A Further Study of Timing With LSO on XP20D0 for TOF PET

In our previous work, a very good time resolution recorded with a 4 times 4 times 20 mm³ LSO crystal coupled to a Photonis XP20D0 photomultiplier was shown, which suggests that TOF-PET based on the LSO is a realistic proposition. A further improvement of the time resolution, due to the common light readout by the cluster of PMTs in the block detector, proposed by Kuhn et al., triggered our further study of the time resolution obtainable with the LSO crystals. In the present work, the optimization of timing using a leading-edge fast discriminator and a constant fraction discriminator was done and the results of the leading-edge timing were discussed in terms of the Hyman theory. In the second part of the study, the time resolution measurements and an optimization of the system realizing the light readout by two XP20D0 PMTs coupled to a common light diffuser were carried out for the 4 times 4 times 20 mm³ LSO crystal placed in various positions with respect to the PMTs. The results show that using a sum of signals and, in consequence, a better light collection from several PMTs improves timing and should allow good homogeneity of the time resolution over a whole surface of the detectors to be achieved.