Intrinsic radiation background of LaBr 3 (Ce) detector via coincidence measurements and simulations
Hao Cheng, Bao-Hua Sun, Li-Hua Zhu, Tian-Xiao Li, Guang-Shuai Li, Cong-Bo Li, Xiao-Guang Wu, Yun Zheng
aa r X i v : . [ phy s i c s . i n s - d e t ] J u l Intrinsic radiation background of LaBr (Ce) detector via coincidencemeasurements and simulations *Hao Cheng , , Bao-Hua Sun , , Li-Hua Zhu , , Tian-xiao Li Guang-Shuai Li Cong-Bo Li Xiao-Guang Wu Yun Zheng
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;and Key Laboratory of Big Data-Based Precision Medicine (Beihang University), Ministry of Industry and Information Technology China Institute of Atomic Energy, Beijing 102413, China
Abstract:
The LaBr (Ce) detector has attracted much attention in recent years for its superior characteristics toother scintillating materials in terms of resolution and efficiency. However, it has relatively high intrinsic radiationbackground due to the naturally occurring radioisotope in lanthanum, actinium and their daughter nuclei. This limitsits applications in low counting rate experiments. In this paper, we identified the radioactive isotopes in the φ ′′ × ′′ Saint-Gobain B380 detector by a coincidence measurement using a Clover detector in a low-background shieldingsystem. Moreover, we carried out a Geant4 simulation to the experimental spectra to evaluate the activities of themain internal radiation components. The activity of radiation background of B380 is determined to be 1.480 (69)Bq/cm , the main sources of which include La of 1.425 (59) Bq/cm , Bi of 0.0136 (15) Bq/cm , Rn of 0.0125(17) Bq/cm , Ra of 0.0127 (14) Bq/cm , and Th of 0.0158 (22) Bq/cm . Key words:
LaBr (Ce) detector, Coincidence measurement technique, Intrinsic radiation, GEANT4 simulation As a new type of inorganic scintillation, theLaBr (Ce) crystal has a high density of 5.08 g/cm , ahigh light output of about 63 photons/keV γ , a fast decaytime of about 16 ns [1] and a good temperature response.These superior characteristics make LaBr (Ce) ideal formany applications [2–6] in environmental monitoring, oilwell logging, nuclear safeguards, and medical imaging.LaBr (Ce) thus is often taken as a substitution to thewidely used NaI(TI) crystal when high performance isdemanded. The integrated LaBr (Ce) detector consistsof a crystal coupled directly to a selected photomulti-plier tube (PMT). Previous studies of LaBr (Ce) detec-tors [1, 7–14] show excellent linearity in γ ray response, agood energy resolution of less than 3% (FWHM) for the662 keV γ ray for the size of up to φ ′′ × ′′ , and an ex-cellent time resolution of about ∼
300 ps (FHWM). Thelatter has make a fast timing detector array composed ofLaBr (Ce) [15, 16] pursued worldwide for nuclear struc-ture studies.On the other hand, LaBr (Ce) detector has a rel-atively high intrinsic radiation background [11, 17–21],which is typically more than 1 to 2 orders of magnitudehigher than that of NaI(Tl) detector. The self-radiationsroot in La and the five short-lived progeny of
Ac im- purities, may cause a non-negligible effect in the energyspectrum as a result. This would seriously limit its ap-plication in low count rate experiments such as space γ rays. Therefore, it is valuable to quantify the intrinsicradiation of LaBr (Ce) and moreover to understand theirinfluence.The present study aims to identify the componentsof internal radiation in LaBr (Ce) and furthermore todeduce their activities. The detector of interest is Saint-Gobain B380 with the size of φ ′′ × ′′ . This is done bycombining the coincidence measurement with the dedi-cated Geant4 simulation. The paper is organized as fol-lows. Section presents the coincidence measurement ofLaBr (Ce) vs. a Clover detector, and the correspondingresults. In Sec. we made Geant4 simulations to theexperimental spectra of both LaBr (Ce) and Clover de-tector, and deduce the activity of La ,
Bi,
Rn,
Ra and
Th. A summary is given in Sec. . (Ce) andcoincidence measurement (Ce) detector La is the only naturally occurring radioactive iso-tope of lanthanum with 0.09% abundance and a half-life ∗ Supported by National Natural Science Foundation of China (11575018, 11375023, 11305269, 11375267, 11405274, 11205245,10927507, 10975191, 11075214, 11175259, 11205068)1) E-mail: [email protected]) E-mail: [email protected]
1f 1 . × years, which affects the energy spectrumbelow 1.5 MeV. La decays in two parallel processes,as shown in Fig. 1. 34.4% of the isotope undergoes β − decay, with a maximum energy of 263 keV, eventuallyto the first excitation state of Ce. The process is as-sociated with the emission of a 788.742 keV γ ray. Theremaining 65.6% of La disintegrates by electronic cap-ture (EC). This process results in stable
Ba with theemission of a 1435.795 keV γ ray and the characteristicX-rays of Ba with energies ranging from 31 to 38 keV. (cid:28581)(cid:28583)(cid:28588) (cid:28608)(cid:28629) (cid:28581)(cid:28583)(cid:28588) (cid:28598)(cid:28629) (cid:28581)(cid:28583)(cid:28588) (cid:28599)(cid:28633) (cid:28582) (cid:28575) (cid:28585) (cid:28575) (cid:28582) (cid:28575) (cid:28580) (cid:28575) (cid:28580) (cid:28575) (cid:28601) (cid:28599) (cid:28586)(cid:28585) (cid:28578) (cid:28586) (cid:28569) (cid:29000) (cid:28577) (cid:28583) (cid:28584) (cid:28578) (cid:28584) (cid:28569) Fig. 1. The decay scheme of
La. The data arefrom NNDC [22].
Ac is the grand-daughter nuclide in the
U decaychain. Due to the similarity in chemistry to lanthanum,it presents as a contaminator with a half-life of of 21.77years in LaBr (Ce). Fig. 2 shows the decay chain downto the stable Pb by emitting α , β and γ rays. Hereincludes 6 α emitters ( Ac,
Th,
Ra,
Rn,
Poand
Bi), and 4 β emitters ( Ac,
Pb,
Bi and
Tl).
Ac and its daughter nuclei produce the higherenergy background by emitting α particles, and also con-tribute to the β continuum up to about 1400 keV due tothe β decay of Pb and
Tl in the decay chain of thisnucleus. 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Fig. 2. Actinide decay chain. Listed are the half-life, energies of α and characteristic γ rays withrelatively high intensities and β -decay end-pointenergy for each nuclide. Data are taken fromNNDC [22]. The coincidence measurement using a HPGe was con-ducted in a low-background counting system (LBS). The environmental background counting rate is 58 per sec-ond. The LBS is a cylinder with a radius of 64 cm anda height of 66.1 cm, and consists of four layers: iron,lead, copper and plexiglass from the outside to the in-side. The schematic diagram of the entire detection sys-tem is shown in Fig. 3. The lead layer can shield most ofthe low energy environment background, and the copperlayer aims to absorb the characteristic X-rays of lead.
Liquid NitrogenPreamplifierLaBr3Clover
Crystal
Crystal
Bracket
PMT
Fig. 3. (Color online) Schematic diagram of exper-imental setup. It contains LaBr and Clover crys-tals, and a low-background shielding room. Theshielding room is composed of plexiglass, copper,lead and iron from the inside to the outside. TheLaBr detector is supported by a bracket in theshielded room. The LaBr (Ce) detector is the Saint-Gobain B380with a φ ′′ × ′′ crystal. A high-purity germanium(HPGe) Clover detector of Canberra was placed directlyfacing to the LaBr (Ce). The high voltage applied for theLaBr (Ce) detector was set to be 520 V. Higher voltagemay cause the electron saturation in the photomultiplier,thus can affect linearity [23] in the energy determination.The Clover consists of four coaxial N-type high-purityGermanium detectors, each with a diameter of 60 mmand length of 60 mm. The energy resolutions for theLaBr (Ce) and Clover are measured to be 2.1% (FWHM)and 0.166% (FHWM) for the 1.332 MeV γ ray, respec-tively. The relative efficiency is 38% for each crystal.The energy and time signals of the two detectorswere acquired by the VME data acquisition system col-lected for 37,187 seconds in total. Dead time correc-tion and time stamp were added in the data acquisi-tion software. Standard radioactive sources Co,
Cs,and
Am were used for the γ -ray energy calibration upto around 1.33 MeV. The calibration accuracy has been2ross-checked by the characteristic γ -rays of La. More-over, to calibrate the high energy spectrum of LaBr (Ce),we have used the recoil-electron energies from the Comp-ton scattering process of 2.615 MeV γ ray of Tl, a nat-urally radioactive nuclide in environmental background.Such calibration is only possible by using the coincidencemeasurement with the Clover.
300 600 900 1200 1500 1800 2100 2400 2700 300010 -1 LaBr X-ray-ray . ke V La La & K particles a La 788.7keV L a . V K . V b C oun t s Energy / keV
Clover Coincidence
Environmental Background
Fig. 4. (Color online) Radiation background spec-tra measured in 37,187 seconds by the LaBr (Ce)detector (a) and the Clover (b). The coincidence β spectrum (red) and X-ray spectrum (green) inthe LaBr (Ce) detector are shown in (a). Co-incidence spectrum (red) and environment back-ground spectrum (black) of Clover are also shownin (b). For details, refer to the text. The intrinsic radiations of the LaBr (Ce) scintilla-tor were identified by the coincidence measurement ofthe LaBr (Ce) and Clover detectors. The relevant back-ground spectra after energy calibration are shown inFig. 4. In the self-counting spectrum of LaBr (Ce), wesee first a low-energy peak centered at around 35.5 keV.It 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 electronresponse in the EC decay process, by referring to thetheoretical calculation [20]. The energy shift is due tothe non-proportional response of the LaBr (Ce) crystal.Then we see a β continuum with an end point of 263 keVmixed with the Compton continua from mainly 788.7,1435.8 keV of La and the 1460.8 keV of K. Withthe increasing energy, the 788.7 keV bump is shown toextend to higher energies and end at around 1 MeV. It isdue to the coincidence of 788.7 keV γ with the β − con-tinuum. The 1435.8 keV γ -rays produced by the EC of La coincided with the 32 keV X-rays of
Ba and the1460.8 keV γ -rays of K, result in a double peak near1461 keV, as shown in Fig. 4(a). The spectrum above 1.5 MeV shows a three-peakstructure, revealing the presence of α emitter contami-nants. Although the α energies from Ac and its daugh-ter nuclei are as high as 5.0 ∼ γ -rays to be in the energy range of 1.5 and 2.5 MeVdue to the well-known light quenching effect (see, e.g.Ref. [18]). The species will be further explored later bythe coincidence measurement.The energy spectrum of the Clover detector as wellas the coincidence spectrum are shown in the Fig. 4(b).It is clearly seen that the characteristic γ -rays of 788.7keV and 1435.8 keV of La decays have been enhancedand the environmental background has been further re-duced in the coincidence spectrum. Setting gates in theClover spectrum on 788.7 keV and 1435.8 keV of
Badecays, allows us to pick up the β spectrum and theX-ray spectrum in the LaBr (Ce) detector, as shown inFig. 4(a).The coincidence β spectrum has triggered a pre-cise study of La decay [20, 24–27], which is a secondforbidden unique decay. The β and X-ray distributionsare shown for comparison in the LaBr (Ce) spectrum. (cid:28614)(cid:28629)(cid:28577)(cid:28582)(cid:28582)(cid:28583)(cid:28585)(cid:28584)(cid:28583)(cid:28584)(cid:28572)(cid:28999)(cid:28573)(cid:28584)(cid:28584)(cid:28585)(cid:28433)(cid:29001)(cid:28434) (cid:28614)(cid:28629)(cid:28577)(cid:28582)(cid:28582)(cid:28583)(cid:28585)(cid:28585)(cid:28584)(cid:28580)(cid:28572)(cid:28999)(cid:28573)(cid:28583)(cid:28583)(cid:28588)(cid:28578)(cid:28583)(cid:28433)(cid:29001)(cid:28434) (cid:28614)(cid:28629)(cid:28577)(cid:28582)(cid:28582)(cid:28583)(cid:28585)(cid:28585)(cid:28584)(cid:28580)(cid:28572)(cid:28999)(cid:28573)(cid:28583)(cid:28582)(cid:28583)(cid:28578)(cid:28589)(cid:28433)(cid:29001)(cid:28434) (cid:28616)(cid:28636)(cid:28577)(cid:28582)(cid:28582)(cid:28587)(cid:28585)(cid:28587)(cid:28581)(cid:28583)(cid:28572)(cid:28999)(cid:28573)(cid:28583)(cid:28580)(cid:28580)(cid:28433)(cid:29001)(cid:28434) (cid:28616)(cid:28636)(cid:28577)(cid:28582)(cid:28582)(cid:28587)(cid:28585)(cid:28587)(cid:28585)(cid:28587)(cid:28572)(cid:28999)(cid:28573)(cid:28582)(cid:28588)(cid:28586)(cid:28578)(cid:28581)(cid:28433)(cid:29001)(cid:28434) (cid:28614)(cid:28629)(cid:28577)(cid:28582)(cid:28582)(cid:28583)(cid:28585)(cid:28587)(cid:28581)(cid:28586)(cid:28572)(cid:28999)(cid:28573)(cid:28581)(cid:28585)(cid:28584)(cid:28578)(cid:28582)(cid:28433)(cid:29001)(cid:28434) (cid:28614)(cid:28629)(cid:28577)(cid:28582)(cid:28582)(cid:28583)(cid:28585)(cid:28587)(cid:28581)(cid:28586)(cid:28572)(cid:28999)(cid:28573)(cid:28581)(cid:28584)(cid:28584)(cid:28578)(cid:28582)(cid:28433)(cid:29001)(cid:28434) (cid:28614)(cid:28629)(cid:28577)(cid:28582)(cid:28582)(cid:28583)(cid:28585)(cid:28586)(cid:28580)(cid:28587)(cid:28572)(cid:28999)(cid:28573)(cid:28582)(cid:28586)(cid:28589)(cid:28578)(cid:28585)(cid:28433)(cid:29001)(cid:28434) (cid:28614)(cid:28642)(cid:28577)(cid:28582)(cid:28581)(cid:28589)(cid:28586)(cid:28584)(cid:28582)(cid:28585)(cid:28572)(cid:28999)(cid:28573)(cid:28584)(cid:28580)(cid:28581)(cid:28578)(cid:28588)(cid:28433)(cid:29001)(cid:28434) (cid:28598)(cid:28637)(cid:28577)(cid:28582)(cid:28581)(cid:28581)(cid:28586)(cid:28582)(cid:28587)(cid:28588)(cid:28572)(cid:28999)(cid:28573)(cid:28583)(cid:28585)(cid:28581)(cid:28578)(cid:28581)(cid:28433)(cid:29001)(cid:28434) (cid:28616)(cid:28636)(cid:28577)(cid:28582)(cid:28582)(cid:28587)(cid:28585)(cid:28587)(cid:28581)(cid:28583)(cid:28572)(cid:28999)(cid:28573)(cid:28583)(cid:28582)(cid:28589)(cid:28433)(cid:29001)(cid:28434) (cid:28614)(cid:28642)(cid:28577)(cid:28582)(cid:28581)(cid:28589)(cid:28586)(cid:28585)(cid:28585)(cid:28583)(cid:28572)(cid:28999)(cid:28573)(cid:28582)(cid:28587)(cid:28581)(cid:28578)(cid:28582)(cid:28433)(cid:29001)(cid:28434) (cid:28616)(cid:28636)(cid:28577)(cid:28582)(cid:28582)(cid:28587)(cid:28585)(cid:28587)(cid:28585)(cid:28587)(cid:28572)(cid:28999)(cid:28573)(cid:28582)(cid:28585)(cid:28586)(cid:28578)(cid:28583)(cid:28433)(cid:29001)(cid:28434) (cid:28616)(cid:28636)(cid:28577)(cid:28582)(cid:28582)(cid:28587)(cid:28585)(cid:28587)(cid:28585)(cid:28587)(cid:28572)(cid:28999)(cid:28573)(cid:28582)(cid:28583)(cid:28586)(cid:28578)(cid:28580)(cid:28433)(cid:29001)(cid:28434) (cid:28614)(cid:28629)(cid:28577)(cid:28582)(cid:28582)(cid:28583)(cid:28570)(cid:28570)(cid:28616)(cid:28636)(cid:28577)(cid:28582)(cid:28582)(cid:28587)(cid:28620)(cid:28577)(cid:28646)(cid:28629)(cid:28653)(cid:28647) (cid:28608)(cid:28629)(cid:28598)(cid:28646) (cid:28583) (cid:28601)(cid:28642)(cid:28633)(cid:28646)(cid:28635)(cid:28653)(cid:28564)(cid:28579)(cid:28564)(cid:28639)(cid:28633)(cid:28618)(cid:28599)(cid:28640)(cid:28643)(cid:28650)(cid:28633)(cid:28646) (cid:28601)(cid:28642)(cid:28633)(cid:28646)(cid:28635)(cid:28653)(cid:28564)(cid:28579)(cid:28564)(cid:28639)(cid:28633)(cid:28618) Fig. 5. (Color online) Matrix of the LaBr (Ce) vsClover. Listed are also the events of α − γ cor-relations and the identified radioactive isotopes.Both detectors are calibrated with characteristic γ -rays. The events between 1.5 and 2.4 MeV ofLaBr α particlesof the Ac decay chain. See text for details.
Part of the coincidence events in the Clover andLaBr (Ce) detectors is displayed in Fig. 5, while the rel-evant projection to the Clover in Fig. 6. The horizontalbands in Fig. 5 are traced back to the α - γ cascades. Thecorrelated γ ray energies and their relative intensities arekey to identify the α emitters.The α emitters are identified to be Th,
Ra,
Rn, and
Bi. This is consistent to previous inves-tigations on the LaBr (Ce) with a size φ ′′ × ′′ [9] andLaCl (Ce) with a size φ × α par-ticles in Fig. 5 can be classed into 2 groups by sorting3he energies, i.e., at 5.5-5.7 MeV (5.540, 5.434, 5.607 and5.716 MeV α from Ra, 5.713 MeV α from Th), and6.2-6.4 MeV (6.278 MeV α from Bi, and 6.425 MeV α from Rn). The third peak in the single spectrumof LaBr (Ce) in Fig. 4(a) is the 7.386 MeV α line fromthe ground state of Po to the ground state of
Pb,in which there is no cascade γ -ray.
150 200 250 300 350 400 450 500050100150200250300350 . k e V k e V Rn Bi Th . k e V . k e V . k e V k e V k e V k e V . k e V k e V . k e V . k e V . k e V C oun t s Energy/keV . k e V Ra Fig. 6. Projected γ -ray energy spectrum of Fig. 5in the Clover. Labeled are the identified nuclei. In this section, we will deduce the activity of variousradioactive contaminations embedded in the LaBr (Ce).Since the radioactive contaminators are evenly dis-tributed in the crystal, it is practically impossible tomake a direct determination because of a lack of an ac-curate efficiency calibration to both detectors.Instead, in this work we develop a Monte Carlomodel based on the GEANT4 version 10.4 [28–30] tookit.The setup includes the Clover, LaBr (Ce) detectors aswell as its bracket, and all components of shieldingsystem as shown in Fig. 3. La isotopes are evenlydistributed in the φ ′′ × ′′ cylindrical LaBr crystal.The density of LaBr (Ce) crystal is set to be 5.08g/cm [1]. We employed the physics constructor classof G4EmStandardPhysics [30]. It includes various pro-cesses like the deposition of β and γ rays in the sensi-tive volumes of the detectors, the occurrence of Comp-ton scattering in the shield, and the characteristic X-rays induced from the shield material. The shieldingmaterials are summarized in Table 1. The activities of La and
Ac decay chain contaminators are deter-mined by reproducing the experimental spectra in boththe LaBr (Ce) and Clover detectors. (Ce)
200 400 600 800 1000 1200 1400110100100010000 C oun t s Energy / keV exp_LaBr Geant4
Fig. 7. (Color online)Comparison of experimental(black line) and simulated (red line) self-countingenergy spectrum in the LaBr detector. The sam-pling of simulation has been scaled to the experi-mental data collected in 3200 seconds. In our experiment, we set up the time stamp for eachevent in our DAQ system. This allows us to comparedirectly the simulation with the experimental data thatare acquired in same measurement time, i.e., 3200 s. Thebest fit to the experiment spectrum is found using theleast square method when the activity of
La 482(19)Bq, corresponding to 1.386 (55) Bq/cm . This corre-sponds to 1,543,500 decays of La in 3200 seconds intotal. The uncertainty quoted here is due to the experi-mental statistics, detection efficiency and branching ratioof the γ -rays. The experimental and simulated spectraare plotted in Fig. 7. Nice agreement is seen except thelow energy part up to about 150 keV.It should be noted that in the above simulation we didnot consider the contribution from Ac and its daugh-ter nuclei. This may partially account for the differencebetween the simulation and experiment spectra. Anotherpossible reason for the low energy deficiency in the simu-lation could be related to the insufficient understandingof β decay of La. This has been discussed in Ref. [24–27].
An alternative way to deduce the
La activity is touse the coincidence γ rays information at 788.7 keV and1435.8 keV in the Clover. This would require an accu-rate efficiency calibration to the Clover detector usinga volume source of the same volume as the LaBr (Ce)detector. This is practically impossible.In reality, we made a two-step optimization for thecalibration [31]. In step one, we follow the standard rou-tine for an efficiency calibration using standard radioac-tive point sources, Cs,
Am, Mn, Y, Cd, Zn4 able 1. Details of shielding materials of the LBS. Listed are the material layer, its inner radius, outer radius, andmaterials defined in GEANT4.
Layer Inner radius/cm Outer radius/cm MaterialIron 30 32 G4 FeLead 21.65 30 G4 PbCopper 21.45 21.65 G4 CuPlexiglass 20.95 21.45 G4 PLEXIGLASSand
Eu. These sources are selected to avoid possi-ble true summing effect. The point source was placed3 cm from the front surface of the Clover detector. Weused the EFFIT program in the Radware package [32]to describe the efficiency curve at the low-energy andhigh-energy regions separately.
200 400 600 800 1000 1200 1400 1600
200 400 600 800 1000 1200 1400 Eu C oun t s exp. sim. ab Energy / keV
511 keV
La 788.7keV L a V k V Fig. 8. (Color online) Comparison of Clover spec-trum (black line) with the simulation (red line)for the
Eu source (a) and the LaBr (Ce) crys-tal (b). The sampling of simulation for LaBr (Ce)has been scaled to the experimental data collectedin 3200 seconds Energy / keV E ff i c i en cy ( % ) Exp. data wiht point sourceFitted curve of point sourceSimmulated curve of point sourceSimmulated curve of volume source
Fig. 9. (Color online) The γ -ray detection efficien-cies of Clover. The red dash line is the fittingcurve to the experimental data (solid triangle) ofstandard point source, while the black dash lineis the simulation efficiency under same detectionconfiguration. The black solid line represents the detection efficiency when the source evenly dis-tributed in the LaBr (Ce) crystal. In this step, we optimized the thickness of thedead layer encapsulating the crystal by the least squaremethod to reproduce best the Clover spectrum of
Eu.The best simulation results of
Eu spectrum togetherwith the experimental data are presented in Fig. 8(a).The simulated detection efficiency curve is comparedwith the experimental results using standard pointsource, as shown in Fig. 9.In step two, we modeled the detection efficiency of theClover with sources evenly distributed in the LaBr (Ce)crystal instead of point sources. The simulated efficiencycurve is shown in Fig. 9. The efficiency of 788.7 keV and1435.8 keV γ rays are determined to be 0.00347(11) and0.00257(10), respectively. In the simulation, the interac-tion of characteristic γ rays with both the LaBr (Ce) andClover detectors and even the shielding material havebeen taken into account. As a result, we found that theClover spectrum collected in 3200s is best reproducedwhen the total count of the 788.7 keV γ ray is 1932 (46).The simulation together with the experimental data isshown in Fig. 8(b). The activity of La is thus ex-tracted to be 505 (20) Bq, corresponding to 1.451 (58)Bq/cm . The uncertainty takes into account the contri-butions from statistic, detection efficiency and branchingratio of the γ rays in the decay of La. In the sameway, the activity of
La has been also deduced fromthe 1435.8 keV γ -ray, to be 500 (22) Bq, correspondingto 1.437 (63) Bq/cm . Both are consistent with that de-termined from the self-counting of LaBr (Ce) detector,as shown in Table 2.The above procedure can also be applied to evaluatethe activities of the Ac decay chain contaminators. InFig. 6, we identified the characteristic γ rays at 351.1keV ( Bi), 271.2 keV and 401.8 keV (
Rn), 269.5 keVand 445.0 keV (
Ra), 236.0 keV (
Th). The deducedactivities for
Rn,
Ra,
Bi and
Th are summa-rized in Table 2. The sum activity of α contaminatorsis 0.0545(35) Bq/cm , which is a factor of 25 smallerthan that of La. More specifically, this resulted in acounting rate (including environmental background) of237 counts/s for the γ -rays energy between 20 and 500keV, 181 counts/s between 500 and 1.5 MeV, 27 counts/s5 able 2. Activities of Bi,
Rn,
Ra.
Th and
La. Listed are the isotopes, the most distinct γ rays, thededuced activities. Results from Ref. [17, 20] are shown whenever available. Isotope γ -energy (keV) Absolute efficiency(%) Activity (Bq/cm ) Reference value (Bq/cm ) Bi 351.1 0.4347 0.0136(15) 0.032 a Rn 271.2 0.4117 0.0134(17) /
Rn 401.8 0.4317 0.0116(16) 0.032 a Ra 269.5 0.4130 0.0126(13) 0.025 a Ra 445.0 0.4283 0.0127(15) /
Th 236.0 0.3840 0.0158(22) 0.037 a Sum activity of α contaminator 0.0545(35) 0.126 a , 0.0443 b La 788.7 0.3467 1.451(58) /
La 1435.8 0.2567 1.437(63) /
La self-counting method 1.386(55) /Avergage activity of
La 1.425(59) 1.488 b Total activity 1.480(69) 1.532 ba Date from Ref. [17] b Date from Ref. [20] above 1.5 MeV in our LaBr (Ce) detector. The self-irradiation affects its application to low production ex-periments, in particular for potential cases with a countrate of less than 450 counts/s.A pioneering study [17] of the LaCl (Ce) detectorwith a crystal size of φ
25 mm ×
25 mm shows a contam-ination level of 1.3 × −
13 227
Ac decay chain atoms/Laatom. The identified total α activity ( Th ,
Bi,
Rnand
Ra) is 0.126 Bq/cm and each contribution is sum-marized in Table 2. A recent study [20] further investi-gated the internal radiation of LaBr (Ce) detectors withvarious sizes of crystals. Average activities of La and
Ac decay chain are determined to be 1.488 and 0.044Bq/cm , respectively. In the present study of the B380type, the Ac decay chain atoms/La atom amounts tobe 5.8 × − . This is almost two orders of magnitudesmaller than that in Ref. [17]. The activities of Acdecay chain contaminators are of typically 40% of that in Ref. [17] and are similar to that in Ref. [20]. This in-dicates that the actinium impurity has been significantlyreduced in the last decade.
In this work, we performanced a coincidence measure-ment using a Clover detector, and identified the internalradioactive nuclei of the B380 LaBr (Ce) detector. Wedetermined the activity of La,
Bi,
Rn,
Ra and
Th by combining the coincidence spectra with Geant4simulations.The sum activities of
La and
Ac decay chain arearound 1.425 (59) and 0.0545 (35) Bq/cm , respectively.These data are useful in designing detector setups basedon the LaBr (Ce), in particular for the purpose of lowcount rate experiments. References , 359-363 (2006)DOI:10.1016/j.nima.2006.02.1923 J.H. Liu, D.W. Wu et all, Using a LaBr :Ce scintillator forpositron annihilation lifetime spectroscopy. Chin. Phys. C ,380-383 (2012) DOI:10.1088/1674-1137/36/4/0164 Y.Y. Ji, H.Y. Choi, W. Lee et al. Application of aLaBr (Ce) Scintillation Detector to an Environmental Radi-ation Monitor. IEEE Trans. Nucl. Sci. , 2021-2028 (2018)DOI:10.1109/TNS.2018.2823322 5 I. Mouhti, A. Elanique, M.Y. Messous et all, Validationof a NaI(Tl) and LaBr (Ce) detector’s models via mea-surements and Monte Carlo simulations. Journal of Radia-tion Research and Applied Sciences. , 1687-8507 (2018)DOI:10.1016/j.jrras.2018.06.0036 C. Cheng, W.B. Jia et al, Determination of thickness of wax de-position in oil pipelines using gamma-ray transmission method.Nuclear ence and Techniques. Nucl. Sci. Technol. , 16-20(2018) DOI:10.1007/s41365-018-0447-47 E.V.D van Loef, P. Dorenbos et all, High-energy-resolutionscintillator: Ce activated LaCl . Appl. Phys. Lett. , 1467(2000) DOI:10.1063/1.13080538 K.S. Shah, J. Glodo, M. Klugerman et all, LaBr :Ce scintilla-tors for gamma-ray spectroscopy. IEEE Trans. Nucl. Sci. ,2410-2413 (2003) DOI:10.1109/TNS.2003.820614 R. Nicolini, F. Camera, N. Blasi, S. Brambilla, R. Bassini,C. Boiano, A. Bracco, F.C.L. Crespi, O. Wieland, G. Ben-zoni, Investigation of the properties of a LaBr :Ce scin-tillator. Nucl. Instrum. Methods A , 554-561 (2007)DOI:10.1016/j.nima.2007.08.22110 F. Quarati, A.J.J. Bos, S. Brandenburg, C. Dathy et all, X-ray and gamma-ray response of a 2 ′′ × ′′ LaBr :Ce scintilla-tion detector. Nucl. Instrum. Methods A , 115-120 (2007)DOI:10.1016/j.nima.2007.01.16111 A. Lavagno, G. Gervino, A. Scarfone, Study of linearityand internal background for LaBr3:Ce gamma ray scintilla-tion detector. Nucl. Instrum. Methods A , 504-505 (2013)DOI:10.1016/j.nima.2012.11.02412 M.S. Alekhin, J.T.M. de Haas, I.V. Khodyuk et al, Improve-ment of gamma-ray energy resolution of LaBr :Ce scintilla-tion detectors by Sr and Ca co-doping. Appl. Phys. Lett. , 161915 (2013) DOI:10.1063/1.480344013 R. Shi, T.X. Guo, H.L. Lu, Y.Y Xu, F.R. Shi, J.B. Yang,Unfolding analysis of LaBr :Ce gamma spectrum with a de-tector response matrix constructing algorithm based on en-ergy resolution calibration. Nucl. Sci. Technol. : Ce fast-timing arrayfor DESPEC at FAIR. Nucl. Instrum. Methods A , 91-95(2014) DOI:10.1016/j.nima.2014.02.03716 B. Longfellow, P.C. Bender, J. Belarge et al, Commissioning ofthe LaBr (Ce) detector array at the National SuperconductingCyclotron Laboratory. Nucl. Instrum. Methods A , 141-147(2018) DOI:10.1016/j.nima.2018.10.21517 B.D. Milbrath, R.C. Runkle, T.W. Hossbach et al, Characteri-zation of alpha contamination in lanthanum trichloride scintil-lators using coincidence measurements. Nucl. Instrum. Meth-ods A , 504-510 (2005) DOI:10.1016/j.nima.2004.11.05418 B.D. Milbrath, J.I. Mcintyre, R.C. Runkle, L.E. Smith, Con-tamination Studies of LaBr (Ce) Scintillators. IEEE Trans.Nucl. Sci. , 3031-3034 (2006) DOI:10.1109/TNS.2006.88106419 R. Rosson, J. Lahr, B. Kahn, Radation background in aLaBr γ -ray scintillation detector. Health Physics. , 703- 708 (2011) DOI:10.1097/HP.0b013e318221117220 F.G.A Quarati, I.V. Khodyuk, C.W.E Eijk, P.Quarati,Study of La radioactive decays using LaBr scin-tillators. Nucl. Instrum. Methods A , 46-52 (2012)DOI:10.1016/j.nima.2012.04.06621 A. Camp, A. Vargas et all, Determination of LaBr (Ce)internal background using a HPGe detector and MonteCarlo simulations. Appl Radiat Isot. (Ce) scintillator. In-struments and Experimental Techniques. , 151-155 (2013)DOI:10.1134/S002044121301031424 Giaz, Agnese and Gosta et all, Measurement of β − decay con-tinuum spectrum of La. Europhysics Letters. , 42002(2015) DOI:10.1209/0295-5075/110/4200225 F.G.A. Quarati, P. Dorenbos, X. Mougeot, Experiments andtheory of
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