Development of highly radiopure NaI(Tl) scintillator for PICOLON dark matter search project
K. Fushimi, Y. Kanemitsu, S. Hirata, D. Chernyak, R. Hazama, H. Ikeda, K. Imagawa, H. Ishiura, H. Ito, T. Kisimoto, A. Kozlov, Y. Takemoto, K. Yasuda, H. Ejiri, K. Hata, T. Iida, K. Inoue, M. Koga, K. Nakamura, R. Orito, T. Shima, S. Umehara, S. Yoshida
PProg. Theor. Exp. Phys. , 00000 (11 pages)DOI: 10.1093 / ptep/0000000000 Development of highly radiopure NaI(Tl)scintillator for PICOLON dark matter search pro ject
K. Fushimi ∗ , Y. Kanemitsu , S. Hirata † , D. Chernyak , R. Hazama , H. Ikeda ,K. Imagawa , H. Ishiura , H. Ito , T. Kisimoto , A. Kozlov , Y. Takemoto ,K. Yasuda , H. Ejiri , K. Hata , T. Iida , K. Inoue , M. Koga ,K. Nakamura ‡ , R. Orito , T. Shima , S. Umehara , and S. Yoshida Department of Physics, Tokushima University, 2-1 Minami Josanajima-cho,Tokushima city, Tokushima , 770-8506, Japan ∗ E-mail: [email protected] Graduate School of Integrated Arts and Sciences, Tokushima University, 1-1Minami Josanajima-cho, Tokushima city, Tokushima , 770-8502, Japan Department of Physics and Astronomy, University of Alabama, Tuscaloosa, Alabama35487, USA and Institute for Nuclear Research of NASU, 03028 Kyiv, Ukraine Department of Environmental Science and Technology, Osaka Sangyo University,3-1-1 Nakagaito, Daito city, Osaka, 574-8530, Japan Research Center for Neutrino Science, Tohoku University, 6-3 Aramaki Aza Aoba,Aobaku, Sendai city, Miyagi, 980-8578, Japan I. S. C. Lab. , 5-15-24 Torikai Honmachi, Settsu city, Osaka, 566-0052, Japan Department of Physics, Graduate School of Science, Kobe University, 1-1Rokkodai-cho, Nada-ku, Kobe city, Hyogo, 657-8501, Japan Institute for Cosmic Ray Research, The University of Tokyo, 5-1-5 Kashiwanoha,Kashiwa city, Chiba, 277-8583, Japan Department of Physics, Osaka University, 1-1 Machikaneyama-cho, Toyonaka city,Osaka 560-0043, Japan National Research Nuclear University “MEPhI” (Moscow Engineering PhysicsInstitute), Moscow, 115409, Russia Kavli Institute for the Physics and Mathematics of the Universe (WPI), 5-1-5Kashiwanoha, Kashiwa city, Chiba, 277-8583, Japan Research Center for Nuclear Physics, Osaka University, 10-1 Mihogaoka Ibarakicity, Osaka, 567-0042, Japan Faculty of Pure and Applied Sciences, University of Tsukuba, 1-1-1 Tennoudai,Tsukuba city, Ibaraki, 305-8577, Japan . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
The highly radiopure NaI(Tl) was developed to search for particle candidates of darkmatter. The optimized methods were combined to reduce various radioactive impurities.The K was effectively reduced by the re-crystallization method. The progenies ofthe decay chains of uranium and thorium were reduced by appropriate resins. Theconcentration of natural potassium in NaI(Tl) crystal was reduced down to 20 ppb.Concentrations of alpha-ray emitters were successfully reduced by appropriate selectionof resin. The present concentration of thorium series and
Ra were 1 . ± . µ Bq/kgand 13 ± µ Bq/kg, respectively. No significant excess in the concentration of Pb © The Author(s) 2012. Published by Oxford University Press on behalf of the Physical Society of Japan.This is an Open Access article distributed under the terms of the Creative Commons Attribution License(http://creativecommons.org/licenses/by-nc/3.0), which permits unrestricted use,distribution, and reproduction in any medium, provided the original work is properly cited. a r X i v : . [ phy s i c s . i n s - d e t ] F e b as obtained, and the upper limit was 5.7 µ Bq/kg at 90% C. L. The achieved level ofradiopurity of NaI(Tl) crystals makes construction of a dark matter detector possible. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Subject Index H20, F40
1. Introduction
Producing a high-sensitivity radiation detector for dark matter is currently the most crucialsubject. Many groups are trying to find a WIMPs signal by various methods and targetnuclei. Although a large volume liquid xenon (LXe) detector set stringent limits on the exis-tence of WIMPs dark matter candidates[1–3], DAMA/LIBRA group reported the significantsignal of WIMPs by applying highly radiopure and large volume NaI(Tl) scintillator. Theyobserved the annual modulating signal of WIMPs rate due to the earth’s revolution aroundthe Sun [4]. The event rate and deposited energy of WIMPs modulate with their maximumat the beginning of June and a minimum at the beginning of December. The DAMA/LIBRAgroup developed a highly radiopure NaI(Tl) scintillator whose total mass was 250 kg. Theyreported a significant annual modulating amplitude between 2 keV ee and 6 keV ee , wherekeV ee is the unit of energy scale calibrated by the kinetic energy of electron [5].Many groups are trying to verify the DAMA/LIBRA’s result by NaI(Tl) scintillator [6–9]. The COSINE group started the low background measurement from 2018 by high-purityNaI(Tl) scintillator with its total 106 kg [8]. The event rate in the low energy region wasabout 2 ∼ Pb and K. The beta rays from
Pb,
Bi, and K make a continuous background in the energy region of interest (below10 keV). The Compton continuum due to 1462 keV gamma-ray emitted after electron captureof K also contributes to the background. These background events obscure the dark mattersignals. Recently, the COSINE reported the improved NaI(Tl) crystal, which was producedby re-crystallization [10]. They proved the effectiveness of the re-crystallization method toreduce the radioactive contamination in NaI(Tl).We have tried further purification by applying a cation exchange resin, in addition tothe re-crystallization method. The appropriate selection of the resin enabled the effectivereduction of the lead ion in the NaI solution. We developed an extremely high-purity NaI(Tl)crystal whose concentration of
Pb is less than 6 µ Bq/kg.
2. Purification methods
The re-crystallization (RC) method helps remove the radioactive impurities that are wellsoluble in water. The solubility of NaI in water is 75.14 at 100 ◦ C, and it decreases 64.76 at25 ◦ C. The solubility is defined as the mass of solute in 100 g of water solution. The differencein the solubility between 100 ◦ C and 25 ◦ C enables us to get the pure NaI sediment. † Present address: Otsuka Pharmaceutical Factory, Inc. Tokushima Itano Factory, Matsunami, Itano-cho, Itano-gun,Tokushima, 779-0195, Japan ‡ Present address: Butsuryo College of Osaka, 3-33 Ohtori Kitamachi, Nishi ward, Sakai city, Osaka, 593-8328, Japan he COSINE group showed the significant K suppression in the RC method[11]. Potas-sium in the aqueous solution of sodium iodide forms a well soluble potassium iodide (KI)or potassium hydroxide (KOH). The solubility of KI and KOH in water are as high as 59.7and 54.2 at 25 ◦ C, respectively, as shown in Table 1. Since the concentration of potassiumis much lower than its solubility, potassium ions remain in the water during the NaI RCprocess. The COSINE group reported the effectiveness of the RC method [10, 11].
Table 1
The solubility of NaI, KI, KOH, and PbI in water [12].Temp.( ◦ C) 0 20 25 100NaI 61.54 64.1 64.76 75.14KI 56.0 59.0 59.7 67.4KOH 49.2 53.8 54.2 64.0PbI K activity in NaI beforeand after this procedure. The activity of K in the NaI was determined using an ultra-lowbackground HPGe detector which is installed at the KamLAND underground facility of theKamioka neutrino observatory. The HPGe detector was constructed by joint efforts of KavliIPMU (Tokyo Univ.) and RCNS (Tohoku Univ.) research groups. More information aboutthe HPGe detector and underground clean room facility can be found in [13, 14]. We foundan apparent reduction of gamma-ray intensity by the RC method. The initial radioactivityof K in the original (non-purified) NaI powder was in the range of 18 ∼
32 mBq/kg at 90%CL. After a single RC cycle, the activity of K was reduced to 0 . ∼
10 mBq/kg at 90%CL.We prepared 100 ◦ C NaI saturated solution in a bottle filled with pure nitrogen. The bottlewas slowly cooled down to room temperature to avoid adhering of NaI crystals to the bottle’ssurface. The NaI crystals were separated from the water solution by suction filtration. Thefiltration was done in a glove box flushed with pure nitrogen to prevent contact with aircontaining radon.
The COSINE group reported the re-crystallization worked well to reduce
Pb [10, 11]. Ourresults also showed a significant reduction by double and triple RC methods, as listed inTable 6. Nevertheless, we need a higher reduction to get a pure NaI(Tl) crystal below a fewtens of µ Bq/kg. We tried to combine another strategy to remove Ra and Pb ions.We extensively searched for the source of
Pb and found it was not only
Rn in purewater but also
Pb in original NaI powder. We combined several methods to remove both
Rn and
Pb from the water solution of NaI. The gaseous
Rn in water was removedby bubbling the pure water with pure nitrogen gas. The longer bubbling, typically one hour,gives a significant reduction. We tried to catch Pb in the water solution of NaI by bothcation exchange resin and crown ether.We searched for the best combination of the resins by processing the NaI aqueous solu-tion, which was initially added 4.9 ppm of lead ion. We tested four resins delivered by two ompanies. The resin A is suitable to remove strontium, uranium, and lead ions. The resinB is designed to remove the lead ion. The resins C and D, which another company delivers,are applied to remove uranium and iron ions.We measured the concentration of the Pb ion in the sample by ICP-MS, Agilent 7900 atOsaka University and Agilent 7700 at Osaka Sangyo University. The measured values of thereduction factor of lead ion are listed in Table 2. According to the result, we selected theresin B to reduce lead ion. In addition to resin B, we apply the resins A to reduce not only
Pb but also U, Ra, and Th.
Table 2
The reduction factors for concentration of Pb ions in the NaI water solutionachieved by use of various resins.ID Sample Reduction factor1 Resin A 1/342 Resin B 1/643 Resin C 1/144 Resin D 1/3
We took necessary precautions to avoid contacts between the room’s air and NaI powderduring drying process. We dried the purified NaI solution by a rotary evaporator. We selecteda flask made of synthetic silica glass to avoid the possible pollution from it. We filled purenitrogen gas into the flask to break the vacuum when the drying finished. Additional dryingwas done by using a vacuum oven before growing NaI(Tl) crystal.The large volume NaI(Tl) crystal was produced by the Bridgeman-Stockbarger technique.The purified and dried NaI powder and thallium iodide were filled into a high-purity graphitecrucible. Certain details describing purification of the crucible material can be found in[13, 14]. The crucible was installed in a quartz vessel filled with inert gas at the center ofthe furnace. The crucible was heated to 700 ◦ C to melt the NaI powder, and it was slowlymoved to crystallize. The crystallization takes at least two weeks of cooling and annealingto make a transparent, colorless, and large single crystal without any inclusion. The meltedNaI does not interact and infiltrate the pure graphite crucible because of the special coatingon the surface of the crucible. In the end of crystal formation the NaI(Tl) ingot detachesfrom the crucible. We see no evidence of interaction between the crucible and melted NaI.We think that there is no significant pollution of the NaI(Tl) ingot occurs by the cruciblematerial.
3. Data taking and analysis
The original NaI(Tl) ingot was used to make a NaI(Tl) crystal shaped as a cylinder (a76.2 mm in diameter and 76.2 mm long). An optical window made of synthetic quartzwith 10 mm in thickness was attached on one side of the crystal edge. Other surfaces ofthe crystal were covered with an enhanced specular sheet (ESR) made of polyethylene.The whole crystal was encapsulated into a 4 mm-thick black acrylic container. We used the crylic housing to minimize absorption of low energy gamma and X-rays emitted by externalcalibration sources.An acrylic housing enables a low energy calibration down to 6.4 keV ee . The K β X-raysemitted after the electron capture of
Ba are between 34.9 keV and 35.8 keV with totalintensity of 11.5 % [15]. After the photoelectronic effect on iodine, the ionized iodine emits itsK α X-ray, 28.6 keV and 28.3 keV [15]. If these K α X-rays escape from the crystal a residualenergy of approximately 6.4 keV is deposited in a NaI(Tl) scintillator. This low energy peakis useful to test the stability of low energy calibration.Acrylic housing has a risk of moisture permeability. We tested deliquesce when the NaI(Tl)detector was installed in the shield with pure nitrogen. The NaI(Tl) detector has not sufferedany deliquescence for at least 1.8 years.We made several trials of purification. The details of the NaI(Tl) detectors are listed inTable 3. Hereafter, we identify the NaI(Tl) detector by crystal ID shown in Table 3.
Table 3
The list of NaI(Tl) detectors and purification techniques used in their production.Crystal ID Purification method
We measured activity of radionuclides remained in the crystals µ sec long wave-forms. Signal selection as well as rejection of the remaining noise pulses from PMT weredone by offline pulse shape analysis (PSD).In the surface laboratory, the current pulse from a PMT was divided into three routes anddigitized by CAMAC ADC [17]. The one was used for a trigger. We introduced one currentsignal of the PMT to a charge-sensitive ADC (CSADC: REPIC RPC-022) channel througha 200 ns cable delay. The CSADC integrates the total charge of the current pulse for 1 µ s.The other current signal was introduced into the other ADC channel directly to integrate itpartially.The PSD was performed to extract the alpha-ray event. The decay time of the NaI(Tl)for an alpha-ray, τ α is 190 ns, on the other hand, the one for a beta or gamma-ray, τ β is 230 ns[18]. This difference enables clear discrimination of an alpha-ray and others. The arameter R was calculated as R ≡ (cid:82) t t I ( t ) dt (cid:82) t t I ( t ) dt , (1)where I ( t ) was the input current. The upper limit and the lower limits of the integrationare defined as t , t , and t , respectively. The present values of t , t , and t were listed inTable 4. Table 4
Integration parameters t , , nsec in two measurement systems. t t t MoGURA 0 200 1200CAMAC 0 200 1000 K in underground laboratory
We analyzed dependence of the concentration of K in the NaI(Tl) detectors
Fig. 1
The detector and the shield of the present measurement in Kamioka.We measured the concentration of K in the NaI(Tl) crystal at Kamioka Underground lab-oratory, Tohoku University. The K decays both beta decay (89.26%) and electron capture(10.72%). We need a low background environment to measure the tiny amount of radioac-tivity because of the enormous external background. The Kamioka Underground laboratoryis suitable to reduce the background from cosmic rays. To suppress environmental gamma-rays, the detectors were placed inside of an ultra-low background passive shielding made ofa 15 ∼
20 cm-thick lead and a 5 cm-thick 99.99% pure copper layers. Structure and compo-sition of the passive shielding were also reported in the earlier publications [13, 14]. Figure1 shows crystal eta+alpha (BiPo)alphabeta/gamma
Fig. 2
Left: PSD analysis result in case of crystal ee due to the saturation of PMT output. The beta/gamma-ray energy spectrum wasobtained without distortion to check the existence of the beta-ray ( E β max = 1311 .
09 keV)and gamma-ray ( E γ = 1461 keV) [15]. We selected the beta- and the gamma-ray events byPSD analysis. The energy spectra observed by the crystals ee of crystal K. The prominent beta-ray spectrum insisted that the K was contained inthe NaI(Tl) crystal of nat
K in the crystal K innatural potassium is 0 . K in the energy spectrum of the crystal nat
K was less than 20 ppb at 90% C. L.The reproducibility and the effectiveness of the triple RC method were tested by ingot nat
K was 30 ppb at 90% C. L. We found that the double RC methodwas enough to reduce potassium contamination. We confirmed the double RC method’seffectiveness by later ingots
We measured the alpha concentration in purified NaI(Tl) crystals. After establishing thepotassium reduction, we optimized the reduction method of alpha-ray emitters. The presentcrystal × .
279 kg for × .
279 kg for . < R < .
45. A largelocus around R = 0 .
53 is the events of beta-rays, gamma-rays, and cosmic-rays. We com-pared the energy spectrum of alpha-rays with one of our previous best NaI(Tl) crystal,
000 2500 3000 3500 4000 4500 5000 5500 6000ENERGY(keVee)00.511.522.53 C O UN T S / da y / k e V Fig. 3
Left: The PSD plot with crystal ± µ Bq/kg of
Ra, 149 ± µ Bq/kg of
Th, and 86 ± µ Bq/kg of
Pb. Weanalyzed the constituent of the alpha-ray spectrum of crystal ee ELECTRON EQUIVALENT ENERGY (keV012345678 ee C O UN T S / k e V Fig. 4
The energy spectrum of alpha-raytaken by crystal ee ), and the other is the higher energyregion (above 4000 keV ee ). Both compo-nents have no prominent structure. Weconsidered that the events above 4000keV ee are an accidental spread of R of highenergy cosmic rays. We assumed thereis a constant background of the cosmic-ray events below 4000 keV ee . The energyspectrum below 4000 keV ee consists of con-stant cosmic-ray, alpha-ray emitted in theNaI(Tl) crystal, and alpha-ray emitted onthe ESR reflector sheet.We measured the surface concentration of alpha-ray emitters on the ESR sheet to deter-mine the alpha-ray intensity. We applied the low-background and position-sensitive alpha-raytracker, µ − PIC[20, 21]. No significant excess beyond the background was observed, as shownin Figure 5. The upper limit of the surface contamination was calculated as 1 . × − alpha/hr/cm (90% C. L.).We simulated the energy spectrum from the ESR sheet by Geant4.10 Monte Carlo simula-tion [22]. We assumed the origin of the alpha-ray was Po because the ESR sheet attractsthe progeny of
Rn.
Pb terminates the decay chain from
Rn, and accumulated
Pbgenerates the
Po. A red line in Figure 4 shows the calculated energy spectrum of Po ig. 5 Left: The energy spectra inside (red) and outside (black shaded) the sample region.Right: The distribution of emitting points of alpha-ray events. The ESR sheet is placed inthe sample region.from the ESR sheet. There is significant excess in the event rate in the low energy regionabove the red line. We concluded the excess is due to the alpha-rays emitted in the NaI(Tl)crystal.The energy spectrum of alpha-ray from the NaI(Tl) crystal showed no prominent structure.The event number of alpha ray was too small to perform further analysis such as time-correlation analysis. We calculated the yield by integrating the number of events instead ofthe peak fitting. We set the integration interval according to the quenching factor of alpha-rayand energy resolution. The alpha-rays which have near energies make a cluster in the energyspectrum because of low energy resolution. We analyzed the alpha-ray intensity according tothe clusterized energy ranges was calculated according to the quenching factor of alpha-rayin NaI(Tl) scintillator [19]. The energy listed in Table 5 is the FWHM (Full Width Half-Maximum) interval of the clusterized peak. The chemical symbol in the parenthesis indicatesthe decay chains of uranium and thorium. The electron equivalent energy of alpha-ray, E ee ,is derived by E ee = f α E α . (2)Where f α is the quenching factor of alpha-ray. We determined f α = 0 .
58 from the shape ofthe energy spectrum.The alpha-ray of
Ra lies between (C) and (D) clusters. The alpha-rays of
Ra con-tributes both event rates of (C) and (D) because of low energy resolution. We divided twoclusters at the peak energy of the alpha-ray of
Ra. The event rates of the clusters (C) and(D) contain half of the
Ra events. We confirmed the result of this analysis by the datafrom crystal × Ra,
Rn,
Po, and
Po were in secular equilibrium. We did not include
Po in thepresent analysis since we cannot take all the events of alpha-rays from
Po. The half-life of
Po is as short as 164 µ sec, and the dead-time of the present data acquisition system was able 5 The RIs of alpha-ray emitters. The energy range (keV ee ) and the event numberin the regions are listed. Detailed treartment of Ra in described in the text.ID RIs Energy range Events(A)
U (U),
Th (Th) 2180 ∼ ± U (U),
Th (Th),
Ra (U) 2580 ∼ ± Th (Th),
Ra* (Th),
Rn (U),
Po (U) 2970 ∼ ± Po (U),
Bi (Th),
Ra* (Th),
Rn (Th) 3300 ∼ ± Po (Th) 3820 ∼ ± Table 6
The concentration of nat
K and alpha-ray emitters in NaI(Tl) scintillators [13].Units of nat
K is in ppb, and others are in µ Bq/kg.ID of NaI/group nat K Ra Pb Th ± . ± . <
20 120 ±
10 1500 6 . ± . <
30 44 ± . ± . ± < . . ± . < < < < <
42 8 ∼
60 10 ∼
420 7 ∼ <
20 8 . ∼
124 10 ∼
30 2 ∼ Po was easily derivedfrom the result of (D) as 14 ± µ Bq/kg. The one of
Ra was derived from the result of(B) as 13 ± µ Bq/kg; both values are consistent with each other.No significant excess was found in the concentration of
Po. We set the upper limit onthe concentration as 5.7 µ Bq/kg at 90%C. L.
4. Prospects - - - - -
10 110 B a ck g r ound C oun t s [/ da y / k g / k e V ] Fig. 6
The expected contribution of RIsfrom the NaI(Tl) crystal. Black: Total. Red: K. Orange:
Pb and
Bi. Blue:
Ra.Magenta: Th series RIs.We successfully developed the highly-radiopure NaI(Tl) scintillator. The severebackground origins, K, Pb, and
Ra,were successfully removed by combiningthe RC method and resin method. All val-ues of radioactive imurities in the crystal . . rom external RIs. They will be selectedand removed by anti-coincidence of arrayed NaI(Tl) detector system.We will start the dark matter search experiment by large NaI(Tl) scintillators, PICOLONphase-I, from 2021 spring. It consists of at least four modules of NaI(Tl) scintillator, whosetotal mass is 23.4 kg. We plan the phase-II and phase-III with the total mass of 100 kg and250 kg of NaI(Tl) crystal to verify and search for new fundamental processes in nuclear andparticle physics. Acknowledgement
We acknowledge the support of the Kamioka Mining and Smelting Company. This work wassupported by JSPS KAKENHI Grant No. 26104008, 19H00688, 20H05246, and Discretionaryexpense of the president of Tokushima University. This work was also supported by theWorld Premier International Research Center Initiative (WPI Initiative). We acknowledgeProfs. H. Sekiya and A. Takeda of ICRR University of Tokyo, and Prof. Y. Takeuchi of KobeUniversity for continuous encouragement and fruitful discussions.
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