Photon Detection Efficiency Measurements of the VERITAS Cherenkov Telescope Photomultipliers after four Years of Operation

TL;DR

This study measures the photon detection efficiency of Hamamatsu photomultiplier tubes used in VERITAS telescopes after four years, finding slight sensitivity changes but overall consistent detection yield.

Contribution

It provides the first detailed comparison of PMT efficiency degradation in Cherenkov telescopes over four years, highlighting minimal impact on detection performance.

Findings

Marginal increase in sensitivity below 300 nm

10-30% decrease in sensitivity above 500 nm

Consistent detection yield of approximately 19%

Abstract

The photon detection efficiency of two sets of R10560-100-20 superbialkali photomultiplier tubes from Hamamatsu were measured between 200 nm and 750 nm to quantify a possible degradation of the photocathode sensitivity after four years of operation in the cameras of the VERITAS Cherenkov telescopes. A sample of 20 photomultiplier tubes, which was removed from the telescopes was compared with a sample of 20 spare photomultiplier tubes, which had been kept in storage. It is found that the average photocathode sensitivity marginally increased below 300 nm and dropped by 10% to 30% above 500 nm. The average photocathode sensitivity folded with the Cherenkov spectrum emitted by particles in air showers, however, reveals a consistent detection yield of 18.9+/-0.2% and 19.1+/-0.2% for the sample removed from the telescope and the spare sample, respectively.