1

Abstract—CeBr3 is emerging as one of the best scintillators

having properties almost similar to Cerium doped lanthanum

halide scintillators. We have measured, for the first time, the

intrinsic energy resolution of Compton electrons in a cylindrical

1 1 CeBr3 detector using the sources, namely, 137Cs, 22Na and 60Co employing Compton Coincidence Technique (CCT). We have

used PIXIE-4 data acquisition system which makes the

measurement setup quite compact. The results have shown that

non-proportionality is the major factor in limiting the overall

energy resolution of CeBr3 and the intrinsic resolution in CeBr3

arises due to processes other than the scattering of electrons inside

the scintillator. We have also studied the dependence of intrinsic

energy resolution on the coincidence window and optimized its

value for a given source.

Index Terms—scintillators, Compton scattering, intrinsic

resolution, coincidence techniques.

I. INTRODUCTION

mong Cerium halide scintillators, CeBr3 is proved to be

promising scintillator in recent times [1]. The great

importance of these detectors is associated with their properties

such as good energy resolution (~4% at 662 keV), excellent

timing resolution (~93ps at 511 keV), high light output (~68000

photons/MeV), high stability in light output during temperature

changes, high effective atomic density (~5.2 g/cm3), possibility

for growing in large volume, etc. [2]-[7]. Another attractive

property of CeBr3 is its higher radio-purity when compared to

Lanthanum halide scintillators. The small amount of internal

radioactivity in CeBr3 is mainly due to the 227Ac contamination

in the raw material of the detector. The overall internal

radioactivity of CeBr3 was reported to be 7-8 times less than that

in LaBr3:Ce and LaCl3:Ce detectors, making CeBr3 more

sensitive to low count rate applications [8]. All these properties

are responsible for its application in well logging devices [6],

sub-nanosecond nuclear half-life using time-of-flight

measurements [7], 3-D imaging in gamma ray astronomy [9],

radioactive aerosol monitoring devices [10], Naturally

Occurring Radioactive Materials (NORM) measurements [11],

solar gamma-ray spectrometer GRIS [12], PING-M experiment

to investigate solar X-ray activity [13], remote sensing

Manuscript submitted on June 24, 2017.

“This work was supported in part by the DST Govt. of India under fast track

grant SR/FTP/PS-032/2011”.

The authors are with the Radiation Detectors and Spectroscopy lab,

Department of Physics, Indian Institute of Technology Roorkee, Roorkee-

applications [14], space missions [15],[16], water treatment

facilities [17], steelworks [18], studying fast ions at JET [19],

high energy gamma spectroscopy [20], etc. A huge amount of

testing and characterization work is in progress by several

groups to fully understand the limitations and potentials of these

crystals. The present work aims to understand the factors

affecting the energy resolution of CeBr3 detectors.

Energy resolution is one of the important parameters

which greatly affects the productivity of any detector. Finding

out the ways to improve the resolution of detectors are still in

progress. It is well established that the major limitation to the

overall energy resolution of a detector is the intrinsic resolution,

which arises due to non-proportionality of light output of the

crystal by means of scattering of electrons (δ-rays) and landau

fluctuations [21]. The origin of intrinsic resolution, however, is

an open problem and yet to be fully understood [22].

In order to measure the intrinsic energy resolution of a

detector, a Compton spectrometer based method was first

proposed, in 1994, by Valentine and Rooney [23]. The method,

benchmarked as Compton Coincidence Technique (CCT) in the

year 1996, provides accurate characterization of light yield non-

proportionality with the measurement of electron response of

the detector [24]. Since then, CCT has been widely used to

determine the non-proportionality in scintillators [25]-[33]. The

technique is based on the detection of Compton scattered γ-rays

and the basic principle of the method is to register in

coincidence the signals from scattering of γ-ray inside the tested

detector followed by absorption of the scattered ray inside the

reference detector. Significant number of experimental studies

were done by Swiderski et al. [34]-[38] to determine the non-

proportionality and intrinsic resolution of Compton electrons in

LaBr3:Ce, LYSO:Ce, CaF2:Eu, BC408, EJ301, NaI:Tl, CsI:Tl,

CsI:Na and Xe gas detectors. In case of CeBr3, although few

detailed studies on the non-proportionality response of CeBr3

and ways to improve it are available in the literature [2], [39]-

[41], no experimental measurements on intrinsic resolution of

Compton electrons in CeBr3 detectors were reported in the

literature. In this paper, we report our studies on intrinsic energy

resolution of Compton electrons in CeBr3 detectors by

employing CCT. The measurements of Compton electrons were

done using a PIXIE-4 multi-channel digital gamma processor

247667, Uttarakhand, India (e-mail: himasnig@gmail.com,

1viru1994@gmail.com, rawatsheetal1991@gmail.com, physics.monalisha3@

gmail.com, anilgfph@iitr.ac.in).

Intrinsic Resolution of Compton Electrons in

CeBr3 Scintillator using Compact CCT

Snigdha Sharma, V. Ranga, S. Rawat, M. Dhibar, Student Member, IEEE, G. Anil Kumar, Member,

IEEE

A

2

which makes whole experimental setup relatively more

compact. The optimization of the coincidence window value,

which should be set for a gamma source of given activity, is

also discussed.

II. EXPERIMENTAL DETAILS

Fig.1 shows the schematic of the experimental setup used in

the present work. The tested detector is a 1″1″ CeBr3 crystal

optically coupled to a 2″ Hamamatsu R6231 PMT operated at

+600 V and the reference detector is a 2.18ʺ×2.36ʺ coaxial type

reverse electrode HPGe detector biased with 4500 V. The

detectors were kept face-to-face at a distance of 4 cm. A 137Cs

source was placed between them. For processing the signals

from detectors, we have used PIXIE-4 multichannel data

acquisition system (supplied by XIA LLC) which provides

digital spectrometry and waveform acquisition for four input

signals per module [42]. Some of the main features of PIXIE-4

system include coincident data acquisition across channels and

modules, pulse heights measured with up to 16 bits accuracy on

each of the four channels, programmable gain, input offset,

trigger and energy filter parameters, etc. It works with common

resistive feedback preamplifiers of either signal polarity.

Detailed technical information about PIXIE-4 system can be

found in Ref. [43]. The present PIXIE-4 module with PCI-PXI

based architect clearly meet the requirements of CCT resulting

in a compact experimental setup for measuring the Compton

electrons. The signals from both detectors through their

respective preamplifiers were directly fed to channels 0 and 1

of PIXIE-4 module. The hit pattern of PIXIE-4 was so adjusted

that only the signals detected in coincidence in channel 0 and

channel 1 were recorded.

III. DATA ANALYSIS

We have recorded the events registered in coincidence in

both detectors using PIXIE-4 data acquisition system. Fig. 2

shows a typical spectrum of 137Cs source measured with HPGe

detector in coincidence mode with a coincidence window of

1 µs. The spectrum shows prominent peak at Compton edge

along with a photo peak and a backscattered peak. Similarly,

Fig. 3 shows the spectrum of 137Cs source measured with CeBr3

detector in coincidence mode. The photo peaks in Fig. 2 and Fig. 3 are due to the accidental coincidences of two gamma rays

detected in both the detectors.

Fig. 1. Schematic of the experimental setup.

0 200 400 600 800 10000

2000

4000

6000

8000

10000

Co

un

ts

Energy(keV)

137

Cs

(with coincidence)

Photopeak

661.6 keV

Compton edge

477.3 keV

Backscatter peak

184.3 keV

HPGe

Fig. 2. Energy spectrum of 137Cs source measured with HPGe in

coincidence with CeBr3.

0 200 400 600 800 10000

1000

2000

3000

4000

5000

Co

un

ts

Energy(keV)

137Cs

(with coincidence)

CeBr3

Photopeak

661.6 keV

Compton edge

477.3 keV

Backscatter peak

184.3 keV

Fig. 2. Energy spectrum of 137Cs source measured with CeBr3 in

coincidence with HPGe.

Fig. 3. 2D spectrum of coincident events in both HPGe (y-axis) and

CeBr3 (x-axis).

3

We are interested in only those events in which gammas are

backscattered from CeBr3 detector at an angle of 180◦ into the

HPGe detector, thus giving off maximum energy to the

electrons in CeBr3. As 137Cs source emits gammas of 661.6 keV

energy, the energies deposited in CeBr3 and HPGe are 477.3

and 184.3 keV, respectively. Fig. 4 shows the 2-dimensional

spectrum of the coincident events registered in both the

detectors. The figure shows a vertical line in CeBr3 (x-axis) and

a horizontal line in HPGe (y-axis). These lines consists of the

registered events corresponding to accidental coincidences of

two gamma rays fully detected in both detectors. The figure also

shows the events that give sum of energies equal to 661.6 keV.

The measurement of energy resolution of Compton electrons in

CeBr3 detector demands the gating on the events that deposit

energy in HPGe detector corresponding to the backscattered

peak. The excellent energy resolution and precise calibration of

HPGe detector enabled us to pick up the exact events of our

interest. We have used RADWARE [44] software for offline

gating. In order to understand the effect of the gate width or

energy window width on the energy resolution, we have

measured the energy resolution of Compton electrons for

different values of gate width starting from 2 keV. Fig. 5 shows

the plot of energy resolution of Compton electrons in CeBr3

versus gate width in HPGe for 137Cs source. The increase in the

gate width causes the widening of scattering angles resulting in

increase in the FWHM of the peak. This leads to worsening of

the energy resolution as evident from the figure. The gating was

done on HPGe energy axis at 184.3 keV with a gate width of

2 keV and the projection was taken onto the CeBr3 axis. The

resulting gated spectrum of CeBr3 is shown in Fig. 6. The figure

clearly shows a Gaussian shaped peak at Compton edge

corresponding to the energy of 477.3 keV. The resolution (∆𝐸

𝐸)

of Compton electrons can be easily determined from this peak.

The intrinsic resolution (δint) was calculated, neglecting the

transfer component [25], by using the equation [27]:

δint = √(∆𝐸

𝐸)2 − 𝛿𝑠𝑡

2 (1)

where δst denotes the photoelectron statistical contribution

given by

𝛿𝑠𝑡= 2.35× √1+Ɛ

𝑁𝑝ℎ𝑒 (2)

where 𝑁𝑝ℎ𝑒 is the number of photoelectrons and Ɛ is the gain

variance of the PMT. For R6231 PMT, the gain variance was

taken to be 0.28 [45]. 𝑁𝑝ℎ𝑒 was calculated from the average

value of absolute light yield reported for encapsulated sample

of the same size [5]. The quantum efficiency of PMT was taken

to be 30% as provided by the manufacturer [46].

IV. RESULTS AND DISCUSSION

A. Intrinsic resolution

In order to understand the effect of intrinsic resolution on

overall energy resolution of CeBr3 detector, we have estimated

the intrinsic resolution of Compton electrons in CeBr3 using

Equation (1) considering the sources 137Cs, 60Co and 22Na. For 60Co, the intrinsic resolution of both 963.4 keV and 1118.1 keV

Compton electrons were calculated, corresponding to gamma

energies 1173 and 1332 keV respectively. For 22Na, in order to

reduce the counts of directly detected 511 gammas, the source

was slightly shifted from the original position. So, HPGe was

gated on 184.8 keV, corresponding to the scattering angle of

140 from CeBr3, giving rise to a Compton peak at 326.3 keV.

The results are presented in Fig. 7. The figure also shows the

intrinsic resolution of two other types of events. One, full

energy peaks due to gamma rays backscattered in HPGe and

then absorbed in CeBr3. These peaks corresponds to the

Compton electron energies, namely, 184.3 keV, 209.8 keV and

214.4 keV. Two, full energy peaks due to gamma rays coming

directly from the source and deposited full energy in CeBr3. The

Fig. 5. Resolution of Compton electrons in CeBr3 versus energy window

width in HPGe for 137Cs source.

0 200 400 600 800 10000

40

80

120

160

200

Co

un

ts

Energy(keV)

137

Cs

Gated spectrum477.3 keV

CeBr3

Fig. 6. Projected spectrum of CeBr3 with HPGe gated on backscattered peak

4

data presented in Fig. 7 clearly confirms the larger contribution

of non-proportionality component in overall energy resolution

of CeBr3 detector, as already reported by Quarati et al. [2]. The

difference between intrinsic resolution of Compton events and

photo peak events of almost similar energy is clearly evident.

For example, the intrinsic resolution of Compton electrons of

energy 1118 keV is 2.2% and of full energy peak corresponding

to 1173 keV is 3.5%. Similarly, the intrinsic resolution of

Compton electrons of energy 477 keV is 4.1% and of full

energy peak corresponding to 511 keV is 5.3%. This confirms

that intrinsic resolution not only arises due to the scattering of

electrons inside the scintillator but also due to the other

processes such as self-absorption and re-emission processes

[40]. Measurements with different sizes of CeBr3 are required

to better understand the factors contributing to the non-

proportionality in these scintillators.

B. Optimization of the coincidence window value

From the activity of the 137Cs source, we could estimate

the time difference between the emissions of two successive

gamma rays from the source. It was calculated to be around

21 μs. If the coincidence window value is set above 21 μs then

there would be an increased number of accidental coincidences

due to more number of gamma rays being emitted during the

time interval. Also, some of the actual coincident events would

be lost. For example, if a gamma scatters at an angle in HPGe,

depositing some energy, the acquisition system will wait for a

coincident event in CeBr3. If the coincident window value is

large enough, another gamma may enter CeBr3 depositing the

required energy of 477.6 keV and backscattering to HPGe,

deposing an energy of 184.3 keV. The system will consider the

first two events as coincident ones rather than the last two,

which are the coincident events we actually require. To avoid

this, window value should be set well below the time required

for the emission of two or more gamma rays. To prove this

statement we extended our work and recorded the data for

870 μs which is the maximum coincidence window value

possible to set in PIXIE-4. We get a better understanding when

we compare the gated spectrum for coincidence window values

ranging from 60ns to 870μs as shown in Fig. 8. Clearly, in order

to get a well pronounced Compton edge peak with considerable

number of counts, one should select a coincidence window

value which is neither too high nor too low and is well below

the time required for the emission of two or more gamma rays.

V. SUMMARY AND CONCLUSION

CeBr3 scintillators are proved to have advantages over

equivalently sized LaBr3:Ce detectors. We could successfully

employ Compton Coincidence Technique in order to measure

the intrinsic resolution of Compton electrons in CeBr3 of

volume 12.87 cm3. The use of PIXIE-4 makes the complete

measurement setup quite compact. The results clearly

suggested that the intrinsic resolution in CeBr3 arises from

processes other than the scattering of electrons inside the

scintillator. Also, an optimization of the coincidence window

value has been done based on the activity of the source in order

to get the best possible Compton edge peak. As the effect of

non-proportionality of light output on the energy resolution is

more prominent for large volume crystals, further work is in

progress to study Compton electrons in large volume CeBr3

detectors which have recently become commercially available.

ACKNOWLEDGMENT

Authors would like to thank Dr. Ajay Y. Deo for his support

during this work.

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