Investigation of response behavior in CdTe detectors versus interelectrodic charge formation position

Some important features of semiconductor detectors (pulse height, energy resolution, photopeak efficiency) are strongly affected by charge collection efficiency; therefore low charge mobility and trapping/detrapping phenomena can more or less degrade the CdTe based detectors performance, depending on the distance between the charge formation location and the collecting electrodes. Using a narrow photon beam, obtained by a 20 mm thick W collimator having a 0.2×2 mm² collimating channel, we have studied the response of a 3×5×2 mm³ CdTe(Cl) planar detector, having the electrodes deposited on the 3×5 mm² sides. In order to investigate the behavior of the above parameters vs. the charge formation position induced by the incoming radiation we have performed a fine scanning of the interelectrodic region irradiating laterally the detector, that is in the configuration that we indicate as a planar transverse field (PTF). For comparison the same detector was irradiated also with a non collimated beam both in lateral (PTF) and in the classical configuration, that is with the beam entering through the cathode, a planar parallel field (PPF). When irradiated in PTF way, a region of the detector having a very good spectroscopic performance can be identified close to the cathode; this allows one to select region sizes with definite energy resolution values, suitable for different applications