8 inch diameter PSPMT for gamma ray imaging

This work presents preliminary measurements taken with the first prototype Hamamatsu (R6970) 8-inch Position Sensitive Photomultiplier Tube (PSPMT). These measurements are compared with similar ones obtained using a 5 inch PSPMT (Hamamatsu R3292). The new 8 inch PSPMT has 12 dynode stages, 11 with a proximity mesh structure whilst the last one is a back reflector. The entrance window is 7.5 mm thick and has an active area of 180 mm diameter. The crossed-wire anode of the 8 inch tube consists of 36×36 wires on a 4 mm pitch. These are paired together to give an 18×18 wire outputs. Both PSPMTs were coupled to a 110 mm diameter, 3 mm thick CsI(Tl) scintillating array in which each pixel has dimensions of 2×2 mm2. Two read-out methods are compared in this paper. The first being the conventional resistive-divider technique. The second method uses a new multi-wire readout technique in which, the charge on each anode wire is individually read out and digitized. Measurements of the spatial resolution, position linearity, energy resolution and intrinsic charge distribution were carried out for both tubes using both read-out systems. Spatial resolution values of approximately 2 mm FWHM were obtained using the 8-inch PSPMT and the multiwire read-out technique. The other measured characteristics were similar to those obtained using the 5-inch PSPMT. These results obtained using the prototype 8-inch PSPMT underline the potential of this detector in the field of imaging in Nuclear Medicine.