A Method To Determine Detector Response Functions In A Heavily Shielded Environment And Application To Spent Fuel Measurements With Cadmium Zinc Telluride Detectors

SCK•CEN is developing an instrument for the measurement of neutron and gamma signatures from a spent fuel element. To design and optimize the system the MCNPX code is used. One of the quantities that are being studied is the expected gamma-ray energy spectrum from a Cadmium Zinc Telluride detector in presence of spent fuel element. Due to the presence of a highly shielded configuration, a small detector and an extended source, standard Monte Carlo methods are very inefficient and time consuming. To tackle this problem, an original approach was developed and applied. We de-coupled the transport problem in two sub-problems; in a first simulation the gamma-ray spectrum impinging the detector as a function of the source gamma-ray energy is determined; in a separate simulation, the intrinsic detector response function was computed; the results from these independent simulations were then combined to calculate the expected detector response to a generic gamma-ray source. We show that the computational time is reduced by a factor 40 with good agreement with the results that are obtained when the full particle transport is carried out. The proposed methodology is also applied to determine the detector response when a Compton suppression system is used.