Executive SummaryThis project seeks to identify a new signature for actinide element detection in active interrogation. This technique works by exciting and identifying long-lived nuclear excited states (isomers) in the actinide isotopes and/or primary fission products. Observation of isomers in the fission products will provide a signature for fissile material. For the actinide isomers, the decay time and energy of the isomeric state is unique to a particular isotope, providing an unambiguous signature for SNM.This project entails isomer identification and characterization and neutron population studies. This document summarizes activities from its third year -completion of the isomer identification characterization experiments and initialization of the neutron population experiments.The population and decay of the isomeric state in 235 U remains elusive, although a number of candidate gamma rays have been identified in earlier work. In the course of the experiments, a number of fission fragment isomers were populated and measured [Ressler 2010]. The decays from these isomers may also provide a suitable signature for the presence of fissile material.Several measurements were conducted throughout this project. This report focuses on the results of an experiment conducted collaboratively by PNNL, LLNL and LBNL in December 2010 at LBNL. The measurement involved measuring the gamma-rays emitted from an HEU target when bombarded with 11 MeV neutrons. This report discussed the analysis and resulting conclusions from those measurements.There was one candidate, at 1164.9 keV, of an isomeric signature of 235 U. The half-life of the state is estimated to be 42 µs. The measured time dependence fits the decay time structure, however the statistics of the measurement prevent the ambiguous declaration that the gamma-ray has the anticipated time structure. Preliminary investigations into alternative explanations of this gamma line, i.e. fission products, neutron scattering, neutron absorption and room background, did not support those alternative origins. In order to unambiguously declare the 1164-keV gamma ray a signature of isomeric transition of 235 U, a measurement with more statistic and better systematic controls would be necessary..
Executive SummaryThis project seeks to identify a new signature for actinide element detection in active interrogation. This technique works by exciting and identifying long-lived nuclear excited states (isomers) in the actinide isotopes and/or primary fission products. Observation of isomers in the fission products will provide a signature for fissile material. For the actinide isomers, the decay time and energy of the isomeric state is unique to a particular isotope, providing an unambiguous signature for SNM.This project entails isomer identification and characterization and neutron population studies. This document summarizes activities from its third year -completion of the isomer identification characterization experiments and initialization of the neutron population experiments.The population and decay of the isomeric state in 235 U remains elusive, although a number of candidate gamma rays have been identified in earlier work. In the course of the experiments, a number of fission fragment isomers were populated and measured [Ressler 2010]. The decays from these isomers may also provide a suitable signature for the presence of fissile material.Several measurements were conducted throughout this project. This report focuses on the results of an experiment conducted collaboratively by PNNL, LLNL and LBNL in December 2010 at LBNL. The measurement involved measuring the gamma-rays emitted from an HEU target when bombarded with 11 MeV neutrons. This report discussed the analysis and resulting conclusions from those measurements.There was one candidate, at 1164.9 keV, of an isomeric signature of 235 U. The half-life of the state is estimated to be 42 µs. The measured time dependence fits the decay time structure, however the statistics of the measurement prevent the ambiguous declaration that the gamma-ray has the anticipated time structure. Preliminary investigations into alternative explanations of this gamma line, i.e. fission products, neutron scattering, neutron absorption and room background, did not support those alternative origins. In order to unambiguously declare the 1164-keV gamma ray a signature of isomeric transition of 235 U, a measurement with more statistic and better systematic controls would be necessary..
The detection of illicit weapons-grade nuclear material is a significant technical challenge receiving a great deal of attention in the nuclear community. A proposed solution is to actively bombard the I would like to offer Dr. Jennifer J. Ressler my sincere gratitude, for without her this research would not have been possible. From bringing me onto her team, arranging the funding that supported this work, revising my documents, to answering countless questions, she has been immensely helpful. Equally deserving of special recognition is Dr. Todd S. Palmer, who has served as my adviser for the past five years. Dr. Palmer has given me the inspiration, guidance, motivation, and professional advice I needed in order to be successful both academically and professionally. I would also like to recognize Dr. David M. Hamby, whose engaging teaching style and course materials were invaluable to the research performed for this thesis.
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