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 48Ca using a CaF2 scintillator as the detector and source. Photomultiplier tubes (PMTs) collect scintillation photons. At the $Q$ value of 48Ca, the current energy resolution (2.6%) exceeds the ideal statistical fluctuation of the number of photoelectrons (1.6%). Because of CaF2’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 48Ca. 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 40K) and 3.3%–2.9% at 2614.5 keV ( $\gamma $ -ray of 208Tl). The energy resolution at the $Q$ value is estimated to be improved from 2.6% to 2.2%, and the detector sensitivity for the $0\nu \beta \beta $ half-life of 48Ca can be improved by 1.09 times.