sizes and hence in-

crease the resolution of the image produced after scan-

ning.

When taking count readings, the experiment was set up

initially to merely take a reading of total photon interac-

tion events (counts) at each point in the scanning array,

and not the number of counts in the either one of the

Co60 peaks. It was assumed that the total count rate

would be dominated by the peaks, and background

counts would remain constant throughout the experi-

ment. As only the relative change in counts was being

observed when scanning, a constant background reading

would not be of interest.

Fig. 5 Simple flow diagram showing electrical connections for

the imaging set up.

The physical parameters of the experiment were kept

constant for the main duration of the investigation, with

a distance of 75.0±0.1mm from radioactive source to

object scanning area and a distance of 20.0±0.1cm from

the front face of the X-Y stage to the face of the detector

(with a further 3mm from the detector cap to the crystal

itself, as mentioned previously). It was assumed that no

gamma rays were attenuated by the air between the X-Y

stage and the detector.

II.V Gamma Ray Imaging and Experimental Devel-

opment

After completion of the gamma ray imaging set up, test-

ing of its proficiency could be completed, focusing

mainly on how well the detector could image small ob-

jects using different resolutions. Whether these images

could be improved with the addition of other electrical

components was also investigated. A number of separate

2D scans were taken with different experimental objec-

tives:

i. Initially, to investigate how the X-Y stage would affect

any gamma ray images taken, a scan was completed (of

scanning array size 20 x20, giving a resolution of 0.29

pixels/mm2) with no object present in the scanning area,

the objective being to see if the mechanism of the X-Y

stage would be seen in the scans. A counting time of 30s

at each scanning point was made, as long as possible to

reduce error in total counts received.


ii. A simple scan was then taken with a small lead strip

of dimensions 36.07±0.07mm x 11.77±0.14mm x

2.92±0.01mm. A scanning array size of 20 x 20 was

chosen for the sole reason that the resultant scan could

have the ‘background scan’ (scan i) taken from it to re-

move any effects of the X-Y stage scanning mechanism.

A counting time of 30s at each point was made allowing

for simple comparison with scan i.

iii. To test how an increased resolution would affect the

image produced by the scanning set up, another scan of

the lead strip from scan ii was taken, but with a resolu-

tion of 0.64 pixels/mm2 (scanning array size of 30 x 30).

This increase in resolution dramatically increased the

time to complete the scan (i.e. from 400 scanning points

to 900 scanning points). The counting time at each point

was chosen to be 60s.

In an attempt to improve the scanning process, it was

decided that a discriminator would be added to the imag-

ing system (between amplification and computation of

voltage pulses) to only accept voltage pulses from the

germanium detector above a certain threshold voltage.

This being the case, only photons registered in the detec-

tor producing voltage pulses over this threshold voltage

would be acknowledged as counts when a scan was

completed.

The set-up of the discriminator was completed using a

voltage pulse generator set at the desired threshold volt-

age. This was completed using the Maestro® software to

visualize the voltage pulses as photon energies. To avoid

tampering with the amplification settings set in part II.II,

an attenuator was added to the system to bring incoming

voltage pulses into the working range of the discrimina-

tor.

For the Co60 source the discriminator was set to remove

any photon events in the detector below an energy of

1.1Mev, as seen in figure 6. This was to ensure that the

total counts at each scanning point were more so domi-

nated by the Co60 energy peaks than previous scans.

Fig. 6 Co60 energy spectrum with dashed line representing the

location of discrimination (1.1Mev) all counts below this value

were not registered in the scanning process.

iv. A scan to test the discriminator’s effect was complet-

ed. This would take the same form as scan iii, a 30 x 30

scan of the same lead strip, the only modification being

the addition of the discriminator.

v. To investigate how apt the detector was in imaging a

less uniform object, a large defect was cut into the side

of the lead strip used in scans iii and iv, the resultant

object can be seen in figure 7. A scan was then complet-

ed (with the discriminator from iv still present) with an

increased scanning array size of 50 x 50 (1.8 pixels/mm2

resolution) in an attempt to see the defect. The counting

time at each point was set to be 37s due to time con-

straints.

vi. In an attempt to see the defect in scan v in more de-

tail, a scan of the top left quadrant of the original scan-

ning area was taken. As before, a 50 x 50 scan was com-

pleted, meaning the resolution in physical space was

increased by 4, with a counting time of 30s at each scan-

ning point.

Fig. 7 Lead strip with defect imaged in scans v and vi with

dimensions shown.

II.VI X-rays and X-ray Fluorescence Scanning

It was hypothesized that if gamma ray imaging with the

germanium detector was successful, that alterations with

the scanning process could lead ultimately to an X-ray

fluorescence scan of an object. Incoming gamma rays

would interact with the object in the scanning area, with

fluorescent X-rays produced by any X-ray producing

materials present in the object for detection.

Before any X-ray fluorescent scans could be taken, in-

vestigations into whether the germanium detector was

capable of observing the lower energies of X-ray pho-

tons were concluded. Due to the very low voltage pulses

from the germanium detector produced by incident X-

rays, a much higher amplification was required to allow

the X-rays to be analysed computationally. A second

amplifier was added to the electrical system in figure 5

and set to a gain of 20, giving an overall gain of 400 for

the whole system.


To investigate whether the detector was able to detect

energies down to those of the hard X-ray part of the

spectrum, a variable X-ray fluorescence source was

used. The source contained a gamma ray source irradiat-

ing a selected metal, which would in turn emit X-rays

characteristic to that metal with energies displayed in

table 2 [22]. For each metal, a spectrum was taken in

Maestro® to see if X-ray peaks could be observed. Met-

als from the variable X-ray source investigated were

terbium, barium and silver bombarded by an americium-

241 gamma ray source emitting photons of 59.5keV,

with respectively decreasing characteristic X-ray ener-

gies.

Target Kα Energy (keV) Kβ Energy (keV)

Cu 8.04 8.91

Rb 13.37 14.97

Mo 17.44 19.63

Ag 22.10 24.99

Ba 32.06 36.55

Tb 44.23 50.65

Table 2. Characteristic X-ray energies of elements contained

within variable X-ray source.

Once it was determined that the detector was capable of

observing X-ray events its ability to observe fluorescent

X-rays emitted from lead, under the bombardment of the

Co60 gamma ray source, was tested. A simple experi-

mental procedure was completed that involved placing a

strip of lead of thickness 0.65±0.2mm right up to the

face of the germanium detector, with a Co60 source di-

rectly behind it. A photon energy spectrum was then tak-

en in Maestro® with a counting time of 500s to ensure

that if any X-ray fluorescent peaks were present that

there was enough time for them to become statistically

significantly large. The amplification of signals was sub-

stantial enough that the gamma ray peaks from the Co60

were not displayed within the energy range of the spec-

trum. A spectrum was then taken with just the Co60

source present to ensure any observed peaks were not

due to unforeseen experimental anomalies, again for

500s.

To implement the newly found ability of the detector to

image fluorescent X-rays it was decided to use it, as in

part II.V, to image objects. As opposed to gamma ray

imaging, which merely displays relative attenuation of

the object scanned, an X-ray fluorescence scan would

solely produce characteristic fluorescent X-rays corre-

sponding to the element present in the object. If the X-

ray fluorescent peaks could be isolated, then the relative

counts detected in these peaks could determine the rela-

tive the composition of an object scanned. To investigate

the current experimental set up’s ability to identify an

element, a thin piece of lead of width 14.0±0.2mm and

average depth 0.65±0.2mm was cut, which would then

be scanned in an attempt to produce an X-ray fluores-

cence image.

To isolate the fluorescent X-ray peak to be observed

when scanning the lead strip a window discriminator

was required. This was configured in an identical way to

the discriminator in part II.V of the investigation, but

with two threshold voltages set. One of which would

discriminate voltages under the threshold, the other volt-

ages above the threshold. Voltages relating to energies

below 72keV were removed, as were voltages pulses

relating to energies over 78keV.

The X-ray fluorescence scan completed was of array size

30 x 30 with a counting time of 500s at each point in the

scanning area, homogeneous to the scans to attempt to

see the lead X-ray fluorescence peaks. To ensure the

highest amount of gamma rays possible would fall on

the lead strip, the Co60 source was placed 3.4mm away

from the scanning area. The scanning area was hence

reduced to a 35mm x 28mm due to obstruction of part

of the X-Y mechanism, leading to a resolution of 0.92

pixels/mm2. To reduce the amount of X-rays attenuated

by the air between the scanning object and detector, the

X-Y stage and scanning object were placed as close to

the germanium detector as possible.

III. Results and Discussion

III.I Germanium Detector Configuration

In part II.II, methods described were to investigate the

optimum configuration of a germanium detector for ana-

lysing energy spectra, with the eventual goal of scanning

and imaging objects using gamma rays. Initial investiga-

tions involved the gain selection of the amplifier in the

electronic set up of the imaging system. A gain selected

would place the peaks from the selected Co60 source in

the center of the Maestro® spectral display. As observed

in figure 8, the position of both Co60 peaks increased

linearly with increased gain. The higher energy of peak 2

lead to a slightly higher rate of change of position as the

channel axis in Maestro® was stretched with increasing

gain.

Fig. 8 Linearly increasing energy peak position with increasing

amplifier gain.


As mentioned previously an increase in gain would

spread out measured energy peaks between channels.

Although this would make the peaks less defined, it was

required to make use of the full range of Maestro®’s

analysis software, leading to a more accurate energy

calibration. Figure 9 shows the effect of a small gain, the

peaks are well defined but very close together.

Fig. 9 Energy Spectrum of Co60 in Maesto® with a gain of 5.

Calibration has not been completed as in figure 6.

It was hence determined that a gain of 20 would be suit-

able for Co60 spectral observations. The subsequent

spectrum taken can be seen in figure 6, but with an ener-

gy calibration completed, converting the channel number

of each peak to an energy using the previously known

energies of Co60 gamma energy peaks.

Following the set-up of the gain for future spectroscopic

measurements, the bias voltage across the germanium

detector was investigated, with results displayed in fig-

ure 10.

Fig. 10 Effect of Bias Voltage across the germnaium detector

of Co60 peak gross area

At very low voltages photon crystal events were missed

by the detector, hence the large difference in peak area at

a voltage of 0.5. As voltage was increased, gross areas of

both peaks increased and subsequently levelled off. A

voltage of 3.5kV was selected to ensure no photon crys-

tal events were missed. Although it was important to

obtain information on how the detector behaved under

different input voltages, it was not crucial for the main

purpose of the investigation due to the fact only relative

drops in count rates would be measured. It can be seen

from figure 10 that a change in bias voltage has a similar

effect on each peak gross area, implying that the voltage

has a similar effect at different photon energies.

III.II Gamma Ray Imaging

After the germanium detector was configured to detect

gamma rays proficiently from the selected Co60 source,

and the X-Y stage pictured in figure 4 was set up to

move as in part II.IV, preliminary scanning could take

place. With scans corresponding to those described in

part II.V:

i. The first completed scan was a ‘background scan' with

no object present in the scanning area. A 20 x 20 scan

led to a resolution of 0.29 pixels/mm2.

Fig. 11 (Scan i) ‘Background’ Scan with no object present in

the scanning area.

Figure 11 clearly shows an attenuating effect of the

mechanism of the X-Y stage. Fortunately it is only in a

small part of the scanning area. The reduction in counts

received by the detector is significant, but it remains to

be seen whether it will be significant enough to affect

subsequent scans.

ii. The primary scan of an object was that of the lead

strip of dimensions 36.07±0.07mm x 11.77±0.14mm x

2.92±0.01mm. With the resultant scan displayed in fig-

ure 12.


Fig. 12 (Scan ii) First scan of a lead strip

It can be observed in the image that the object scanned is

approximately 10mm in width, similar to that of the

physical object in question. A more precise determina-

tion of width would require a higher resolution scan, due

to one pixel taking up a physical size of 2mm x 1.75mm.

The large drop in count rates of scanning points in the

top of the image was due to the X-Y stage attenuating

gamma rays, homogeneous to scan i. This being the

case, it was determined that a ‘relative’ count scan would

be produced by taking total counts at each scanning

point of the scan ii from the ‘background’ scan i. The

resultant image can be seen in figure 13.

Fig. 13 Combination of scan i and scan ii to create a ‘corrected

scan in an attempt to remove the attentuation of photons due to

the mechanism of the X-Y stage

It appears that the ‘corrected scan’ has been partially

successful in removing the effects of the X-Y stage, es-

pecially in the top left quadrant of the scan. There is still

a though discrepancy between the top of the corrected

scan and the lead strip itself.

After further investigation into the movement of the X-Y

stage, it was determined that an electronic interference

effect was responsible for firstly the slight difference in

the image of the X-Y stage mechanism in scans i and ii,

leading to the ‘corrected scan’ not correctly displaying

the lead strip; and secondly the slight curve in the imag-

es of the lead strip. The interference effect was located in

the wiring of the separate X and Y stepper motors, when

X was programmed to be stationary (whilst Y stepped),

it would step without command, and vice versa. Addi-

tional programming with LabVIEW™ corrected the ef-

fect.

iii. After demonstration that the imaging process could

see the lead strip at low resolution, and the interference

effect from initial scans was dealt with a more in detail

scan of the lead strip could be completed, with a 30 x 30

image produced, leading to a 0.64 pixel/mm2 resolution.

Fig. 14 (Scan iii) Higher resolution scan of lead strip with an

increased resolution and removal of interference effect.

Figure 14 gives an improved image of the lead strip,

demonstrating the success of the interference correction.

The X-Y stage can still though seen to be affecting the

image, and to take another ‘background scan’ as in scan i

would take a large amount of time (due to the difference

in resolution, a simple comparison is not possible as in

figure 13). This being the case it was determined that

any in detail analysis of the image should neglect the top

12mm (≈10 scanning points in y) of the scan, hence pro-

ducing an image with the same resolution but of smaller

physical size. This would give a better lead/background

contrast due to the absence of the X-Y mechanism pho-

ton attenuation.


Fig. 15 Scan iii with the top of the image removed to eliminate

the image of X-Y stage mechanism.

Figure 15 displays the desired increase in contrast be-

tween the lead and air, the width of the lead strip was

determined from the image to be within the range of

8.0mm to 13.3mm by observation, comparable with the

11.77±0.14mm width of the lead strip.

iv. As described in part II.V, scan iii was repeated with

the same conditions, but now with an addition of a dis-

criminator to the imaging system to remove low energy

photons being counted and subsequently displayed in the

scanning image. The resultant scan is displayed in figure

16.

Fig. 16 (Scan iv) Lead strip scan with discriminator added to

imaging sytem.

The addition of the discriminator has successfully re-

moved the effect of the X-Y mechanism as seen in pre-

liminary scans due to the fact low energy photons pro-

duced by the mechanism’s attenuating effect were not

measured. The removal of low energy photons will have

also removed a large fraction of the assumed constant

‘background’ counts (i.e. photons only partially attenuat-

ed by the lead and any background radiation). Although

figure 16 seems to display the lead strip with a less de-

fined edge than that of figure 14, the lack of gradient

between the background and the lead strip allows for a

more accurate determination (again by observation) of

the width of the strip as within the range of to 9.3mm to

12mm. A higher resolution scan would be needed to pre-

cisely determine the width of the strip.

As a comparison of the scan with and without a discrim-

inator, both images were plotted with the same colour

bar in an attempt to identify whether the discriminator

scan returns an image with better contrast (for this to be

completed, the count totals in scan iv were scaled up by

a factor of seven to bring them in the range of scan iii).

The resultant images are displayed in figure 17, showing

a higher contrast between counts within and outside the

lead strip in the discriminator scan.

Fig. 17 Scans iii (right) and iv (left) plotted with same relative

colour bar to show the increased contrast produced by the addi-

tion of the discriminator.

Due to the lower counts recorded using the discrimina-

tor, the percentage error in these count rates will be larg-

er than the scan without the discriminator present. It was

determined though that the increase in percentage error

did not have a large effect on the colour gradient of the

scan when observed in figure 16. It must be noted

though that if a quick scan were to be completed (one

with a small counting time at each scanning array point),

the larger errors on count totals in the discriminator scan

would make the set-up without the discriminator prefer-

able.

It was determined that for the duration of scanning with

gamma rays that the discriminator would remain in the

imaging system due to its ability to achieve superior

contrast and remove the X-Y stage mechanism from

images, neglecting the need to cut down scans, as with

scan iii.

v. To test further the abilities of the scanning set up, a

scan of the lead strip with a defect cut (figure 7) was

completed. An increased resolution of 1.8pixels/mm2

was configured in an attempt to see the defect, with the

resultant scan in figure 18.


Fig. 18 (Scan v) Scan of lead strip with defect (figure 7).

Dashed area scan displayed in figure 19.

Examining figure 18 gives little detail of the defect, if it

is in fact visible at all. A wider base suggests one is pre-

sent but one cannot say with conviction it is there (the

low counts at the top of the scan represent the edge of

the scanning area). It was hence decided a more detailed

scan would be attempted.

vi. The final gamma ray scan built on the work of scan v.

All parameters were the same although the scan was

completed in the top left quadrant of figure 18 in an at-

tempt to display the defect in the lead strip. A 50 x 50

scan produced a 7.1pixels/mm2 resolution image.

Fig. 19 (Scan vi) High resolution scan of the top left quadrant

of scan v. (Dashed area in figure 18.)

The detailed scan gives more insight into the fact the

defect is present, but not a defined edge to the defect.

This does not allow in detail analysis into the defects

size to be completed. The lower count readings at the top

of the scan are due to the overlap of the lead strip with

the outside of the scanning area, leading to a large atten-

uation of counts. It is hypothesized that the source’s

spherical distribution of gamma rays does not allow for

precise scanning using the set up implemented in this

investigation, a more proficient collimation of the gam-

ma source would be required for more detailed gamma

ray scanning. Although the set-up is more than capable

of observing lead based objects with well-defined edg-

es.

III.III X-rays and X-ray Fluorescence Scanning

The use of a variable hard X-ray source determined that

the detection system was capable of detecting photon

crystal events down to the hard X-ray part of the elec-

tromagnetic spectrum. Spectra taken displayed charac-

teristic X-ray peaks for terbium but not conclusively for

barium and silver. With calibration of Maestro® com-

pleted using the known energies of the terbium peaks.

[22] For low energy peaks, it was determined that an

insufficient amount of photon crystal events occurred to

ensure that a peak was distinguishable from the Comp-

ton background from inside the detector itself, i.e. pho-

tons colliding with electrons, varying their energies, cre-

ating a random ‘background’ of photon energies picked

up by the detector. Figure 20 displays the spectra taken.

Fig. 20 Spectra taken with variable X-ray source with elements

terbium (top), barium (middle) and silver (bottom) present. The

Am-241 gamma ray peak is constant in all spectra at 59.5keV.

With comparison of figure 20 and table 2, one can clear-

ly identify the characteristic X-ray peaks from the terbi-

um present at energies of 50.65keV and 44.23keV re-

spectively. With regards to the barium spectrum, one

small peak could be identified at around 36keV which

may correspond to a barium peak, but cannot be conclu-

sively identified due to the uncertainty in the total counts

of the peak maximum overlapping the uncertainty of the

background counts (see appendix). No characteristic

peaks are present in the silver spectrum.

The determination that the germanium detector was able

to identify characteristic X-ray peaks, if only down to a

certain energy, allowed the progression from the variable

X-ray source to the Co60 source bombarding the lead

strip described in part II.VI. Before scanning could be

completed an attempt to see the X-ray fluorescence of

lead was completed.


Fig. 21 Spectra of Co60 without strip of lead present (top) and

with lead (bottom) displaying characteristic X-ray fluorescence

peaks, error bars of the summit of peak values have been dis-

played, with errors on the background counts too small to be

identified. Red dashed lines display the location of the window

discriminator set up.

Figure 21 conclusively displays two characteristic X-ray

peaks of lead, which can be attributed to the K-alpha and

K-beta peaks at approximately 75keV and 85keV respec-

tively. [23] It was decided, due to its larger area, that the

K-alpha peak would be observed when completing scan-

ning and imaging to observe X-ray fluorescence, this

being the case all other energy photons were discrimi-

nated against. The completion of the window discrimina-

tor set up, removing photons of energies below 72keV

and above 78keV, allowed the final scan, described in

part II.VI to be completed.

Fig. 22 Scan of K-alpha fluorescent X-rays from lead strip.

A study of figure 22 shows an increase in counts in the

vicinity of the lead strip’s location, although not a uni-

form or distinct increase in distribution of counts, the

fact that the number of X-rays of energies within the

discrimination range implies that lead is indeed present.

The gradient between the implied center of the lead strip

in the scan and the empty scanning area is attributed to

the fact that the gamma ray beam is not fully collimated,

and X-ray fluorescence photons are emitted in random

directions from target atoms. The lack of a straight

stream of first gamma photons to the fluorescent object,

and then X-rays from it creates difficulties in determin-

ing a well-defined edge in a scanned object.

The varied distribution of counts recorded in the lead

strip itself is due to the non-uniform depth of the lead

strip, hence a different number of lead atoms in different

locations of the strip, investigations into the number of

fluorescent photons and lead depth would need to be

completed to verify this hypothesis.

IV. Further Investigation

This investigation merely scratches the surface of possi-

bilities with nuclear imaging. Detailed underlying set up

and basic imaging have been researched, but many fur-

ther experimental avenues that could be explored.

Firstly, for example, detailed investigation into the phys-

ical parameters of the experiment could be conducted.

Comparison of scans with the X-Y stage at different dis-

tances from the detector may infer an optimum distance

for scanning. Also with the distance from the source to

scanning area, the addition of (or indeed research into)

further collimation of the gamma ray beam is recom-

mended for far distances.

The main focus of this investigation has been with the

use of Co60 as a radioactive gamma ray source. To fur-

ther develop the imaging set up, it is encouraged that a

variety of gamma ray sources are investigated, with

scans taken to establish at which gamma ray energies the

highest resolution and contrast images can be produced.

The investigation described here has concluded that the

detection of X-ray fluorescence photons from lead is

possible with a germanium detector, allowing for further

study in the field. For example, the possibility of deter-

mining the lead content of an object is suggested, with

even in-vivo studies conceivable with refinement of the

experimental set-up. [18]

Lead is not the only X-ray fluorescent material available

for detection by the scanning system, any photons of

energies larger than around 40keV (an investigation into

the lowermost energy limit if the detector is also sug-

gested) could in theory be detected and analysed. This

being the case, a less trivial experimental set up includ-

ing multiple window discriminators and BNC ports for

analysis could allow for the detection of different char-

acteristic X-rays. This in turn would allow objects com-

posed of different, unknown materials to be scanned and

identified.

V. Conclusions

Detailed investgation into the set up of a germanium

detector determined the most preferable electronic set up

for imaging lead using a gamma ray source, with Co60 a

more than acceptable source for imaging lead based

objects. The set up inolved a bias voltage supply, the

detector itself, amplification, and analysis software for

trivial spectral display and manipulation. The optimum

bias gain setting for detection of photon cyrstal events

was determined, as well as the optimum amplification

setting for studying photons in the gamma ray part of the


spectrum in the Maestro® spectral analysis software.

The addition of a discriminator was also investgiated,

which removed some scanning artefacts and led to more

defined images. The addtion of the discriminator lowers

the amount of counts recorded by the detector per unit

time, hence a quick scan will yield high errors on count

rates. It may be preferable for a scan requiring

completion in a limited amount of time to not have the

discrimintor present.

The physical set up of the scanning system was kept

constant through primary scanning, investigations into

the effect of altering the set-up is suggested as possible

further study.

Primary scanning of a simple object implied the

scanning set up was capable of producing images

consistent with the object placed in the scanning area,

allowing for an estimate of the object’s physical size,

with the addition of the discriminator refining this

process. There were limitations to the method, testing

with an artefact cut into a small lead strip led to

difficulties in imaging the defect, increasing image

resolution allowed the presence of the artefact to be

detected but not its physical size to be determined. It is

hence implied that the system described here can be used

to identify well defined objects, with small imperfections

deemed too small to be imaged accurately, although their

presence may be identifiable.

Development from imaging simply with gamma rays led

to investigations into whether the germanium detector

was capable of observing photons in the hard X-ray part

of the spectrum, with photon energies down to approxi-

mately 40keV detected. Although it is suggested that in

detail study could more accurately determine the lower-

most energy capable of observation with a germanium

detector.

The observation of fluorescent X-rays was applied to a

similar process used to image using gamma rays in the

primary part of the experiment. Bombarding a thin strip

of lead was with Co60 and discriminating against pho-

tons not in its K-alpha characteristic X-ray peak allowed

an X-ray fluorescence scan to be produced. This in turn,

due to the specificity of the energy of the X-ray peak,

allowed the item to be determined as lead. Many devel-

opments of this notion are suggested, with the main aim

to determine the relative composition of an unknown

object.

To conclude, the use of gamma ray and hard X-ray pho-

tons allow for a huge depth of study into the size, struc-

ture and composition of materials, of which the basic

principles have been studied within this investigation.

Further study refining the processes described here

would allow for the set-up of an imaging system more

than appropriate for location in academic study and in-

dustrial environments.

Acknowledgements: The author would like to thank Dr.

Ian Terry and Paul Branch for their help and guidance

for the duration of the experiment.

References

[1, 3] K. S. Krane, Introductory Nuclear Physics, 2nd

edition , John Wiley & Sons Inc. , USA (1988) p.i, p.327

[2] M. N. Wernick, J. N. Aarsvold, Emission Tomogra-

phy: The Fundamentals of PET and SPECT, Elsevier

Inc., UK (2004) p.3

[4] G. Nelson, D. Reilly, Gamma-Ray Interactions with

Matter, Passive Nondestructive Analysis of Nuclear Ma-

terials, Los Alamos National Laboratory, LA-UR 90-

732, (1991) p.32

[5] F. Ajzenberg-Selove(edited), Nuclear Spectroscopy –

Part A, Academic Press Inc. , UK (1960) p.211

[6, 7, 14, 19] G. Gilmore, Practical Gamma-ray Spec-

trometry, 2nd

Edition, John Wiley & Sons Ltd., England

(2008) p.22, p.41, p.22, p.5

[8] R. Bourne, Fundamentals of Digital Imaging in Med-

icine, Springer Science & Business Media, UK (2010)

p.35

[9] T.C. Weekes, Very High Energy Gamma-Rau As-

tronomy, IOP Publishing Ltd, UK (2003) p.3

[10] H. Karttunen et al., Fundamental Astronomy, Fifth

Edition, Springer, USA (2007) p.76

[11, 12] H. Aichinger et al., Radiation Exposure and

Image Quality in X-ray Diagnostic Radiology, Second

Edition, Springer, Germany (2012) p.13, p.14

[13] C. Zhang, Fundamentals of Environmental Sam-

pling and Analysis, John Wiley & Sons Inc., USA

(2007) p.227

[15, 17] W.Bambynek, B. Crasemann et al. , X-Ray Flu-

orsecence Yeilds, Auger, and Coster-Kronig Transition

Probabilities, Reviews of Modern Physics, Vol. 44,

Number 4 (1972) p.718, p.718

[16] R. Klockenkamper, A. von Bohlen and L. Moens,

Analysis of Pigments and Inks on Oil Paintings and His-

torical Manuscripts Using Total Reflection X-Ray Fluo-

rescence Spectrometry, X-Ray Spectrom. 29, 119–129

(2000) p.119

[18] L. Ahlgren, K. Liden et al. X-ray fluorescence anal-

ysis of lead in human skeleton in vivo, Scand. j. work

environ. & health 2 82 – 86 (1976) p.82

[20] Ortec®, Maestro®-32 MCA Emulator Software

User’s Manual, Advanced Measurement Technology,

Inc. USA (2006),

(http://web.mit.edu/8.13/8.13d/manuals/Ortec-

MAESTRO-software-manual.pdf) p.76

[21] Laboratoire National Henri Becquerel,

http://www.nucleide.org/DDEP_WG/Nuclides/Co-

60_tables.pdf, 2014

[22] MIT Department of Physics, X-ray Physics, Id:

31.xrays.tex,v 1.22 2007/08/29 (2014) p.6

[23] Ametek®, http://www.amptek.com/charge-

trapping-in-xr-100t-cdte-cadmium-telluride-detectors-

application-note/, 2014

[24, 25, 26] I. G. Hughes and T. P. A. Hase, Measure-

ments and Their Uncertainties, Oxford University Press,

Oxford (2010) p.30, p.44, p.20


Errors Appendix

For the duration of the experiment it was assumed the

use of Poisson statistics was appropriate for analysing

counting events. This being the case it was taken that if

the experiment yielded a mean count of N, the best esti-

mate of the error was taken to be the root of this value

[24], reporting the measurement as,

𝑁 ± √𝑁. (A.1)

On the determination of spectral peaks, it was hypothe-

sied that a peak could be said to be distinguished from

the ‘background’ photon energies if the error bars of the

reading of the peak maximum and a subsequent back-

ground count reading in the vicinity of the peak did not

overlap.

When analysing spectra in the Maestro® software the

determination of errors came straight from the Oretc®’s

Maestro® software manual or were displayed by the

program itself. For example, an uncertainty of 5 was

quoted by the measurement software when determining

the channel position of peaks in part II.II of the investi-

gation. When a measurement of peak area was taken

subsequently in this part of the experiment, the gross

area of the peak was quoted to have an error identical to

that of a single count reading, the square root of the

measured value (i.e. √𝑁 ). [20]

The lack of detailed error analysis made study of gamma

ray images quick and trivial, as mentioned previously, it

was assumed all total count readings had a simple square

root error. The one exception to this was in part II.V

where scans i and ii were compared to produce a ‘cor-

rected’ scan in an attempt to remove the effects of the X-

Y stage used in the investigation. The subtraction of two

count totals at each scanning point required the use of

propagation of errors through a function of subtraction

[25], given by

𝛼𝑐 = √(𝛼𝑏)2 + (𝛼𝑙)2, (A.2)

where αb is the error in the counts at each point in the

‘background’ scan (scan i) and αl the error in counts at

each point in the lead strip scan (scan ii). αc hence is the

combined error of counts at each scanning point in the

corrected scan (figure 13).

Diverging away from errors relating to radioactive count

rates and to more trivial matters, when determining the

error on physical measurements, for example, lead strip

dimensions. The error on mean measurements was simp-

ly taken as the standard error of the mean [26],

𝛼 =𝜎𝑁−1

√𝑁, (A.3)

with σN-1 taken to be the standard deviation of measure-

ments, and N the number of measurements taken.

The final values that require justification of error values

was those of gain and bias voltage in part II.II of the

experiment. These were taken simply to be double the

smallest division on the analogue dials on the electronic

systems to account for any electrical inaccuracy. These

were taken to be 0.2 and 0.02V respectively.

Level 3 Laboratory Project Report - Cameron Anderson

Documents

Transcript of Level 3 Laboratory Project Report - Cameron Anderson