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 mm3) 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.