Characterizing IMARAD CZT detectors with time resolved anode and cathode signals

We present the results of using standard IMARAD CZT detectors with a 100 MHz readout of the anode and cathode pulses. The detectors, 2 cm x 2 cm large and 0.5 cm thick, have 64 indium pixellated anode contacts at a pitch of 2.5 mm. We investigate the possibilities to improve on the detector's photo-peak efficiency and energy resolution using two depth of interaction (DOI) indicators: (i) the total charge induced on the cathode and (ii) the drift time of the electron cloud determined from the anode and/or the cathode pulses. The DOI correction with the cathode charge gives better results, increasing the 662 keV photo-peak efficiency by 57% and improving the energy resolution from 2.33% to 2.15% (FWHM, including the electronic noise). The information on the time dependence of the induced charge can be used as a diagnostic tool to understand the performance of the detector. Detailed comparison of the pulse shapes with detector simulations gives the electron mobility and drift time and makes it possible to assess the weighting potential and electric field inside the detector.