A study of event position dependence for the AMoRE-II R&D detectors with Li2MoO4 crystal absorbers

Abstract

The AMoRE collaboration conducts experiments to search for the neutrinoless double beta decay of 100Mo using massive Li2MoO4 (LMO) crystals with cryogenic sensors. The forthcoming phase (AMoRE-II) of the experiment will include 6 cm (diameter) × 6 cm (height) LMO cylindrical crystals, and this has been chosen to reduce the number of crystals and sensors. Additionally, these crystals will have diffusive surfaces rather than polished ones, which helps to reduce the crystal preparation time significantly. The signal of crystals with diffusive surfaces is slower than that of polished crystals. However, due to the mitigated position dependence, diffusive crystals exhibit better discrimination between alpha and beta/gamma signals by pulse shape analysis. We also found that muon events show two bands in the rise time of the large LMO crystal with polished surface, indicating the muon passage at the edge of the crystal and the mitigation of the band structure in the crystals with the diffusive surface. To study the position dependence in the crystal absorber further, we irradiated some R&D detectors with localized alpha sources. In this paper, we discuss the particle identification and position dependence of gamma, alpha, and muon events for the large AMoRE-II type detectors based on the pulse shape analysis