Study on degradation of VUV-sensitivity of MPPC for liquid xenon scintillation detector by radiation damage in MEG II experiment

TL;DR

This study investigates how radiation damage affects the VUV sensitivity of MPPCs used in a liquid xenon detector in the MEG II experiment, revealing surface damage and recovery through thermal annealing.

Contribution

It provides new insights into the radiation-induced degradation and recovery mechanisms of MPPCs' VUV sensitivity in high-radiation environments.

Findings

VUV PDE significantly degrades under radiation.

Degradation is localized to the MPPC surface.

Thermal annealing can restore PDE levels.

Abstract

In the MEG II experiment, the liquid xenon gamma-ray detector uses Multi-Pixel Photon Counters (MPPC) which are sensitive to vacuum ultraviolet (VUV) light under a high-intensity muon beam environment. In the commissioning phase of the detector with the beam, a significant degradation in the photon detection efficiency (PDE) for VUV light was found, while the degradation in the PDE for visible light was much less significant. This implies that the radiation damage is localized to the surface of the MPPC where incoming VUV photons create electron-hole pairs. It was also found that the PDE can recover to the original level by thermal annealing.