Digital Pulse Processing for CdZnTe Detectors

Segmented Cadmium Zinc Telluride (CdZnTe) detectors are promising candidates for a high resolution radiation detection system due to their good energy and spatial resolution. They are also well suited for medical imaging in a Compton camera application. This first Compton camera prototype is set up with a double sided strip detector in the scatter layer. Such an electrode layout enables a fast timing at the expense of efficiency losses. As a Compton camera needs an adequate number of valid scattering events with precise timing, a pixelated electrode layout performs best. The analog readout electronics for a 8 x 8 pixeldetector (20 x 20 x 5mm3) has been replaced by a digital FPGA-based system. All detector signals are sampled by charge sensitive preamplifiers with 100MS/s and 14 bit resolution. The pulse heights are extracted with an adjustable Moving Window Deconvolution in combination with a slow shaping IIR filter (Recursive Moving Average). To generate a trigger, we implemented a digital constant fraction discriminator (DCFD) on the fast shaped signal. This signal is derived by applying a FIR filter to the deconvoluted cathode signal. The algorithm uses a symmetric [−1, 1] coefficient structure for fast computation. Our results show that the accuracy of the DCFD can be improved by optimizing the filter length. Both algorithms for energy and timestamp calculation have been evaluated with measurements of a 22Na source and at the ELBE accelerator with a pulsed beam. The initial implementation results in an energy resolution of about 5% (FWHM at 511 keV) for a randomly selected pixel and a timing resolution better than 12 ns FWHM for all events up to 10MeV.