MCNPTM simulations for identifying environmental contaminants using prompt gamma rays from thermal neutron capture reactions

The primary purpose of the Multispectral Neutron Logging (MSN) project was to assess the effectiveness of existing neutron-induced spectral gamma-ray logging techniques for identifying environmental contaminants along boreholes, to further improve the technology, and to transfer that technology to industry (Conaway and Frankle, 1993). The logging instrument incorporated a pulsed 14 MeV neutron source and HPGe detector. Gamma-ray spectra from thermal neutron capture reactions were used to identify contaminants in the borehole environment. This type of technique complements physical sampling and is useful in environmental restoration projects where characterization of contaminated sites is required and long-term monitoring may be needed. We used an enhanced version of the Monte Carlo N-Particle computer code (MCNPTM) to investigate many more potential contaminants than could be accomplished experimentally. MCNP was tested and benchmarked with experimental data for Cl provided by our collaborators (Myers, 1988; Frankle, 1994) and experimental data for Cl, Cd, Sm, Gd and Hg obtained as part of this project (Conaway and Duray, 1994). A number of enhancements were developed for MCNP as a result of this project, some of which will be part of the next major code release. The results from the benchmarking effort, estimates of minimum detection thresholds for our instrument based on MCNP simulations, and some of the MCNP code enhancements will be discussed.