Characterisation of cadmium zinc telluride detector spectra - application to the analysis of spent fuel spectra
Room-temperature semiconductor detectors such as CdTe and CdZnTe have been used in the field of nuclear safeguards to characterise spent fuel (SFAT, Gamma-Taucher), to determine the uranium enrichment and to verify the unirradiated nuclear fuel. The energy resolution of these detectors is lower than for a typical germanium detector but significantly better than for a sodium iodide detector. An incomplete charge collection leads to spectral distortion manifested by asymmetric full-energy peaks and long low-energy tails. The interpretation of the spectra obtained with a CdTe or a CdZnTe detector requires correspondingly accurate knowledge of the analytical line shape. In the present work, the spectral response of a hemispherical CZT (10×10×5 mm 3) detector is described with a set of functions (GPP code, J. L. Campbell). The fit parameters have been determined with standard γ sources thus covering the energy range 121-1332 keV. Point sources in different experimental conditions (background, shielding) have been used to simulate spent fuel spectra. Applying the fixed fit parameters, the ratio 134Cs/ 137Cs has been determined with a good precision.