Fast Photomultipliers for TOF PET

Presently, a majority of detectors for PET systems is based on scintillator crystals read by photomultipliers. In our previous work, very good time resolution recorded with a 10times10times5 mm³ LSO crystal coupled to Photonis XP20D0 and Hamamatsu R5320 photomultipliers was shown. Results were almost identical for both detectors and close to 170 ps but the properties such us quantum efficiency of the photocathode and transit time jitter were significantly different. The XP20D0 possessed high QE and the measured photoelectron number was 40% higher than that of the R5320. The R5320 had the time jitter of 140plusmn7 ps at FWHM, three times better than that of the XP20D0. The fact, that despite of the large differences in the parameters of the used PMTs comparable time resolution was achieved, triggered our further study of the most important properties of the photomultipliers and their influence on timing and energy resolution, with a goal of optimizing time-of-flight (TOF) PET systems. Thanks to a close cooperation with Photonis, during last few years we gathered large amount of information and experimental data of various types of PMTs. The aim of this paper is to present general conclusions and dependencies that were derived from these multiple experiments. This should help to develop an ultimate, PMT based detector, for TOF PET systems.