Rejection of pile-up events in Li2MoO4 scintillating bolometers for neutrinoless double-beta-decay detection

摘要: Neutrinoless double-beta-decay (0νββ) detection necessitates an exceptionally low background as this hypothesized process has not been observed at half-life of O(10 ²⁶ ) years. When the intrinsic background from the detector and its passive support materials is low, the primary sources of this background in 0νββ experiments of 100Mo are the pile-up events of two-neutrino double beta-decay and alpha events. This work presents methods that contribute to further reducing pile-up events, thereby supporting the feasibility of 0νββ detection. One is a time-over-threshold (TOT) method,        wherein the signal’s amplitude is compared against a threshold, and the duration of the signal exceeding the threshold is measured. This method aims to discern pile-up events by analyzing the time duration discrepancy between pile-up events and single events of equivalent amplitude. Using MATLAB, the double exponential ideal waveform is first generated, followed by the addition of white Gaussian noise to simulate realistic signal conditions. The method is evaluated in two scenarios: a constant threshold and a dynamically changing threshold. Compared to a constant threshold, a dynamic threshold that adjusts according to the waveform can effectively improve discrimination efficiency. The results demonstrate that the TOT method with a dynamically changing threshold achieves a discrimination efficiency exceeding 92.5% at a signal-to-noise ratio of 30. Notably, at higher signal-to-noise ratios, this method’s discrimination efficiency rivals that of the traditional mean-time method. This paper also tries to improve the mean-time method, including the generalization of the mean-time definition and the combination with the TOT method. More
over, this paper performs simulation calculations of the achievable background index for projected light detectors under different rise times and signal-to-noise ratios. In an actual experiment, the background index is influenced by various factors beyond the simulation work. Nevertheless, the background suppression scheme proposed in this work provides a solid foundation for the next generation CUPID experiment to achieve the physics goal, reducing the background index below
1 × 10 ⁻⁴ counts/(keV·kg·yr).