A new position reconstruction method for position sensitive photomultipliers

A new position reconstruction method for position sensitive photomultiplier tubes is proposed in this work. The algorithm is based on a mathematical model operating on the charge signals recorded from the anode wires of a multi-wired anode system. This method overcomes the usual irregularities produced by the center of gravity algorithm near the edges of the field of view, especially when a homogeneous scintillation crystal is used. According to this method, the amount of the detected charge on a multi-wired anode system is calculated from the light distribution on the photo-cathode assuming a superimpose of analytically defined Gauss curves and a constant amplification of the photomultiplier tube. The parameters of this expression are experimentally defined on an event-by-event analysis by performing all required transformations. Data are obtained from a small field, high resolution γ-Camera system with a 16X+16Y multi-wired crossed anode using the Position Sensitive Photomultiplier Tube (PSPMT R2486, HAMAMATSU). The optical photon distribution for each type of the scintillation crystal used in the experiment is calculated with the photon transport system DETECT2000. Systematic measurements for a group of inorganic scintillations crystals of CsI(Tl) with 2mm-4mm-8mm-12mm and 20mm in thickness, as well as of BGO with 2mm-3mm-5mm and 8mm in thickness, have been performed for different radiation sources ( 60 Co, 137 Cs, 99m Tc). The experimentally obtained parameters for the produced light distribution inside the various crystals are expressed and categorized according to the crystal geometrical characteristics. The developed method seems to drastically improve the resolution of the reconstructed planar information, even when homogeneous scintillation crystals are used.