Speaker
Description
Photomultiplier tubes (PMTs) are used in Imaging Atmospheric Cherenkov Telescopes (IACTs), to detect Cherenkov light produced by air showers induced by gamma rays. In the PMTs, accelerated photoelectrons occasionally collide with residual gas inside the tube, producing positive ions that strike the photocathode and generate additional electrons. This ion feedback produces afterpulses, which may cause a hindrance to observation. The afterpulsing rate of the PMTs for the Large-Sized Telescopes (LSTs) of the Cherenkov Telescope Array Observatory, which is a next-generation IACT, has been found to increase in PMTs that were kept unused in storage, likely due to an increase of residual gas. In contrast, PMTs that had been operated in the first LST showed a slight decrease in afterpulsing rate. This decrease was considered to result from a reduction of residual gas caused by ion feedback, but the details had not been clear until this study. In this study, to investigate factors responsible for the change in the afterpulsing rate, we operated several PMTs under different high voltage conditions, with some were illuminated by LED light while the others were not. We kept the PMTs under this condition for three weeks repeating an afterpulse measurement every day. As its result, we confirmed that the reduction of afterpulses requires both illumination of the photocathode and application of high voltage to PMTs. Remarkably, the reduction strongly depends on the applied high voltage and is closely correlated with the integrated anode current, which is proportional to the number of the multiplied secondary electrons at the last stage. Therefore, we conclude that the reduction of residual gas is mainly caused by ionization occurring at later dynodes of the PMTs, with the ions adhering to the dynodes, establishing a key mechanism for the afterpulse reduction. We thus establish a key mechanism for the afterpulse reduction of PMTs.
| Speaker Confirmation | Yes |
|---|
