A CdZnTe detector for space research

The potential of X-ray and gamma-ray detectors based on heavy element semiconductor materials, such as CdZnTe (CZT), is now widely recognised. The CZT detector has the advantage of a high quantum efficiency for hard X-rays and in addition there is no need for cryogenic cooling. Consequently, the CZT detector system can be made very compact with a low power consumption. Because of these properties, the CZT detector is very suitable for small satellites used in space research, but it should also be interesting for many other industrial and scientific applications. Although the quality of the material has improved considerably in recent years, it is still a drawback that most CZT semiconductors contain defects, which can trap the charge carriers generated by the ionising radiation. This will perturb the signal generation and reduce the energy resolution of the detector. The problem is most severe for holes, which have trapping probabilities more than an order of magnitude higher than those of electrons. We report here a technique developed at DSRI for reducing the sensitivity to holes. It is based on the use of micro strip electrodes, and it leads to a dramatic improvement of the energy resolution of the detector. Moreover, it is possible to compensate for the loss of holes. Especially for space research applications, it is important that the technique does not require any event rejection. The prototype structure consists of 14 drift strip electrodes and a single anode readout strip. The drift strips provide an electrostatic shield so that the movement of the positive charge carriers (holes) only will induce a minor signal at the anode strip. The signal of the planar cathode electrode is influenced strongly by the holes. Combining the signals from the planar electrode and the anode strip, it is possible to correct for the hole contribution still present in the anode strip signal. The next step will be the development of a large-area CZT detector and a readout technique needed for missions within the Danish Micro Satellite Program. (author)