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

摘要

The photon detection efficiency of two sets of R10560-100-20 superbialkaliphotomultiplier tubes from Hamamatsu were measured between 200 nm and 750 nm toquantify a possible degradation of the photocathode sensitivity after fouryears of operation in the cameras of the VERITAS Cherenkov telescopes. A sampleof 20 photomultiplier tubes, which was removed from the telescopes was comparedwith a sample of 20 spare photomultiplier tubes, which had been kept instorage. It is found that the average photocathode sensitivity marginallyincreased below 300 nm and dropped by 10% to 30% above 500 nm. The averagephotocathode sensitivity folded with the Cherenkov spectrum emitted byparticles in air showers, however, reveals a consistent detection yield of18.9+/-0.2% and 19.1+/-0.2% for the sample removed from the telescope and thespare sample, respectively.