A study of the spectroscopic response of planar CdTe detectors when irradiated at various angles of incidence

The behaviour of the spectroscopic response of CdTe detectors has been investigated with a novel irradiation geometry, i.e. with the impinging angle of the incoming photons, referred to the normal to the cathode surface, larger than 0°. In fact, the classical Planar Parallel Field irradiation geometry, i.e. with photons impinging perpendicularly to the cathode surface, limits the spectroscopic performance at high photon energies (E>60 keV), if thick detectors are used, in order to obtain high detection efficiency. This is due to the large variation in the amount of the charges collected for different paths of the charge carriers, owing to their low mobility and high trapping/detrapping and recombination probability. The collecting distances for the holes can be greatly reduced (according to the cosine law), by increasing the value of the impinging angle. This improves the energy resolution without losses in detection efficiency. Several sets of measurements were carried out by changing the impinging angle of a narrow photon beam, coming from collimated 57Co and 241Am sources, to the cathode surface of CdTe detectors; the values of the angle ranged from 0° (orthogonal to the cathode) to about 88° (beam grazing the cathode). The spectroscopic responses of the detectors for 14, 60 and 122 keV photon energies, at different angles, were evaluated and analysed. The experimental results show that an actual improvement in the spectroscopic performance is obtained, mainly for the high-energy X-rays, by this tilting procedure.