Spectroscopic response of CdTe planar detectors, irradiated at various impinging angles, as a function of thickness

A new irradiation configuration for planar X- and gamma-ray detectors, where the angle between the normal to the cathode surface and the direction of the incident photons can be suitably set, has been recently proposed. This configuration allows one to enhance the detection efficiency, which depends on the absorbing thickness, minimizing at the same time the spread of the collecting distances for the charge carriers, thus improving the detector spectroscopic performance in the energy range 10/spl divide/150 keV. The possible dependence of the spectroscopic response of CdTe detectors, used in this configuration, on detector thickness has been investigated. Several detectors obtained from different ingots with inter-electrode distances of 3, 4, 5 mm, and 3/spl times/10 mm/sup 2/ electrode area, were irradiated with a narrow beam of /sup 57/Co photons at various angles of incidence. The detectors have been characterized observing the behavior of the photopeak centroid, the energy resolution, the photopeak efficiency and the peak-to-valley ratio. The results of this study confirm that the used configuration really improves the spectroscopic performance at high energies, where most of the photons are absorbed deeply in the crystal. Conversely, no a definite dependence on the devices thickness has been observed, showing that possible non-homogeneity in the electric field and/or in the dislocation of crystal defects is overcome already when using 3 mm thick devices. Thus the indication is toward the use of 2 mm thick detectors, as this would allow one to use lower bias voltage and to save material.