  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.
