Scintillators for the detection X-rays, gamma rays, and thermal neutrons
Dr. P. Dorenbos
Section Radiation Detection & Matter
Scintillators are materials that convert the energy of ionizing radiation into a flash of light.
Scintillation material can be gaseous, liquid, glass-like, organic (plastics), or inorganic. In each
case the material should be transparent to its own scintillation light. Inorganic wide band gap
ionic crystals are the most widely used scintillators for detection of X-rays, gamma rays, and
thermal neutrons.
Gamma rays interact with a scintillator by means of 1) photoelectric interaction (dominant below
500 keV and interaction probability is proportional to Zeff3-4 where Zeff is the effective atomic
number of the atoms in the compound), 2) Compton scattering (dominant around 1MeV and
interaction probability proportional to the density), or 3) pair creation (dominant well above the
threshold at 1.02 MeV). For efficient detection of gamma rays of energies 100 keV to 10 MeV, a
scintillator should contain high atomic number elements (e.g. Ba, La, I, Lu, Cs, Pb, Bi) and
posses a high density. Depending on application crystals of sizes that may range from a few cm3
up to 1 dm3 may be required. For the detection of thermal neutrons, thermal neutrons need to be
captured by isotopes with high capture cross section. Most popular is 6Li with 6% natural
abundance
n + 6Li
7
Li
3
H + α + 4.79 MeV
(1)
The reaction energy of 4.79 MeV is shared between the triton and alpha particle, and both particle
create an ionization track.
The history on inorganic scintillator discovery is shown in Figure 1. ZnS played an important role
in the discovery of the alpha-particle by the famous experiments of Ernest Rutherford. Around
1900 photon detectors were not yet available and for the detection of the scintillation flashes the
eyes of young students were used. Matters changed after the development of the photomultiplier
tube around 1945. Very soon NaI activated with Tl+ was discovered. Even today, NaI:Tl is still
the most widely applied scintillator. CsI:Tl has higher density than NaI:Tl but it is much slower,
6
LiI:Eu was developed for thermal neutron detection and BGO is popular for its very high density.
New scintillation research activities arose after the discovery of a sub-nanosecond fast
scintillation decay component in BaF2 in 1982. The fast emission is caused by a then new
luminescence phenomenon, i.e., core valence luminescence (CVL). PbWO4 was developed at
CERN in Geneva. More than 75 thousand 23 cm long PbWO4 scintillator crystals are required for
the electromagnetic calorimeter of the CMS (Compact Muon Selenoid) detector at LHC (Large
Hadron Collider). In terms of light output (100 photons/MeV) it is a very poor scintillator.
However, considering the very high energies of the gamma rays involved, the high density and
the short decay time are much more important parameters. Past 15 years many Ce3+ activated
scintillators have been developed. Lu2SiO5:Ce combines a high density with a fast scintillation
response and is used in scanners for medical diagnostics. The newest scintillators LaCl3:10%Ce
and LaBr3:5%Ce3+ were discovered at Delft University and provide record high energy
resolution and ultrafast detection of gamma rays. It is available under the trade mark BriLanCe
and generates much interest in the radiation detection world.
1
LaBr3:Ce
LaCl3:Ce
RbGd2Br7:Ce
LuAlO3:Ce
Lu2SiO5:Ce
PbWO4
CeF3
(Y,Gd)2O3:Ce
BaF2( fast)
YAlO3:Ce
Bi4Ge3O12
BaF2 (slow)
CsI:Na
CdS:In
ZnO:Ga
CaF2:Eu
silicate glass:Ce
LiI:Eu
CsI
CsF
CsI:Tl
CdWO4
NaI:Tl
ZnS:Ag
CaWO4
1900
1920
1940
1960
1980
2000
2020
year
Fig, 1. The history of inorganic scintillator discovery (reproduced from M.J. Weber, J. Lumin 100 (2002)
p.35)
Table 1: Compilation of scintillation properties (density ρ , refractive index n, scintillation decay τ,
emission wavelength λ, photon yield Y, energy resolution (FEHM) at 662 keV) of some well known
scintillators.
scintillator
ρ (g/cm3)
NaI(Tl)
CsI(Tl)
BaF2 slow comp.
BaF2 fast comp,
Bi4Ge3O12 (BGO)
PbWO4
3.67
4.51
1.85
1.80
4.89
1.56
7.13
8.28
7.4
5.37
3.86
5.07
Lu2SiO5:Ce (LSO)
YAlO3:Ce
LaCl3:10%Ce
LaBr3:5%Ce
n
λ (nm)
Y (ph/MeV)
230
3340
415
540
42000
65000
2.15
2.20
1.82
1.95
630
0.6
300
10
47
27
310
220
480
470
420
370
9500
1400
8200
100
25000
18000
1.9
1.95
22
17
350
380
42000
70000
τ(ns)
R @ 662 keV
(%)
6.5
7
8
-7.7
-8
4
3.3
2.8
Scintillation light output and scintillation mechanisms
2
Figure 2 demonstrates the general principle of scintillation. An ionizing particle creates an
ionization track in the host crystal, and electrons from the filled valence band are excited to the
empty conduction band (arrow 1). The average energy to create one ionization is about 2.5Eg
where Eg is the band gap of the scintillator. The value for β is larger than unity because in the
electron-electron collisions during track creation momentum conservation does not allow for the
creation of electron and holes of zero kinetic energy (=momentum). This means that for a wide
band gap oxide crystal with Eg=8 eV, approximately 50,000 free electrons and free holes are
created upon total absorption of 1 MeV gamma ray energy.
2
6
5
L
×
1
4
6
3
Fig. 2 Principle of scintillation in activated wide band gap materials.
After ionization, the hot electrons relax to the bottom of the conduction band (arrow 2) and the
hot holes relax to the top of the uppermost valence band (arrow 3). Next the free electrons should
recombine with the hole to emit a photon. Figure 2 illustrates the situation for an impurity
activated scintillator. The activator ion creates energy levels within the forbidden gap of the host
material, and the free electrons and holes recombine radiatively via an excited state of the
impurity ion (arrow 4). The total light output is given by
Yph =
106 SQ
photons/MeV
β Eg
(2)
where Yph is the number of photons emitted by the scintillator per unit of energy absorbed
(usually photons/MeV). β is a constant that appears approximately 2.5. For the ideal situation, the
transfer efficiency S and the quantum efficiency Q of the activator ion are 100% and then with
Eg=8 eV the light yield will be 50000 ph/MeV. In the ideal situation also the transfer speed of
free electrons and free holes to the impurity ion is instantaneous, i.e., faster than 1 ns. In that case
the rise time of the scintillation pulse is very short and the scintillation decay time τs is
determined by the life time τν of the activator excited state only.
Figure 3 shows the light output of known scintillators and luminescent phosphors as function of
the band gap of the host material. The solid curve is the theoretical maximal output obtained
when S=Q=1. For the available oxide scintillators the yield is limited to below 30000 ph/MeV
whereas based on Eq. (2) values of 50,000 ph/MeV should be possible.
3
The situation is much better for LaCl3:Ce and LaBr3:Ce. They appear, in terms of light output,
ideal scintillators with yields close to the theoretical maximum. Proceeding to increasingly
smaller band gap materials we arrive at the most popular scintillator NaI:Tl. With 42000 ph/MeV,
NaI:Tl appears only 50% as efficient as the theoretical maximum. Pure NaI at liquid nitrogen
temperature is known to yield much higher light output of 80000 ph/MeV. The highest light
outputs are reported for the sulfides which have the smallest band gap. ZnS:Ag, used for α
particle detection by Rutherford, has a light yield of 90000 ph/MeV, which is about the same as
for CaS:Ce used in the first generation of cathode ray tubes for TV screens.
140
Yield (photons/keV)
120
bromides
oxides
fluoride
sulfides
chlorides
iodides
100
ZnS:Ag
80
NaI:(80K)
60
LaBr3:Ce
40
NaI:Tl
CsI:Tl
K2LaCl5:Ce
LaCl3
Lu2S3:Ce
20
β=2.5
YAlO3:Ce
Lu2SiO5:Ce
BaF2
0
2
3
4
5
6
7
8
9
CaF2:Eu
10 11 12 13 14
Egap
Fig. 3 Scintillator light output of various scintillators as function of the band gap of materials. The solid
curve indicates the theoretical limit.
Most scintillators and phosphors do not reveal the theoretical maximum light yield. There are
many causes for electron-hole losses in scintillators. Figure 2 shows that electrons and holes may
recombine without emitting a photon (arrow 5). This may occur in the intrinsically pure lattice
but also due to the presence of defects or unintended impurities that are sometimes called ``killer
centers'' (arrows 6).
Other mechanisms competing with the wanted scintillation process are illustrated in Figure 4. It
shows that a hole in the valence band is trapped in the activator ion (arrow 1), but the electron is
trapped somewhere else (arrow 2). When the electron trap is shallow (< 0.5 eV), the trapping is
not stable at room temperature. Thermal activation of the electron back to the conduction band
and subsequent transfer to the hole trapped on the activator ion (arrow 3) may lead to delayed
luminescence (arrow 4). Depending on the depth of the electron trap, this afterglow may last
several ms or it may persist much longer. Lu2SiO5:Ce3+, for example, shows a strong afterglow
lasting for several hours. For materials with even large trapping depth the trapping is permanent
at room temperature. Those types of materials can be used for dosimeters. The number of filled
traps is proportional to the amount of radiation dose received. By heating the material the
electrons can be liberated from their traps. Recombination of the electron with the hole then
yields luminescence (thermo-luminescence). The TL intensity is a direct measure for the received
dose.
A hole in the valence band tends to be shared between two adjacent anions and a molecular like
defect is created in the lattice. In alkali-halides the molecular complex is known as a Vk center.
By thermal activation the Vk center may jump from one site to an adjacent site. It tends to trap an
4
electron. If this occurs, a self trapped exciton (STE) is created. The STE is a neutral defect and
may also migrate relatively easily by thermal activation through the lattice. The self trapped
exciton can also decay under the emission of a photon.
3
2
4
1
Fig. 4. The role of trapped holes in the scintillation process
a)
c)
b)
LaC 3
l %
0.6
Ce
LaC 3
l %
10
Ce
LaC 3
4%
Ce
400 K
400 K
400 K
200 K
175 K
135 K
250
300
350
400
450
wavelength (nm)
500
550
100 K
250
300
350
400
450
wavelength (nm)
500
100 K
550
250
300
350
400
450
500
550
wavelength (nm)
Fig. 5. X -ray excited emission in LaCl3 with 0.6%, 4%, and 10% Ce3+ as function of temperature in K
Figure 5 shows X-ray excited emission spectra of LaCl3 with Ce concentration of 0.6%, 4%, and
10% Ce. At 135 K and 0.6%, Ce3+ emission is observed as the double peaked emission at 337 nm
and 358 nm. In addition a 0.70 eV broad band emission is observed peaking at 400 nm. This
emission is caused by STEs. Upon heating to 400 K, the STE emission disappears and the Ce
emission gains intensity. The explanation is as follows. At low temperature the free holes have
two options: 1) they self-trap to form a Vk center or 2) they are trapped by Ce3+ to form Ce4+.
This all happens on the sub-nanosecond timescale. At 135 K, the Vk center mobility is low and it
5
will trap the electron to from the STE that provides the broad band emission. Electrons trapped by
Ce4+ give the characteristic Ce3+ emission doublet. Upon heating the crystal, the STE becomes
mobile and transfers its energy to Ce3+. Ce3+ emission with an effective lifetime dictated by the
transfer rate from the STE is observed. When the concentration increases the role of STEs
becomes less and for 10% Ce3+ almost all emission is as Ce3+ emission. Figure 6 (left panel)
shows that the scintillation pulse of LaCl3 contains a fast component and a slow component.
Scintillation decay curves of NaI:Tl and Lu2SiO5:Ce3+ (LSO) are also shown for comparison.
intensity (arb. units)
Intensity (a.u.)
10000
3+
LaCl3:10%Ce
3+
LaCl3:30%Ce
NaI:Tl
LaBr3:0.5%Ce
1000
NaI:Tl
100
LaBr3:4%Ce
10
Lu SiO :Ce
2
5
1
0
200
400
600
800
1000
time (ns)
0
50
100
150
200
250
300
time (ns)
Fig. 6. γ excited decay curves of various scintillators.
Energy resolution and non-proportionality
Figure 7 shows pulse height spectra of a 137Cs source, emitting 662 keV gamma rays and 32 keV
X-rays, measured with a NaI:Tl scintillator and a LaBr3:0.5% Ce3+ on a standard photomultiplier
tube. One of the most important properties of a scintillator applied for gamma ray spectroscopy is
the resolution with which the energy of gamma rays can be determined. Figure 7 shows that it is
much better for LaBr3:Ce than for the traditional NaI:Tl.
1.2
(5)
counts (arb. units)
1.0
(1)
0.8
(3)
0.6
(4)
0.4
(2)
b)
0.0
EC(662)
477 keV
0.2
a)
0
100
200
300
400
500
600
700
800
energy (keV)
Fig. 7. 137Cs pulse height spectra with a) LaBr3:0.5%Ce3+ and b) with NaI:Tl+
6
The energy resolution R is usually specified as
R=
∆E
= 2.35σ ( E )
E
(3)
where ∆E is the full width at half maximum intensity (FWHM) of the total absorption peak at
gamma energy E and σ(E) is the standard deviation in the pulse height.
The energy resolution achievable with a NaI:Tl photomultiplier combination is about 6.4%. The
scintillator LaBr3:0.5% Ce3+ reveals a much better resolution of 3.3%. This together with the
much faster decay time of the LaBr3:Ce3+ scintillator, makes LaBr3:Ce3+ and also LaCl3:10%
Ce3+ a scintillator that is superior to NaI:Tl+.
Formally the energy resolution can be written as
2
2
2
R 2 = Rstat
+ Rnp2 + Rinh
+ Rdet
(4)
where Rstat is the contribution from the statistics in the number Ndph of detected photons. Rnp is a
contribution connected with non-proportionality in the scintillation light yield with gamma ray or
electron energy. Rinh is a contribution from in-homogeneities or non-uniformities in the
scintillator, the light reflector or the quantum efficiency of the photon detector. Rdet is a
contribution from noise and variance in the gain of the photon detector. The last two contributions
are related to crystal growth and detector technology. The first two are fundamental in nature and
intrinsic to the scintillator.
Rstat follows Poisson statistics
Rstat = 2.34
1+ v ( M )
N dph
(5)
Where v(M) is the variance in the gain of the photomultiplier tube and is about 0.1.
Figure 8 shows the energy resolution of scintillators at 662 keV as function of Ndph . These are the
number of generated photoelectrons in the case of PMT readout. The solid curve represents Rstat
given by Eq. (1). As required by Eq. (4) energy resolutions are always larger than Rstat. Data on
YAlO3:Ce3+, LaCl3:10% Ce3+ and LaBr3:0.5% Ce3+ are very close to Rstat indicating that the
other three contributions in Eq. (4) are insignificant. The situation for the well known scintillators
NaI:Tl, CsI:Tl, and Lu2SiO5:Ce3+ is much different. The observed resolution appears twice as
large as Rstat indicating important other contributions.
The poor resolution of NaI:Tl, CsI:Tl, and Lu2SiO5:Ce3+ is caused by a response of the
scintillator that is not proportional with the energy of the gamma ray. Figure 9 shows the
proportionality curves for several scintillators. The light yield in photons/MeV at gamma ray
energy Eγ relative to the light at energy 662 keV is shown as function of Eγ. For a proportional
response, the curve should be a constant line at value 1. That of YAlO3:Ce3+ and LaBr3:Ce3+
are indeed close to one between 30 keV and 1 MeV. On the other hand for Lu2SiO5:Ce3+,
7
gamma rays or X-rays of 10 keV are 40% less efficient in producing scintillation light than at 662
keV energy. For NaI:Tl 30 keV gammas are 20% more efficient than at 6 MeV.
1.2
NaI:Tl
9
BaF2
1.1
Lu2SiO5:Ce
YAlO3
1.0
7
6
CsI:Tl
relative yield
Energy resolution at 662 keV
8
NaI:Tl
5
YAlO3:Ce
4
0.9
LaBr3
0.8
LaCl3:Ce
3
0.7
LaBr3:Ce
Lu2SiO5
2
CdZnTe
0.6
1
Ge
1000
10000
100000
0.5
Ndq
10
100
1000
energy (keV)
Fig. 8. Left panel. The energy resolution (FWHM) of scintillators at 662 keV. The solid curve is the
calculated Poisson statistical contribution. Ge and CdZnTe are semiconductor detectors. For Ge the energy
resolution of 0.2% at 662 keV is 5 times better than expectations from Poisson statistics. The is because the
variance in the number of generated and detected electron hole pairs in the ionization track does not follow
Poisson statistics.
Fig. 9 Right panel. Proportionality curves of scintillators, i.e., normalized scintillation yield as function of
gamma ray energy.
The non-proportionality with gamma ray energy is directly related with non-proportionality with
electron energy. Suppose Y(Ee) is the light output of a scintillator as function of electron energy
Ee. After the absorption of a gamma ray with energy 662 keV, a cascade of events takes place
both in the atom that the gamma particle interacted with and in the ionization track formed by the
primary electron. For a gamma particle labeled 1, the cascade eventually results into a collection
of n secondary electrons (also called δ-rays) with energies E1, E2, .., En that each create a branch
n
of the ionization track with .
∑E
i
= 662keV . Another gamma particle labeled 2 creates another
i
m
collection of m secondary electrons E1, E2, .., Em. Again the sum
∑E
i
= 662keV . Since the
i
light yield is not proportional with electron energy, the two gamma’s of the same energy do not
produce equal amount of photons. The creation of energetic secondary electrons is a statistical
process and leads to fluctuating light output and therefore to a contribution Rnp in Eq. (3). Only
when a scintillator is proportional the light output does not depend on secondary electron
distribution. This is the situation for YAlO3:Ce3+, LaCl3:Ce3+, and LaBr3:Ce3+ at energies
above 30 keV.
Scintillation decay time and the luminescence centers
Decay time of the scintillation pulse is the most important parameter when a scintillator is used
for fast timing. It depends on the transfer speed of charge carriers from the ionization track to the
luminescence center and by the lifetime of its excited state. In the ideal case of infinitely fast (i.e.
8
faster than 1 ns) energy transfer to the luminescence center, the scintillation decay is determined
by the decay rate Γν =1/τν of the excited state. The decay rate is given by
Γν =
1
τν
∝
n ( n2 + 2)
λ
2
3
f µ i
2
(6)
The decay rate decreases with the third power of the wavelength λ of emission and increases with
the refractive index n of the host material. The last factor in Eq. (6) is the matrix element that
connects the initial state (this is the excited state of the activator ion) with the final state via the
electric dipole operator µ. This is non-zero only for electric dipole allowed transitions.
The number of activator ions that show electric dipole allowed transitions suitable for scintillation
applications is limited. These are the transitions between the 5d and the 4f orbitals in the
lanthanides, Ce3+, Pr3+, and Eu2+ and the transitions between the 6s6p and 6s2 configurations in
the so-called 6s2-elements Tl+, Pb2+, and Bi3+. These are precisely the activator ions
encountered in applied scintillators.
Transitions in Tl+, Pb2+, and Bi3+ are from a 6s6p spin triplet 3PJ state to the 6s2 spin singlet 1S0
ground state. Those transitions are spin forbidden, yet partly relaxed by the spin-orbit interaction,
leading to relatively long scintillation decay times. The scintillation decay time in NaI:Tl is 230
ns (see Figure 6) and 300 ns for BGO. For fast timing purposes the 6s2-elements are not suitable
as activator. Eu2+ is also relatively slow (~1 µs).
Figure 10 shows X-ray excited emission spectra of Ce3+, Pr3+, and Nd3+ in various compounds.
Ce3+ emits in a characteristic doublet at wavelengths depending on the type of compound. It is
usually around 300 nm in fluorides and it tends to shift to longer wavelengths with smaller value
for the band gap of the host crystal. For Lu2S3:Ce3+ the emission is in the red at 600 nm. Pr3+
shows a more complex emission with four main bands. When in the same compound as Ce3+ the
emission to the ground state is always at 1.5 eV higher energy than Ce3+. That of Nd3+ emits at
2.8 eV higher energy.
wavelength [nm]
800 600
0.8
Ce
400
200
3+
Lu2S3
LiYF4
3+
Pr
3+
Y3Al5O12
2
4f -4f
LiLuF4
2
0.4
0.8
Pr
Nd
quantum efficiency [%]
yield [arb. units]
0.4
0.8
Ce
80
Nd
3+
LaF3
3
3
4f -4f
30
40
3
50
-1
wavenumber [10 cm ]
60
silicon photodiode
TMAE
40
20
0
20
3+
60
photomultiplier
tube
0.4
10
3+
200
400
600
800
1000
wavelength [nm]
Figure 10. Left panel. X-ray excited luminescence in Ce3+, Pr3+, and Nd3+ doped compounds.
Figure 11 Right panel. Wavelength range of Ce3+, Pr3+, and Nd3+ 5d-4f emission in compounds
compared with typical quantum efficiency curves of a photomultiplier tube, a photodiode, and the
photosensitive gas TMAE.
9
In addition to the relatively broad 5d-4f emissions in Pr3+ and Nd3+, these ions also reveal
narrow emission bands caused by transitions between 4f levels. These emissions are dipole
forbidden and very slow (ms). The life time of the Ce3+ 5d state depends on the type of
compound and is found between 15 and 60 ns. Because of the shorter wavelength of emission, see
Eq. (6), the lifetime of Pr3+ is roughly two times shorter and that of Nd3+ is four times shorter
than that of Ce3+.
Figure 11 shows the observed range of values for the 5d-4f emission of Ce3+, Pr3+, and Nd3+ in
compounds together with the quantum efficiency curves of a photosensitive gas TMAE, a bialkali photomultiplier and an photodiode (PD). Depending of the type of compound the emission
of Ce3+ may match nicely with the maximum quantum efficiency of a PMT or of a (A)PD.
Altogether Ce3+ is an unique activator ion. It combines: 1) a fast 5d-4f emission, 2) absence of
slow 4f-4f emission, 3) an almost unity Q=1 internal quantum efficiency, 4) the proper
wavelength of emission, 5) it is also an excellent hole trap, and finally 6) it can be substituted on
La3+, Gd3+, and Lu3+ sites that are constituents of high density host crystals.
Anatomy of a pulse height spectrum
Figure 12 shows the pulse height spectrum of 24Na measured with LaBr3:Ce, the highest energy
resolution scintillation available today, coupled to a photomultiplier tube (PMT). Although the
source emits only gamma photons of 2.75 MeV and 1.37 MeV, the spectrum is very rich in
features that are, in order of decreasing energy, numbered 1 to 15. Starting on the high energy
side we first observe the total absorption peak (1) at 2.75 MeV caused by a) photoelectric
absorption, b) Compton scattering followed by photoelectric absorption of the scattered gamma
ray, c) pair creation followed by absorption of the two 511 keV annihilation quanta. The
maximum energy EC(Eγ) transferred to the electron in Compton scattering is
EC =
2 Eγ2
511 + 2 Eγ
(7)
and it gives the Compton edge at EC(2.75)=2.52 MeV which is denoted by feature (3) in Fig. 13.
When a gamma ray undergoes multiple Compton scattering interaction in the scintillator it
contributes to the counts in the tail at (2).
Part of the 2.75 MeV gamma rays is absorbed by pair creation. The created positron looses its
kinetic energy and subsequently annihilates with an electron creating two 511 keV gamma rays
that may or may not escape from the crystal. Features (7) and (5) at 1732 keV and 2243 keV are
the double and single 511 keV escape peaks. 511 keV annihilation quanta Compton scattered in
the scintillator with escape of the scattered gamma ray lead to features (6) and (4) on top of the
Compton background from 2.75 MeV gamma rays. Those features extend to 2073 keV, i.e., 170
keV below the single 511 keV escape peak (5), and to 2584 keV, i.e., 170 keV below the total
absorption peak (1).
The pulse height spectrum is flat between 1450 keV and 1650 keV (8). This is the only part of the
spectrum that is composed of one single contribution, i.e., from Compton scattering of 2.75 MeV
gamma rays only. Peak (9) is the total absorption peak of 1.37 MeV gamma rays. Pair creation
leads to the faint double (14) and single (12) 511 keV escape peaks.
10
6
counts (arb.units)
9
4
15
11
14
12
2
3
7
13
10
1
8
0
0.0
0.5
1.0
1.5
6
2.0
5
4
2.5
2
3.0
Energy (MeV)
Fig. 12. The 24Na gamma ray pulse height spectrum measured with LaBr3:Ce. Features 1 to 15 are
discussed in the text. The horizontal dashed line indicates the Compton background from 2.75 MeV gamma
rays. Dashed vertical lines indicate the location of Compton edges.
The Compton edge (11) starts below at EC(1.37)=1.16 MeV with a tail (10) due to multiple
Compton scattering. Absorption of 1.37 and 2.75 MeV gamma rays outside the scintillator either
by pair creation or Compton scattering and subsequent detection of 511 keV annihilation gamma
ray or the Compton scattered gamma ray leads to the 511 keV back scatter peak (13) and the
Compton back scatter events (15) at around 250 keV.
11