Dr. Khai Bui, Osaka Univ., JP

Neutrino-less double beta decay (0νββ) is useful for determining the neutrino mass and confirming the Majorana nature of neutrino. In a 0νββ experiment, an irremovable two-neutrino double beta decay (2νββ) background surrounds the Q-value of the double-beta-decay isotope. The energy resolution must be improved to differentiate between 0νββ and 2νββ events. CAlcium fluoride for studies of Neutrino and Dark matters by Low Energy Spectrometer (CANDLES) aims to obtain the 0νββ of ⁴⁸Ca by using CaF₂(un-doped, non-enriched) scintillator crystals as detectors and sources. Scintillation photons are collected by surrounding photomultiplier tubes (PMTs). Ideally, the energy resolution should equal the statistical fluctuation of the number of photoelectrons. At the Q-value of ⁴⁸Ca, the current energy resolution (2.6%) exceeds this fluctuation (1.6%). Because of CaF₂’s long decay constant of 1000 ns, a signal integration in 4000 ns is used to calculate the energy. The baseline fluctuation (σ_baseline) is accumulated in the signal integration, degrading the energy resolution. Therefore, this paper studies the σ_baseline in the CANDLES III detector, which has a severe effect (1%) at the Q-value of ⁴⁸Ca. To avoid σ_baseline, photon counting can be used to obtain the number of photoelectrons in each PMT; however, a significant photoelectron signal overlapping probability in each PMT causes missing photoelectrons in counting and reduces the energy resolution. “Partial photon counting” reduces the σ_baseline and minimizes photoelectron loss. We thus obtain improved energy resolutions of 4.5--4.0% at 1460.8 keV (γ-ray of ⁴⁰K), and 3.3--2.9% at 2614.5 keV (γ-ray of ²⁰⁸Tl). The energy resolution at the Q-value shows an estimated improvement of 2.2%, with improved detector sensitivity by factor 1.09 for the 0νββ half-life of ⁴⁸Ca. In this talk, I also discuss a possibility to improve CANDLES energy resolution when applying photon counting for CaF₂(un-doped) operated at low temperatures. It is estimated to achieve the sensitivity close to (or can be better than) the current world-best limit of effective neutrino mass.