Bi background in CANDLES experiment

The neutrino-less double beta decay (0νββ) is a process that occurs only when neutrinos and antineutrinos are identical (Majorana). This process is very important for the determination of Majorana nature and absolute scale of neutrino mass, but it has not yet been observed. In the 0νββ decay search experiment, the decay rate is quite rare thus it is required to be extremely reduced the background events (BG) near the Q-value of 0νββ.

In the CANDLES experiment, 96 CaF2 scintillation crystals with natural Ca (containing 350 g of ⁴⁸Ca) were used to search for 0νββ. The Q-value of ⁴⁸Ca is 4.27 MeV, which is highest among double beta decay isotopes, so that BG rate is strongly suppressed. 

  One of the major BGs is the pile up waveform, The half-life of ²¹²Po beta decay(T_1/2 = 299 nsec) is shorter than the decay-time of CaF2 scintillation (1 μsec); hence, the delayed ²¹²Po α-decay piles up the prompt ²¹²Bi β decay (²¹²BiPo event).

The ²¹²BiPo event is observed as one event when the time-lag of the decays is relatively short (Double Pulse: DP). 

This ²¹²BiPo events are difficult to distinguish from the waveform of double beta decay (Single Pulse: SP) by conventional fitting, which reduces the sensitivity to 0νββ.

We developed the new analysis tool using machine learning to identify this pile up waveform and evaluated the BG reduction rate. The removal efficiency of ²¹²BiPo event by discriminating SP and DP waveforms using machine learning is presented.