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

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.


Introduction
Underground astroparticle physics experiments, such as searches for dark matter or neutrino-less double beta decay, require either a low energy threshold or a high energy resolution. Due to their high light yields, inorganic scintillators are being widely adopted for use in astroparticle detectors.
There is abundant indirect evidence that strongly implies the existence of non-luminous dark matter [1]. Based on cosmological observations, it is estimated that only 5% of the universe is made of ordinary matter. However, despite 27% of the universe being composed of dark matter, we still know little of its nature. Weakly Interacting Massive Particles (WIMPs) are one of the most promising candidates for particulate dark matter [2]. The DAMA/LIBRA experiment has observed an annual event rate modulation with high statistical significance using NaI(Tl) scintillator crystals [3], and has claimed to have discovered dark matter. That said, several other highly sensitive experiments have ruled out the DAMA result [4]. These results should be investigated for a range of target nuclei. In order to observe an annual signal modulation caused by WIMP-induced nuclear recoil, it is very important to lower the energy threshold.
Searches for neutrino-less double beta decay (0νββ) are considered to be the most practical way of determining whether neutrinos are of Majorana or Dirac type and of discovering lepton number violation. If neutrinos are of Majorana type, i.e., neutrinos and anti-neutrinos are identical, then the leptogenesis scenario gives a good explanation of the matter-antimatter asymmetry in the present universe [5]. To distinguish the monoenergetic electron pairs produced by 0νββ from the continuousenergy electron pairs produced by two-neutrino double beta decay (2νββ) in 0νββ experiments, it is important to have high energy resolution. The most stringent lower limit so far on the half-life of neutrino-less double beta decay is T 1/2 > 10 26 years (90% C.L.), given by the KamLAND-Zen experiment [6]. The CANDLES experiment is currently searching for 48 Ca double beta decay using undoped CaF 2 scintillators in the Kamioka Observatory in Japan [7]. The main advantage of 48 Ca is that it has the highest Q-value (4.3 MeV) among all the candidate isotopes for 0νββ. We are aiming to develop an inorganic scintillator involving 48 Ca with a light yield higher than the 10,000 ph./MeV produced by undoped CaF 2 , for use in future 0νββ searches.
In this paper, we evaluate the performance of a calcium iodide (CaI 2 ) scintillator involving 48 Ca, with a particular focus on its pulse shape discrimination capability.

Calcium Iodide (CaI 2 ) Scintillator
CaI 2 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 did not come into widespread use due to limitations in early crystal growth and processing techniques.
Since 2016, the University of Tsukuba and the Institute for Material Research (IMR) at Tohoku University have been jointly developing CaI 2 crystals using updated facilities and leading-edge techniques. Initially, it was difficult to vaporize CaI 2 due to the similarity of its melting and boiling points. We eventually overcame this problem by sealing off a quartz tube to prevent evaporation, finally succeeding in growing the half-inch CaI 2 crystal shown in Figure 1 by the Bridgman-Stockbarger method [9]. Here, the pink color seen on the crystal's surface comes from SiCl 4 , which we used as a reactive-gas atmosphere to eliminate water and oxygen.
We cut and polished the CaI 2 crystal in a dry room (maintained at below 3% humidity) to create one 5 × 5 × 1 mm in size. Then, we measured the crystal's energy response when coupled to photomultiplier tubes (Hamamatsu R7600U-200) with ultra-bialkali photo-cathodes using a 137 Cs gamma-ray source. As Figure 2 shows, using 662-keV gamma rays the photoelectric peak of CaI 2 was 2.7 times higher than that of NaI(Tl), and its estimated light yield was 107,000 ph./MeV. We also obtained good resolution, namely 3.2% @ 662 keV. Table 1 and [10] summarize the measurement results and compare them with those for other scintillators.

Pulse Shape Discrimination Performance of CaI 2
In dark matter search experiments, we need to separate the nuclear recoil signal produced by WIMP scattering from the environmental gamma-ray background. Double beta decay searches use tagged alpha rays produced by U/Th series elements to identify the background from internal radioactive impurities. The detector's particle identification performance is therefore of great importance in astroparticle experiments. Since inorganic scintillator detectors use pulse shape discrimination to separate the signal from the background and improve Figure 2: Energy spectra for CaI 2 and NaI(Tl), produced with 662-keV gamma rays from a 137 Cs source [10]. Here, the photoelectric peak of CaI 2 is 2.7 times higher than that of NaI(Tl), and its estimated light yield is 107,000 ph./MeV. the signal-to-noise ratio, we now evaluate the pulse shape discrimination performance of our CaI 2 crystals.
We took measurements using an 241 Am alpha-ray source and a 137 Cs gamma-ray source and compared the resulting waveforms. The signal from the photomultiplier tube was digitized and acquired at a sampling rate of 400 MHz by a WaveCatcher module [11] connected to a Windows PC. Ten thousand events were recorded by this setup for each source. For each recorded event, we determined the pedestal over the first 100 ch, then took the start channel, i.e., the pulse's rising point, as the first point where the signal continuously exceeded 5 mV from the pedestal for 5 ch. Figure 3 shows a typical 137 Cs waveform.  Figure 4 shows the average waveform for ten thousand events. Here, the red line represents alpha-ray events from 241 Am, while the black line represents beta rays from Compton scattering by 137 Cs gamma rays. These curves have been normalized to enclose the same area. The slow components are essentially the same for both the alpha and beta rays, but there is a substantial difference over the first 300 ns or so. For our pulse shape discrimination analysis, we therefore defined a ratio parameter based on the ratio of the area for the first 300 ns to the total area. Figure 5 shows the ratio parameter as a function of energy, based on data for 662-keV gamma rays from 137 Cs, for beta (blue) and alpha (red) rays, respectively. This approach enabled us to achieve very good separation, at least above 100 keV.  Here, the blue, red, and green points represent beta rays, alpha rays, and alpha rays after crystal deliquescence, respectively. CaI 2 has strong hygroscopic properties and reacts with water in the air to form CaI 2 ·(H 2 O) n . Even in a dry room, the surface of a CaI 2 crystal deliquesces and becomes white within an hour. Because alpha rays are stopped by the surface, we believed that differences between the alpha and beta rays would appear over time due to the impact of surface deliquescence. In order to investigate this hypothesis, we took measurements for a deliquesced CaI 2 crystal that had been left in a dry room for more than an hour; the results are shown in Figure 5 in green. These results indicate that deliquescence does not change the pulse shape, but it does decrease light yield. Thus, the strong pulse shape discrimination capability of CaI 2 is unaffected by deliquescence.

Conclusion and Prospects
Inorganic scintillators are being used for dark matter and double beta decay experiments. We are thus developing CaI 2 crystals that contain 48 Ca from nuclei that have undergone double beta decay and can generate high luminescence yields. We prepared half-inch CaI 2 crystals by the Bridgman-Stockbarger method at IMR, then sliced off samples a few millimeters square to measure their scintillation characteristics. The amount of light they emitted was 107,000 ph./MeV, 2.7 times more than NaI(Tl) and 10 times more than undoped CaF 2 . In addition, the light was emitted at a wavelength of 410 nm, within the photomultiplier's sensitivity range. When we measured the crystal's waveform discrimination characteristics using 241 Am alpha rays and 137 Cs gamma rays, we found substantial differences in the waveforms over the first 300 ns. Thus, even a simple ratio analysis achieved very good separation, meaning that CaI 2 shows great potential for pulse shape discrimination.
At present, it is difficult to grow and process large crystals due to the rapid deliquescence and frequent cleavage of CaI 2 .
Research is underway to reduce cleavage by changing the crystal growth parameters and crystal composition. In future work, we will also develop an analysis method that can achieve better pulse shape discrimination and use it to create a detector.