A Study on Energy Resolution of CANDLES Detector

In a neutrinoless double-beta decay ( 0\nu \beta \beta ) experiment, energy resolution is important to distinguish between 0\nu \beta \beta and background events. CAlcium fluoride for studies of Neutrino and Dark matters by Low Energy Spectrometer (CANDLES) discerns the 0\nu \beta \beta of 48 Ca using a CaF 2 scintillator as the detector and source. Photomultiplier tubes (PMTs) collect scintillation photons. At the Q value of 48 Ca, the current energy resolution (2.6%) exceeds the ideal statistical fluctuation of the number of photoelectrons (1.6%). Because of CaF 2 's long decay constant of 1000 ns, a signal integration within 4000 ns is used to calculate the energy. The baseline fluctuation ( \sigma _{\mathrm{ baseline}} ) is accumulated in the signal integration, thus degrading the energy resolution. This article studies \sigma _{\mathrm{ baseline}} in the CANDLES detector, which severely degrades the resolution by 1% at the Q value of 48 Ca. To avoid \sigma _{\mathrm{ 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 \sigma _{\mathrm{ baseline}} and minimizes photoelectron loss. We obtain improved energy resolutions of 4.5%-4.0% at 1460.8 keV ( \gamma -ray of 40 K) and 3.3%-2.9% at 2614.5 keV ( \gamma -ray of 208 Tl). The energy resolution at the Q value is estimated to be improved from 2.6% to 2.2%, and the detector sensitivity for the