Finite element model based spatial linearity correction for scintillation detectors that use position sensitive avalanche photodiodes

We have developed a method using polynomial correction derived from a finite element model (FEM) to correct spatial linearity of scintillation detectors that use position sensitive avalanche photodiodes (PSAPD). A PSAPD is a planar avalanche photodiode with a resistive coating that allows continuous, but non-linear, positioning of scintillation light pulses over the entire active area. The spatial response of a scintillation crystal array coupled to a PSAPD shows a characteristic pincushion distortion when using a linear center of mass positioning calculation. A finite element simulation was used to derive the spatial response function for the PSAPD detector, and then used to calculate a polynomial linearity correction. We performed spatial linearity correction of several flood histograms from a ²²Na point source on discrete, scored and continuous crystal arrays of lutetium orthosilicate (LSO). For discrete arrays, spatial linearity correction improves crystal segmentation by correcting the pincushion distorted peaks in the 2-D crystal positioning histogram. For continuous sheets, spatial linearity correction allows for linear positioning over the central part of the crystal array without the use of time consuming spatial calibration measurements