Competition between Increasing and Decreasing Effects of the Afterpulsing Rate of PMTs during Night-Sky Observations

TL;DR

This study investigates the competing effects of increasing and decreasing afterpulsing rates in photomultiplier tubes used in atmospheric Cherenkov telescopes, showing that operational conditions can mitigate long-term performance degradation.

Contribution

It reveals that high voltage and light exposure during observations reduce afterpulsing, counteracting the increase caused by atmospheric molecules, thus maintaining PMT performance over years.

Findings

No increase in afterpulsing after five years of operation.

Operational conditions decrease afterpulsing, offsetting atmospheric effects.

PMT performance remains stable over long-term use.

Abstract

Photomultiplier tubes (PMTs) have been widely used in imaging atmospheric Cherenkov telescopes (IACTs). The Large-Sized Telescopes (LSTs) of the Cherenkov Telescope Array Observatory (CTAO), the latest-generation IACTs, are optimized for challenging observations of low-energy gamma rays, specifically in the 20 to 150 GeV range. To this end, PMTs with an exceptionally low afterpulsing probability have been developed and installed. However, the afterpulsing rate increases over time due to the infiltration of atmospheric molecules, particularly helium, into the tube. Interestingly, we found that the afterpulsing rate decreases when PMTs are operated at high voltage and exposed to light -- a condition naturally met during IACT observations. To evaluate the latest instrument response, after five years of operation, we removed several PMTs from the first LST, which is currently the only operational telescope among the CTAO instruments. Our laboratory measurements showed no increase in afterpulsing compared to pre-installation values. This suggests that the decrease in afterpulsing during operation offsets the increase, thereby maintaining the long-term performance of the PMTs.