CANDLES Project for the Neutrinoless Double Beta Decay of 48Ca

Neutrinoless double beta decay(0νββ) is currently known to be an only experiment to verify whether lepton number is conserved or not. Its observation will indicate that lepton number is not conserved, a neutrino is identical to its anti-neutrino, called Majorana particle. Hence the left-handed and the right-handed neutrinos could have different masses. In order to observe 0νββ, we have been studying ββ of Ca. Our first stage experiment using the ELEGANT VI detector system gave the best lower limit of the have life of 0νββ of Ca. Now we have been working on CANDLES experiment to reach much longer lifetime region lead to much lower effective Majorana neutrino mass region. The current status of CANDLES experiment will be presented. The CANDLES experiment is the project to search for 0νββ of Ca with undoped CaF2(pure) scintillators immersed the liquid scintillator (LS) worded as 4π active veto. The time constants of CaF2 and LS are 1μsec and a few nsec, respectively. So events from outside the scintillators deposit energy in the LS and are rejected by the pulse shape difference. The detector system, CANDLES III(underground) is installed in the Kamioka Underground Observatory, Japan and has been operated since June, 2011. The ββ nuclei Ca has the largest Q value of 4.3MeV among ββ nuclei. Thus background from natural radioisotopes are free except for two distinctive radioactive decays. One is the sequential decays of Bi (Th-chain isotope, Qβ = 2.2MeV) and Po (α, visible energy = 3MeV by quenching in CANDLES) with a relatively short half life of 300ns. The other is Tl (Th-chain isotope, Qβ = 5.0MeV) In order to remove these backgrounds, we use pulse shape information. The scintillation pulse is recorded by 500 MHz Flash ADC (FADC), thus the sequential decay signals,BiPo with the time difference (∆T) of longer than 10 nsec can be rejected. As for the signals with ∆T is shorter than 10 nsec, the main components of the pulse is formed by an α signal and rejected by the pulse shape difference between α and β events. Tl event is identified by the detected preceding Bi α decay event. I will present the pulse shape difference of α, β events in CaF2 crystal and events in the LS. The capabilities of the pulse shape discrimination between α and β events in CaF2, between events in CaF2 and events from outside(CaF2+LS) are also shown. The method for identifying Bi and Tl delayed coincidence events will be also presented.