Spectroscopic performance of newly designed CdTe detectors

An analysis of the variation in size of good spectroscopy region as a function of the inter-electrode distance for planar CdTe detectors operated at room temperature is reported. Four detectors with inter-electrode distance ranging from 1.0 to 2.5 mm, in steps of 0.5 mm, were investigated. The measurements were carried out by scanning the detectors, in the configuration with the electric field perpendicular to the incoming radiation, Planar Transverse Field (PTF), with a narrow beam of 122 keV photons, obtained by using a 20 mm thick tungsten collimator having a 0.2×2 mm2 collimating channel. A “best charge collection region” has been identified, close to the cathode and ∼0.4 mm wide, almost independent of the detector thickness in the above range, in which the photopeak amplitude and the energy resolution assume the best and sufficiently constant values. Outside this region, both these parameters rapidly worse, with a behavior well described by the Hechtʹs relationship. These characteristics appear essentially in the energy range 30<Ex<250 keV, a very important one for space astrophysical applications, as well as in medical radiology. On the basis of these results, 0.5 mm thick detectors were envisaged, as single elements to be stacked in linear arrays, in order to set up larger-area detectors with optimum spectroscopic performance; further, coupled detectors obtained by mounting two similar detectors with a common anode (back-to-back configuration) have been designed and produced, in order to reduce the electronics associated with large detector arrays. The above observed features are lost when using these thin detectors, as in this case a wide non-linear near-anode region overlaps the “best spectroscopy region” producing a quick degradation of the spectroscopic performance after only 0.2 mm from the cathode; the proposed coupled detectors configuration does not introduce further significant noise sources. Larger statistics with detectors obtained by crystal slices cut from different parts of the same ingot is needed to confirm the observed features; in any case, changes in the detector size (both thickness and electrode areas) and better contacting techniques could result in the improvements of the spectroscopic performance of thin detectors.