The Impact of Helium Exposure on the PMTs of the SuperNEMO Experiment

TL;DR

This study investigates how helium gas exposure affects the performance of Hamamatsu PMTs used in the SuperNEMO experiment, finding negligible impact on energy resolution but increased after-pulsing rates.

Contribution

It introduces methods to measure helium partial pressure inside PMTs and determine helium breakdown points, enhancing understanding of helium effects on PMT performance.

Findings

Energy resolution remains stable with helium exposure.

After-pulsing rate increases with helium exposure.

Methods for helium pressure reconstruction and breakdown detection are proposed.

Abstract

The performance of Hamamatsu 8" photomultiplier tubes (PMTs) of the type used in the SuperNEMO neutrinoless double-beta decay experiment (R5912-MOD), is investigated as a function of exposure to helium (He) gas. Two PMTs were monitored for over a year, one exposed to varying concentrations of He, and the other kept in standard atmospheric conditions as a control. Both PMTs were exposed to light signals generated by a Bi-207 radioactive source that provided consistent large input PMT signals similar to those that are typical of the SuperNEMO experiment. The energy resolution of PMT signals corresponding to 1 MeV energy scale determined from the Bi-207 decay spectrum, shows a negligible degradation with He exposure; however the rate of after-pulsing shows a clear increase with He exposure, which is modelled and compared to diffusion theory. A method for reconstructing the partial pressure of He within the PMT and a method for determining the He breakdown point, are introduced. The implications for long-term SuperNEMO operations are briefly discussed.