The performance of photomultipliers exposed to helium

We report results of a study to determine how the performance of photomultipliers is affected by exposure to He. In our tests we monitor two 5 in. diameter EMI hemispherical photomultipliers while they are operated in He environments. Initially we observed He + afterpulses, at an approximately constant delay relative to the primary anode pulse. As the He gas pressure in the tubes increases however, strings of pulses typical of Townsend discharges occur. For the glass composition and geometry of the photomultipliers used in our tests, the internal gas concentration as a function of exposure to He is calculated using Fick's law for the permeation of solids by gases. The He permeation constant for the photomultiplier glass is obtained from a semiempirical formula developed by Altemose. We calculate the internal He concentration resulting from the He exposure which is observed to cause the regular occurrence of discharges and find that it is consistent with that required for production of discharges in the Townsend model. Guidelines are presented for using our results to estimate lifetimes of photomultiplier's of different geometries and glass types when operated in He environments.