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The semiconductor nuclear detector structure is usually a superposition of several different layers: the metallic electrode, the interface, the depleted zone, the nondepleted one, and finally the second electrode. All inactive layers, and particularly the 'dead layer' region below the metallic electrodes, give rise to absorption for photons (X, [beta]) and to energy loss for charged particles. As a consequence, the detectors measure lower count rates for photons, particularly at low energies, and lower energies for charged particles. The amount of this effect can be only partially attributed to the inactive metallic electrode; the main part has to be attributed to the highly defected zone below the electrode in the interfacial region. This is a structural drawback which needs to be studied to reduce this effect, e.g., by developing new polishing and deposition techniques, or to perform the appropriate corrections. We have measured the thickness of this dead layer for various solution dilution electroless depositions of Pt electrodes on CdTe devices. These measurements were carried out by using collimated (X, [lambda]) rays beams impinging at different angles, especially the grazing ones, and comparing the count rates registered at two selected angles, and by measuring Rutherford backscattering spectra and energy loss of [sup.4] He ions. Index Terms--CdTe detectors, dead layer, energy loss, Rutherford backscattering spectrometry (RBS) measurements, various irradiation angles, X-ray measurements.

In the classic irradiation configuration of solid-state X- and Gamma-ray detectors, where the photons impinge normally to the cathode, the electrode and detector dead layer thickness affect the detection efficiency and the spectroscopic performance of the device, mainly at the lowest incident photon energies. The concentration of defects in the near-electrode regions greatly depends on the mechanical and chemical treatments used for the surface preparation (polishing and etching), before electrode deposition, as well as on metallic diffusion in the crystal, which result in differently thick dead layers. In this paper, we evaluate, in an easy, experimental way, the dead layer thickness, irradiating a detector by a narrow photon beam, at various incident angles. The areas relevant to the 14-keV ([sup.57]Co) and to 22-keV ([sup.109]Cd) photopeaks, at different angles of incidence, are independently used to solve a linear equation depending on the photons absorption in the electrodes (Pt) and the dead layer (CdTe) material. As an example, data obtained with a CdTe detector 2-mm-thick and 3 X 10 [mm.sup.2] electrode area are presented and discussed.

The spectroscopic response of CdTe detectors has been investigated with an unusual irradiation geometry, i.e., with the beam impinging at various angles through the cathode surface. In fact, the classical planar parallel field (PPF) irradiation geometry, i.e., with photons impinging perpendicularly to the cathode, limits the spectroscopic performance at high energies (E > 60 keV), if thick detectors are used, owing to the large spread in the paths of the charge carriers. These paths for the holes can be greatly reduced by using larger impinging angles, thus improving the energy resolution without losses in detection efficiency. Several sets of measurements have been carried out with a narrow photon beam, coming from collimated [sup.57]Co, [sup.109]Cd and [sup.241]Am sources; measurement angles ranged from 0[degrees] (orthogonal to the cathode) to 87[degrees] (beam grazing the cathode). The spectroscopic responses of the detectors for 14, 22, 60 and 122 keV photon energies have been analyzed. Detectors equipped with both full anodes and microstrip anodes were used. In this last case the rectangular collimated spot of the beam was used with the long side both parallel and orthogonal to the strip direction. The experimental results show that an actual improvement in the spectroscopic performance is obtained, mostly for the high-energy X-rays, by this tilting procedure. Further, this technique allows the evaluation of the detector dead layer thickness. The obtained performance and possible medical and space applications are discussed. Index Terms--CdTe detectors, CCE, spectroscopy, various irradiation angles.