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

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Abstract

We developed a CANDLES-III system to study the neutrinoless double beta ($0\ensuremath{\nu}\ensuremath{\beta}\ensuremath{\beta}$) decay of $^{48}\mathrm{Ca}$. The proposed system employs 96 ${\mathrm{CaF}}_{2}$ scintillation crystals (305 kg) with natural Ca ($^{\mathrm{nat}}\mathrm{Ca}$) isotope which corresponds 350 g of $^{48}\mathrm{Ca}$. External backgrounds were rejected using a $4\ensuremath{\pi}$ active shield of a liquid scintillator surrounding the ${\mathrm{CaF}}_{2}$ crystals. The internal backgrounds caused by the radioactive impurities within the ${\mathrm{CaF}}_{2}$ 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 ${\mathrm{CaF}}_{2}$ crystals and the rate of high energy $\ensuremath{\gamma}$-rays caused by the $(\mathrm{n},\ensuremath{\gamma})$ reactions induced by environmental neutrons. The expected background rate was in a good agreement with the measured rate, i.e., approximately ${10}^{\ensuremath{-}3}$ events/keV/yr/(kg of $^{\mathrm{nat}}\mathrm{Ca}$), in the $0\ensuremath{\nu}\ensuremath{\beta}\ensuremath{\beta}$ 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 $0\ensuremath{\nu}\ensuremath{\beta}\ensuremath{\beta}$ decay in a low background condition using a detector on the scale of hundreds of kg of nonenriched Ca. Deploying scintillators enriched in $^{48}\mathrm{Ca}$ will increase the sensitivity strongly. $^{48}\mathrm{Ca}$ has a high potential for use in $0\ensuremath{\nu}\ensuremath{\beta}\ensuremath{\beta}$ decay search, and is expected to be useful for the development of a next-generation detector for highly sensitive measurements.

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