文部科学省 [先端研究基盤共用・プラトフォーム形成事業 成果報告]
兵庫県立大学 高度産業科学技術研究所 ニュースバル放射光施設

ガンマ線検出器試験
所属

甲南大学理工学部

ビームライン

ＢＬ０1

利用者氏名

宇都宮弘章

利用分野

量子ビーム技術

利用年度

平成２６年度活用技術

ガンマ線利用

利用成果の概要
LaBr3:CeとCeBr3無機シンチレーション検出器は、NaI(Tl)検出器に比べて早いガンマ線応答、向上したエネ ルギー分解能と検出効率を持ち、高エネルギーガンマ線検出において優れた性能を有している。現在、ブカレスト近郊(ルーマニア)に欧州連合の研究所ELI-NPが建設中であり、利用代表者が
convenerを務める研究グループ(Gamma Above NeutronnThrehold)では、巨大共鳴およびピグミー共鳴のガンマ崩壊を研究するのためのガンマ線検出器候補として、LaBr3:CeとCeBr3検出器を検討している。ニュースバル放射光施 設で生成するレーザー逆コンプトンガンマ線ビームを用いて、両シンチレー
ターの高エネルギーガンマ線に 対する性能比較評価を行った。
以下のことが判明した。
(1) エネルギー分解能ではLaBr3:Ce検出器がCeBr3よりやや優れている。
(2) エネルギー直線性は同等である。.
(3) LaBr3:Ce検出器はよく知られているように138La由来の低エネルギーバックグランドガンマ線が存在する が、巨大共鳴の高エネルギーガンマ崩壊の検出には干渉しない。CeBr3検出器には検出器由来のバック グランドガンマ線は存在しない。
(4)LaBr3:Ce検出器とCeBr3検出器はどちらもELI-NPプロジェクトのガンマ線検出器として適している。

<利用目的>
ELI-NPプロジェクトで巨大共鳴のガンマ崩壊を測定するガンマ線検出器の候補であるLaBr3:Ce検出
器とCeBr3検出器の性能比較評価をレーザー逆コンプトンガンマ線を用いて行うことを目的とした。

<実験方法>NeｗSUBARU蓄積リング中の電子ビームとNd:YVO4レーザー光子(波長1064nm)の逆コンプトン散乱によってガンマ線ビームを発生させた。3.5"×4"LaBr3:Ce検出器と3×3" CeBr3検出器の逆コンプトンガ
ンマ線ビームに対する応答関数を測定した。発生させたガンマ線の最大エネルギー(16.93, 15.19,
13.03, 12.30, 10.87, 9.54, 8.29, 7.12, 6.04 MeV)は電子エネルギーの絶対エネルギー較正値[1]から求めた。逆コンプトン散乱によるガンマ線生成過程を組み込んだモンテカルロシミュレーションコード
GEANT4を用いて、測定した応答関数を解析した。逆コンプトン散乱のシミュレーションには電子ビームのエミッタンスを含めている。また、標準ガンマ線源(60Co, 137Cs)を用いて2種類のシンチレーション検出器の低エネルギーガンマ線に対するエネルギー分解能を測定した。

<実験結果>
得られたデータは、一緒に実験を行ったIoana Gheorghe(ELI-NPリサーチアシスタント、ブカレスト大学
大学院生)によって解析され、レポートが作成されている。モンテカルロシミュレーションによって測定した応答関数をよく再現することができた。レポートを資料として添付する。

1/2

文部科学省 [先端研究施設共用促進事業 トライアルユース 成果報告]
兵庫県立大学 高度産業科学技術研究所 ニュースバル放射光施設<成果の波及効果、今後の見通し>
本実験で得られた結果は、ELI-NPの当該研究グループのTDR(Technical Design report) に反映された。
最終的なガンマ線検出器の選択は、業者見積りとELI-NPプロジェクトの国際評価委員会の評価によって決定される。

文献[1] H.Utsunomiya et al., IEEE Transactions on Nuclear Science 61, 1252 (2014).

<図面等>

お問い合わせ先 兵庫県立大学 高度産業科学技術研究所
ニュースバル放射光施設 共用促進室〒678-1205 兵庫県赤穂郡上郡町光都1-1-2
TEL : 0791-58-2543 FAX : 0791-58-2504
E-mail : kyoyo@lasti.u-hyogo.ac.jp
http：//www.lasti.u-hyogo.ac.jp/NS/

2/2

Comparative study of energy resolution and linearity of CeBr3 andLaBr3:Ce using Laser Compton Scattered γ-ray beams
December 18, 2014

1

Introduction

The unique energy resolution and intensity parameters of the future ELI-NP Gamma Beam System (GBS) willallow a thorough investigation of the excitation and particle and gamma decay of Giant Resonances. Using thebrilliant GBS at ELI-NP, the Gamma Above Neutron Threshold (GANT) research group proposes to study the
GDR excitation and the competition of its various decay channels as well as that of direct and statistical decay
processes to improve our understanding of the GDR structure. A multi purpose neutron and gamma radiation
detection setup consisting in a flexible array up to 40 large volume scintillator detectors and liquid scintillation
neutron detectors each is purposed to be developed.
Cerium doped Lanthanum Bromide - LaBr3 :Ce - detectors are forseen to be used for gamma ray detection in
the multipurpose setup because of their good energy resolution, fast response and linear response with energy.
But we are also interested in other types of detectors having the required working parameters. Such is the caseof Cerium Bromide - CeBr3 - scintillation detectors, which are known to have poorer energy resolution than the
LaBr3 :Ce ones, but considerably superior to other scintillation detectors such as NaI.
We report here on our comparative study between the working parameters of Cerium doped Lanthanum
Bromide (LaBr3 :Ce) and Cerium Bromide scintillation detectors investigated with quasimonochromatic γ-ray
beams produced by the scattering of laser photons on relativistic electrons (LCS) at the synchrotron radiation
facility NewSUBARU.

2Experimental procedure

2.1

Detectors

We investigated the energy resolution of two cylindrical scintillation detectors, a 3” × 3” CeBr3 detector from
Scionix and a 3.5” × 4” LaBr3 :Ce detector, BrilLanCe380 89S102/3.5, Saint Gobain. High positive voltage wasapplied on the LaBr3 :Ce (630 V) and on the CeBr3 detector (555 V), the two voltage values being chosen to
use the entire ADC input range for minimum gain on the amplifier. Both detectors had photomultiplier tubes
dedicated to energy spectroscopy measurements. The LaBr3 :Ce photomultiplier signal was sent to an Ortec
preamplifier and NIM amplifier module with 0.5 µs shaping time. The amplifier signal was inserted into a 100
MHz 16 bit ADC digital MCA. The CeBr3 detector had the TB-5 digital base from AMPTEK, containing a 14
pin photomultiplier base tube, a digital pulse processor with charge sensitive preamplifier and MCA and both
(low and high voltage) power supplies.

2.2

Energy calibration with standard sourcesStandard calibration sources 60 Co, 137 Cs and 152 Eu were used for low energy calibration and for investigating
the energy resolution in the energy range of 661 to 1332 keV. Energy spectra of 60 Co and 137 Cs γ-ray sources
taken with the LaBr3 :Ce and the CeBr3 detectors are displayed in Figure 1. The spectra are taken in the same
conditions as the LCS γ-ray beam measurements - the same applied high voltage and the same amplification
gain. Relative energy resolution values in full width at half maximum (FWHM) are displayed in Table 1 fordifferent amplification conditions given by the applied high voltage. For the investigated energy range, theenergy resolution of the LaBr3 :Ce detector is superior but comparable to the CeBr3 one. By increasing with ∼170 V the applied high voltage chosen to optimize the ADC input range usage on both detectors, the energy
resolution of the LaBr3 :Ce is considerably improved by ∼ 10 %, while the CeBr3 one is remains constant within
1 %.

1

4

4

10

60

10

Co source

137

Cs source
LaBr (Ce)LaBr (Ce)

3

CeBr

10

Counts / 10 keVCounts / 10 keV

3

3

3

2

10
15000
12000

1

10

3

CeBr

10

3

2

10
18000

12000

9000

1

10

6000

6000

3000
0

0

1.0

1.2

1.4

0

1.6

0

0.0

0.55

0.60

0.65

0.70

0.75

10

10
0.5

1.0

1.5

2.0

2.5

3.0

0.0

0.5

1.0

1.5

2.0

2.5

3.0

Energy [MeV]

Energy [MeV]

(a)

(b)

Figure 1: Energy spectra of standard calibration sources (a) 60 Co and (b) 137 Cs recorded with the LaBr3 :Cedetector (black line) and with the CeBr3 detector (red line) in the same conditions as the LCS γ-ray beammeasurements - the same applied high voltage and the same amplification gain.
Table 1: Relative energy resolution values in FWHM for the LaBr3 :Ce and the CeBr3 detectors. Eγ and ∆E
are the gamma ray energy and the detector’s relative energy resolution in FWHM.
Emax
γ
[keV]
344.29661.657
778.900
963.380
1173.28
1332.490
661.657
1173.28
1332.490

2.3∆E [%]
3.53
3.30
3.07
3.48
2.99
2.96

LaBr3 :CeHigh voltage [V]
+ 630
+ 630
+ 630
+ 630
+ 630
+ 630
+ 800
+ 800
+ 800

∆E [%]
5.95
4.39
4.05
3.44
3.51
3.39
4.45
3.49
3.37

CeBr3
High voltage [V]
+ 555
+ 555
+ 555
+ 555
+ 555
+ 555
+ 730
+ 730
+ 730

LCS γ-ray beam measurements

Quasimonochromatic γ-ray beams with typical energy resolution values of 1.2 - 1.6 % in FWHM are produced
at the synchrotron radiation facility NewSUBARU in the inverse Compton scattering of laser photons from
relativistic electrons circulating in a storage ring. For this measurement, a 40 W Nd:YVO4 laser (λ = 1064nm) and electron beams with energies between 584 and 982 MeV were used to generate LCS γ-ray beams
with maximum energies between 6 and 17 MeV. For each energy point, the γ-ray beam energy spectrum wasalternatively measured with each scintillation detector.
The experimental setup is displayed in Figure 2. The 3.5” × 4” LaBr3 :Ce and the 3” × 3” CeBr3 detectors
were alternatively mounted at the end of the γ-ray beam line in the experimental Hutch 2, GACKO (Gamma
collaboration hutch of Konan University), to record the incident LCS γ-ray beams.
Fine tunings of the laser optics and the collimator alignment along the horizontal (x), vertical (y) androtational (θ) axes were carried out by irradiating the LaBr3 :Ce with LCS γ-ray beams. The optimal aligment
was obtained by maximizing the γ-ray flux incident on the detector. After completing the collimator aligmentprocedure, both detectors were aligned to the gamma ray beam axis with synchrotron radiation. In the caseof the NewSUBARU facility, the laser beam is sent head-on against the electron beam, therefore the electron
beam axis coincides with both the laser and the γ-ray beam axis.
We measured the energy spectra of LCS γ-ray beams for 9 energy values of the electron beam. Figure 3
shows the energy spectra recorded with both detectors for each LCS γ-ray beam having the maximum energyof: (a) 6.042 MeV, (b) 7.121 MeV, (c) 8.286 MeV, (d) 9.536 MeV, (e) 10.872 MeV, (f) 12.289 MeV, (g) 13.027
MeV, (h) 15.187 MeV and (i) 16.931 MeV. For each energy point we measured for the same amount of time bothLCS γ-ray spectra and background synchrotron radiation spectra. The electron beam energies and current, thelaser power, the maximum γ-ray beam energies along with the measurement times and count rates for eachdetector at each energy point are listed in Table 2.

2

(a)

(b)

(c)

Figure 2: (a) Schematic image of the experimental setup in the γ-ray beam line GACKO at NewSUBARU
facility. (b) and (c) Images of the experimental setup consisting in a 3.5” × 4” LaBr3 :Ce and a 3” × 3” CeBr3
detector. For each energy point, the LCS beam is recorded alternatively using the two detectors which are inturns aligned to the electron beam axis.
A top-up operation was performed for the 982.427 MeV electron beam energy measurements, having theelectron beam current constant at 300 mA. For the lower energy values of the electron beam, the electrob beam
current varied between 288 and 143 mA. A 5 cm thick lead absorber was placed at ∼ 60 cm in front of themeasuring detector to prevent pile-up events and possible saturation of phototube. The laser power was varied
between 4 to 26 % of the maximum 40 W value to obtain detector count rates between 3 and 9 kcps.
We can observe in the LCS γ-ray beam energy spectra shown in Figure 3 that the LaBr3 :Ce detector has ahigher full energy deposition probability than the CeBr3 detector. The difference between detectors full energy
peak - first escape peak ratio is because of the different crystal sizes. Also, the second escape peak is significantly
higher in the CeBr3 spectra because of the smaller crystal size.
The γ-ray beams are produced at NewSUBARU by the collision of a Nd:YVO4 laser in the first harmonicand electron beams of highly precised energy. The NewSUBARU electron nominal beam energy resolution is of
0.04 % and the uncertainty in the energy calibration over the energy range from 550 to 974 MeV is (5.5 - 9.4)
× 10−5 . Therefore, as will be the case of ELI-NP, the γ-ray beams have a precisely defined, sharp high energy
front. After collimation, the γ-ray beam presents a low energy tail with an small energy width of ∼ 1.2 to 1.6 %in FWHM values, depending on collimator thickness and aperture1 . An example of incident γ-ray beams along
1 A complex collimation system consisting of four main collimators is employed at NewSUBARU. A large apeture (2 cm) firstcollimation stage consisting in a water cooled Copper collimator placed near the interaction point is used for protecting the electron
pipes from high flux γ-ray beams. A second collimator stage of large aperture (5 cm) consisting in a 30 cm thick lead block placed
at approximately 13 meters from the interaction point is used for absorbing γ-rays scattered at large angles. The fine tuning
collimation sistem consists of two 10 cm thick lead blocks of variable apertures (1 to 6 mm).

3

1500012000
EE= 584.847 MeV

ele

12000

E

10000

= 6.042 MeV
LaBr

3

CeBr

7500

Counts / 10 keVCounts / 10 keV

max

3

5000

ele

0

E

= 7.121 MeV
LaBr

9000

3

CeBr

3

6000

4.0

4.5

5.0

5.5

6.0

6.5

4.5

5.0

5.5

6.0

6.5

7.0

E

ele

Counts / 10 keVCounts / 10 keV

E

3

3

4000

= 785.871 MeV

E= 10.872 MeV

9

LaBr

4000

3

CeBr

ele

= 835.911 MeV

max

3

2000

E

3000

= 12.289 MeV
LaBr

3

CeBr
2000

3

1000

2000

0

0
5

6

7

8

9

0

6

10

7

8

9

10

11

6

(d)
ele

E

= 860.838 MeVCounts / 10 keV

3

CeBr

2000

3

1000

11

12

13

(f)

ele

= 930.030 MeV

E

max

E

= 13.027 MeV

10

1500

max

LaBr

9

(e)

1500
LaBr

3

CeBr

ele

= 982.427 MeV

max= 15.187 MeVCounts / 10 keVEE

8Energy [MeV]

2000

3000

7

Energy [MeV]

Energy [MeV]Counts / 10 keV

8

(c)

max

= 9.536 MeV

CeBr

7

Energy [MeV]4000= 735.699 MeV

LaBr

6

(b)

max

6000

3

6000

5

Counts / 10 keV

ele

3

CeBr

7.5

6000
E

LaBr

Energy [MeV]

(a)

E

9000

0

4.0

Energy [MeV]8000= 8.286 MeV

E

3000

0

3.5

= 685.477 MeV

ele

max

3000

2500

E

= 635.213 MeV

max

Counts / 10 keV

12500

3

1000

= 16.931 MeV

E

LaBr

1000

3

CeBr

3

500

500

0

0
6

7

8

9

10

11

12

13

14

0
6

8

10

12

Energy [MeV]

Energy [MeV]

(g)

(h)

14

16

6

8

10

12

14

16

18

Energy [MeV](i)

Figure 3: Laser Compton Scattered γ-ray beam spectra recorded with a 3.5” × 4” LaBr3 detector (black line)
and with a 3” × 3” CeBr3 (red line). The detectors’ response functions have been investigated using LCS γ-ray
beams with maximum energies of: (a) 6.042 MeV, (b) 7.121 MeV, (c) 8.286 MeV, (d) 9.536 MeV, (e) 10.872
MeV, (f) 12.289 MeV, (g) 13.027 MeV, (h) 15.187 MeV and (i) 16.931 MeV. See text for details.
with experimental and simulated LaBr3 :Ce detector response is shown in Figure 4 taken from the PRC article
published this year ”Photoneutron cross sections for samarium isotopes: toward a unified understanding of (γ,n) and (n, γ) reactions in the rare earth region” by Filipescu et al.
Therefore, the high energy front of the LCS γ-ray beam spectra is given entirely by the energy resolution of
the detector and it is the main indicator of energy resolution differences between the LaBr3 :Ce and the CeBr3
detector. As can be seen in Figure 3, the LCS γ-ray beam high energy front measured with the CeBr3 detector
is steaper than the one recorded with the LaBr3 :Ce detector.

4

Photon Yields / 10 keV

6×103
5×103
4×103
3×103
2×103
103
0

4

6

8

10

12

14

Energy [MeV]
Figure 4: Typical spectra of the NewSUBARU LCS γ-ray beams recorded with the LaBr3 :Ce detector (solidlines), the simulations of the response function (dotted lines) and of the incident γ-ray beam (gray lines). Figure
taken from D.M. Filipescu et al PRC paper (2014).
, Eele , tm and Iele
Table 2: Details regarding each LCS γ-ray beam measurement with each detector. Emaxγ
represent the LCS γ-ray beam maximum energy, the electron beam energy, the measurement time for each
irradiation and the electron beam current.

2.4

Emax
γ
[MeV]

Eele
[MeV]

6.042
7.121
8.286
9.536
10.872
12.28913.02715.18716.931584.847635.213685.477735.699785.871835.911860.838
930.030
982.427

LaBr3 :CeCount rate tm[kcps]
[s]
7.5
200
8.9
200
8.8
200
7.4
200
5.4
200
5.5
200
4.8
200
4.7
200
3.3
300

Iele
[mA]
143
168
178
203
216
245
255
288
300

CeBr3
Count rate tm[kcps]
[s]
6.1
300
6.9
300
8.0
300
5.8
300
4.7
300
3.8
300
3.8
300
3.3
300
2.3
300

Iele
[mA]
151
163
185
198
224
238
264
279
300

Laser power
[W]
10.4
9.68
8.36
5.84
3.52
2.8
2.36
1.68
<1.68

Linearity of detector response with energy

The response linearity with energy was investigated for both scintillation detectors. The energy calibration was
performed using the low energy γ-ray transitions of the 60 Co source and the maximum energy of each LCS γ-ray
beam. Although simulations are necessary to obtain the precise spectrum position of the γ-ray beam maximum
energy, calitative information can obtained by examining the experimental spectra. The energy calibration
was performed with a third degree polynomial. The degree of the polynomial was chosen to obtain the mostsimilar correspondance between energy and channel number for both detectors. As can be observed in Figure
5, in the present configurations consisting in photomultiplier tube types, crystal size, applied high voltage andamplification gain, etc., the CeBr3 detector in the present configuration has a more linear response than the
LaBr3 :Ce.

5

18000

LaBr3:

16000

Experimental
Polynomial fit

y = -102.988 + 2.147

14000

Energy [keV]

x+

2

3

x + 1.764E-8 x- 9.4687E-5

12000

10000

8000

6000

CeBr3:

4000

Experimental
Polynomial fit

y = -22.350 + 2.134

2000

x-

2

- 1.584E-5

3

x + 6.840E-9 x

0
0

1000

2000

3000

4000

5000

6000

7000

8000

ChannelFigure 5: Linearity in the response of a LaBr3 :Ce detector - black line, and of a CeBr3 detector to γ rays in
the energy range 1173 - 16931 keV. The data for low-energy photons were taken with the standard γ-ray source
60
Co. The data for 6 -17 MeV γ rays were taken with LCS γ-ray beams produced with a Nd:YVO4 laser. The
solid lines are the 3rd -order polynomial fits to the data.

2.5

Intrinsic radioactivity of LaBr3 :Ce crystals
6000
Electron energy: E

ele

5000

Nd:YVOCounts / 10 keV

4laser,= 982.427 MeV= 1064 nm

max

E= 16.931 MeV

4000

LaBr , 3.5 x 4 inch
3

Acquisition time: 300 sec,
CeBr , 3 x 3 inch

3000

3

Acquisition time: 300 sec

2000

1000

0
0

2

4

6

8

10

12

14

16

18

Energy [MeV]

Figure 6: Energy spectra of 16.931 MeV maximum energy LCS γ-ray beam recorded with the LaBr3 :Ce detector(black line) and with the CeBr3 detector (red line). The large volume LaBr3 :Ce detector presents a considerable
instrinsic radioactivity component at energies between 0.7 - 2.5 MeV.
Since the LaBr3 crystals are grown from natural lanthanum, the LaBr3 :Ce detector contains 0.09 % of theradioactive isotope 138 La. With a half life of 1.02 × 1011 years, 138 La decays either by electron capture (65.5 %)to 138 Ba or by β − (34.4 %) to 138 Ce. Also a small amount of contamination with radioactive 227 Ac is present
in the scintillator. The radioactive decay products of these impurities contribute to the natural background
spectrum of the LaBr3 :Ce scintillation detector. The prominent peak at 1436 keV which can be clearly seen in
Figure 1(b) results from the electron capture branch of 138 La. The group of peaks at energies between 1.8 MeV
and 2.8 MeV are α-decays from the 227 Ac decay chain. Additional features in the spectrum can be accounted
for by decay products of the 238 U and 232 Th decay chains. We also show in Figure 6 an energy spectra of a
16.931 MeV maximum energy γ-ray beam measured with the LaBr3 :Ce and with the CeBr3 detector, without
subtracting background.

6

3Conclusions

We have investigated the energy resolution and response linearity for two types of scintillation detectors, alarge volume LaBr3 :Ce detector and a CeBr3 one. While for an investigation made with nuclei emitted gamma
rays of precisely determined energy the difference in the energy resolution of the two detectors is obvious,
LaBr3 :Ce detector being superior to the CeBr3 one, when investigating with an ∼ 1 % energy resolution γ-ray
beam and considering the ability to observe separately the full energy, first and second escape peaks, the two
detectors appear to have similar energy resolution. When investigating the detector response to the sharp highenergy front characteristic to LCS γ-ray beams produced at NewSUBARU, the CeBr3 detector in the previous
configuration has slightly better resolution than the LaBr3 :Ce one. The linearity of response with energy is also
comparable for the two detectors in the present configuration. A clear disadvantage of the LaBr3 :Ce detector
is its intrinsic radioactivity, which is not present in the CeBr3 crystal.
We conclude that both detectors are suitable to be used in the multidetector array proposed by the Gamma
Above Neutron Threshold ELI-NP Research Group.

7