A high light yield calcium iodide (CaI2) scintillator is being developed for
an astroparticle physics experiments. This paper reports scintillation
performance of calcium iodide (CaI2) crystal. Large light emission of 2.7 times
that of NaI(Tl) and an emission wavelength in good agreement with the sensitive
wavelength of the photomultiplier were obtained. A study of pulse shape
discrimination using alpha and gamma sources was also performed. We confirmed
that CaI2 has excellent pulse shape discrimination potential with a quick
analysis.Comment: 5 pages, 5 figures, Proceeding of the 15th Vienna Conference on
  Instrumentation (VCI2019

0000003982

Ichimura, Koichi

IIDA, Takashi

Kamada, Kei

Kim, Kyoung Jin

Yoshikawa, Akira

Yoshino, Masao

Nuclear Instruments and Methods in Physics Research Section A Accelerators Spectrometers Detectors and Associated Equipment

English

arXiv

Tsukuba Repository  (Tulips-R)

Nuclear Inst. and Methods in Physics Research, A 958 (2020) 162629Contents lists available at ScienceDirectNuclear Inst. and Methods in Physics Research, Ajournal homepage: www.elsevier.com/locate/nimaHigh-light-yield calcium iodide (CaI2) scintillator for astroparticle physicsTakashi Iida a,∗, Kei Kamada b, Masao Yoshino c, Kyoung Jin Kim c, Koichi Ichimura d,e,Akira Yoshikawa b,ca Faculty of Pure and Applied Sciences, University of Tsukuba, 1-1-1 Tennodai, Tsukuba, Ibaraki 305-8571, Japanb New Industry Creation Hatchery Center, Tohoku University, 6-6-10 Aoba, Sendai, Miyagi 980-8579, Japanc Institute for Material Research, Tohoku University, 2-1-1, Sendai, Miyagi 980-8577, Japand Kamioka Observatory, Institute for Cosmic Ray Research, University of Tokyo, 456, Higashi-Mozumi, Kamioka, Hida, Gifu 506-1205, Japane Kavli Institute for the Physics and Mathematics of the Universe (WPI), the University of Tokyo, Kashiwa, Chiba, 277-8582, JapanA R T I C L E I N F OKeywords:Scintillation detectorDouble beta decayDark matterCalcium IodideA B S T R A C TWe are developing a high-light-yield calcium iodide (CaI2) scintillator for astroparticle physics experiments.This paper reports our CaI2 scintillator crystal’s scintillation performance. It achieved high light emissions (2.7times those of NaI(Tl)) at a wavelength that was well within the photomultiplier’s sensitivity region. We alsoperformed a pulse shape discrimination study using alpha and gamma-ray sources, confirming after a briefanalysis that CaI2 has excellent pulse shape discrimination potential.1. IntroductionUnderground astroparticle physics experiments, such as searchesfor dark matter or neutrino-less double beta decay, require either alow energy threshold or a high energy resolution. Due to their highlight yields, inorganic scintillators are being widely adopted for use inastroparticle detectors.There is abundant indirect evidence that strongly implies the ex-istence of non-luminous dark matter [1]. Based on cosmological ob-servations, it is estimated that only 5% of the universe is made ofordinary matter. However, despite 27% of the universe being composedof dark matter, we still know little of its nature. Weakly InteractingMassive Particles (WIMPs) are one of the most promising candidatesfor particulate dark matter [2]. The DAMA/LIBRA experiment hasobserved an annual event rate modulation with high statistical sig-nificance using NaI(Tl) scintillator crystals [3], and has claimed tohave discovered dark matter. That said, several other highly sensitiveexperiments have ruled out the DAMA result [4]. These results shouldbe investigated for a range of target nuclei. In order to observe anannual signal modulation caused by WIMP-induced nuclear recoil, itis very important to lower the energy threshold.Searches for neutrino-less double beta decay (0𝜈𝛽𝛽) are consideredto be the most practical way of determining whether neutrinos are ofMajorana or Dirac type and of discovering lepton number violation.If neutrinos are of Majorana type, i.e., neutrinos and anti-neutrinosare identical, then the leptogenesis scenario gives a good explanationof the matter–antimatter asymmetry in the present universe [5]. Todistinguish the mono-energetic electron pairs produced by 0𝜈𝛽𝛽 from∗ Corresponding author.E-mail address: tiida@hep.px.tsukuba.ac.jp (T. Iida).the continuous-energy electron pairs produced by two-neutrino doublebeta decay (2𝜈𝛽𝛽) in 0𝜈𝛽𝛽 experiments, it is important to have highenergy resolution. The most stringent lower limit so far on the half-life of neutrino-less double beta decay is T1∕2 > 1026 years (90% C.L.),given by the KamLAND-Zen experiment [6]. The CANDLES experimentis currently searching for 48Ca double beta decay using undoped CaF2scintillators in the Kamioka Observatory in Japan [7]. The main advan-tage of 48Ca is that it has the highest Q-value (4.3 MeV) among all thecandidate isotopes for 0𝜈𝛽𝛽. We are aiming to develop an inorganicscintillator involving 48Ca with a light yield higher than the 10,000ph./MeV produced by undoped CaF2, for use in future 0𝜈𝛽𝛽 searches.In this paper, we evaluate the performance of a calcium iodide(CaI2) scintillator involving 48Ca, with a particular focus on its pulseshape discrimination capability.2. Calcium Iodide (CaI𝟐) scintillatorCaI2 crystals were discovered by Hofstadter et al. in the 1960s [8]and are known to have high light yields, double those of NaI(Tl)scintillators. However, despite their high light yields, these crystals didnot come into widespread use due to limitations in early crystal growthand processing techniques.Since 2016, the University of Tsukuba and the Institute for MaterialResearch (IMR) at Tohoku University have been jointly developing CaI2crystals using updated facilities and leading-edge techniques. Initially,CaI2 was vaporized due to the similarity of its melting and boilingpoints. We eventually overcame this problem by sealing off a quartzhttps://doi.org/10.1016/j.nima.2019.162629Received 31 March 2019; Received in revised form 25 July 2019; Accepted 22 August 2019Available online 26 August 20190168-9002/© 2019 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY license(http://creativecommons.org/licenses/by/4.0/).T. Iida, K. Kamada, M. Yoshino et al. Nuclear Inst. and Methods in Physics Research, A 958 (2020) 162629Fig. 1. Photographs of a CaI2 scintillator crystal grown by the Bridgman–Stockbargermethod (top) and the measurement setup (bottom). Here, a CaI2 crystal a fewmillimeters in size was placed on a photomultiplier tube in a dry room at IMR.Table 1Summary of the scintillation properties of NaI(Tl) and CaI2 scintillators.NaI(Tl) CaI2Light yield 39,000 ph./MeV 107,000 ph./MeVEnergy resolution 6.4%@662 keV 3.2%@662 keVDecay time 230 ns 834 nsEmission wavelength 420 nm 410 nmDensity 3.67 g/cm3 3.97 g/cm3tube to prevent evaporation, finally succeeding in growing the half-inchCaI2 crystal shown in Fig. 1 by the Bridgman–Stockbarger method [9].Here, the pink color seen on the crystal’s surface comes from SiCl4,which we used as a reactive-gas atmosphere to eliminate water andoxygen.We cut and polished the CaI2 crystal in a dry room (maintained atbelow 3% humidity) to create one 5 × 5 × 1 mm in size. Then, we mea-sured the crystal’s energy response when coupled to photomultipliertubes (Hamamatsu R7600U-200) with ultra-bialkali photo-cathodes us-ing a 137Cs gamma-ray source.As Fig. 2 shows, using 662 keV gamma rays the photoelectric peakof CaI2 was 2.7 times higher than that of NaI(Tl), and its estimated lightyield was 107,000 ph./MeV. We also obtained good resolution, namely3.2% @ 662 keV. Table 1 and [10] summarize the measurement resultsand compare them with those for other scintillators.3. Pulse shape discrimination performance of CaI𝟐In dark matter search experiments, we need to separate the nuclearrecoil signal produced by WIMP scattering from the environmentalgamma-ray background. Double beta decay searches use tagged alpharays produced by U/Th series elements to identify the backgroundfrom internal radioactive impurities. The detector’s particle identifi-cation performance is therefore of great importance in astroparticleexperiments. Since inorganic scintillator detectors use pulse shape dis-crimination to separate the signal from the background and improve thesignal-to-noise ratio, we now evaluate the pulse shape discriminationperformance of our CaI2 crystals.Fig. 2. Energy spectra for CaI2 and NaI(Tl), produced with 662 keV gamma rays froma 137Cs source [10]. Here, the photoelectric peak of CaI2 is 2.7 times higher than thatof NaI(Tl), and its estimated light yield is 107,000 ph./MeV.Fig. 3. Typical CaI2 scintillation light waveform, produced by our setup under 137Csgamma-ray excitation.We took measurements using an 241Am alpha-ray source and a137Cs gamma-ray source and compared the resulting waveforms. Thesignal from the photomultiplier tube was digitized and acquired at asampling rate of 400 MHz by a WaveCatcher module [11] connectedto a Windows PC. Ten thousand events were recorded by this setupfor each source. For each recorded event, we determined the pedestalover the first 100 ch, then took the start channel, i.e., the pulse’s risingpoint, as the first point where the signal continuously exceeded 5 mVfrom the pedestal for 5 ch. Fig. 3 shows a typical 137Cs waveform.Fig. 4 shows the average waveform for ten thousand events. Here,the red line represents alpha-ray events from 241Am, while the blackline represents beta rays from Compton scattering by 137Cs gamma rays.These curves have been normalized to enclose the same area. The slowcomponents are essentially the same for both the alpha and beta rays,but there is a substantial difference over the first 300 ns or so. Forour pulse shape discrimination analysis, we therefore defined a ratioparameter based on the ratio of the area for the first 300 ns to the totalarea. Fig. 5 shows the ratio parameter as a function of energy, basedon data for 662 keV gamma rays from 137Cs, for beta (blue) and alpha(red) rays, respectively. This approach enabled us to achieve very goodseparation, at least above 100 keV.CaI2 has strong hygroscopic properties and reacts with water in theair to form CaI2⋅(H2O)𝑛. Even in a dry room, the surface of a CaI2crystal deliquesces and becomes white within an hour. Because alpha2T. Iida, K. Kamada, M. Yoshino et al. Nuclear Inst. and Methods in Physics Research, A 958 (2020) 162629Fig. 4. Average pulse shapes for alpha (red) and beta (black) ray events. (Forinterpretation of the references to color in this figure legend, the reader is referredto the web version of this article.)Fig. 5. Two-dimensional plot of the ratio parameter as a function of energy. Here, theblue, red, and green points represent beta rays, alpha rays, and alpha rays after crystaldeliquescence, respectively. (For interpretation of the references to color in this figurelegend, the reader is referred to the web version of this article.)rays are stopped by the surface, we believed that differences betweenthe alpha and beta rays would appear over time due to the impact ofsurface deliquescence. In order to investigate this hypothesis, we tookmeasurements for a deliquesced CaI2 crystal that had been left in a dryroom for more than an hour; the results are shown in Fig. 5 in green.These results indicate that deliquescence does not change the pulseshape, but it does decrease light yield. Thus, the strong pulse shapediscrimination capability of CaI2 is unaffected by deliquescence.4. Conclusion and prospectsInorganic scintillators are being used for dark matter and doublebeta decay experiments. We are thus developing CaI2 crystals thatcontain 48Ca from nuclei that have undergone double beta decay andcan generate high luminescence yields. We prepared half-inch CaI2crystals by the Bridgman–Stockbarger method at IMR, then sliced offsamples a few millimeters square to measure their scintillation char-acteristics. The amount of light they emitted was 107,000 ph./MeV,2.7 times more than NaI(Tl) and 10 times more than undoped CaF2.In addition, the light was emitted at a wavelength of 410 nm, withinthe photomultiplier’s sensitivity range. When we measured the crystal’swaveform discrimination characteristics using 241Am alpha rays and137Cs gamma rays, we found substantial differences in the waveformsover the first 300 ns. Thus, even a simple ratio analysis achieved verygood separation, meaning that CaI2 shows great potential for pulseshape discrimination.At present, it is difficult to grow and process large crystals due to therapid deliquescence and frequent cleavage of CaI2. Research is under-way to reduce cleavage by changing the crystal growth parameters andcrystal composition. In future work, we will also develop an analysismethod that can achieve better pulse shape discrimination and use itto create a detector.AcknowledgmentsThis work was performed as part of the Inter-University Coopera-tive Research Program of the Institute for Materials Research, TohokuUniversity (Proposal No. 18K0061 and 19K0089). It was supportedby JSPS KAKENHI Grant-in-Aid for Scientific Research (B) 18H01222and Grant-in-Aid for Young Scientists (B), Japan 16K17700. The au-thors also appreciate the support of the University of Tsukuba’s BasicResearch Support Program Type S.References[1] F. Zwicky, On the masses of nebulae and of clusters of nebulae, Agron. J. 86(1953) 217, http://dx.doi.org/10.1086/143864.[2] G. Jungman, M. Kamionkowski, K. Griest, Supersymmetric dark matter, Phys.Rep. 267 (1996) 195–373, http://dx.doi.org/10.1016/0370-1573(95)00058-5,arXiv:hep-ph/9506380.[3] R. Bernabei, et al., Final model independent result of DAMA/LIBRA-phase1, Eur.Phys. J. C 73 (2013) 2648, http://dx.doi.org/10.1140/epjc/s10052-013-2648-7,arXiv:1308.5109.[4] E. Aprile, et al., XENON Collaboration, First dark matter search results fromthe XENON1T experiment, Phys. Rev. Lett. 119 (18) (2017) 181301, http://dx.doi.org/10.1103/PhysRevLett.119.181301, arXiv:1705.06655.[5] M. Fukugita, T. Yanagida, Baryogenesis without grand unification, Phys. Lett.B174 (1986) 45–47, http://dx.doi.org/10.1016/0370-2693(86)91126-3.[6] A. Gando, et al., KamLAND-Zen Collaboration, Search for majorana neutrinosnear the inverted mass hierarchy region with KamLAND-Zen, Phys. Rev. Lett. 117(8) (2016) 082503, http://dx.doi.org/10.1103/PhysRevLett.117.082503, arXiv:1605.02889; Phys. Rev. Lett. 117 (10) (2016) 109903, http://dx.doi.org/10.1103/PhysRevLett.117.109903, (Addendum).[7] T. Iida, et al., Status and future prospect of 48Ca double beta decay searchin CANDLES, in: Proceedings, 14th International Conference on Topics inAstroparticle and Underground Physics (TAUP 2015), in: J. Phys. Conf. Ser.,Torino, Italy, 7–11 September 2015, vol. 718, no. 6, 2016, 062026, http://dx.doi.org/10.1088/1742-6596/718/6/062026.[8] R. Hofstadter, E.W. O’Dell, C.T. Schmidt, CaI2 and CaI2(Eu) scintillation crystals,IEEE Trans. Nucl. Sci. NS 11 (1964) 3.[9] M. Manutchehr-Danai (Ed.), Bridgman–Stockbarger technique, in: Dictionary ofGems and Gemology, Springer Berlin Heidelberg, Berlin, Heidelberg, 2009, p.111, URL https://doi.org/10.1007/978-3-540-72816-0_2926.[10] K. Kamada, et al., Single crystal growth and scintillation properties of Ca(Cl, Br,I)2 single crystal, Ceram. Int. 43 (2016) S423–S427, http://dx.doi.org/10.1016/j.ceramint.2017.05.249.[11] The wave catcher family of SCA-based 12-bit 3.2-GS/s fast digitizers, in: IEEE-NPSS Real Time Conference (RT) 15106931, 2014, pp. 1–8, http://dx.doi.org/10.1109/RTC.2014.7097545.3

High-light-yield calcium iodide (CaI2) scintillator for astroparticle physics

High-light-yield calcium iodide (CaI2) scintillator for astroparticle
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High-light-yield calcium iodide (CaI2) scintillator for astroparticle physics

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