EEur. Phys. J. C manuscript No. (will be inserted by the editor)
Understanding NaI(Tl) crystal background for dark mattersearches
G. Adhikari , P. Adhikari , C. Ha , E.J. Jeon a,1 , N.Y. Kim , Y.D. Kim ,S.Y. Kong , H.S. Lee , S.Y. Oh , J.S. Park , K.S. Park Center for Underground Physics, Institute for Basic Science (IBS), Daejeon 34047, Republic of Korea Department of Physics and Astronomy, Sejong University, Seoul 05006, KoreaReceived: date / Accepted: date
Abstract
We have developed ultra-low-background NaI-(Tl) crystals to reproduce the DAMA results with theultimate goal of achieving purity levels that are com-parable to or better than those of the DAMA/LIBRAcrystals. Even though the achieved background leveldoes not approach that of DAMA/LIBRA, it is cru-cial to have a quantitative understanding of the back-grounds. We have studied background simulations to-ward a deeper understanding of the backgrounds anddeveloped background models for a 9.16-kg NaI(Tl) crys-tal used in the test arrangement. In this paper we de-scribe the contributions of background sources quan-titatively by performing Geant4 Monte Carlo simula-tions that are fitted to the measured data to quan-tify the unknown fractions of the background composi-tions. In the fitted results, the overall simulated back-ground spectrum well describes the measured data witha 9.16-kg NaI(Tl) crystal and shows that the back-ground sources are dominated by surface
Pb andinternal K in the 2 to 6-keV energy interval, whichproduce 2.4 counts/day/keV/kg (dru) and 0.5 dru, re-spectively.
Keywords
Geant4 · simulations · backgrounds · NaI(Tl) · dark matter Numerous astronomical observations have led to theconclusion that the majority of the matter in our uni-verse is invisible, exotic, and nonrelativistic dark matter[1,2]. However, it is still unknown what the dark matteris. Weakly interacting massive particles (WIMPs) are a Corresponding author: [email protected] one of the most attractive dark matter particle candi-dates [3,4]. The lightest supersymmetric particle (LSP)hypothesized in theories beyond the standard modelof particle physics is a suitable candidate for a darkmatter WIMP. There have been numerous experimentsthat directly search for WIMPs in our galaxy by lookingfor nuclear recoils that are produced by WIMP–nucleusscattering [5,6].To date, no other experiments, except for the DAMAexperiment [7,8,9], have found an annual modulationsignal interpreted as WIMP interactions with a signifi-cance of 9.2 σ . However, this finding has spurred a con-tinuing debate since the WIMP–nucleon cross sectionsinferred from the DAMA modulation are in conflictwith limits from other experiments that directly mea-sure the nuclear recoil signals, such as XENON100 [10],LUX [11], and SuperCDMS [12].The Korea Invisible Mass Search (KIMS) is an ex-periment that aims at searching for dark matter atan underground laboratory located in Yangyang, SouthKorea (Y2L). We are performing a high-sensitivity searchfor WIMP interactions in an array of NaI(Tl) crystalsin an attempt to reproduce the DAMA/LIBRA’s ob-servation of an annual modulation signal [24,25].There are several groups, such as DM-Ice [13,14],ANAIS [16,18], and SABRE [19], developing ultra-low-background NaI(Tl) crystals with the goal of reproduc-ing the DAMA/LIBRA results and currently, KIMSand DM-Ice have agreed to operate a single experi-ment, COSINE, at Y2L using NaI(Tl) crystals and atotal mass of 106 kg is being used in the first-stageexperiment, COSINE-100. As part of this program wehave developed ultra-low-background NaI(Tl) crystalsand studied their properties in a variety of test se-tups with the ultimate goal of achieving purity lev-els that are comparable to or better than those of the a r X i v : . [ a s t r o - ph . I M ] J un DAMA/LIBRA crystals. Even though current backgroundlevels achieved by the research and development arehigher than those of DAMA/LIBRA, it is crucial tohave a quantitative understanding of the backgrounds.For further understanding of the backgrounds, wehave performed Monte Carlo simulations based on Geant4and compared their results with measured data (seeSect. 3.2). To build concrete background models fora 9.16-kg NaI(Tl) crystal used in one of test setups,we studied background simulations of internal radioac-tive contaminants, such as natural radioisotopes insideNaI(Tl), cosmogenic radionuclei, and surface contami-nations in NaI(Tl) crystal (see Sect. 3.2.2 and 3.2.3),and external background sources from the exterior ofcrystals (see Sect. 3.2.1). We quantified their contribu-tions by treating them as floating and/or constrainedparameters in the data fitting (see Sect. 3.2.4). In addi-tion, our evaluation of background prospects, based onthis study, is described in section 4.
Pb contamination in the powderand crystal growing process. As listed in Table 1 [25],the total α rate of NaI-005 was reduced by more thana factor of three compared to NaI-002.In this paper, thus, we focused on the backgroundmodel for the NaI-005 crystal and studied Geant4-basedsimulations of the background sources internal/externalto NaI-005 in this test arrangement. Internal backgrounds of the two crystals, NaI-001 and NaI-002, were simu-lated for coincidence data of NaI-005 and backgroundsfrom the PMTs were also simulated for external back-grounds of NaI-005.2.2 Background measurements in the NaI(Tl) crystaltest setupNaI-002 and NaI-005 crystals have an identical cylin-drical shape with a diameter of 4.2 inches, a length of11 inches, and a mass of 9.16 kg. The light yield andthe measured background rates from internal radioac-tive contaminants in the NaI(Tl) crystals are listed inTable 1. The details of the three crystals are discussedin reference [25].Three different types of PMTs were used in thetest arrangement: a metal-packed R11065SEL, a glass-packed R12669SEL, both manufactured by HamamatsuPhotonics, and 9269QA of Electron Tubes, Ltd..R12669SEL PMTs were coupled with NaI-002 and NaI-005; R11065SEL and 9269QA were coupled with NaI-001 and CsI(Tl) crystals. The radioactivity levels of thePMTs were measured underground with a high-purityGe (HPGe) detector and their measurements are listedin Table 2 [24].We used the measured activities inside the crystalsand from the PMTs for the simulation study of the NaI-005 backgrounds. In addition to natural radioisotopesmeasured, there are also backgrounds from cosmic ex-citation that are continuously decreasing as a functionof time. We considered I, Te, and Na isotopes fromcosmic radiation as background sources and they areincluded in the simulations and their contribution de-scribed later.
U and
Th were simulated,assuming that decay chains of
U and
Th are eachin equilibrium, thus all related activities within thechains are simply equal to the
U and
Th activ-ities multiplied by the branching ratios for decay of the (a) Detector shielding (b) Configuration of the test arrangement
Fig. 1
Schematic view of detector shielding (a): muon detector (MD), polyethylene (PE), and copper shield (CuShield).Configuration for three NaI(Tl) crystals with the CsI(Tl) crystal array (b).
Table 1
Light yield and measured background rates from internal radioactive contaminants in the NaI(Tl) crystal [25].Crystal nat K( K) U Th α Rate Light yield(unit) (ppb) (ppt) (ppt) (mBq/kg) (PE/keV)NaI-001 40 . ± . < . < .
19 3 . ± .
01 15 . ± . . ± . < .
12 0 . ± . . ± .
01 15 . ± . . ± . < .
04 0 . ± .
01 0 . ± .
01 12 . ± . Table 2
Specifications for PMTs used in this study [24]. (a) The radioactivities were measured with a HPGe detector at Y2L.(b) SEL means “selected for high quantum efficiency”.PMT R12669SEL b R11065SEL b a U( Bi ) 25 ± ± ± Ac) 12 ± ± ± K) 58 ± ± ± daughter isotopes. However, it needs to specify all long-lived parts of U and
Th daughters to consider bro-ken decay chains when they are fitted to the measureddata to quantify their unknown background fractionsand, thus, we grouped daughter isotopes from the fulldecay chains of
U and
Th according to their half-lives. We used five groups for
U and three groups for
Th, as listed in Table 3, in the data fitting. K istreated as its own, additional group.Each simulated event includes all energy depositedin the crystals within an event window of 10 µ s from thetime a decay is generated, to account for the conditionsin the data acquisition system (DAQ) of the experimen-tal setup [24]. Sometimes decays with relatively shorthalf-life such as Po decay (with a half-life of 300 ns) and the following decays will appear in the same event,called pileup events, and they were treated within oneevent in simulated event.The simulated spectrum was smeared by the en-ergy resolution, as a function of energy, that was mea-sured during the calibration. Calibration points weremeasured using γ –ray sources, Am and Co; theycontribute peaks at 59.5 keV(
Am), 1170 keV, and1332 keV. Measured background spectra from severalradioactivities were used for additional calibrations; thereare peaks at 3.2 keV and 1460 keV from K, 30.8 keVfrom
I, 67.2 keV from
I, and 609.3 keV and 1120.3keV from
Bi, respectively. We also used the peak at0.8 keV from Na.
Table 3
Isotopes grouped by half-life: five groups for
U (a) and three groups for
Th (b).(a)Group Decay chain Half-life U Th 4.47 × yr1 Th Pa 24.1 days Pa U 6.70 h2 U Th 2.46 × yr3 Th Ra 7.54 × yr Ra Rn 1.60 × yr Rn Po 3.82 days4 Po Pb 3.10 min Pb Bi 26.8 min Bi Po 19.9 min Po Pb 1.64 × − s Pb Bi 22.2 yr5 Bi Po 5.01 days Po Pb 138 days (b)Group Decay chain Half-life6 Th Ra 1.40 × yr7 Ra Ac 5.75 yr Ac Th 6.15 hr Th Ra 1.91 yr Ra Rn 3.63 days Rn Po 55.6 s Po Pb 0.145 s8 Pb Bi 10.6 hr Bi Po 60.6 min Po Pb 2.99 × − s Bi Tl 60.6 min Tl Pb 3.05 min
Energy (keV)1 10 C oun t s / da y / k e V / k g - - -
10 110 data total MCK NaI-005, Pb bulk
NaI-005, U238
PMTs, Th232
PMTs, K PMTs,
Fig. 2
Comparison of measured background spectra to spec-tra generated by Monte Carlo simulation for single-hit eventsof NaI-005
To build the concrete background model for NaI-005it needs to compare the measured background level inNaI-005 with the simulated background spectra thatare each composed of different background composi-tions. Initially, we studied background simulations ofnatural radioisotopes in NaI(Tl) crystals and PMTsand compared the total of the simulations with thedata. In this comparison, we found there remains sig-nificant backgrounds to be modeled and, thus, we con-sidered additional background components to improve the model. The details of the simulation studies are de-scribed in subsections.For the initial comparison, we simulated daughterisotopes from full decay chains of U, Th, and Klocated inside NaI(Tl) crystals and 26 PMTs assuminga chain equilibrium. By using the measured activities ofradioactive sources listed in Tables 1 and 2, event rateswere normalized to the units of counts/day/keV/kg (dru).Fig. 2 shows the normalized background energy distri-butions in NaI-005 for single-hit events in which thereis an energy deposit in NaI-005 only. The total of thesimulations (solid red line) is compared with the mea-surement (open black circles). In the simulations, thebackground sources for energies below 10 keV are dom-inated by K (solid blue line) and
Pb (solid cyanline) internal to the NaI(Tl) crystal, and for high ener-gies above ∼
100 keV external backgrounds from PMTsare dominant. The details are itemized in the following.However, as shown in Fig. 2, the overall measuredbackground level is higher than that of backgroundsimulations and, thus, there are significant remainingbackgrounds. For the contributions of the backgroundsources not included in Fig. 2, we studied backgroundsimulations of the external sources, cosmogenic radionu-clei, and surface contaminations of the NaI(Tl) crystal.To quantify the contributions of all background sourcesin the simulation they are fitted to the measured databy floating and/or constrained the unknown fractionsof the background spectra as fit parameters. In thefit, daughter isotopes from full decay chains of U, Th, and K are grouped into 9 background spectra:5 background spectra for
U, 3 background spectrafor
Th, and a spectrum for K. The fitted resultsfor the contributions of those background sources to the total background level are described in sections 3.2.1,3.2.2, and 3.2.3, quantitatively. All the fit results of ac-tivities of background sources and background eventsin the 2 to 6 keV energy region are listed in Tables 4and 5 in section 3.2.4. • Internal backgrounds of NaI(Tl) crystalsTo normalize the backgrounds from internal radioac-tive contaminants, we assumed a chain equilibrium.Therefore, all related activities within the chains areequal to U, Th, and K activities, in Table 1,multiplied by the branching ratios for decay of thedaughter isotopes. We also added the backgroundsimulation of internal
Pb by considering the mea-sured α rate. The resultant background contribu-tions, except for those from K and
Pb, werenegligible ( < − dru). • External backgrounds from PMTsWe used 26 3-inch PMTs of three different PMTtypes in the test arrangement. We thus consideredmeasured activities in terms of different PMT types,as listed in Table 2. The normalized backgroundcontributions from the PMTs are represented in threeradioactive sources :
U (solid magenta line),
Th(solid khaki line), and K (dotted blue line).
The majority of the external γ background comes fromthe radioactive decay of isotopes in the surroundingrocks. To block such an environmental background weinstalled several shielding layers, as described in sec-tion 2.1, and measured the background reduction withan ultra-low background 100% HPGe detector at Y2L.By using the full shielding structure and N gas flowinginto the inside of the copper shield to avoid backgroundsfrom Rn in the air at Y2L, we could reduce the envi-ronmental background by a factor of 10,000 (measuredto be 1 . ± .
49 pCi/L [23]), thus ensuring that thosecontributions would be negligible.However, there exist some background from radioac-tive sources in detector components inside the shield-ing. Typically, PMTs and materials—such as the cop-per shield, the copper structure to which the PMT isaffixed to the crystal, the PMT base, connectors, andbunches of cables—would contain radioactive sourcesthat can contribute to the background as an exter-nal background source inside the shielding. For PMTs,even though we measured the radioactivity levels, back-ground contributions will be different for the main com-ponents in the PMTs, such as the PMT window, body,and stem, where radioactive sources were generated. Although it is difficult to reproduce accurately back-ground contributions from external sources without know-ing well about the external background contamination,it is possible to estimate the effects of such radioactivesources, external to the NaI(Tl) crystal, by consideringthem as parameters in the fit. To take it into accountwe simulated the background spectra in such a way thatradioisotopes contained in U, Th, and K weregenerated randomly in the whole PMT body includingthe PMT window and stem, and we grouped the re-sults into nine background spectra, instead of perform-ing each of the simulations of all the external sourcesbesides PMTs. It is because that the background en-ergy distribution in NaI-005 is very similar to that fromPMTs when we simulated external background sourcesfrom the inside space of the copper shield. We also sep-arately considered background contributions from thetwo PMTs attached to NaI-005, apart from the otherPMTs attached to NaI-001, NaI-002, and CsI(Tl) crys-tals, because the external background contributions toNaI-005 will be different for the distance from the ex-ternal sources to the crystal. Consequently, we have twosets of nine background spectra for testing the effectsof the external background sources. They were fittedto the measured data, as floating and/or constrainedparameters, to estimate their fractions contributed tothe total background and, in the fit, we used single-hit events and multiple-hit events, the latter of whichhave energy deposits in two or more crystals, simulta-neously. We estimated the external background contri-butions by assuming that the fitted results for PMTssimulations include the backgrounds from all the exter-nal sources. As shown in Fig. 3, the fitted results wellreproduce the measured data for both single-hit eventsand multiple-hit events for energies above ∼
100 keV.The black circles represent the data and the solid redline represents the sum of simulated background spec-tra with the fitted fractions of groups 1 (dotted ma-genta line), 4 (solid magenta line), 7 (solid khaki line),and 8 (dotted khaki line) and K (dotted blue line);the biggest contribution is from
Ra (group 4). Thoseexternal backgrounds are expected to be vetoed by anactive veto detector in the new detector design.
We installed NaI-005 in the test setup in December2014, soon after that crystal was delivered to Y2L, andwe recorded data for about a month. Therefore, somebackgrounds are expected from cosmogenic activations.We checked backgrounds over the specified time intervaland observed a clear reduction of the peak at 30.8 keVfrom
I within the first 10 days. We thus did not use
Energy (keV)1 10 C oun t s / da y / k e V / k g - - -
10 110 data sum of MC externalU (group 1)
PMTs, Ra (group 7)
PMTs, Ra (group 4)
PMTs, Th (group 8)
PMTs, K PMTs,
Energy (keV)1 10 C oun t s / da y / k e V / k g - - -
10 110 data sum of MC externalU (group 1)
PMTs, Ra (group 7)
PMTs, Ra (group 4)
PMTs, Th (group 8)
PMTs, K PMTs, (a) Single-hit events (b) Multiple-hit events
Fig. 3
Simulated background spectra of U, Th, and K from external sources inside the copper shield.
Energy (keV)1 10 C oun t s / da y / k e V / k g - - -
10 110 data I NaI-005, Te
NaI-005, Te
NaI-005, Te
NaI-005, Na NaI-005,
Fig. 4
Cosmogenic backgrounds for the NaI(Tl) crystal withthe CsI(Tl) crystal array. the first 16 days data to exclude the background from
I in this study.We simulated backgrounds from cos-mogenic isotopes, which are expected to be producedby cosmic ray exposure [15]. The simulated backgroundspectra are used in the data fitting, by floating theirunknown fractions, and the fitted results are shown inFig. 4. The dominant isotopes are itemized in detail inthe following. • I (dotted gray line)
I has a half-life of 59.43 days and it decays to
Te by electron capture from shell K and uppershells, emitting 35.5 and 67.2-keV γ rays and/or in-ternal conversion electrons, respectively, producingthe two big energy peaks shown by the dotted gray line in Fig. 4. • Te (solid gray line)The half-lives of
Te and
I are 19.17 and 12.93days, respectively, and a clear reduction of the peakfrom
I within the first 10 days was observed.However, there is still peak at 573 keV identifiedwith γ rays emitted by electron capture in the de-cay of Te that is produced as a result of the decayof the longer-lived cosmogenic isotope m Te (half-life = 164.2 days), which is reported in reference toANAIS [17]. • m Te (dotted brown line) and m Te (solid brownline) m Te and m Te are long-lived metastable statesand their half-lives are 57.4 and 119.2 days, respec-tively. They contribute peaks at 145 and 248 keV. • Na (solid cyan line) Na can be produced through the ( n, n ) reactionon Na by energetic cosmic neutrons at sea level.It decays via positron emission (90%) and electroncapture (10%), followed by 1270-keV γ -ray emissionwith a mean lifetime of 3.8 yr. The electron cap-ture decay produces ∼ ∼
10% of the Na decay will produce 0.8-keV X-raysand 1270-keV γ rays simultaneously. Meanwhile, thepositron will be converted to two 511-keV annihila-tion γ rays. If one of the two 511 and 1270 keV γ rays escapes from a crystal, the energy deposited inthe crystal will be 650–1000 keV. We looked for acoincident event that deposits 1270 keV of energyin NaI-002, resulting in a γ -ray hit in the 650 to . ± . Na in the fit.
Energy (keV)0 10 20 30 40 50 60 70 80 90 A r b i t a r y un i t m surface depth m m m m m Fig. 5
Comparison of background spectra simulated at var-ious surface depths.
Energy (keV)1 10 C oun t s / da y / k e V / k g - - -
10 110 data Pb surface
NaI-005, Pb bulk
NaI-005,
Fig. 6
Simulated background spectra from bulk
Pb (solidblue line) and a
Pb surface (solid green line).
The levels of
U and
Th contamination mea-sured in NaI-005 are too low to account for the total observed α -particle rate, which suggests that they aredue to decays of Po nuclei that originate from
Rncontamination that occurred sometime during the pow-der and/or crystal processing stages. This is confirmedby the observation of a 46-keV γ peak that is char-acteristic of Pb. There is also the possibility of sur-face contamination, which is expected to affect the low-energy spectrum in a different way from bulk
Pb.To clarify the effect of surface
Pb contaminationwe simulated the background spectra at various surfacedepths from 0.1 to 10 µ m by generating Pb randomlywithin the depth. Fig. 5 shows a comparison of back-ground spectra simulated in different surface depths.The low-energy background looks different from thatdue to bulk
Pb (cyan color), and the heights of the ∼ Pb.In the simulation, we added a background contribu-tion from surface
Pb generated randomly within asurface depth of 10 µ m. Background fractions of bulkand surface Pb are obtained in such a way that theyare treated as floating parameters in the fit to the mea-sured data. And they are found to be 0.05 ± ± ± α activity from decays of Pofrom crystals grown by AS-WSII powder (0 . ± . We simulated full decay chains of U, Th, and Kfrom NaI(Tl) crystals and 26 PMTs that are groupedinto three sets of nine background spectra and addi-tionally we simulated five cosmogenic isotopes and bulkand surface
Pb, including their progenies. Using allof the simulated background spectra, we fit the modelto the measured data for both single-hit events andmultiple-hit events of NaI-005, in the 2 to 1510-keVfitting range, to determine the unknown backgroundfractions; we used a maximum likelihood fit using Pois-son statistics [28]. In the fitting, nine of the internalbackground groups were constrained by the measuredactivity errors and the other groups for external back-grounds were treated as floating and/or constrained pa-rameters. All the cosmogenic background spectra andbulk/surface Pb210 spectra were considered as floatingparameters in the fit. The fitted results of activities ofbackground sources in NaI-005 and PMTs are listed inTable 4.Fig. 7 shows the fitted results for all the simulatedbackground spectra plotted as various lines with differ-
Energy (keV)1 10 C oun t s / da y / k e V / k g - - -
10 110 data total MC U (group 1) PMTs, K NaI-005, Pb surface
NaI-005, Ra (group 4)
PMTs, Na NaI-005, Pb bulk
NaI-005, Ra (group 7)
PMTs, Te
NaI-005, K PMTs, Th (group 8)
PMTs, I
NaI-005, Te
NaI-005, Te
NaI-005,
Energy (keV)1 10 C oun t s / da y / k e V / k g - - -
10 110 data total MC U (group 1) PMTs, K NaI-005, Pb surface
NaI-005, Ra (group 4)
PMTs, Na NaI-005, Pb bulk
NaI-005, Ra (group 7)
PMTs, Te
NaI-005, K PMTs, Th (group 8)
PMTs, (a) (b)
Fig. 7
Measured background spectrum (open black circles) fitted with all of simulated background spectra for single-hit events(a) and multiple-hit events (b). ent colors and styles for both single-hit events (a) andmultiple-hit events (b) in the 0.5 to 2000-keV energyregion. The overall energy spectrum summed over allsimulations (solid red line) is well matched to the data(open black circles) not only for single-hit events butalso for multiple-hit events. However, there are showndiscrepancies between measurements and simulationsfor high energies above ∼ ∼ Pb and K contaminations inside the crys-tal, the contributions of which are found to be 2.4 and0.5 dru, respectively. Remnants of cosmogenic activa-tion of Te (0.95 dru) still persist but will quickly bereduced with a lifetime of <
100 days. We also deter-mined the external backgrounds, which amounted to0.59 dru. In the low energy region below 0.5 keV thedata is suppressed by the requirement for low energynoise rejection and there is shown less number of eventsin the data.
Energy (keV)0 2 4 6 8 10 12 14 16 18 20 C oun t s / da y / k e V / k g data K NaI-005, Pb surface
NaI-005, Na NaI-005, Te
NaI-005, Ra (group 4)
PMTs, K PMTs, total MC
Fig. 8
Comparison of data and Monte Carlo simulation ofthe low-energy background spectra.
We have studied NaI(Tl) crystal backgrounds based onMonte Carlo simulation using the Geant4 toolkit. Thestudies show that all the simulated background spectra,normalized by measured activities and fitted fractions,describe the data well not only for single-hit events butalso for multiple-hit events. According to the compari-son between the Monte Carlo simulation and the datain the low-energy spectra, the background is found tobe dominated by
Pb, mainly surface
Pb, which isdue to
Rn exposure during crystal growing and/orhandling procedures, and K within the NaI(Tl) crys-tal, the background level of which is consistent with
Table 4
Summary of the fitted radioactive contaminants in NaI-005 (a) and PMTs (b).(a) Activities[mBq/kg]Background sources Group Isotopes Measured Fitted1 U < . × − (5 . ± . × − U (5 . ± . × − Th (5 . ± . × − Ra (5 . ± . × − Internal 5
Pb (4 . ± . × − Th (7 . ± . × − (8 . ± . × − Ra (7 . ± . × − Th (8 . ± . × − K 1 . ± .
13 1 . ± . Pb 0 . ± . Pb 0 . ± . I 5 . ± . Na 0 . ± . . ± . Te 2 . ± . m Te 0 . ± . m Te 0 . ± . U 25 ± . ± . U 22 . ± . Th 22 . ± . Ra 150 ± Pb 22 . ± . Th 12 ± . ± . Ra 18 ± Th 14 . ± . K 58 ± . ± . U 78 . ± . . ± . U 70 . ± . Th 70 . ± . Ra 156 . ± . Pb 82 . ± . Th 25 . ± . . ± . Ra 140 . ± . Th 140 . ± . K 504 ±
72 2772 ± Table 5
Simulated background events of NaI-005 in dru unit[/day/keV/kg] in the (2–6) keV energy interval.Single-hit eventsBackground sources [/day/keV/kg]Energy [keV] 2-6
Pb surface 2 . ± . K 0 . ± . . ± . . ± . . ± . . ± . ANAIS’s expectation evaluated by assuming the activ-ities of their characterized NaI(Tl) crystals [18].External background contributions are expected tobe tagged by an active veto detector with a liquid scin-tillator (LS) surrounding crystals in the new detectordesign. We already observed such a reduction, 0 . ± .
04 in the 6 to 20-keV energy region, with a prototypeactive veto system using an LS [26]. An LS veto de-tector can also reduce the contribution of the internal K background, by tagging 3-keV X-ray events withrequired conditions for the LS veto signal, by a factorof 2 with the optimized thickness of the LS in the newdetector design [26]. In addition, we are studying the suppression of the Pb crystal-surface background toachieve a background level as low as bulk
Pb, whichcontributes 0.04 dru in the 2 to 6-keV energy region.Moreover, improving the purity of NaI(Tl) crystals withsmall concentrations of
Pb, ∼ µ Bq/kg, is possible,as reported in reference to KamLAND-PICO [20].As a result, we are expecting that we can reach abackground level of < . Acknowledgments
This research was funded by the Institute for Basic Sci-ence (Korea) under project code IBS–R016–A1.
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