  We report results from studies of the helium poisoning of a 16-anode
photomultiplier tube R5900-00-M16 manufactured by Hamamatsu Photonics. A tube
was immersed in pure helium for a period of about four months and was
periodically monitored using a digital oscilloscope. Our results are based on
the analysis of waveforms triggered by the dark noise pulses. Collected data
yield evidence of after-pulses due to helium contamination of the tube. The
probability of after-pulsing increased linearly with the exposure time to
helium but the phototube suffered only a small drop in gain, indicating
generally strong resilience to helium poisoning.
