Evemt Localization in a Continuous Scintillation Detector Using Digital Processing

We have designed, constructed and begun to evaluate a two-dimensional position-sensitive detector which will be used in a multi-slice PET scanner. We have measured 5.2 mm transverse and 6.0 mm axial spatial resolution, with an array of thirty 5-cm PMTs (10-columns by 3 rows). The pulses are shortened and integrated for 240 nsec, which results in a countrate capability of up to 1 Mcps without significant loss of spatial resolution. Although the one-dimensional detector upon which this one is based used analog processing to determine the position of an event, we now use digital processing because of the potential advantages it offers. We can implement variable integration which allows one to choose the optimal setting for high countrate capability and best spatial resolution. Resolution did not improve with longer integration times to the degree expected; however, at 240-nsec integration time resolution was only 0.6 mm worse than the best resolution achieved with longer integration time. This shorter integration time considerably extends our countrate capability and may obviate the need for variable integration for all but the highest countrates. The ability to perform position arithmetics more complicated than a simple centroid calculation might also help localizing the event in two dimensions. Digital processing allows us to use different biases for the transverse and axial centroid calculations, and to vary the bias as a function of position, which helps to extend the useful area.