Model calculations of the response of CZT strip detectors

Position-sensitive CZT detectors for research in astrophysics in the five - several hundred keV range are being developed by several groups. These are very promising for large area detector arrays in coded mask imagers and small-area focal plane detectors for focusing X-ray telescopes, We have developed detectors with crossed-strip readout and optimized strip widths and gaps to improve energy resolution. A "steering electrode" is employed between the anode strips to improve charge collection. A model of charge drift in the detectors and charge induction on the electrodes has been developed to allow us to better understand these types of detectors and improve their design. The model presently accounts for the electric held within the detector, the charges' trajectories, mobility and trapping of holes and electrons, and charge induction on all electrodes including their time dependence. Additional effects are being added. The model is described and its predictions are compared with laboratory measurements. Results include (1) the dependence of anode, cathode and steering electrode signals on interaction depth, transverse (to the strips) position, electron and hole trapping, strip width and gap, and bias, (2) trajectories of charges for various anode and steering electrode bias voltages, (3) a method to improve energy resolution by using depth of interaction information, and (4) an electrode geometry and bias optimized for the improved energy resolution. In general, the model provides good agreement with the measurements.