The Woodruff School

SUMMARY

Neutron radiation detectors are an integral part of the Department of Homeland Security (DHS) efforts to detect the illicit trafficking of radioactive or special nuclear materials into the U.S. In the past decade, the DHS has deployed a vast network of radiation detection systems at various key positions to prevent or to minimize the risk associated with the malevolent use of these materials. The greatest portion of this detection burden has been borne by systems equipped with He-3 because of its highly desirable physical and nuclear properties. However, a dramatic increase in demand and dwindling supply combined with a lack of oversight regarding the existing 3He stockpiles has produced a critical shortage of this gas which has virtually eliminated the viability of the gas in detector applications. And, although a number of research efforts have been undertaken to develop suitable replacements, none of these efforts are attempting to closely match a He-3 detector response across different neutron energy spectra. Therefore, the objective of the proposed research will be to produce spectrally matched and validated equivalent neutron detectors for the direct replacement of He-3 based neutron detector systems in two typical scenarios.

The initial effort will go toward developing the baseline computational models associated with commercial off-the-shelf He-3 (4 atm) and BF3 tubes (0.96 atm). These models will be applied through a combination of forward Monte Carlo and forward and adjoint 3-D Sn (discrete ordinates) transport methods to calculate the applicable detector responses to the standard neutron source terms of plutonium-beryllium (PuBe) and spherical plutonium. The computer codes MCNP5 and PENTRAN will be used for these calculations, respectively, and the results from the codes will be compared with direct measurements obtained using the gas-filled detectors.

The comparison of the calculations and measurements will be used to establish the fidelity of the computational approach. And once this process has been validated, it will be applied toward the final task of computationally evaluating and designing spectrally-matched replacements for specific He-3 geometries using BF3 and polyvinyl toluene (PVT) detectors.

SUBJECT:	Ph.D. Proposal Presentation

BY:	Scottie Walker

TIME:	Friday, December 16, 2011, 3:00 p.m.

PLACE:	Boggs, 3-39

TITLE:	Spectrally-Matched Neutron Detectors Designed Using Computational Adjoint Sn for Plug-In Replacement of He-3

COMMITTEE:	Dr. Glenn Sjoden, Chair (NRE) Dr. Farzad Rahnema (NRE) Dr. Chaitanya Deo (NRE) Dr. Adam Stulberg (INTA) Dr. James Petrosky (Air Force Institute of Technology)

SUMMARY Neutron radiation detectors are an integral part of the Department of Homeland Security (DHS) efforts to detect the illicit trafficking of radioactive or special nuclear materials into the U.S. In the past decade, the DHS has deployed a vast network of radiation detection systems at various key positions to prevent or to minimize the risk associated with the malevolent use of these materials. The greatest portion of this detection burden has been borne by systems equipped with He-3 because of its highly desirable physical and nuclear properties. However, a dramatic increase in demand and dwindling supply combined with a lack of oversight regarding the existing 3He stockpiles has produced a critical shortage of this gas which has virtually eliminated the viability of the gas in detector applications. And, although a number of research efforts have been undertaken to develop suitable replacements, none of these efforts are attempting to closely match a He-3 detector response across different neutron energy spectra. Therefore, the objective of the proposed research will be to produce spectrally matched and validated equivalent neutron detectors for the direct replacement of He-3 based neutron detector systems in two typical scenarios. The initial effort will go toward developing the baseline computational models associated with commercial off-the-shelf He-3 (4 atm) and BF3 tubes (0.96 atm). These models will be applied through a combination of forward Monte Carlo and forward and adjoint 3-D Sn (discrete ordinates) transport methods to calculate the applicable detector responses to the standard neutron source terms of plutonium-beryllium (PuBe) and spherical plutonium. The computer codes MCNP5 and PENTRAN will be used for these calculations, respectively, and the results from the codes will be compared with direct measurements obtained using the gas-filled detectors. The comparison of the calculations and measurements will be used to establish the fidelity of the computational approach. And once this process has been validated, it will be applied toward the final task of computationally evaluating and designing spectrally-matched replacements for specific He-3 geometries using BF3 and polyvinyl toluene (PVT) detectors.