Computer Simulation of a Germanium Detector Response to Highly Radioactive Waste

The Defense Waste Processing Facility (DWPF) at the Savannah River Site will prepare high-level radioactive waste for disposal through the use of a glass solidification process. Westinghouse Savannah River Laboratory personnel are supporting the DWPF in the area of process instrumentation. One type of monitoring system would use a germanium detector positioned at the canyon wall to monitor the waste canister as it is being filled. The ratio of photopeaks from appropriate fission product gamma rays will be determined to provide information on the uniformity of fill of the waste canister and to indicate process performance. Los Alamos National Laboratory personnel are supporting the Westinghouse Savannah River Laboratory by performing computer simulations of the germanium detector response. The Los Alamos Monte Carlo transport code, MCNP, was used to simulate the spectra obtained from a coaxial germanium detector that was nominally 30% efficient. The computer model included the germanium detector, the concrete wall collimator, and the canister itself. A pulse-height detector tally was used to bin the energy deposited in the germanium crystal. The ability of MCNP to accurately model the geometry and composition of the detector and surrounding materials is an important aspect in the accuracy of spectral simulation because much of the spectrum is due to scattered radiation. Effects of the high radiation level and subsequent distortions in the electronic processing of detector pulses were not included in the simulation. Results of the MCNP calculations and comparisons with experimental data will be presented.