文章编号：2096-3424（2021）01-0001-07DOI：10.3969/j.issn.2096-3424.20076 Scintillation Properties of LaCl3∶Ce Crystal with
Low Radioactivity Background
HOU Yueyun， YANG Lei， GUI Qiang， ZHANG Chunsheng， ZHANG Mingrong
（Beijing Glass Research Institute, Beijing 101111, China）
Abstract：10% Ce3+-doped lanthanum chloride (LaCl3∶Ce) crystals with low-radioactive background have been prepared using raw materials with low content of U-series isotopes. The radioactive background was estimated to be about 0.713 counts·s–1·cm–3 resulted from the natural U-series radioactive isotopes in the prepared LaCl3∶Ce crystal. The energy resolution (FWHM) of the LaCl3∶Ce crystal was 3.3% for 662 keV γ-rays. Moreover, the peak-to-valley ratio, relative light output and peak count rate of the LaCl3∶Ce crystal were approximately 2.61 times, 87.88% and 1.96 times of that of NaI∶Tl crystal with the same size of Ø25 mm× 50 mm, respectively. The effects of crystal size on the scintillation properties of the prepared LaCl3∶Ce crystal were discussed.
Key words：LaCl3∶Ce；scintillation crystal；background count rate；energy resolution
CLC numbers：O734 Document code：A
Cerium-doped lanthanum chloride (LaCl3∶Ce) crystal is a type of excellent scintillator discovered by Guillot et al. in 1999[1]. It has a hexagonal structure with P63/m space group, a density of 3.86 g/cm3 and a melting temperature of 859 ℃[2-4]. So low melting temperature allows the growth of LaCl3∶Ce crystals in sealed quartz ampoules by using the Bridgman crystal growth technique. Before LaCl3∶Ce crystal grow, the raw materials must be subjected to anaerobic dehydration treatment to prevent forming of oxide impurities. The scintillation properties of LaCl3 crystals with different Ce3+ concentrations have fully studied, and it was found that the LaCl3 crystal doped with 10% Ce3+ shows higher light output, shorter decay time and good energy resolution[2,5]. The best energy resolution reported was 3.2% for 662 keV γ-rays obtained for a small sample with a volume of 1 cm3[3]. In generally, it is considered that the higher isotopes of 138La and U-series contained in the raw materials, which limited the application of the LaCl3∶Ce crystals in 1 460 keV and above 1.6 MeV, due to its high radioactive background. Therefore, high-performance LaCl3∶Ce crystals could be obtained by reducing the content of uranium(U)-series isotopes in raw materials. Recently, high-quality LaCl3∶Ce crystals were obtained with Bridgman method by using starting materials, LaCl3·7H2O and CeCl3·7H2O, with low-radioactive background at Beijing Glass Research Institute.
In this work, the scintillation properties of the LaCl3∶Ce crystal with low-radioactive background
Received：2020-11-16
Author：
Corresponding author：Zhang Mingrong (1964-), male, professor, focuses on the inorganic materials, scintillation crystal and radiation
Citation：HOU Yueyun，YANG Lei，GUI Qiang，et al. Scintillation Properties of LaCl3∶Ce Crystal with Low Radioactivity Background[J]. Journal of Technology，2021, 21（1）：1-7.
引文格式：侯越云，杨蕾，桂强，等. 低放射性本底LaCl3∶Ce晶体闪烁性能（英）[J]. 应用技术学报，2021，21（1）：1-7. 第21卷 第1期应 用 技 术 学 报Vol. 21 No. 1 2021年 3月JOURNAL OF TECHNOLOGY Mar. 2021
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were measured.
模切刀1 Experimental Setup and Results
Two pieces of LaCl3∶Ce crystal with different sizes, Ø25 mm×25 mm and Ø25 mm×50 mm, were cut from one crystal ingot. One piece of Ø25 mm×50 mm NaI∶Tl crystal was also prepared as a reference. Fig.1 shows photographs of above three crystal samples. The energy resolution, photofraction, relative light output, linearity and count rate of the samples were measured and compared. Moreover, the effect of the thickness of LaCl3∶Ce crystal on its scintillation properties was also discussed.
Ø25 mm × 50 mm LaCl
3
: CeØ25 mm × 25 mm LaCl3: CeØ25 mm × 50 mm NaI: Tl
Fig. 1 Three LaCl3∶Ce and NaI∶Tl crystal samples
A high-voltage power supply (Ortec 556), an amplifier (Ortec 672), a preamplifier (Ortec 113), and a multichannel analyzer (Ortec 926 MCB) are used to set up an energy spectrum measurement system. The crystal samples were optically coupled with a 50 mm -diameter Hamamastu R6231-100 super bialkali photomultiplier (PMT) with a quantum efficiency of 35% at 350 nm[6]. 5 mm-thick white polytetrafluoroethylene film was used to cover the surfaces of the crystal samples except the faces coupled to the PMT to ensure the effective collection of the scintillation light from the samples. Furthermore, the light emitting surface of sample was optically coupled to the optical window of PMT with a silicone oil with a dynamic viscosity of 20 000 mm2/s (20 000 cSt) at 25 ℃. To prevent the crystal from hydrating, all handling was done in a dry nitrogen glove and the ambient temperature was 20 ℃. The working voltage of the PMT is positive 700 V. Four radioactive sources (241Am, 22Na, 137Cs, and 60Co) are used in the measurements.
1.1 Energy resolution and photofraction
Fig.2 shows the distribution of the energy resolution (full width at half maximum over the peak position) of full absorption peaks of 59.5, 511,662 keV, 1 173.2 keV and 1 332.5 keV γ-rays. At less than 200 keV, the energy resolution of LaCl3∶Ce crystal is not as good as that of NaI∶Tl crystal. However, the energy resolution of LaCl3∶Ce crystal has a significant advantage compared with NaI∶T
l in the γ-ray energy range of 200 ~ 1 332.5 keV.
Ø25 mm × 50 mm LaCl3: Ce
Ø25 mm × 25 mm LaCl3: Ce
Ø25 mm × 50 mm NaI: Tl
241Am
22Na
137Cs
60Co60Co
Energy/keV
200400600800 1 0001 2001 400
Fig. 2 Energy resolution (FWHM) of LaCl3∶Ce and NaI∶Tl samples versus energy for 241Am, 22Na, 137Cs and 60Co γ-
rays
Fig.3 shows the energy spectra of three samples under irradiation of the γ-rays from 241Am, 57Co, 22Na and 137Cs radioactive sources. The 59.5 keV γ-ray full-energy peak of the 241Am source is shown in Fig.3(a). The energy resolution (E.R.) of the Ø25 mm×50 mm LaCl3∶Ce sample is 11.39%, while the Ø25 mm×50 mm NaI∶Tl sample can reach 8.51%, which is better than that of LaCl3∶Ce. The energy resolution of the Ø25 mm×
25 mm LaCl3∶Ce sample is 10.80%. As can be seen
a smaller crystal size resulted in a better energy resolution. The 662 keV γ-ray full-energy peak of the 137Cs source is shown in Fig.3(b). The energy resolution of the Ø25 mm×25 mm LaCl3∶Ce sample is 3.33%, and those of LaCl3∶Ce and NaI∶Tl samples with the same size of Ø25 mm×50 mm are 3.37% and 6.37%, respectively. The results are better than that reported by Balcerzyk[7] and Alexiev[8], which may be attributed to the improved quality of the obtained LaCl3∶Ce crystal. As shown in Fig.3 (c) and 3(d), the energy resolutions of the Ø25 mm×50 mm LaCl3∶Ce for 511 keV, 1 173.2 keV and 1 332.5 keV γ-rays are 3.37%, 2.52% and 2.39%, respectively.
The peak-to-valley (p/v) ratio is a convenient
2应 用 技 术 学 报第21卷xuebao.sit.edu
parameter for evaluating the energy resolution of a scintillation detector. It does not affected by any possible shift in the signal. It can be calculated by obtaining the p /v ratio between two γ peaks or by taking the ratio between the low energy peak and the PMT (electronic noise). For further analysis of the measured energy spectra, the p /v ratio of the samples can be calculated from the energy spectra for 60
Co source in Fig.3(d). For LaCl 3∶Ce and NaI ∶Tl crystal samples with the same size of Ø25 mm×50 mm, the count ratio of the peak (1 332.5 keV) to the valley between 1 332.5 and 1 173.2 keV is 40.5 and 15.5, respectively. The p /v ratio of the Ø25 mm×25 mm LaCl 3∶Ce crystal sample is 42.8. There exists a correlation between the obtained results and the energy resolutions of the samples. Therefore, The difference in energy resolution between different samples can be evaluated by examining the p /v ratio conveniently.Photofraction is an important parameter describing the γ-ray response of the detector. It is defined as
the ratio of the full-energy peak count to the total spectrum count measured at a specific γ-ray ener-gy
[7,9-10].
. The photofraction results of three crystal
samples for 511 keV and 662 keV γ-rays are given in Tab. 1. For comparison, Tab. 1 also gives the σ-ratio of the photoelectric absorption cross section to the total cross section including coherent scattering,which are calculated by XCOM program [11]
. The results of two scintillation crystals are very close,but the σ-ratio of NaI ∶Tl crystal is higher than that of LaCl 3∶Ce crystal. The photofraction of LaCl 3∶Ce is related to the size of the crystal, the value of larger crystal is higher. This may due to a more
significant
Compton
scattering
and photoelectric
absorption,
which
makes
the
contribution of the full-energy peak in the energy spectrum more obvious. The photofraction and σ-ratio of two scintillators have the same trend,which is higher at 511 keV γ-rays, due to increasing in the photoelectric absorption cross section with the decrease of the energy of the rays.
Ø25 mm × 50 mm LaCl 3: Ce E.R.@59.5 keV = 11.39%
Ø25 mm × 25 mm LaCl 3: Ce E.R.@59.5 keV = 10.80%
Ø25 mm × 50 mm Nal: Tl E.R.@59.5 keV = 8.51%
241
rfid simAm
22
Na
137
Cs
60
Co
Ø25 mm × 50 mm LaCl 3: Ce E.R.@661.65 keV = 3.37%
Ø25 mm × 25 mm LaCl 3: Ce E.R.@661.65 keV = 3.33%Ø25 mm × 50 mm Nal: Tl E.R.@661.65 keV = 6.37%
Ø25 mm × 50 mm LaCl 3: Ce E.R.@511 keV = 3.77%
Ø25 mm × 25 mm LaCl 3: Ce E.R.@511 keV = 3.73%
Ø25 mm × 50 mm Nal: Tl E.R.@511 keV = 7.01%
Ø25 mm × 50 mm LaCl 3: Ce E.R.@1 173.2 keV = 2.52%E.R.@1 332.5 keV = 2.39%
Ø25 mm × 25 mm LaCl 3: Ce E.R.@1 173.2 keV = 2.54%E.R.@1 332.5 keV = 2.44%Ø25 mm × 50 mm Nal: Tl E.R.@1 173.2 keV = 4.35%E.R.@1 332.5 keV = 4.42%
70605040302010 4.54.03.53.02.52.01.51.00.5
03.02.52.01.51.00.5
102030
40506070200400
600800 1 000 1 200 1 4000200400
600800 1 0001 2001 400800
100200300400500600700800
90Energy/keV
Energy/keV
(a)
Energy/keV
Energy/keV
(c)
(d)
(b)
Fig. 3 Energy spectra of 241
Am, 22
Na, 137
Cs, and 60
Co sources measured with three samples
第1期HOU Yueyun, et al ：Scintillation Properties of LaCl 3∶Ce Crystal with Low Radioactivity Background
3
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Tab. 1 Photofraction and σ ratio for three samples
Crystal Size/mm
22Na 511 keV137Cs 662 keV Photofraction /%σ- ratio/%Photofraction/%σ- ratio/%
NaI∶TlØ25×5030.117.423.311.4
LaCl3∶Ce Ø25×5025.1
14.8
22.0
9.6Ø25×2522.720.2
1.2 Relative light output and linearity
The charged particles or γ-rays with energy E are incident on the scintillator, and the energy is deposited in the scintillator, and N photons with an average energy of ε are excited[12]. The luminous efficiency of the scintillator is:
In actual measurement, the photon number N and the average photon energy ε are difficult to measure at the same time. Considering the sensitivity of the measuring instrument and other factors, the light output of the scintillator is usually evaluated by the relative light output. For calculating the value of relative light output, the full-energy peak pulse height (corresponding to the channel) of three crystal samples for 59.5 keV, 511 keV, 662 keV, 1 173.2 keV, and 1 333.5 keV γ-rays were measured.
The results of relative light output are shown in Tab. 2. When the 137Cs source was used, the relative light output of the Ø25 mm×25 mm LaCl3∶Ce sample is 108% higher than that of the Ø25 mm×50 mm LaCl3∶Ce sample. At 511 ~ 1332.5 keV, the relative light output of the Ø25 mm×50 mm sample is 86% ~ 88% of the NaI∶Tl reference sample. Due to the self-absorption of fluorescence of t
he crystal, the thin crystal with the same diameter has weaker self-absorption, and its relative light output is thus higher.
Tab. 2 Relative light output statistics of LaCl3∶Ce and NaI∶Tl crystals
Crystal Size/mm
纸币识别器
Relative light output/%
@59.54 keV@511 keV@661.65 keV@1 173.24 keV@1 332.5 keV
NaI∶TlØ25×50100.00100.00100.00100.00100.00 LaCl3∶Ce
Ø25×5074.2786.0487.8887.8388.04
Ø25×2580.4793.6495.1696.1096.33
Linearity means that the total light output of the scintillation crystal is proportional to the energy of the absorbed γ-ray photons[7,13]. The linear response can be defined as Eq.(2)：
where E is the energy of γ ray, PH is the pulse height corresponding to the ray of energy E, and the measurement data is normalized with the data points of 137Cs source 662 keV γ-ray[14].
Fig.4(a) presents the light yield linearity of the samples. The results plotted show very linear energy behaviour for the samples in the test. It can be intuitively judged that the light output of LaCl3∶Ce crystal, which measures the same energy, is lower than that of NaI∶Tl reference crystal by comparing the slope of each line.
Fig.4(b) shows the calculation results of linear response. The pulse height of 662 keV γ-ray from 137Cs is defined as 1. The nonlinearity that appear at measured energy range is noted. The dashed straight line in Fig.4(b) indicates the ideality of the scintillation crystal. The measured values are distributed around the ideal value, showing a certain linearity. From 511 keV to 1 332.5 keV, the linearity of the crystal samples are all within 3%, and there is no significant difference between LaCl3∶Ce and NaI∶Tl scintillation crystals. For energy less than 511 keV, all samples show obvious
4应 用 技 术 学 报第21卷xuebao.sit.edu
nonlinearity. The linearity of NaI ∶Tl crystal at 59.5 keV is 15% higher than that at 662 keV. It is worth noting that LaCl 3∶Ce crystal has more stable linearity and more suitable for low energ
y ray detection with higher linear requirements. The
obvious non-linear response appears under low energy rays, and which is mainly because of the non-radiative electron-hole pair recombination in the high ionization density part of the ionization track
of the γ rays. And that means close to the track (＜5 nm), with the decrease of the electron velocity (or energy E), the energy loss of the ionized electrons is increasing so the composite loss is highest at the end of the main track [15-16]
.
1.3 Count rate and background
Tab. 3 shows the statistical results of the count rates of the samples. Ratio LC/NI represents the ratio of the count rates of the Ø25 mm×50 mm LaCl 3∶Ce sample to that of the Ø25 mm×50 mm NaI ∶Tl. Comparing the total energy spectra count rates from low energy to high energy, the spectrum count rates of LaCl 3∶Ce crystal is 3% ~ 25%higher than that of NaI ∶Tl reference crystal, and the ratios of the peak count rate of the two samples are 1.09 ~ 2.31.
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Tab. 3 Count rate of the samples for different radioactive source
Crystal
Size/mm Spectrum count rates / (counts·s –1
·cm –3
)
Peak count rates / (counts·s –1·cm –3
)
241
Am
22
Na
137
Cs
60
Co
59.5 keV 511 keV 662 keV 1 173.2 keV
1 332.5 keV
LaCl 3∶Ce Ø25 × 256 243.43474.098 450.34808.7032.20 2.2828.200.830.63Ø25 × 506 415.52530.628 863.691 014.1635.25 3.8355.23 1.83 1.41NaI ∶Tl
Ø25 × 50
6 077.39482.86
7 973.67744.1133.69 3.5945.05 1.330.91Ratio LC/NI
1.03
1.12
1.05
1.25
1.09
1.68
1.96
2.20
2.31
LaCl 3∶Ce contains natural radionuclides, of which La contains about 0.09% of 138
La. 66.4% and 33.6% of
138
La, after half-life of 1.02×1011
years,
undergo two types of decay processes, electron capture and β-, respectively, and decay to steady-state 138
Ba and 138
Ce, respectively [17]
. γ-rays with an energy of 1 436 keV and characteristic X-rays of 32 keV are released by the electron-capture decay,and γ-rays with an energy of 789 keV are released through the β-decay process.
Fig.5 presents the background spectra of LaCl 3∶Ce and NaI ∶Tl with the same size measured by PMT in a lead chamber with a wall thickness of 15 cm. For the background spectrum of LaCl 3∶Ce crystal, the count rate of （20 ~ 255） keV X and β rays is 1.038 counts·s –1
·cm –3
, the count rate of 760
keV ~ 1 MeV γ and β rays is 0.051 counts·s –1
·cm –3
,and 138La and 40
K (in PMT and aluminum) interact at 1 468 keV γ and X rays count rate is 0.075counts·s –1·cm –3
. The count of α particles produced
Ø25 mm × 50 mm LaCl 3: Ce Ø25 mm × 25 mm LaCl 3: Ce Ø25 mm × 50 mm Nal: Tl
Ø25 mm × 50 mm LaCl 3: Ce Ø25 mm × 25 mm LaCl 3: Ce Ø25 mm × 50 mm Nal: Tl
137
Cs
Ideal response
0200400
600800 1 0001 2001 400200
400
600800 1 0001 2001 400Energy/keV
Energy/keV
(a)
Fig. 4 Linear response of LaCl 3∶Ce and NaI ∶Tl crystals
500
1 000 1 500
2 000 2 500
3 000
b301
Energy/keV
LaCl 3: Ce Nal: Tl
X-ray
Background spectrum
138
La β-rays
(138La)
788.7 keV
+ β-rays (138La, 40K)
1 436 keV
(+ 32 keV X-rays)215
culterPo
211
Bi, 219Rn 227
Th, 223Ra α particles
138
La 57138
Ba 56138Ce 56
52
2
000
0EC 66.4%1 436 keV 789 keV β−
33.6%Stable
Fig. 5 Background spectra of LaCl 3∶Ce and NaI ∶Tl crystals
第1期
HOU Yueyun, et al ：Scintillation Properties of LaCl 3∶Ce Crystal with Low Radioactivity Background 5
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