用CANDLES-III低本底测量研究$^{48}$ Ca的无中微子双β衰变
Low background measurement in CANDLES-III for studying the neutrino-less double beta decay of $^{48}$Ca
摘要：
我们建立了CANDLES-III系统来研究$^{48}$ Ca的无中微子双β（0 $νββ$）衰变。所提出的系统使用96 $_{2}$闪烁晶体（305 kg），其具有天然Ca（$^{\rm nat.}$ Ca）同位素，其对应于350 μ g的$^{48}$ Ca。使用围绕CaF $_2$晶体的液体闪烁体的4 $π$有源屏蔽来拒绝外部背景。通过对信号脉冲波形的分析，可以有效地降低CaF $_2$晶体中放射性杂质引起的内本底。为了评价CANDLES-III探测器低本底测量的可行性，我们分析了在神冈地下获得的130.4天寿命的数据。利用Monte Carlo模拟，我们估算了CaF $_{2}$晶体中放射性杂质的本底率和环境中子引起的（n，$γ$）反应所产生的高能$γ$ -射线率。预期的背景速率与测量的速率非常一致，即，在0 $νββ$窗口中，约10起$^{-3}$事件/keV/年/（$^{\rm nat.}$ Ca kg）。总之，通过将测量的能谱与背景模拟进行比较，适当地估计了背景候选者。使用这种测量方法，我们在低背景条件下使用具有Ca同位素的检测器进行了0 $νββ$衰变的第一次搜索，其中Ca不富集，规模为数百kg。的$^{48}$钙同位素具有很高的潜力，用于0 $νββ$衰变搜索，并预计将用于开发下一代探测器的高灵敏度测量。
英文摘要：
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.
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