PART II LABORATORY
12 GAMMA RAY SPECTROSCOPY
12.1 MOTIVATION
This practical is primarily about detector characterisation. An ideal detector will give you just the
signal you want and nothing else, but in the real world detectors have finite resolution and pick
up all sorts of other signals including noise. The purpose of this lab is to give you experience in
characterising a detector, and in sifting out which features are due to the signal, which are due to
other interactions in the detector, and which are properties of the detection system. As well as
characterising the detector you will also see how characteristic gamma ray spectra can be used
to identify the radioactive elements present in an unknown sample.
The plan for this experiment is thus as follows. First the detector must be characterised, which
involves determining the relationship between incident gamma ray energy and channel number
on the multi-channel analyser using a set of known calibration samples (don’t panic - more about
the equipment later!). We will notice that there are more features on the collected spectra than
can be accounted for from the known gamma ray energies - these are due to secondary
detection events which you will have to characterise in order to interpret the spectra. Armed with
this information we then go on to analyse the spectra of two unknown sources in order to
determine their composition.
Remember that the primary concern is showing an understanding of what is going on. Getting
the right results is nice, but we are more concerned with developing a mature understanding of
the detector interactions and showing an understanding of what is going on with the equipment,
and your report should reflect this.
12.2 THE DETECTOR SYSTEM
The purpose of this section is to provide an overview of the detection system, plus some
information about the way in which gamma rays interact with matter necessary for interpreting
the spectra.
12.2.1 OVERVIEW
The basic idea of the detection process is relatively simple: in order to measure the gamma ray
spectrum it is necessary to convert the gamma rays emitted by the source into a signal which
can be used to produce a quantitative graph of the energy spectrum. The conceptual layout of
the equipment used to do this is illustrated in Figure 12.1.
 Gamma rays are incident on a sodium iodide (NaI) scintillator crystal which
produces flashes of visible light in response to gamma rays 1. The light output of the
scintillator is directly proportional to the energy of the incident gamma ray so that a 1
MeV photon will produce twice the light of a 0.5MeV photon, provided that all of the
incident photon energy is deposited in the crystal (more about this later).
 This flash of light is converted into a pulse of electrons by a photomultiplier tube
located immediately behind the scintillator crystal. The photomultiplier tube used in
this setup consists of a series of 10 plates (dynodes) in an evacuated glass tube held
at approximately 100V between each plate. Photons incident on the front of the
photomultiplier release electrons through the photoelectric effect, and these free
electrons are accelerated from plate to plate gaining energy from the voltage applied
between the plates. Each time an electron hits one of the dynodes it releases a
number of electrons, all of which are accelerated to the next dynode by the potential
difference between the dynodes, where each of the incident electrons releases a
number of further electrons. Thus an electron cascade is formed resulting in a
detectable pulse of charge on the final dynode in response to the incident photon2.
 This charge pulse is converted into a voltage pulse by the preamplifier, which
produces a voltage spike proportional to the amount of charge in the input pulse, and
1
The term scintillator comes from scintillation, which refers to the flashes of light produced by the crystal
when it is hit by high-energy radiation.
2 This is a fairly sketch description - a more detailed description of the way in which a photomultiplier tube
works can be found in the appendix to this chapter.
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GAMMA RAY SPECTROSCOPY
Radioactive
source
NaI Scintillator
Photomultiplier tube
(Gamma ray to visible photons)
(converts visible photons to charge pulse using
the photoelectric effect and an electron cascade)
High energy photon
(gamma ray)
Charge pulse
CRO
(to directly observe the
pulses from the amplifier)
Voltage pulse
Preamplifier
Voltage pulse
(charge pulse to voltage pulse)
Amplifier
(amplifies voltage pulse)
Multi-channel analysier card in PC
(converts voltage pulses into
a spectrum to analyse)
Figure 12.1 Schematic layout of the measurement system
Gamma rays are incident on a NaI scintillator crystal which produces flashes of light in response to
incident gamma rays, with the light output directly proportional to the incident gamma energy. These
flashes of visible light are converted into a charge pulse by a photomultiplier tube, with this charge pulse
being converted into a voltage pulse suitable for input into the multi-channel analyser card in a desktop
PC by a preamplifier and amplifier pair.
the voltage pulse from the pre-amp is then further amplified by a voltage amplifier
into a signal suitable for detection using the multi-channel analyser (MCA) in the PC.
 The MCA card looks at the magnitude of each pulse arriving at the input and
increments an internal counter according the voltage of the incoming pulse. The MCA
we are using has 1024 channels (pigeon holes) ranging from 0 volts to 10V input, with
the width of each channel being approximately 10mv. Each time a pulse arrives at the
input the MCA determines its magnitude and increments the counter corresponding to
that voltage by one, thus the number of counts in any channel represents the number
of events detected within the voltage range of that channel - the greater the number of
events, the greater the height of that channel on the display. In order to interpret this
spectrum, which really displays the useless information about counts in each channel,
it is necessary to determine the relationship between channel number and incident
gamma ray energy. In other words it is necessary to calibrate the detector, and this is
the purpose of the first part of this lab session3, section 12.4.
12.2.2 THE DETECTION OF GAMMA RAYS
The incident gamma rays are converted into flashes of light by the sodium iodide (NaI)
scintillator crystal, thus it is necessary to understand something about the way in which gamma
rays interact with the scintillator in order to interpret the spectrum data. A -ray can interact with
matter via a number of atomic processes but by far the most probable are the photoelectric
3
Ideally the relationship between channel number and incident gamma ray energy will be a liner one, with
the channel number being directly proportional to incident energy. For this to be the case all of the
elements in the detection system shown in Figure 12.1- scintillator, photomultiplier tube and amplifiers must be linear in their response. Fortunately this is pretty well the case for the equipment in use here, but
if it were not the case this would show up in the calibration and could be corrected for using a non-linear
calibration of energy to channel number.
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PART II LABORATORY
effect, Compton scattering and pair production 4. Each of these processes produce a slightly
different energy at the detector for a given incident gamma ray, giving rise to a peaks at different
locations on the spectrum. It is therefore necessary to understand these interaction processes in
order to interpret the spectra collected.
a)
Photoelectric effect
In the photoelectric effect a -ray of energy E interacts with an atomic electron with binding
energy  b , with the energy of the -ray being completely absorbed by the electron5 which is then
ejected from the atom. To balance momentum, the nucleus also recoils taking with it some of
the photon momentum, whilst it’s mass means that it has very little recoil energy. By energy
conservation the ejected electron has a kinetic energy of
Ee  E  b
Eq. 12.1
which is carried off by the free electron. Another electron then replaces the free electron with
the binding energy of  b released as an x-ray, which is in turn absorbed by further photoelectric
interactions and all of the incident -ray energy is absorbed. If the ejected electron comes from
the innermost shell of the atom the x-ray produced is called a K x-ray, so called because it
results from a transition into the K-shell of the atom. The energy of this K x-ray is a function of
the binding energy of the atom, which in turn is a function of atomic number: the higher the
atomic number the higher the energy of the K x-ray. A plot of the relationship between atomic
number and K x-ray energy is shown in Figure 12.2. The probability of the photoelectric effect
increases with atomic number Z as Z 4 , thus detector efficiency is improved through the use of
heavy elements.
b)
Compton scattering
In Compton scattering the -ray undergoes an elastic collision with an electron which is so
loosely bound to the atom that it can be considered to be a free electron. Without the much
heavier nucleus to carry off the recoil momentum the full energy of the -ray can not be
absorbed, thus the -ray is scattered off the electron with reduced energy, the electron taking the
balance of the energy with it as kintetic energy. The energy of the recoil electron, referred to as
the Compton electron, depends upon the angle of scattering and is given by
  1  cos  
Ee  E 

 1   1  cos  
Eq. 12.2
where
E = energy of the incident -ray,
Ee = energy of the recoil electron,
 = photon scattering angle,
 = E mo c2 ,
mo c2 = electron rest mass (511keV).
This reaction produces a distribution of electron energies from zero up to some maximum value
depending on the scattering angle  with the maximum energy of the recoil electron being when
the scattering angle is   180 . As with the photoelectric effect the recoil electrons ultimately
convert their kinetic energy into optical photons through subsequent atomic interactions.

c)
Pair production
4
All of these processes produce an energetic electron as a result of the interaction. This energetic
electron is rapidly stopped by matter and is converted into a large number of low energy (visible) photons
by ionisation and atomic excitation, thereby producing the flashes of light detected by the photomultiplier
tube with the intensity of the light directly proportional to the energy of the electron produced by interaction
of the -ray with the crystal.
5 The nucleus is capable of accepting some of the momentum of the photon, enabling both momentum
and energy to balance.
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GAMMA RAY SPECTROSCOPY
Figure 12.2 K x-ray energy as a function of atomic number.
If the -ray has sufficient energy pair production may occur when the -ray passes within the field
of the nucleus. In pair production the -ray energy is converted into the rest mass and kinetic
energy of an electron-positron pair; the positron then annihilates with an electron producing two
annihilation photons of energy 511keV (the rest mass of an electron) which can then further
interact by either Compton scattering, the photoelectric effect, or can be lost from the detector
altogether. By energy conservation, pair production can not occur unless the -ray energy is
greater than 1022keV, E  2me c2 .
d)
Combined effect of all three interaction processes
When a -ray source is placed in front of the detector, the detector is bathed in a continuous flux
of -rays, each of which can interact with the scintillator by any of the above processes. The
relative probability of each of these processes is a function of -ray energy, and the form of this
relationship is shown in Figure 12.3.
The final spectrum recorded by the detector is simply the probability weighted sum of the above
three processes, that is to say the actual spectrum recorded is a combination of all energetically
possible events for every -ray the source produces. Note that it is possible for the -ray or x-ray
produced in any of these processes to further interact: for example, the recoil gamma ray from
Compton scattering may further interact via the photoelectric effect. However, the probability of
two interactions is much lower than the probability of a single interaction, thus for the purposes
of the analysis here only single processes will be considered.
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PART II LABORATORY
Figure 12.3 Interaction probability for an NaI detector.
The relative probability of photoelectric, Compton and pair production interactions as a function of incident
Gamma ray energy for an NaI scintillator crystal.
12.3 INTRODUCTORY EXERCISES
These calculations are designed to help you interpret the spectra and should be done before you
start the first days’ work.
Pre-lab Question (a) Calculate the maximum energy imparted to the recoil electron in the
case of Compton scattering of a -ray of energy E. What scattering angle does this
maximum energy correspond to? Calculate the energy of the electron for the case of a
622keV -ray, and at what energies do the events for other scattering angles appear?
Pre-lab Question (b) Now consider what happens when the scattered photon escapes from
the detector: what is the maximum energy deposited in the detector when the scattered
photon escapes? Draw a diagram of this process, including the scintillator crystal, indicating
where all of the reaction products end up. How much energy would be deposited in the
detector if the scattered photon were also absorbed within the detector?
Pre-lab Question (c)
Calculate the energy of the scattered photon for an incident -ray of
energy E when the scattering angle is 180 , and then calculate the energy of the scattered
photon for an incident 622keV -ray. Once again draw a simple diagram of this and suggest
a process by which only the scattered photon could be measured by the detector.
Hint: Could the -ray scatter from anything behind the scintillator so that only the scattered
photon is measured by the scintillator crystal?
Pre-lab Question (d)
Show that for high energy -rays, E   , the energy of the photons
scattered through 180 approaches 255keV, and that this result becomes independent of ray energy as E   .
Pre-lab Question (e)
Consider pair production by a -ray of energy E  1022 keV . How
much energy is deposited in the detector if both the electron and positron deposit all of their
energy in the scintillator? How much energy is deposited if one of the annihilation photons
escapes from the detector, and how much is deposited if both annihilation photons escape?
Once again diagrams may help you here. What does this say about the nature of the peaks
you expect to see from pair production?
Pre-lab Question (f)
Consider a mono-energetic -ray source.
In the light of your
calculations above draw on a calibrated scale the spectrum you would expect to see for a
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GAMMA RAY SPECTROSCOPY
622keV -ray. Remember that the total spectrum is simply the sum of all possible processes
weighted according the probability of that process so it might be useful to draw separate
sketches of the contribution of each process first. See Figure 12.3 for the relative
probabilities of different events at different energies. Similarly draw separate sketches of the
spectrum you would expect to see for a mono-energetic -ray source of 50keV and
2000keV, taking into account the relative probability of detection events in each case.
12.4 DETECTOR CALIBRATION
12.4.1 SETTING UP THE EQUIPMENT
 First of all look around the bench.
Identify all the pieces of equipment,
Caesium 137 (137Cs)
32 keV
and follow the cables to see what is
662 keV
connected to what.
Sodium 22
(22Na)
511 keV
 Then turn on the power supplies to the
1275 keV
equipment one by one: this includes
Cobalt 60
(60Co)
1173 keV
the NIM bin containing the amplifier
1332 keV
modules, the HV supply (which should
Table 12.1 Energies of the calibration
read 100V once turned on), the CRO
sources
and the PC. Let the equipment stand
as is for about 5 minutes to warm up
(this enables the electronics to stabilise
and helps make sure your readings don’t drift during the experiment), during which time you
should find the 137Cs source and place it in front of the scintillator so that it is roughly in line
with the detector’s central axis. Whilst doing this also locate the two other calibration sources,
22Na and 60Co, but leave them some distance from the detector so that they don’t
contaminate the 132Cs spectrum.
 Slowly increase the voltage on the EHT supply to 800V (EHT means Extra High Tension and
is a 1950s term for High Voltage (HV)) and view the output of the preamplifier on the CRO to
make sure that you are getting some signal from the photomultiplier. Then reconnect the preamplifier output to the amplifier input and view the output of the amplifier on the CRO. If you
are having trouble finding any pulses at all try adjusting the triggering and Y-scale on the
CRO, and if you are unsure about what you are seeing talk to your demonstrator after first
thinking about what you’d expect to see.
Question (a)
Do you notice anything interesting about the pulses? For example, are
all the pulses of the same height and shape?
 We now have to set the gain on the amplifier so that the amplifier output falls within a useful
detection range of the MCA card. Recall that the MCA card digitises voltages in the range
from 0 volts to 10V into 1024 channels and that we want to be able to measure gamma rays
of at least 1800 keV (but no more than 2000 keV) in order to identify the unknown sources.
Question (b)
Given that the highest energy gamma emitted by 137Cs is 662 keV (see
Table 12.2) and assuming that the relationship between incident gamma ray
energy and voltage is linear and that 0keV=0V, at what voltage should this peak
appear in order to allow detection of 1800 keV gamma rays by the MCA, and in
what channel number should the peak corresponding to the 662 keV gamma ray
appear?
Now adjust the amplifier gain so that the maximum peak height on the CRO trace is at this
calculated voltage, which should be around 3.5V. Note your final amplifier settings.
 The detector output is highly sensitive to variations in EHT voltage supplied to the
photomultiplier tube. To investigate this, gradually increase the EHT voltage until the output
voltage displayed on the CRO has doubled. Note the change in EHT required to do this and
comment on the sensitivity of the output to EHT fluctuations.
Question (c)
What demands does this place on the stability of the EHT supply, and
what will be the effect of small drifts (eg: 5V) in EHT voltage on the output
pulses and, hence, on the spectrum measured by the MCA?
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PART II LABORATORY
Figure 12.4 Summary of MCA commands
Question (d)
What precautions can you take to minimise the effect of EHT drift on
your measurements?
 For bonus points, try to explain why the photomultiplier might be so sensitive to variations in
supply voltage (think about the design of your photomultiplier tube, which has 10 dynodes,
and about how electron cascade is amplified).
 Return the EHT voltage to 800V and let it stabilise for at least 10 minutes before collecting
any real data, readjusting the supply if necessary. Remember to note the final (stable)
voltage at which your actual data is collected.
12.4.2 SETTING UP THE DATA ACQUISITION SOFTWARE
 Make sure the PC with the MCA card is switched on and start the MCA program by typing
‘mca’ at the command prompt. (yes, this is a DOS based program but it works as well as its
modern counterparts. Who needs a graphical interface anyway?)
 Review the MCA command list (see box) and try the following:
Start collecting a test spectrum (Alt-1). You should see a series of dots
creeping up on the screen - let this run for about a minute or so.
Stop acquisition (Alt-2) and transfer your data from the MCA card into the
computer’s buffer memory (Alt-5), then save the data (Alt-F, Alt-S) into a
temporary file. You can only save data in the buffer, but can only acquire
data from the MCA card, thus you must always transfer the data from the
MCA to the buffer (Alt-5) before saving. Many a student has wasted hours
by saving the wrong spectrum in the wrong file, so be careful.
Move the cursor using the left and right arrow keys. Note that a counter at the
bottom of the screen changes as you do this: this indicates which channel
number the central line is located over. Note that Page-up and Page-down
move the cursor quickly from one place to another.
Adjust the vertical scale from logarithmic to linear (Up arrow): at first the
spectrum will disappear, but keep going until the spectrum reappears. To
return to a logarithmic scale keep hitting the down arrow key.
116
GAMMA RAY SPECTROSCOPY
Change to expanded view (F3) and move the cursor around again. Now change
the size of the expansion region using Keypad +/- and note that you can get
the cursor resolution down to single channel units.
 Note the counter on the left indicating the detector dead time. It takes the MCA card a few
milliseconds to digitise the incoming pulse during which time no further incoming pulses can
be measured. The dead time measures the amount of time the MCA is digitising data
relative to the amount of time it is waiting for incoming data. We want to measure as many
counts as possible, but if the dead time is too high many pulses will be missed and the quality
of the spectrum will decrease. A good compromise is to have a dead time of less than 10%,
which can be achieved by adjusting the source to detector distance. Check that your dead
time is less than 10%, adjust the source if necessary, and record your final dead time value
for all spectra.
 If you are at all unsure about whether your spectrum looks OK check with your demonstrator
now.
12.4.3 COLLECTING KNOWN SPECTRA
We are now in a position to collect spectra of all the known sources. First check that the EHT
supply is still set to 800V and, if necessary, readjust it. Then start the MCA program counting for
approximately 10 minutes. There is nothing magical about this figure: the longer the count time
the better the spectrum, but more time you have to spend waiting for data collection. 10 minutes
is a reasonable compromise between the two, but feel free to chose some other time if you think
it is appropriate.
Make sure only the 137Cs source is infront of the detector and collect a spectrum for the time you
have determined. Save this file, print one copy for each partner by pressing <Print screen>, and
record the channel numbers of all features using the MCA program 6. Repeat this procedure for
both the 22Na and 60Co sources, remembering to check the dead time and EHT voltage before
starting each collection run.
12.4.4 ANALYSIS
a)
Feature identification
Each calibration source gives out only two -rays, but there are many more features than this on
each of your spectra. Identify the origin of all features on all of your spectra, noting which ones
are in the same position for all spectra and which are in different locations. Remember from
your calculations in section 12.3 above that a gamma ray of a single energy incident on the
detector can produce peaks at a number of positions. Use the results of these calculations to
identify all of the peaks on your three spectra, including uncertainties, and comment on how well
the peak location agrees with the result of your calculations (you may need to do the energy
calibration before being able to determine whether all the features are at the energy you expect).
One of the peaks present in all graphs is hard to identify on the basis of gamma ray-detector
interactions alone: it is due to the interaction of a gamma ray with the lead blocks around the
detector.
b)
Energy calibration
We are now in a position to determine the relationship between channel number and energy for
the detector operating with your particular electronic settings. From Table 12.2 we know the
energies of the two gamma rays given off by each source, that is six known gamma rays in total,
and from the results in section 12.4.4(a) we know the channel number corresponding to each of
these energies. One useful feature of the scintillation detector is that the output voltage is
proportional to the energy deposited in the detector, thus we can calibrate the MCA channel
numbers in terms of energy by performing a linear fit to the data. Do this calibration now,
including the uncertainties in measured channel number, and comment on the shape of the
graph.
Question (e)
Does it appear to be linear? Does it pass through the origin? Does it
appear to be what you expect?
6
Remember that you can analyse one data set in the PC memory whilst collecting the next data set in the
MCA buffer.
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PART II LABORATORY
Using your calibration data work out the energies of all other features on your graph and
compare them to the expected energies of secondary features such as Compton edges, pair
production and backscatter peaks. Comment on the extent to which the measured values agree
with the calculated values.
Question (f)
Do these results indicate that you have correctly identified all of the
features on the spectrum?
12.5 IDENTIFICATION OF UNKNOWN SOURCES
Armed with the calibration graph we are now in a position to use the spectrometer to identify
unknown sources. Using the same electronics settings as used in section 12.4.3 (why?) collect
spectra of the two unknown sources and, using your calibration data from section 12.4.4(b)
above, measure the energy of all features across the full range of the spectrum including
uncertainties.
Using the table of known gamma ray energies in Table 12.2 identify the unknown sources.
Question (g)
Which peaks can be dismissed as properties of the system, which are
secondary features such as Compton edges and pair production (amongst
others), and which can be attributed to the sources?
Remember that it’s not so much a correct identification as a good description of your reasoning
that we are after here, so think carefully about what you’re doing. Fully justify your reasoning,
and don’t forget to refer to the ratio column in Table 12.2 which indicates the proportion of
events from a given nucleus which give rise to a gamma ray of that particular energy: the higher
the ratio the higher the peak should be.
12.6 ENERGY RESOLUTION
The amount of detail that can be observed in the spectra is governed by the resolution of the
detector, which essentially measures the width of the peak produced by the equipment for an
incident gamma ray of a single energy. The poorer the resolution, the broader the peak will be
and the harder it will become to distinguish two gamma rays of closely spaced energy. There
are a number of ways to measure resolution depending on how you wish to characterise the
breadth of the peaks, but a standard, commonly used measure of resolution is given by
Resolution 
FWHM
E
Eq. 12.3
where FWHM is the full width at half maximum of an isolated peak and E is the energy of
that peak. Thus this measure of resolution expresses resolution as the ratio of the width of the
peak to the energy represented by that peak.
Return to your calibration spectra, 132Cs, 60Co and 22Na, and measure the full width at half
maximum (FWHM) for the two characteristic peaks in each spectrum and, thence, calculate the
resolution for this set of six peaks (remembering to include uncertainties). Plot this measure of
resolution as a function of gamma ray energy and comment on its shape. Given that the gamma
rays in a full energy peak have all lost an identical amount of energy in the scintillator suggest a
cause for the finite peak width. (Hint: Think about what is happening in the detector chain and
where the most likely causes of variations in peak height might occur. It is unlikely that the
amplifiers and MCA card contribute significantly to the peak broadening.)
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GAMMA RAY SPECTROSCOPY
Energy
(keV)
32
35
53
53
77
80
81
97
121
122
136
176
186
232
242
245
225
265
270
273
280
284
295
Source
137Ba
125Sb
133Ba
Ratio
(%)
7.0
4.5
2.2
226Ra
142Ba
131I
133Ba
75Se
75Se
152Eu
75Se
125Sb
226Ra
142Ba
226Ra
152Eu
142Ba
75Se
56Ni
133Ba
75Se
131I
226Ra
54.0
2.4
33.9
3.5
17.3
28.2
59.0
6.8
3.4
57.2
6.7
7.4
100.0
59.1
35.6
7.1
25.2
5.9
16.9
Energy
(keV)
303
344
353
356
364
364
381
401
425
428
463
511
570
600
601
607
609
636
637
622
723
750
769
Source
133Ba
152Eu
226Ra
133Ba
142Ba
131I
125Sb
75Se
142Ba
125Sb
125Sb
e+ ann
207Bi
142Ba
125Sb
125Sb
226Ra
125Sb
131I
137Cs
131I
56Ni
226Ra
Table 12.2 Known gamma ray energies
119
Ratio
(%)
18.4
26.3
32.0
62.2
22.2
81.1
1.5
11.6
27.5
29.8
10.4
99.7
9.0
17.8
4.9
41.7
11.4
7.2
85.1
1.8
47.8
5.3
Energy
(keV)
779
812
894
898
949
964
1001
1064
1078
1086
1120
1173
1204
1238
1277
1333
1378
1408
1562
1764
1770
1836
Source
152Eu
56Ni
142Ba
88Y
142Ba
152Eu
142Ba
207Bi
142Ba
152Eu
226Ra
60Co
142Ba
226Ra
22Na
60Co
226Ra
152Eu
56Ni
226Ra
207Bi
88Y
Ratio
(%)
12.8
74.1
61.5
93.2
50.0
14.4
44.0
75.5
52.2
10.0
14.3
99.86
76.6
5.0
99.95
99.98
4.8
20.6
13.1
15.9
7.0
99.4
Gamma ray spectroscopy
1