Time Resolution Measurements with Fast Photomultipliers

A comparision of experimental data and theoretical predictions on time resolution limits of fast photomultipliers has been performed. XP 1020 photomultipliers have been used and measurements were done with a fast electronic system built in our laboratory. This system was tested with a Co60 source, the best time resolutions achieved over a period of two hours being 164 psec at f.w.h.m. A hydrogen discharge lamp has been built which produced short light pulses whose shape is determinated by a sampling method. Several light intensities were used to simulate different energy losses in plastic scintillators and time resolution measurements were performed for different threshold settings of the discriminator. The results of these measurements were compared to theoretical data calculated by means of a statistical model of the photomultiplier. Good agreement was found assuming a photoelectronic yield of one photoelectron per 2.5 KeV in Naton 136 plastic scintillator and a single electron response (SER) variance of 500 psec. The necessity of having more accurate data on photomultiplier parameters, mainly concerning the transit time jitter, was pointed out in order to have a better understanding of the photomultiplier's behaviour.