Speaker
Description
Photomultiplier tubes (PMTs) are crucial in photon-counting experiments due to their high detection efficiency and low noise levels. A key application is in imaging atmospheric Cherenkov telescopes (IACTs), which observe Cherenkov light from air showers triggered by high-energy gamma and cosmic rays. They are also employed in the Large-Sized Telescopes (LSTs) and the Medium-Sized Telescopes of the Cherenkov Telescope Array Observatory (CTAO), the latest-generation IACTs currently under construction. The LSTs are optimized for relatively low-energy observations, particularly in the range of 20 GeV to 150 GeV, requiring PMTs with extremely low false-signal pulsing rates to avoid data acquisition issues. Currently, only the first LST (LST-1) is in operation among the CTAO telescopes.
Afterpulsing in PMTs, caused by ionized gas molecules from accelerated electrons (mainly triggered by night-sky photons), generates false signal pulses. To address this, CTAO and Hamamatsu Photonics K.K. developed novel PMTs (R11920-100 for LST-1 and R12992-100 for subsequent telescopes) with an exceptionally low afterpulsing rate of less than 2 × 10^-4 per photoelectron input.
However, the afterpulsing rate increases over time due to atmospheric molecules, particularly helium, penetrating the tube. This increase, measured at roughly 3 × 10^-5 per year, could degrade the LSTs’ energy threshold over their 20-year operation. Conversely, we found that the afterpulsing rate decreases when PMTs are operated under high voltage with light exposure, a condition naturally met during IACT observations.
In 2023, we removed several PMTs from LST-1. These tubes were installed shortly before or after the telescope’s first light at the end of 2018. Our laboratory measurement showed no increase in afterpulsing compared to pre-installation measurements from 2015, suggesting that the decreasing effect quickly cancels the increasing one during operation. In this talk, we report detailed results and discuss the mechanisms behind this behaviour.
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