Interpolation in a pixellated CZT γ-radiation detector

We describe a method of interpolation for a pixellated γ-radiation detector made of cadmium zinc telluride (CZT) with the goal to optimize its intrinsic spatial resolution. A similar method is known from silicon strip sensors where the interpolation improves the intrinsic spatial resolution over the single-strip digital resolution. We used a CZT crystal with a 2-dimensional matrix of anodes (pixels) on one side and a cathode on the other side. Each anode was electrically connected to a charge sensitive amplifier/shaper, which provided a pulse height output proportional to the amount of the electrical charge induced at the anodes. The amplifiers were implemented in a custom made application specific integrated circuit (ASIC) that allowed us to read out the charge from the triggering pixel and, in addition, the charge from its nearest neighbour pixels. We used ²⁴¹Am, ⁵⁷Co and ²²Na radiation sources to acquire energy spectra and shadow images from a tungsten metal edge. We observed charge sharing between pixels in about 10% and 24% of the photon interactions for 59.5 keV and 122 keV, respectively. We found that the method of interpolation did not improve the intrinsic spatial resolution in the CZT detector used, presumably due to the very small gap between pixels. Nevertheless, we consider interpolation important, since it has the potential to improve intrinsic spatial resolution either with a different method or with an anode layout that allows for more charge sharing, e.g., with larger gaps between pixels. Like in silicon sensors the method may benefit from additional electrodes in the gaps between the readout pixels.