Low background measurement in CANDLES-III for studying the neutrino-less double beta decay of $^{48}$Ca
Creators: name: Ajimura, S.
name: Chan, W. M.
name: Ichimura, K.
name: Ishikawa, T.
name: Kanagawa, K.
name: Khai, B. T.
name: Kishimoto, T.
name: Kino, H.
name: Maeda, T.
name: Matsuoka, K.
name: Nakatani, N.
name: Nomachi, M.
name: Saka, M.
name: Seki, K.
name: Takemoto, Y.
name: Takihira, Y.
name: Tanaka, D.
name: Tanaka, M.
name: Tetsuno, K.
name: Trang, V. T. T.
name: Tsuzuki, M.
name: Umehara, S.
name: Akutagawa, K.
name: Batpurev, T.
name: Doihara, M.
name: Katagiri, S.
name: Kinoshita, E.
name: Hirano, Y.
name: Iga, T.
name: Ishikawa, M.
name: Ito, G.
name: Kakubata, H.
name: Lee, K. K.
name: Li, X.
name: Mizukoshi, K.
name: Moser, M.
name: Ohata, T.
name: Shokati, M.
name: Soberi, M. S.
name: Uehara, T.
name: Wang, W.
name: Yamamoto, K.
name: Yasuda, K.
name: Yoshida, S.
name: Yotsunaga, N.
name: Harada, T.
name: Hiraoka, H.
name: Hiyama, T.
name: Hirota, A.
name: Ikeyama, Y.
name: Kawamura, A.
name: Kawashima, Y.
name: Maeda, S.
name: Matsuoka, K.
name: Nakajima, K.
name: Ogawa, I.
name: Ozawa, K.
name: Shamoto, K.
name: Shimizu, K.
name: Shinki, Y.
name: Tamagawa, Y.
name: Tozawa, M.
name: Yoshizawa, M.
name: Fushimi, K.
name: Hazama, R.
name: Noithong, P.
name: Rittirong, A.
name: Suzuki, K.
name: Iida, T.
Publisher: name: arXiv
Year: 2020
Subjects: subject: High Energy Physics - Experiment (hep-ex)
subject: Nuclear Experiment (nucl-ex)
subject: Instrumentation and Detectors (physics.ins-det)
subject: FOS: Physical sciences
subject: FOS: Physical sciences

description: We developed a CANDLES-III system to study the neutrino-less double beta (0$νββ$) decay of $^{48}$Ca. The proposed system employs 96 CaF$_{2}$ scintillation crystals (305 kg) with natural Ca ($^{\rm nat.}$Ca) isotope which corresponds 350\,g of $^{48}$Ca. External backgrounds were rejected using a 4$π$ active shield of a liquid scintillator surrounding the CaF$_2$ crystals. The internal backgrounds caused by the radioactive impurities within the CaF$_2$ crystals can be reduced effectively through analysis of the signal pulse shape. We analyzed the data obtained in the Kamioka underground for a live-time of 130.4\,days to evaluate the feasibility of the low background measurement with the CANDLES-III detector. Using Monte Carlo simulations, we estimated the background rate from the radioactive impurities in the CaF$_{2}$ crystals and the rate of high energy $γ$-rays caused by the (n, $γ$) reactions induced by environmental neutrons. The expected background rate was in a good agreement with the measured rate, i.e., approximately 10$^{-3}$ events/keV/yr/(kg of $^{\rm nat.}$Ca), in the 0$νββ$ window. In conclusion, the background candidates were estimated properly by comparing the measured energy spectrum with the background simulations. With this measurement method, we performed the first search for 0$νββ$ decay in a low background condition using a detector with a Ca isotope, in which the Ca present was not enriched, in a scale of hundreds of kg. The $^{48}$Ca isotope has a high potential for use in 0$νββ$ decay search, and is expected to be useful for the development of a next-generation detector for highly sensitive measurements.
description: 26 pages, 9 figures, submitted to Physical Review D