Investigation of spectral response of monolithic CZT for imaging

Monolithic CZT with multiple anode pads and a common cathode has been used in a number of gamma camera prototypes. The pixelation of the anode pads has been shown to result in high intrinsic resolution that in turn has been shown to yield advantages in both close subject planar imaging and in Single Photon Emission Computed Tomography. A major problem in the way of commercialization of the technology is the spectral response of the semiconductor. It shows a significant loss of detected events from the full energy peak and thus there is unrealized potential for a further improved image quality. This spectrum degradation is caused by a variety of loss mechanisms arising principally from reduced hole mobility and hole trapping. Gamma-ray interactions in regions close to the anodes and in regions between the anode pads ('roads') both contribute significantly but in unknown relative amounts to a slow signal component and a resultant ballistic deficit. To separate the relative contributions and understand further the control variables we have studied the spectral response of monolithic detector samples of different thickness. A strong dependence of the spectral peak-to-tail ratio on the thickness was found, while at the same time the peak width changed consistently.