A Study on Energy Resolution of CANDLES Detector

In a neutrinoless double-beta decay ( 0νββ ) experiment, energy resolution is important to distinguish between 0νββ and background events. Calcium fluoride for studies of Neutrino and Dark matters by Low Energy Spectrometer (CANDLES) discerns the 0νββ of
⁴⁸
Ca using a CaF
₂
scintillator as the detector and source. Photomultiplier tubes (PMTs) collect scintillation photons. At the QQ 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 (σ
_baseliine
) is accumulated in the signal integration, thus degrading the energy resolution. This article σ
_baseliin
studies in the CANDLES detector, which severely degrades the resolution by 1% at the QQ value of 48Ca. To avoid σ
_baseliin
, 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 σ
_baseliin
and minimizes photoelectron loss. We obtain improved energy resolutions of 4.5%-4.0% at 1460.8 keV ( gammaγ -ray of 40K) and 3.3%-2.9% at 2614.5 keV ( gammaγ -ray of 208Tl). The energy resolution at the QQ value is estimated to be improved from 2.6% to 2.2%, and the detector sensitivity for the 0νββ half-life of 48Ca can be improved by 1.09 times.