An experimental method to evaluate the dead Layer thickness of X- and gamma-ray semiconductor detectors

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 (⁵⁷Co) and to 22-keV (¹⁰⁹Cd) 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 3×10 mm² electrode area are presented and discussed.