Investigation of a new readout scheme for high resolution scintillation crystal arrays using photodiodes

We are exploring the possibility of using PIN photodiodes to readout the scintillation crystals used in positron emission tomography (PET) detector designs. Semiconductor photodetectors typically have a lower signal to noise ratio than photomultiplier tubes (PMTs). However, they have the advantage of compactness, and, thus, scintillation crystal readout schemes not available to PMTs because of their size and geometry limitations, are readily available to photodiodes. With current PET detector designs, only a small fraction of the available scintillation light, created from 511 keV gamma ray interactions within the crystal, is collected. Scintillation light collection studies were performed for several crystal geometries and surface treatments using both simulations and measurements. In this report, we present a feasible photodiode readout scheme that allows greater than 90% of the available scintillation light created in either BGO or LSO scintillation crystals to be collected by the photodetector. This improvement in light collection with the photodiode readout somewhat compensates for its lower inherent signal to noise ratios and makes it feasible for use in PET detectors. A coincident timing spectrum resolution of 9.4 ns FWHM was measured for 511 keV interactions with one LSO crystal coupled to a photodiode, the other to a PMT.