Low background measurement in CANDLES-III for studying the neutrinoless double beta decay of ${}^{48}\mathrm{Ca}$

We developed a CANDLES-III system to study the neutrinoless double beta ($0\nu \beta \beta$) 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 $4\pi$ 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 $\gamma$-rays caused by the $(\mathrm{n},\gamma )$ 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 ${}^{\mathrm{nat}}\mathrm{Ca}$), 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 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 $0\nu \beta \beta$ decay search, and is expected to be useful for the development of a next-generation detector for highly sensitive measurements.

Figure 1

Detector setup of CANDLES-III (a: side view and b: top view). Magnified figure in (b) shows details of ${\mathrm{CaF}}_2$ module. The layer number ($N_l=1-6$) is from top to bottom. The module number ($N_m=1-96$) first increases in x-axis, and then in negative y- and z-axes. For top layer ($N_l=1$), $N_m=1-16$. For bottom layer ($N_l=6$), $N_m=81-96$. Here, $N_m$ of zoomed ${\mathrm{CaF}}_2$ module is 14.

Figure 2

Scintillating pulse shape observed using CANDLES-III detector. Recorded waveforms for 62 PMTs are summed up. Data were recorded in 2 nsec time-bin for earlier 768 nsec region and 64 nsec time-bin for later region. (a) Observed pulse shapes for $\beta$-event, (b) $\beta +\mathrm{LS}$-event, and (c) $\alpha$-event. Blue, red, and green lines show reference pulse shapes of $\beta \text{−}$, $\alpha \text{−}$, and LS-events, respectively.

Figure 3

Separation of $\beta$- and $\beta +\mathrm{LS}$-events via PSD analysis: (a) ${\mathrm{PSD}}_{\beta }$ distribution of $\beta$- and $\beta +\mathrm{LS}$-events. Pulse shapes of a 2.6 MeV $\beta$-event and 140 keV LS-event were randomly added to create pulse shape of $\beta +\mathrm{LS}$-event. 2.6 MeV $\beta$-event was the same as that used to create reference pulse shape of $\beta$-events. (b) LS energy dependence on discrimination ability obtained via the same analysis as in (a), at different LS energies.

Figure 4

Two-dimensional distribution of reconstructed position (x versus y directions). Actual distance to adjacent crystal is 20 cm, but reconstructed position is not corrected for that distance.

Figure 5

Average number of photoelectrons for 1.836 MeV $\gamma$-ray of ${}^{88}\mathrm{Y}$ as function of module number. The 16 modules between two vertical dotted lines correspond to one layer.

Figure 6

Last part of ${}^{232}\mathrm{Th}$ series from ${}^{212}\mathrm{Bi}$ to ${}^{208}\mathrm{Pb}$. ${}^{208}\mathrm{Tl}$ always decays into the excited states of ${}^{208}\mathrm{Pb}$, then decays into the ground state with 2.615 MeV $\gamma$-ray emission.

Figure 7

Typical pulse shapes of ${}^{212}\mathrm{BiPo}$ events. (a)Event with time-lag shorter than 10 nsec. Red and blue lines show reference pulse shapes for $\alpha$- and $\beta$-events, pulse heights of which are fitted by integrating pulse shapes between 500 nsec and 4000 nsec. (b)Event at which time-lag is estimated to be 46 nsec. Red line shows fitting result with reference pulse shapes for $\beta$-event (prompt) and $\alpha$-event (delayed). Blue line shows reference pulse shapes for $\alpha$-event normalized as in (a).

Figure 8

Obtained energy spectra from each event selection using (a) 93 ${\mathrm{CaF}}_2$ crystals and (b) 21 high purity ${\mathrm{CaF}}_2$ crystals. Black lines correspond to raw events without any selections. Blue lines correspond to events after applying selection criteria (1) and (2). Red lines correspond to events after applying criteria (1)—(4). Details of the event selection criteria (1)—(4) are described in text. The region between two vertical lines represents $0\nu \beta \beta$ window of 4.17–4.48 MeV. After event selections, no events are observed in the ${\mathrm{Q}}_{\beta \beta }$-value region when 21 high purity crystals are selected.

Figure 9

Obtained energy spectra (plots) and simulated background spectra (lines) obtained using 93 ${\mathrm{CaF}}_2$ crystals in (a) logarithmic scale and (b) linear scale. Blue, magenta, and green lines correspond to simulated background spectra of ${}^{208}\mathrm{Tl}$ and ${}^{212}\mathrm{BiPo}$, $2\nu \beta \beta$ decay, and $(\mathrm{n},\gamma )$ events, respectively. Red line represents summed up spectrum of all three simulated spectra. The experimental energy spectrum in the ${\mathrm{Q}}_{\beta \beta }$-value region is well reproduced by the simulated one.