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 for the existing He-3 stockpile has produced a critical shortage of this gas which has virtually eliminated its viability for 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, which is critical for certain non-proliferation programs and special nuclear material (SNM) assessments. Therefore, the objective of the proposed research was to produce several spectrally matched and validated equivalent neutron detectors for the direct replacement of He-3 in these neutron detection applications. Prior to developing any actual designs, the fidelity of a computational approach was validated by executing radiation transport models for existing BF3 and He-3 tubes and then comparing the results of these models to laboratory measurements conducted with these exact detectors. Both tubes were 19.6 cm in height, with a 1-inch diameter, and operated at 1 atmosphere and 4 atmospheres pressure respectively. The models were processed using a combination of forward Monte Carlo and forward and adjoint 3-D Sn (discrete ordinates) transport methods. The computer codes MCNP5 and PENTRAN were used for all calculations with a nickel-filtered plutonium-beryllium (PuBe) source term that is equivalent to that of weapons-grade Pu. Once the computational methods were validated, several plug-in models were developed that matched the neutron spectral response and reaction rate of a 1-inch diameter He-3 tube with a length of 10 cm and operating at 4 atmospheres pressure. The equivalent designs consist of large singular tubes and dual tubes containing either BF3 gas, B-10 linings, and/or polyvinyl toluene (PVT).

SUBJECT:	Ph.D. Dissertation Defense

BY:	Scottie Walker

TIME:	Friday, May 3, 2013, 9:00 a.m.

PLACE:	Boggs, 3-47

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. James Petrosky (Air Force Institute of Technology) Dr. Adam Stulberg (INTA)

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 for the existing He-3 stockpile has produced a critical shortage of this gas which has virtually eliminated its viability for 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, which is critical for certain non-proliferation programs and special nuclear material (SNM) assessments. Therefore, the objective of the proposed research was to produce several spectrally matched and validated equivalent neutron detectors for the direct replacement of He-3 in these neutron detection applications. Prior to developing any actual designs, the fidelity of a computational approach was validated by executing radiation transport models for existing BF3 and He-3 tubes and then comparing the results of these models to laboratory measurements conducted with these exact detectors. Both tubes were 19.6 cm in height, with a 1-inch diameter, and operated at 1 atmosphere and 4 atmospheres pressure respectively. The models were processed using a combination of forward Monte Carlo and forward and adjoint 3-D Sn (discrete ordinates) transport methods. The computer codes MCNP5 and PENTRAN were used for all calculations with a nickel-filtered plutonium-beryllium (PuBe) source term that is equivalent to that of weapons-grade Pu. Once the computational methods were validated, several plug-in models were developed that matched the neutron spectral response and reaction rate of a 1-inch diameter He-3 tube with a length of 10 cm and operating at 4 atmospheres pressure. The equivalent designs consist of large singular tubes and dual tubes containing either BF3 gas, B-10 linings, and/or polyvinyl toluene (PVT).