Reducing the Distortion in Resistive Layer Positioning Devices: A Simulation Study

Position sensing in some radiation detectors can be achieved by using a resistive layer to divide the charge between multiple readout elements. This was demonstrated experimentally for one-and two-dimensional detection devices, for example in position-sensitive avalanche photodiodes (PSAPDs). Some problems remain however, regarding the ability of these devices to efficiently use their entire surface for position decoding. It is particularly difficult, for instance, to resolve positions near the edges of PSAPDs. The problem resides in part in the linear position decoding scheme employed to read a nonlinear charge dispersion in the resistive layer. In order to explore potential solutions, we have developed a finite-element model of resistive layers and have used it to study the spatial distortion under various conditions. The model consists in a network of resistors where the value and position of each element reflect the layout of the ohmic contacts on the resistive layer. We study, for a square resistive layer, the effect of size, shape and location of the readout ohmic contacts on the spatial distortion. In particular, we show that long ohmic strips along the layer´s edge help in reducing the pincushion distortion. We also demonstrate that a gradient of resistivity across the layer or a diagonal readout can also be used to achieve similar results. These observations suggest that it is possible to modulate the spatial distortion in these devices and to improve the overall linearity of their response.