Light Decay Time/Gain Shift in a LaBr/sub 3/: Ce/LYSO:Ce Phoswich Detector

We have found that scintillation light emitted by LaBr
₃
is totally absorbed as it passes through an optically joined piece of LYSO on its way to the photocathode of a photomultiplier tube. However, scintillation events in the LaBr
₃
reappear as events with a scintillation light decay time similar to the native light decay time of LYSO (40.8 ns) and much greater than the native light decay time of LaBr
₃
(17.8 ns). The conversion of LaBr
₃
events to scintillations with this new light decay time occurs, for this experimental arrangement, with about 41% gain efficiency, i.e. the photopeak energy is approximately 41% that of the LaBr
₃
coupled directly to the phototube. The light decay time of LYSO events is unchanged in this phoswich configuration and the gain for LYSO scintillation events is moderately reduced to about 70% of the value measured with the LYSO crystal alone. Here, we describe experiments that confirm and characterize this effect. We conclude with an explanation consistent with these results.