Development of a planar head PEM system based on an array of PSPMT and YAP crystals

We are developing a PEM detector head based on two planar detector heads each measuring 6/spl times/6cm/sup 2/. Each head is composed of a 3/spl times/3 matrix of R8520-C12 phototubes from Hamamatsu and a YAP:Ce array of 2/spl times/2/spl times/30mm/sup 3/ pixels manufactured by Crytur. The R8520 is a compact 25.7/spl times/25.7mm/sup 2/ square detector that has an active area of 22/spl times/22mm/sup 2/. It has an excellent imaging performance and its square shape lends it well to PEM applications where a large active area is important to maximise sensitivity. However, even when tiled, the R8520-C12 only has an active area of 73%. Therefore, in an effort to recover the pixels lying on the detector boundaries that are not directly viewed by the active area, we have used a technique involving a quartz light diffuser between the crystal matrix and the PSPMT detectors. It is possible to recover virtually 100% of the area, but at the expense of a reduction in the system spatial resolution. Therefore, we have tested two versions of the detector head; one with the quartz diffuser which should provide a better sensitivity through maximised active area, and a second with no quartz that should have a superior imaging performance. Flood images are used to compare these two systems in terms of pixel identification and geometrical efficiency.