Detection and Characterization of Short-Lived Fission Fragments for Post-Detonation Analysis with Continuous and Monoenergetic Fast-Neutron Beams

We propose to characterize a wide range of short-lived fission fragments with half-lives between approximately one second and several hours by using cyclic neutron activation analysis (CNAA) to support DTRA's research and development efforts for pre- and post-detonation nuclear forensics. We also propose to use traditional neutron activation analysis (NAA) in conjunction with tailored radiochemistry procedures for fission fragments with half-lives up to several days. Specifically, we will use monoenergetic 14-MeV and fast-neutron spectrum (~MeV) to measure independent and cumulative fission yields of short-lived fission products (FPs) from fissile and fissionable actinides to enhance precision and expediency of pre- and post detonation nuclear-forensics non-destructive analyses. These fundamental parameters are poorly known for many FPs, especially for short-lived FPs. As a result, the measurement precision of pre- and post-detonation analyses based on detection of neutrons and gammas emitted from a sample is substantially limited. To mitigate this problem, the time-dependent delayed neutron and gamma intensities from short-lived FPs will be measured using neutron detectors and high resolution gamma spectrometers operating in coincidence. The Pennsylvania State Breazeale Reactor (PSBR) and PNNL's Thermo-Scientific D711 deuterium-tritium (D-T) 14-MeV neutron generator will be used to induce fissions in various actinides of interest, namely Th-232, U-233, U-235, U-238, and Am-241. The PSBR's fast-neutron irradiator (FNI) provides a neutron spectrum similar to that expected from a nuclear weapon, thus providing a unique capability for these measurements. Tailored radiochemical separation will be used to isolate problematic isotopes such as Ba-139, Ce-143, Rh-105, Pr-145, and Pm-151 with weak or low-energy gamma signatures for accurate quantification. The use of radiochemistry combined with the fission weapon- like neutron spectrum of the PSBR's FNI provides the unique opportunity to more precisely determine FP data for a realistic neutron spectrum. The final outcome of the project will be the independent and cumulative fission yields of nominally 70 short-lived FPs from each target actinide, including Zr-99, Se-86, As-85, Kr-92, and La-146, to allow for more accurate, robust, and expedient pre- and post-detonation non-destructive analyses. Measurements obtained during the course of this work will be compared with the Evaluated Nuclear Data Files (ENDF), the Joint Evaluated Fission/Fusion Files (JEFF), and theoretical nuclear-physics models. A follow-on work is also proposed to examine the feasibility of using the new measured data in conjunction with PSU's measurements for actinide-mixture assay to estimate the precision of actinide mixture assay using active interrogation.