Development of ultra-pure NaI(Tl) detectors for the COSINE-200 experiment

Abstract

The annual modulation signal observed by the DAMA experiment is a long-standing question in the community of dark matter direct detection. This necessitates an independent verification of its existence using the same detection technique. The COSINE-100 experiment has been operating with 106 kg of low-background NaI(Tl) detectors providing interesting checks on the DAMA signal. However, due to higher backgrounds in the NaI(Tl) crystals used in COSINE-100 relative to those used for DAMA, it was difficult to reach final conclusions. Since the start of COSINE-100 data taking in 2016, we also have initiated a program to develop ultra-pure NaI(Tl) crystals for COSINE-200, the next phase of the experiment. The program includes efforts of raw powder purification, ultra-pure NaI(Tl) crystal growth, and detector assembly techniques. After extensive research and development of NaI(Tl) crystal growth, we have successfully grown a few small-size (0.61-0.78 kg) thalliumdoped crystals with high radio-purity. A high light yield has been achieved by improvements of our detector assembly technique. Here we report the ultra-pure NaI(Tl) detector developments at the Institute for Basic Science, Korea. The technique developed here will be applied to the production of NaI(Tl) detectors for the COSINE-200 experiment.

Study on NaI(Tl) crystal at -35 C for dark matter detection

“…[21]. The crystal was assembled as a detector inside a low-humidity N 2 -gas flushed glove box in which the humidity level was maintained to be less than a few tens of ppm (H 2 O) using a molecular sieve trap [23]. All assembly parts were cleaned using dilute Citranox liquid by sonication, and baked in an oven before being moved into the glove box.…”

“…In addition to reproducing the DAMA results, NaI(Tl) crystals are particularly interesting owing to their high light yield observed up to 22 photoelectrons per keV [21] and the sensitivity of the odd number of protons in both sodium and iodine to spin-dependent WIMP-proton interactions. Because ultra-pure NaI(Tl) crystals are produced well by many groups for reproducing DAMA/LIBRA experiments [22,23,24], future dark matter search experiments using NaI(Tl) crystals can explore the uncovered parameter space of WIMP dark matter [25]. Increasing the light yield and improving the pulse shape discrimination (PSD) power for nuclear recoil events are essential for increasing sensitivity of such experiments.…”

Identification of new isomers in $$^{228}$$Ac: impact on dark matter searches

“…A decreasing rate of twopulse events is evident and an exponential fit with R(t) = A exp(−t/τ ) is overlaid. Because initial purification of NaI powder [31,32] and decomposition of impurities from the crystal growing process [15], amounts of 232 Th and 228 Ra can be in non-equilibrium depending on their chemical properties. A similar non-equilibrium status between 238 U and 226 Ra was previously observed in the NaI(Tl) crystal [15].…”

“…Because initial purification of NaI powder [31,32] and decomposition of impurities from the crystal growing process [15], amounts of 232 Th and 228 Ra can be in non-equilibrium depending on their chemical properties. A similar non-equilibrium status between 238 U and 226 Ra was previously observed in the NaI(Tl) crystal [15]. If crystallization effectively removes 232 Th but not 228 Ra, the initial 228 Ra activity will be reduced with a lifetime of 3028 days that is consistent with the measured rate decrease with τ = 2724 ± 903 days in Fig.…”

“…Because of their long half-lives and large natural abundances, contamination from 238 U and 232 Th as well as their progenies are significant issues for low-background dark matter search experiments [14,15]. Therefore, an accurate understanding of their contamination levels and resultant contributions to detector background is essential.…”