Monte Carlo Calculations Of Low Background Detector Response

An implementation of the Electron Gamma Shower 4 code (EGS4)[ I] has been developed to allow convenient simulation of typical gamma ray measurement systems. Coincidence gamma rays, beta spectra, and angular correlations have been added to adequately simulate a complete nuclear decay and provide corrections to experimentally determined detector efficiencies. This code has been used to strip certain low-background spectra for the purpose of extremely low-level assay. Monte Carlo calculations of this sort can be extremely successful since low background detectors are usually free of significant contributions from poorly localized radiation sources, such as cosmic muons, secondary cosmic neutrons, and radioactive construction or shielding materials. Validation of this code has been obtained from a series of comparisons between measurements and blind calculations. The singles counting efficiency of a particular counting experiment was reproduced within the accuracy of the known dimensions of the detector components. This allowed the complete simulation of a nuclear decay, including two coincident gamma rays with a non-isotropic angular correlation. In addition, text-book efficiency curves have been reproduced for several sizes of sodium iodide detectors. Examples of the use and several spectral results will be presented.