PROSPECTS FOR NEW FAST PHOTOMULTIPLIERS

Comparative studies of the time resolution measured with the XP2020, X P2020UR and the XP2020UR-M modified by a screening grid at the anode w ere carried out for three different scintillators: a small NE111, a me dium size NE213 and for a NaI(Tl) crystal. Compared to the XP2020 an i mportant progress in timing with the XP2020UR was shown, due to its lo wer time jitter, however, mainly observed with the small NE111 plastic s. The modified XP2020UR-M represents a further remarkable improvement of the time resolution over the XP2020UR by a factor of 1.14, indepen dently of a type of the scintillator used (fast plastic or NaI(Tl)). T he analysis of the experimental data suggest strongly that this method can be successfully applied to all fast photomultipliers, independent ly of their time jitter. The observed limitation of the time resolutio n measured with the intensive fast light pulses from the NE111 plastic at 1 MeV energy seems to be associated with the space charge effect i n the dynode structure due to low interstage voltages. It thus suggest s a photomultipliers construction with a reduced number of dynodes. Fi nally, the study defines requirements addressed to the future fast pho tomultipliers which would bring improvement of the time resolution ove r the XP2020UR by a factor of about 1.3.