Modeling and Correction of Spatial Distortion in Position-Sensitive Avalanche Photodiodes

Position-sensitive avalanche photodiodes (PSAPDs) are a promising alternative to photomultiplier tubes for the development of a new generation of gamma imagers. They offer compactness, high gain and superior quantum efficiency. PSAPDs having a sensitive surface of up to 28times28 mm² have been fabricated. However, unlike pixellated imaging devices having a similar configuration, each PSAPD can achieve submillimeter position sensing over its surface with only four readout channels. This key feature is obtained by Anger-logic event positioning from the signals of four corner anodes printed on a resistive layer covering one of the PSAPD surfaces. This readout scheme provides high degree of multiplexing for reading position and energy information from the device, but leads to pincushion distortion in the spatial information due to the nonlinear charge sharing pattern associated with the potential across the resistive layer. We have developed a method to reproduce and correct this distortion based on finite-element simulations of the readout configuration. The resistive layer and the anodes are represented by a two-dimensional array of resistors and this circuit is numerically solved to obtain the signal on the four anodes for different current injection nodes. The relation between the injection positions and the resulting Anger positions is modeled and then used to correct experimental data. The algorithm was tested on ^99mTc flood images of a 16times16 array of 0.4times0.4times4 mm³ CsI(Tl) crystals and successfully restored the regular pattern. The correction procedure is fast and robust, and constitutes a step toward the realization of a low-cost, high-resolution gamma camera based on PSAPDs