Spectroscopic behavior of CdTe detectors as a function of the interelectrode distance

Abstract

An analysis of the variation in size of the best spectroscopy region as a function of the inter-electrode distance for planar CdTe detectors is reported. Four detectors with inter-electrode distance ranging between 1.0 and 2.5 mm, in steps of 0.5 mm, were investigated. The measurements were carried out by scanning the detectors with a narrow beam of 122 keV photons, obtained by using a 20 mm thick tungsten collimator having a 0.2 X 2 mm 2 collimating channel. The detectors were irradiated in the configuration with the electric field perpendicular to the incoming radiation, Planar Transverse Field. A 'best charge collection region' has been identified, close to the cathode and approximately 0.4 mm wide, independent of the detector thickness in the investigated range, in which the photopeak amplitude and the energy resolution assume the best and sufficiently constant values. When going outside this region, both these parameters become quickly worse, with a trend well described by the Hecht's relationship. These feature appear essentially in the energy range 30 x < 250 keV, a very important one for space astrophysical applications, as well as in medical radiology. At higher energies, changes in the prevalent photon interaction mechanism require use of alternative methods to improve the spectroscopic performance. An appropriate selection of the width of the irradiated region can give the possibility to match the requirements of particular applications.