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

• DOI: 10.48550/arxiv.2502.02423
• Publication Date: 2025-02-04
• Authors: Takuto Kiyomoto; T. Nagayoshi; Shunsuke Sakurai; Mitsunari Takahashi; Tokonatsu Yamamoto; Alice Donini; Yusuke Inome; Yukiho Kobayashi; D. Mazin; Razmik Mirzoyan; S. Nozaki; H. Ohoka; A. Okumura; Takayuki Saito; Ryuji Takeishi; Masahiro Teshima
• Keywords: FOS: Physical sciences; Astrophysics - Instrumentation and Methods for Astrophysics; Instrumentation and Methods for Astrophysics (astro-ph.IM)

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