Combined MCNP/GADRAS simulation of HPGe gamma spectra

The ability to accurately simulate gamma spectra from materials that emit both neutrons and gammas is very important to the analysis of special nuclear materials (SNM), e.g., uranium and plutonium. The approach presented in this paper uses MCNP neutron/photon transport simulations coupled to GADRAS detector response calculations to produce realistic estimates of the full gamma spectrum, including full-energy peaks, Compton continua and other features. The approach is expected to generate more accurate gamma ray spectra for complex three-dimensional geometries than can be obtained from one-dimensional deterministic transport simulations (e.g. ONEDANT) like those currently implemented in GADRAS. Although MCNP simulations are generally more time consuming than one-dimensional deterministic transport simulations, MCNP can handle coupled continuous-energy neutron/photon particle transport through complex geometries in three dimensions, and it obviates the need for approximations employed in deterministic multigroup SN calculations. The main advantage of this approach results from (1) how easy it is to simulate complex three-dimensional geometries in MCNP, and (2) GADRAS´s ability to accurately calculate the response of a gamma spectrometer to the incident gamma and neutron field. The approach presented here offers some clear advantages over one-dimensional deterministic transport codes to accurately simulate real-world situations and generate realistic estimates of measured gamma spectra.