Intrinsic background radiation of LaBr₃(Ce) detector via coincidence measurements and simulations Enhanced Publication

Intrinsic background radiation of LaBr 3 (Ce) detector via coincidence measurements and simulations 1 2 3 first-author 1 2 corresp bhsun@buaa.edu.cn bhsun@buaa.edu.cn bhsun@buaa.edu.cn https://orcid.org/0000-0001-9868-5711 1 2 corresp zhulh@buaa.edu.cn zhulh@buaa.edu.cn zhulh@buaa.edu.cn https://orcid.org/0000-0002-9509-7348 3 1 3 3 3 School of Physics, Beihang University, Beijing 100191, China School of Physics, Beihang University, Beijing 100191, China Beijing Advanced Innovation Center for Big Data-Based Precision Medicine, School of Medicine and Engineering, Beihang University, Beijing 100191, China Beijing Advanced Innovation Center for Big Data-Based Precision Medicine, School of Medicine and Engineering, Beihang University, Beijing 100191, China China Institute of Atomic Energy, Beijing 102413, China China Institute of Atomic Energy, Beijing 102413, China The LaBr 3 (Ce) detector has attracted much attention in recent years because of its superior characteristics compared with other scintillating materials in terms of resolution and efficiency. However, it has a relatively high intrinsic background radiation because of the naturally occurring radioisotopes in lanthanum, actinium, and their daughter nuclei. This limits its applications in low-counting rate experiments. In this study, we identified the radioactive isotopes in the ϕ 3" × 3" Saint-Gobain B380 detector by a coincidence measurement using a Clover detector in a low-background shielding system. Moreover, we carried out a Geant4 simulation of the experimental spectra to evaluate the activities of the main internal radiation components. The total activity of the background radiation of B380 is determined to be 1.523 (34) Bq/cm 3 . The main sources include 138 La at 1.428 (34) Bq/cm 3 , 207 Tl at 0.0135 (13) Bq/cm 3 , 211 Bi at 0.0136 (15) Bq/cm 3 , 215 Po at 0.0135 (3) Bq/cm 3 , 219 Rn at 0.0125 (12) Bq/cm 3 , 223 Fr at 0.0019 (11) Bq/cm 3 , 223 Ra at 0.0127 (10) Bq/cm 3 , 227 Th at 0.0158 (22) Bq/cm 3 , and 227 Ac at 0.0135 (13) Bq/cm 3 . Of these, the activities of 207 Tl, 211 Po, 215 Po, 223 Fr, and 227 Ac are deduced for the first time from the secular equilibrium established in the decay chain of 227 Ac. LaBr 3 (Ce) detector Coincidence measurement technique Intrinsic radiation GEANT4 simulation 1 Introduction 1 As a new type of inorganic scintillator, the LaBr 3 (Ce) crystal has a high density of 5.08 g/cm 3 , a high light output of approximately 63 photons/keV γ , a fast decay time of approximately 16 ns [ 1 1 ], and a good temperature response. These superior characteristics make LaBr 3 (Ce) ideal for many applications [ 2 2 - 6 6 2-6 ] in environmental monitoring, oil well logging, nuclear safeguards, and medical imaging. Thus, LaBr 3 (Ce) is often used as a substitute for the widely used NaI(TI) crystal when high performance is required. The integrated LaBr 3 (Ce) detector consists of a crystal coupled directly to a photomultiplier tube (PMT). Previous studies of LaBr 3 (Ce) detectors [ 1 1 , 7 7 - 14 14 7-14 ] show excellent linearity in the γ ray response, a good energy resolution of less than 3% (FWHM) for the 662 keV γ ray for a size of up to ϕ 3" 3", and an excellent time resolution of approximately ∼ 300 ps (FWHM). The latter has made possible a fast timing detector array composed of LaBr 3 (Ce) [ 15 15 , 16 16 ] that is pursued worldwide for nuclear structure studies. Conversely, the LaBr 3 (Ce) detector has a relatively high intrinsic background radiation [ 11 11 , 17 17 - 21 21 17-21 ], which is typically at least 1 to 2 orders of magnitude higher than that of the NaI(Tl) detector. The self-radiation root in 138 La and the five short-lived progeny of 227 Ac impurities may cause a non-negligible effect in the energy spectrum as a result. This would seriously limit its application in low-count rate experiments such as those with space γ rays. Therefore, it is valuable to quantify the intrinsic radiation of LaBr 3 (Ce) and to understand its influence. The present study aims to identify the components of internal radiation in LaBr 3 (Ce) and to deduce their activities. The detector of interest is the Saint-Gobain B380 with a size of ϕ 3" 3". This experiment is performed by combining coincidence measurement with dedicated Geant4 simulation. The remainder of this paper is organized as follows. Section 2 presents the coincidence measurement of LaBr 3 (Ce) vs. a Clover detector, and the corresponding results. In Sec. 3, we performed Geant4 simulations of the experimental spectra of both LaBr 3 (Ce) and the Clover detector, and we deduce the activity of 138 La, 221 Bi, 219 Rn, 223 Ra, and 227 Th. A summary is provided in Sec. 4. 2 Internal radiation of LaBr 3 (Ce) and coincidence measurement 1 2.1 Internal radiation of LaBr 3 (Ce) detector 2 The internal radiation in the LaBr 3 (Ce) crystal has two origins: the naturally occurring radioisotope 138 La and the 227 Ac impurity. Together, they can decrease the detection sensitivity of γ rays with energies up to approximately 2.5 MeV. A better understanding of self-activity is essential for designing experiments. 138 La is the only naturally occurring radioactive isotope of lanthanum, with 0.09% abundance and a half-life of 1.05 × 10 11 years, which affects the energy spectrum below 1.5 MeV. 138 La decays in two parallel processes, as shown in Fig. 1 Fig. 1 . 34.4% of the isotope undergoes β - decay, with a maximum energy of 263 keV, eventually to the first excitation state of 138 Ce. This process is associated with the emission of a 788.742-keV γ ray. The remaining 65.6% of 138 La disintegrates by electronic capture (EC). This process results in stable 138 Ba with the emission of a 1435.795-keV γ ray and the characteristic X-rays of Ba with energies ranging from 31 to 38 keV. 227 Ac is the grand-daughter nuclide in the 235 U decay chain. Because of its similarity in chemistry to lanthanum, it presents as a contaminator with a half-life of 21.77 in LaBr 3 (Ce). Fig. 2 Fig. 2 shows the decay chain down to the stable 207 Pb by emitting α , β , and γ rays. This includes 6 α emitters ( 227 Ac, 227 Th, 223 Ra, 219 Rn, 215 Po, and 211 Bi) and 4 β emitters ( 227 Ac, 211 Pb, 211 Bi, and 207 Tl). 227 Ac and its daughter nuclei produce a higher energy background by emitting α particles, and they also contribute to the β continuum up to approximately 1400 keV as a result of the β decay of 211 Pb and 207 Tl in the decay chain of this nucleus. 2.2 Experimental setup 2 Coincidence measurement using an HPGe was conducted in a low-background counting system (LBS). The environmental background counting rate was 58 per second. The LBS is a cylinder with a radius of 64 cm and a height of 66.1 cm, and it consists of four layers: iron, lead, copper, and plexiglass, from the outside to the inside. A schematic diagram of the entire detection system is shown in Fig. 3 Fig. 3 . The lead layer can shield most of the low-energy environment background, and the purpose of the copper layer is to absorb the characteristic X-rays of lead. The LaBr 3 (Ce) detector is the Saint-Gobain B380 with a ϕ 3" ×3" crystal. A high-purity germanium (HPGe) Clover detector from Canberra was placed directly facing the LaBr 3 (Ce). The high voltage applied to the LaBr 3 (Ce) detector was set to 520 V. A higher voltage may cause electron saturation in the photomultiplier, affecting linearity [ 23 23 ] in the energy determination. The Clover consists of four coaxial N-type high-purity germanium detectors, each with a diameter of 60 mm and length of 60 mm. The energy resolutions for the LaBr 3 (Ce) and Clover were measured to be 2.1% (FWHM) and 0.166% (FHWM) for a 1.332-MeV γ ray, respectively. The relative efficiency was 38% for each germanium crystal. The energy and time signals of the two detectors were acquired using the VME data acquisition system and collected for 37,187 s in total. Dead time correction and time stamps were added to the data acquisition software. Standard radioactive sources 60 Co, 137 Cs, and 241 Am were used for the γ -ray energy calibration up to approximately 1.33 MeV. The calibration accuracy was cross-checked using the characteristic γ -rays of 138 La. Moreover, to calibrate the high-energy spectrum of LaBr 3 (Ce), we used the recoil-electron energies from the Compton scattering process of 2.615-MeV γ rays of 208 Tl, a naturally radioactive nuclide in an environmental background. Such calibration is only possible using the coincidence measurement with Clover. 2.3 Coincidence measurement 2 The intrinsic radiation of the LaBr 3 (Ce) scintillator was identified by coincidence measurement using the LaBr 3 (Ce) and Clover detectors. The relevant background spectra after energy calibration are shown in Fig. 4 Fig. 4 . In the self-counting spectrum of LaBr 3 (Ce), we first see a low-energy peak centered at approximately 35.5 keV. This is attributed to the sum of 95.6% of the 31.83 keV K α X-ray response and 90 % of the 5.6 keV Auger electron response in the EC decay process, by referring to the theoretical calculation [ 20 20 ]. The energy shift is due to the non-proportional response of the LaBr 3 (Ce) crystal. Then, we see a β continuum with an end point of 263 keV mixed with the Compton continua, primarily from the 788.7 and 1435.8 keV of 138 La and the 1460.8 keV of 40 K. With increasing energy, the 788.7 keV bump is shown to extend to higher energies and end at approximately 1 MeV. This is due to the coincidence of the 788.7-keV γ with the β - continuum. The 1435.8-keV γ -rays produced by the EC of 138 La coincided with the 32-keV X-rays of 138 Ba and the 1460.8-keV γ -rays of 40 K, resulting in a double peak near 1461 keV, as shown in Fig. 4(a) Fig. 4(a) . The spectrum above 1.8 MeV shows a three-peak structure, revealing the presence of α emitter contaminants. Although the α energies from 227 Ac and its daughter nuclei are as high as 5.0 ∼ 7.4 MeV (see Fig. 2 Fig. 2 ), the energies are read out in the spectrum calibrated with γ -rays to be in the energy range of 1.5 and 2.5 MeV because of the well-known light quenching effect (see, for example Ref. [ 18 18 ]). The energy spectrum of the Clover detector, as well as the coincidence spectrum, are shown in Fig. 4(b) Fig. 4(b) . It is clear that the characteristic γ -rays of 788.7 keV and 1435.8 keV of 138 La decays were enhanced and the environmental background was further reduced in the coincidence spectrum. Setting gates in the Clover spectrum at 788.7 keV and 1435.8 keV of 138 Ba decays allows us to pick up the β spectrum and the X-ray spectrum in the LaBr 3 (Ce) detector, as shown in Fig. 4(a) Fig. 4(a) ].The coincidence β spectrum has triggered a precise study of 138 La decay [ 20 20 , 24 24 - 27 27 24-27 ], which is a second forbidden unique decay. The β and X-ray distributions are shown for comparison in the LaBr 3 (Ce) spectrum. Part of the coincidence events in the Clover and LaBr 3 (Ce) detectors are displayed in Fig. 5 Fig. 5 , while the projection to the Clover is shown in Fig. 6 Fig. 6 . The horizontal bands in Fig. 5 Fig. 5 are traced back to the α - γ cascades. The correlated γ ray energies and their relative intensities are key to identifying the α emitters. The α emitters identified by the γ-α coincidence are 227 Th, 223 Ra, 219 Rn, and 211 Bi. This is consistent with previous investigations of LaBr 3 (Ce) with a size of ϕ 1" ×1" [ 9 9 ] and LaCl 3 (Ce) with a size of ϕ 25mm × 25mm) [ 17 17 ]. The main γ -rays and α emitted by 227 Th, 223 Ra, 219 Rn, and 211 Bi are shown in Table 1 Table 1 . Some α values listed in the table cannot be seen in Fig. 5 Fig. 5 because their branches are relatively low. The α (6.038 MeV from 227 Th, 6.62 MeV from 211 Bi, and 6.82 MeV from 219 Rn) with relatively high intensity cannot be seen in Fig. 5 Fig. 5 because the parent nucleus decays to the ground state of the daughter nucleus. The third peak in the single spectrum of LaBr 3 (Ce) in Fig. 4(a) Fig. 4(a) is the 7.386 MeV α line from the ground state of 215 Po to the ground state of 211 Pb, in which there is no cascade γ -ray. 1 1 Isotope 1 1 <math id="M2"><mrow><msub><mi>α</mi><mrow><msub><mi>E</mi><mi>γ</mi></msub></mrow></msub><msub><mi>E</mi><mi>α</mi></msub></mrow></math> (keV) 1 1 Shown in Fig. 5. 1 1 227 Th 1 1 α 236 5757 1 1 √ 1 1 227 Th 1 1 α 256 5757 1 1 √ 1 1 227 Th 1 1 α 286 5757 1 1 √ 1 1 227 Th 1 1 α 300 5713 1 1 √ 1 1 227 Th 1 1 α 329 5713 1 1 √ 1 1 227 Th 1 1 α 61 5977 1 1 / 1 1 227 Th 1 1 α 334 5709 1 1 / 1 1 223 Ra 1 1 α 144 5716 1 1 √ 1 1 223 Ra 1 1 α 154 5716 1 1 √ 1 1 223 Ra 1 1 α 270 5607 1 1 √ 1 1 223 Ra 1 1 α 324 5540 1 1 √ 1 1 223 Ra 1 1 α 338 5540 1 1 √ 1 1 223 Ra 1 1 α 445 5434 1 1 √ 1 1 223 Ra 1 1 α 127 5747 1 1 / 1 1 219 Rn 1 1 α 402 6425 1 1 √ 1 1 219 Rn 1 1 α 271 6553 1 1 / 1 1 219 Rn 1 1 α 402 6425 1 1 / 1 1 211 Bi 1 1 α 351 6278 1 1 √ 3 Discussion 1 In this section, we will deduce the activity of various radioactive contaminations embedded in LaBr 3 (Ce). Because radioactive contaminants are evenly distributed in the crystal, it is practically impossible to make a direct determination because of the lack of accurate efficiency calibration for both detectors. Instead, in this work, we develop a Monte Carlo model based on the Geant4 version 10.4 [ 28 28 - 30 30 28-30 ] toolkit. The setup includes the Clover, the LaBr 3 (Ce) detector as well as its bracket, and all components of the shielding system, as shown in Fig. 3 Fig. 3 . 138 La isotopes are evenly distributed in the ϕ 3" ×3" cylindrical LaBr 3 crystal. The density of the LaBr 3 (Ce) crystal was set to 5.08 g/cm 3 [ 1 1 ]. We employed the physics constructor class of G4EmStandardPhysics [ 30 30 ]. It includes various processes, such as the deposition of β and γ rays in the sensitive volumes of the detectors, the occurrence of Compton scattering in the shield, and the characteristic X-rays induced from the shield material. The shielding materials are summarized in Table 2 Table 2 . The activities of 138 La and 227 Ac decay chain contaminators were determined by reproducing the experimental spectra in both the LaBr 3 (Ce) and Clover detectors. 1 1 Layer 1 1 Inner radius (cm) 1 1 Outer radius (cm) 1 1 Material 1 1 Iron 1 1 30 1 1 32 1 1 G4_Fe 1 1 Lead 1 1 21.65 1 1 30 1 1 G4_Pb 1 1 Copper 1 1 21.45 1 1 21.65 1 1 G4_Cu 1 1 Plexiglass 1 1 20.95 1 1 21.45 1 1 G4_PLEXIGLASS 1 1 3.1 Simulation of the self-counting LaBr 3 (Ce) 2 We compare the simulation with the experimental data acquired at the same measurement time, that is, 37,187 s. The best fit to the experimental spectrum is found using the least squares method when the activity of 138 La 482(19) Bq corresponds to 1.396 (55) Bq/cm 3 . This corresponds to 180,030,639 decays of 138 La over 37,187 s in total. The uncertainty quoted here is due to the experimental statistics, detection efficiency, and branching ratio of the γ -rays. The experimental and simulated spectra are plotted in Fig. 7 Fig. 7 . Good agreement is seen, except for the low-energy part up to approximately 150 keV. To reproduce the experimental spectrum, we adopted the following function [ 31 31 ] for the γ -ray energy ( E γ ) resolution ( η ): <math display="block" id="M3"><mrow><mi>ln</mi><mo stretchy="false">(</mo><mi>η</mi><mo stretchy="false">)</mo><mo>=</mo><mo>−</mo><mfrac><mn>1</mn><mn>2</mn></mfrac><mo>×</mo><mi>ln</mi><mo stretchy="false">(</mo><msub><mi>E</mi><mi>γ</mi></msub><mo stretchy="false">)</mo><mo>−</mo><mn>3.82</mn></mrow></math> It should be noted that in the above simulation, we did not consider the contribution from 227 Ac and its daughter nuclei. This may partially account for the difference between the simulation and experimental spectra. Another possible reason for the low-energy deficiency in the simulation could be insufficient understanding of the β decay of 138 La. This has been discussed in Ref. [ 24 24 - 27 27 24-27 ]. 3.2 Activities determined using the Clover data 2 An alternative way to deduce the 138 La activity is to use the coincidence γ ray information at 788.7 and 1435.8 keV in Clover. This would require an accurate efficiency calibration of the Clover detector using a volume source of the same volume as the LaBr 3 (Ce) detector, which is practically impossible. In reality, we performed a two-step optimization of the calibration [ 32 32 ]. In step one, we followed the standard routine for efficiency calibration using the standard radioactive point sources 137 Cs, 241 Am, 54 Mn, 88 Y, 109 Cd, 65 Zn, and 152 Eu. These sources are selected to avoid a possible true summing effect. The point source was placed 3 cm from the front surface of the Clover detector. We used the EFFIT program in the software package [ 33 33 ] to describe the efficiency curve for the low-energy and high-energy regions separately. In this step, we optimized the thickness of the dead layer encapsulating the crystal by the least squares method to best reproduce the Clover spectrum of 152 Eu. The best simulation results of the 152 Eu spectrum together with the experimental data are presented in Fig. 8(a) Fig. 8(a) . The simulated detection efficiency curve was compared with the experimental results using a standard point source, as shown in Fig. 9 Fig. 9 . In step two, we modeled the detection efficiency of the Clover with sources evenly distributed in the LaBr 3 (Ce) crystal, instead of point sources. The simulated efficiency curve is shown in Fig. 9 Fig. 9 . The efficiency of 788.7 keV and 1435.8 keV γ -rays were determined to be 0.00347(11) and 0.00257(10), respectively. In the simulation, the interaction of characteristic γ -rays with both the LaBr 3 (Ce) and Clover detectors and even the shielding material were taken into account. As a result, we found that the Clover spectrum collected for 37,187 s is best reproduced when the total count of the 788.7-keV γ -ray is 22,423 (164). The simulation and experimental data are shown in Fig. 8(b) Fig. 8(b) . The activity of 138 La is thus extracted to be 504 (16) Bq, corresponding to 1.451 (58) Bq/cm 3 . The uncertainty takes into account the contributions from the statistics, detection efficiency, and branching ratio of the γ -rays in the decay of 138 La. In the same way, the activity of 138 La was also deduced from the 1435.8-keV γ -ray to be 500 (20) Bq, corresponding to 1.437 (63) Bq/cm 3 . Both are consistent with those determined from the self-counting of the LaBr 3 (Ce) detector, as shown in Table 3 Table 3 . In fact, the activity of 138 La in the LaBr 3 (Ce) B380 detector can be estimated according to the natural abundance of 138 La, 0.08881(71)% [ 22 22 ], and its half-life T 1/2 =1.0510 11 a. The activity amounts to 1.456 (40) Bq/cm 3 . This is in good agreement with our measurements. [a] Date from Ref. [ 17 17 ], [b] Date from Ref. [ 21 21 ], 1 1 Isotope 1 1 γ -energy (keV) 1 1 Absolute efficiency (%) 1 1 Activity (Bq/cm 3 ) 1 1 Reference value (Bq/cm 3 ) 1 1 211 Bi 1 1 351.1 1 1 0.4347 1 1 1.3610 -2 (15) 1 1 0.032 a 1 1 219 Rn 1 1 271.2 1 1 0.4117 1 1 1.3410 -2 (17) 1 1 / 1 1 219 Rn 1 1 401.8 1 1 0.4317 1 1 1.1610 -2 (16) 1 1 0.032 a 3 1 Average of 219 Rn 1 1 1.2510 -2 (12) 1 1 / 1 1 223 Ra 1 1 269.5 1 1 0.4130 1 1 1.2610 -2 (13) 1 1 0.025 a 1 1 223 Ra 1 1 445.0 1 1 0.4283 1 1 1.2710 -2 (15) 1 1 / 3 1 Average of 223 Ra 1 1 1.2710 -2 (10) 1 1 / 1 1 227 Th 1 1 236.0 1 1 0.3840 1 1 1.5810 -2 (22) 1 1 0.037 a 3 1 Sum activity of the above α contaminators 1 1 5.4510 -2 (30) 1 1 1 1 1 1 207 Tl 1 1 / 1 1 / 1 1 1.3510 -2 (13) 1 1 / 1 1 211 Po 1 1 / 1 1 / 1 1 3.7910 -5 (36) 1 1 / 1 1 215 Po 1 1 / 1 1 / 1 1 1.3510 -2 (13) 1 1 / 1 1 223 Fr 1 1 / 1 1 / 1 1 1.910 -4 (11) 1 1 / 1 1 227 Ac 1 1 / 1 1 / 1 1 1.3510 -2 (13) 1 1 / 1 1 138 La 1 1 788.7 1 1 0.3467 1 1 1.451(58) 1 1 1 1 138 La 1 1 1435.8 1 1 0.2567 1 1 1.437(63) 1 1 1 1 138 La 2 1 Self-counting method 1 1 1.396(55) 1 1 / 3 1 Average activity of 138 La 1 1 1.428(34) 1 1 1.530(70) b 2 1 Total activity 1 1 1.523(34) 1 1 / 1 1 The above procedure can also be applied to evaluate the activities of the 227 Ac decay chain contaminators. In Fig. 6 Fig. 6 , we identified the characteristic γ -rays at 351.1 keV ( 211 Bi), 271.2 keV and 401.8 keV ( 219 Rn), 269.5 keV and 445.0 keV ( 223 Ra), and 236.0 keV ( 227 Th). The activities of 211 Bi, 219 Rn, 223 Ra, and 227 Th are 0.0136 (15) Bq/cm 3 , 0.0125 (12) Bq/cm 3 , 0.0127 (10) Bq/cm 3 , and 0.0158 (22) Bq/cm 3 , respectively. The half-life of the parent radionuclide 227 Ac is much longer than the half-life of the daughter radionuclide in the Ac-decay chain. In principle, secular equilibrium should occur, namely, the activities A of each emitter should be equal: <math display="block" id="M4"><mrow><mi>A</mi><mo>=</mo><msub><mi>λ</mi><mn>1</mn></msub><msub><mi>N</mi><mn>1</mn></msub><mo>=</mo><msub><mi>λ</mi><mn>2</mn></msub><msub><mi>N</mi><mn>2</mn></msub><mo>=</mo><msub><mi>λ</mi><mn>3</mn></msub><msub><mi>N</mi><mn>3</mn></msub><mn>....</mn></mrow></math> where λ i and N i are the decay constant and the number of nuclear species i , respectively. Indeed, as shown in Fig. 10 Fig. 10 , the activities of 211 Bi, 219 Rn, 223 Ra, and 227 Th agree well within the error bars. Here, the decay branchings of the 227 Ac and 211 Bi decay were taken into account. The average activity was computed to be 0.0135(13) Bq/cm 3 . The secular equilibrium allows us to determine the activities of all nuclei in the decay chain, namely, 207 Tl, 211 Po, 215 Po, 223 Fr, and 227 Ac as 1.3510 -2 (13), 3.7910 -5 (36), 1.3510 -2 (13), 1.910 -4 (11), and 1.3510 -2 (13), respectively. The results are summarized in Table 3 Table 3 . We included all of the α emitters in the decay chain, as discussed above, into the Geant4 code, and simulated the 37,187-s energy spectrum. The quenching effect of the α particles and recoil nuclei in the scintillator is not included because of the lack of precise calibration. The results are shown in Fig. 11 Fig. 11 . The energy resolution of each alpha particle was assumed to be 6%. One can easily identify a similar pattern of simulations for the experimental high-energy spectrum. This verifies the 227 Ac contribution to the energy spectrum of 4.5 MeV to 5.2 MeV, 223 Ra and 227 Th to the spectrum of 5.2 MeV to 6.5 MeV, 211 Bi and 219 Rn to the spectrum of 6.5 MeV to 7.2 MeV, and 211 Po and 215 Po to the spectrum of 7.2 MeV to 8.5 MeV. The sum activity of α contaminators is 0.095(4) Bq/cm 3 , which is smaller by a factor of 14 than that of 138 La. A similar intrinsic background exists in all of the La-containing crystals. A pioneering study [ 17 17 ] of the LaCl 3 (Ce) detector with a crystal size of ϕ 25 mm ×25 mm identified the total α activity of 227 Th, 221 Bi, 219 Rn, and 223 Ra as 0.126 Bq/cm 3 , and each contribution is summarized in Table 3 Table 3 . In the present study of the B380 type, the 227 Ac atom/La atom amounts to 2.0× 10 -12 . The sum activities of 227 Th, 221 Bi, 219 Rn, and 223 Ra are typically 40% of that in Ref. [ 17 17 ]. This indicates that the actinium impurity has been significantly reduced in the last decade. 138 La and 227 Ac impurities resulted in a counting rate (including environmental background) of 237 counts/s for γ -ray energies between 20 and 500 keV, 182 counts/s between 500 and 1.5 MeV, and 27 counts/s above 1.5 MeV in our LaBr 3 (Ce) detector. Self-irradiation affects its application to low-production experiments, in particular for potential cases with a count rate of less than 450 counts/s. 4 Summary 1 In this work, we performed a coincidence measurement using a Clover detector and identified the internal radioactive nuclei of the B380 LaBr 3 (Ce) detector. By combining the coincidence spectra with Geant4 simulations, we determined the activity of 138 La, 211 Bi, 219 Rn, 223 Ra, and 227 Th with good accuracy. Moreover, we found that the secular equilibrium of the 227 Ac decay chain is well established in the detector considered here. 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U1832211, U1867210, 11922501, 11961141004, 11575018, 11790322, and U1932209). 2020-07-11 2020-08-20 2020-08-22 2020-10-09 2020-10-01 2021-04-10 © China Science Publishing & Media Ltd. (Science Press), Shanghai Institute of Applied Physics, the Chinese Academy of Sciences, ChineseNuclear Society and Springer Nature Singapore Pte Ltd. 2020 This is a post-peer-review, pre-copyedit version of an article published in Nuclear Science and Techniques. The final authenticated version is available online at: http://dx.doi.org/10.1007/s41365-020-00812-8 http://dx.doi.org/10.1007/s41365-020-00812-8 . 2020 Nuclear Science and Techniques 10 31