SUMMARY

In order to model the energy deposition response of a gamma-ray detector for a broad range of wide area contamination scenarios accurately and in a timely manner, the proposed study seeks to develop a detector response function methodology by coupling the stochastic Monte Carlo radiation transport method with the CADIS hybrid radiation transport method to significantly increase the computational efficiency of thousands of Monte Carlo pulse height simulations. The CADIS hybrid method, available through the newly-developed Monte Carlo code, Shift, coupled with the 3D ad-joint transport solver Denovo, will be harnessed to develop high-fidelity detector response functions for a range of gamma-ray detectors for radionuclides of interest in the environmental assessment process. Capabilities and limitations of the radiation transport codes will be assessed for a breadth of contaminated media scenarios, modeled gamma-ray detectors, and radionuclide photon energies. To demonstrate the rationale behind the proposed methodology, an analysis study will be conducted to address the capabilities and limitations associated with pulse height tally variance re-duction in the following: native MCNP, the CADIS hybrid method in Shift coupled with MCNP, the ADVANTG code developed at Oak Ridge National Laboratory, the MCNP DXTRAN variance re-duction technique, and the MCNP weight-window generator. An investigatory measurement benchmark of the detector response functions for a laboratory-scale setup of environmental contamination and surrogate source terms is proposed to evaluate physical detector efficiency parameters. In addition, a method to determine the contribution of bremsstrahlung radiation from highly-attenuated beta-emitters in various environmental media will be incorporated into the detector response function methodology.