  Photomultiplier tubes (PMTs) are widely used in astroparticle physics
experiments to detect light flashes (e.g. fluorescence or Cherenkov light) from
extensive air showers (EASs) initiated by statistically rare very high energy
cosmic particles when travelling through the atmosphere. Their high
amplification factor (gain) allows the detection of very low photon fluxes down
to single photons. At the same time this sensitivity causes the gain and
signal-to-noise ratio to decrease with collected charge over the lifetime of
the PMT (referred to as "ageing"). To avoid fast ageing, many experiments limit
the PMT operation to reasonably low night sky background (NSB) conditions.
However, in order to collect more event statistics at the highest energies, it
is desirable to extend the measurement cycle into (part of) nights with higher
NSB levels. In case the signal-to-noise ratio remains large enough in the
subsequent reconstruction of the EAS events, lowering the PMT gain in such
conditions can be an option to avoid faster ageing. In this paper, performance
studies under high NSB with Photonis XP3062 PMTs, as used in the fluorescence
detector of the Pierre Auger Observatory, are presented. The results suggest
that lowering the PMT gain by a factor of 10 while increasing the NSB level by
a similar factor does not significantly affect the PMT performance and ageing
behaviour so that detection and offline reconstruction of EASs are still
possible. Adjusting the PMT gain according to a changing NSB level throughout a
night has been shown to be possible and it follows a predictable behaviour.
This allows to extend the measurement cycles of experiments, based on PMTs of
type Photonis XP3062 or comparable and exposed to the NSB, to enhance the
sensitivity especially at the highest energies where events are very rare.
