  The VERITAS gamma-ray telescope array has been operating since 2007 and has
been equipped with Hamamatsu R10560-100-20 PMTs since 2012. A decision to
continue operations into the mid 2020s was taken in 2019 so the question of
whether the PMTs would need replacing became important and a study was
initiated.
  We present results from scanning two groups of 20 Hamamatsu R10560-100-20
PMTs with an LED flasher. One group comprised five PMTs from each of the four
VERITAS telescopes and the other was made up of 20 PMTs of the same type, and
date of manufacture, that had never been used. We measured three test variables
related to gains and high-voltage response and found that there were no
significant differences between the two groups. This indicates that there has
been little ageing in the PMTs that have been used on the telescopes and that
replacement is unnecessary.
