Low background measurement in CANDLES-III for studying the neutrinoless double beta decay of Ca48

S. Ajimura, W. M. Chan, K. Ichimura, T. Ishikawa, K. Kanagawa, B. T. Khai, T. Kishimoto, H. Kino, T. Maeda, K. Matsuoka, N. Nakatani, M. Nomachi, M. Saka, K. Seki, Y. Takemoto, Y. Takihira, D. Tanaka, M. Tanaka, K. Tetsuno, V. T. T. Trang, M. Tsuzuki, S. Umehara, K. Akutagawa, T. Batpurev, M. Doihara, S. Katagiri, E. Kinoshita, Y. Hirano, T. Iga, M. Ishikawa, G. Ito, H. Kakubata, K. K. Lee, X. Li, K. Mizukoshi, M. Moser, T. Ohata, M. Shokati, M. S. Soberi, T. Uehara, W. Wang, K. Yamamoto, K. Yasuda, S. Yoshida, N. Yotsunaga, T. Harada, H. Hiraoka, T. Hiyama, A. Hirota, Y. Ikeyama, A. Kawamura, Y. Kawashima, S. Maeda, K. Matsuoka, K. Nakajima, I. Ogawa, K. Ozawa, K. Shamoto, K. Shimizu, Y. Shinki, Y. Tamagawa, M. Tozawa, M. Yoshizawa, K. Fushimi, R. Hazama, P. Noithong, A. Rittirong, K. Suzuki, T. Iida

Physical Review D 103(9) 2021年5月26日査読有り

We developed a CANDLES-III system to study the neutrinoless double beta (0νββ) decay of Ca48. The proposed system employs 96 CaF2 scintillation crystals (305 kg) with natural Ca (Canat) isotope which corresponds 350 g of Ca48. External backgrounds were rejected using a 4π active shield of a liquid scintillator surrounding the CaF2 crystals. The internal backgrounds caused by the radioactive impurities within the CaF2 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 CaF2 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 Canat), in the 0νββ 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 0νββ decay in a low background condition using a detector on the scale of hundreds of kg of nonenriched Ca. Deploying scintillators enriched in Ca48 will increase the sensitivity strongly. Ca48 has a high potential for use in 0νββ decay search, and is expected to be useful for the development of a next-generation detector for highly sensitive measurements.
Projected WIMP sensitivity of the XENONnT dark matter experiment

E. Aprile, J. Aalbers, F. Agostini, M. Alfonsi, L. Althueser, F.D. Amaro, V.C. Antochi, E. Angelino, J.R. Angevaare, F. Arneodo, D. Barge, L. Baudis, B. Bauermeister, L. Bellagamba, M.L. Benabderrahmane, T. Berger, A. Brown, E. Brown, S. Bruenner, G. Bruno, R. Budnik, C. Capelli, J.M.R. Cardoso, D. Cichon, B. Cimmino, M. Clark, D. Coderre, A.P. Colijn, J. Conrad, J.P. Cussonneau, M.P. Decowski, A. Depoian, P. Di Gangi, A. Di Giovanni, R. Di Stefano, S. Diglio, A. Elykov, G. Eurin, A.D. Ferella, W. Fulgione, P. Gaemers, R. Gaior, M. Galloway, F. Gao, L. Grandi, C. Hasterok, C. Hils, K. Hiraide, L. Hoetzsch, J. Howlett, M. Iacovacci, Y. Itow, F. Joerg, N. Kato, S. Kazama, M. Kobayashi, G. Koltman, A. Kopec, H. Landsman, R.F. Lang, L. Levinson, Q. Lin, S. Lindemann, M. Lindner, F. Lombardi, J. Long, J.A.M. Lopes, E. López Fune, C. Macolino, J. Mahlstedt, A. Mancuso, L. Manenti, A. Manfredini, F. Marignetti, T. Marrodán Undagoitia, K. Martens, J. Masbou, D. Masson, S. Mastroianni, M. Messina, K. Miuchi, K. Mizukoshi, A. Molinario, K. Morå, S. Moriyama, Y. Mosbacher, M. Murra, J. Naganoma, K. Ni, U. Oberlack, K. Odgers, J. Palacio, B. Pelssers, R. Peres, J. Pienaar, V. Pizzella, G. Plante, J. Qin, H. Qiu, D. Ramírez García, S. Reichard, A. Rocchetti, N. Rupp, J.M.F. dos Santos, G. Sartorelli, N. Šarčević, M. Scheibelhut, J. Schreiner, D. Schulte, M. Schumann, L. Scotto Lavina, M. Selvi, F. Semeria, P. Shagin, E. Shockley, M. Silva, H. Simgen, A. Takeda, C. Therreau, D. Thers, F. Toschi, G. Trinchero, C. Tunnell, K. Valerius, M. Vargas, G. Volta, H. Wang, Y. Wei, C. Weinheimer, M. Weiss, D. Wenz, C. Wittweg, Z. Xu, M. Yamashita, J. Ye, G. Zavattini, Y. Zhang, T. Zhu, J.P. Zopounidis

Journal of Cosmology and Astroparticle Physics 2020(11) 031-031 2020年11月16日査読有り

XENONnT is a dark matter direct detection experiment, utilizing 5.9 t of instrumented liquid xenon, located at the INFN Laboratori Nazionali del Gran Sasso. In this work, we predict the experimental background and project the sensitivity of XENONnT to the detection of weakly interacting massive particles (WIMPs). The expected average differential background rate in the energy region of interest, corresponding to (1, 13) keV and (4, 50) keV for electronic and nuclear recoils, amounts to 12.3 ± 0.6 (keV t y)-1 and (2.2± 0.5)× 10-3 (keV t y)-1, respectively, in a 4 t fiducial mass. We compute unified confidence intervals using the profile construction method, in order to ensure proper coverage. With the exposure goal of 20 t y, the expected sensitivity to spin-independent WIMP-nucleon interactions reaches a cross-section of 1.4×10-48 cm2 for a 50 GeV/c2 mass WIMP at 90% confidence level, more than one order of magnitude beyond the current best limit, set by XENON1T . In addition, we show that for a 50 GeV/c2 WIMP with cross-sections above 2.6×10-48 cm2 (5.0×10-48 cm2) the median XENONnT discovery significance exceeds 3σ (5σ). The expected sensitivity to the spin-dependent WIMP coupling to neutrons (protons) reaches 2.2×10-43 cm2 (6.0×10-42 cm2).
Measurement of ambient neutrons in an underground laboratory at the Kamioka Observatory

Keita Mizukoshi, Ryosuke Taishaku, Keishi Hosokawa, Kazuyoshi Kobayashi, Kentaro Miuchi, Tatsuhiro Naka, Atsushi Takeda, Masashi Tanaka, Yoshiki Wada, Kohei Yorita, Sei Yoshida

Progress of Theoretical and Experimental Physics 2018(12) 2018年12月1日査読有り筆頭著者責任著者

Abstract Ambient neutrons are one of the most serious backgrounds for underground experiments searching for rare events. The ambient neutron flux in an underground laboratory at the Kamioka Observatory was measured using a $\mathrm{^3He}$ proportional counter with various moderator setups. Since the detector response largely depends on the spectral shape, the energy spectra of the neutrons transported from the rock to the laboratory were estimated by Monte Carlo simulations. The ratio of the thermal neutron flux to the total neutron flux was found to depend on the thermalizing efficiency of the rock. Therefore, the ratio of the count rate without a moderator to that with a moderator was used to determine this parameter. Consequently, the most likely neutron spectrum predicted by the simulations for the parameters determined by the experimental results was obtained. The result suggests an interesting spectral shape, which has not been indicated in previous studies. The total ambient neutron flux is $(23.5 \pm 0.7 \ \mathrm{_{stat. } } ^{+1.9}_{-2.1} \ \mathrm{_{sys. } }) \times 10^{-6}$ cm$^{-2}$ s$^{-1}$. This result, especially the energy spectrum information, could be a new and important input for estimating the background in current and future experiments in the underground laboratory at the Kamioka Observatory.

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1468 2019年11月3日

Ambient neutrons are one of the most serious backgrounds for underground experiments in search of rare events. The ambient neutron flux in an underground laboratory of Kamioka Observatory was measured using a $\mathrm{^3He}$ proportional counter with various moderator setups. Since the detector response largely depends on the spectral shape, the energy spectra of the neutrons transported from the rock to the laboratory were estimated by Monte-Carlo simulations. The ratio of the thermal neutron flux to the total neutron flux was found to depend on the thermalizing efficiency of the rock. Thus, the ratio of the count rate without a moderator to that with a moderator was used to determine this parameter. Consequently, the most-likely neutron spectrum predicted by the simulations for the parameters determined by the experimental results was obtained. The result suggests an interesting spectral shape, which has not been indicated in previous studies. The total ambient neutron flux is $(23.5 \pm 0.7 \ \mathrm{_{stat. } } ^{+1.9}_{-2.1} \ \mathrm{_{sys. } }) \times 10^{-6}$ cm$^{-2}$ s$^{-1}$. In this paper, we explain our method of the result and discuss our future plan.