Effect of light flash on photocathodes

We have measured the discharge of the bi-alkali (high resistivity) photocathode of the 2 inch diameter Hamamatsu photomultiplier type R1828-01 when its surface is uniformly illuminated with 5 × 108 photons. The gain of the tube (when operated at − 1800 V) is reduced by 7% immediately after the flash, and recovers with a time constant of about 50 μs. Time delays are also affected. Photomultipliers with multi-alkali (low resistivity) photocathodes do not show these effects.