Measurement of Fission Yields Using Cyclic Neutron Activation Analysis and a Fast Fission Spectrum at the Penn State Breazeale Reactor

Open Access
- Author:
- Lani, Chad
- Graduate Program:
- Nuclear Engineering
- Degree:
- Doctor of Philosophy
- Document Type:
- Dissertation
- Date of Defense:
- May 09, 2024
- Committee Members:
- Jon Schwantes, Program Head/Chair
Amanda Johnsen, Major Field Member
Arthur Motta, Major Field Member
Marek Flaska, Chair & Dissertation Advisor
Bruce Pierson, Special Member
Luiz de Viveiros, Outside Unit & Field Member - Keywords:
- Neuton Activation Anaysis
Gamma Spectroscopy
Fission Yields
Short-Lived - Abstract:
- Due to a large number of incidents involving lost or stolen radioactive materials, there is fear that a nuclear weapon may be used by a non-state actor. To combat such scenarios, the field of nuclear forensics is looking for nuclear data that can aid in the determination of the constituents of a nuclear device post-detonation. Such data include the identification of the nuclear isotopes resulting from the fission of the actinide material. With the use of nuclear forensic techniques, experimental data can determine the origin of the material and where in its life cycle the material was diverted. To support nuclear forensics and the effort to analyze isotopes post-detonation, a new pneumatic transport system has been developed and installed at the Penn State Breazeale Reactor to measure fission fragments in an environment similar to a nuclear detonation by using the method of cyclic neutron activation analysis on actinide targets. Neutron activation analysis uses the fission of actinide materials and enables quantification of the fission fragments present. The neutron spectrum used to irradiate these targets is a fast fission spectrum with relatively small neutron thermalization. This spectrum is important since it resembles that of a nuclear detonation, and it focuses on a region of nuclear data where they are sparse or nonexistent. Thorium-232 was the first actinide to be irradiated and measured cyclically, improving the statistics of quickly decaying products. To perform these cyclic irradiations, the samples are pneumatically transferred from the reactor centerline to the detectors with a sub-second transit time. Having such a short transit time allows for the detection of isotopes with half-lives on the magnitude of seconds, which is a unique capability of this system when combined with an experimental reactor as the neutron source. High fidelity simulation models have been created in Geant4, a platform for Monte-Carlo simulations of radiation transport through matter. These models were used to assess the detection systems’ response to irradiated actinide materials and have been benchmarked against experimental measurements. The data gathering effort focused on the detection of short-lived fission fragments with half-lives between 0.5 and 20 seconds. These half-lives were selected because many of the isotopes in this range have not been measured, and this region pushed the limits of the newly built system. To aid in the measurement of nuclear data that can support nuclear forensics, cumulative fission yields have been extracted from the data with a temporal analysis and compared to the values of other experimental work or models where applicable. This work describes the pneumatic transport system in detail along with its characterization by using an indium and zirconium target. The detection system consists of a high-purity germanium detector used in conjunction with a CAEN Hexagon multi-channel and has been modeled and experimentally characterized with a mixed gamma standard. With the use of the pneumatic transport system, seven isotopes were successfully measured from the fission of 232Th, 88Br, 92Kr, 94Rb, 95Sr, 99Zr, 141Xe, and 143Ba. Of these isotopes, four of them have no prior experimental data. The shortest half-live measured was 141Xe at 1.73 seconds and each of the measured isotopes agree within uncertainty to the values published in the nuclear database, ENDF/B-VIII.0.