Search

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.

We developed a CANDLES-III system to study the neutrino-less double beta (0$\nu\beta\beta$) 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$\pi$ 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 $\gamma$-rays caused by the (n, $\gamma$) 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$\nu\beta\beta$ 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$\nu\beta\beta$ 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$\nu\beta\beta$ decay search, and is expected to be useful for the development of a next-generation detector for highly sensitive measurements., Comment: 26 pages, 9 figures, submitted to Physical Review D