  The photon detection system for the LHCb RICH Upgrade consists of an array of
multianode photomultiplier tubes (MaPMTs) read out by custom-built modular
electronics. The behaviour of the whole chain was studied at CERN using a
pulsed laser. Threshold scans were performed in order to study the MaPMT
pulse-height spectra at high event rates and different photon intensities. The
results show a reduction in photon detection efficiency at 900 V bias voltage,
marked by a 20 % decrease in the single-photon peak height, when increasing the
event rate from 100 kHz to 20 MHz. This reduction was not observed at 1000 V
bias voltage.
