A measurement of the 18O(α,n)21Ne reaction
Creators: name: Toomey, Rebecca
Publisher: name: No Publisher Supplied
Year: 2022

description: For nuclear nonproliferation, passive neutron measurements are used to verify and characterize nuclear materials. This characterization requires an understanding of the decay product spectra. The o18an reaction is a dominant neutron source in fuels that are used to produce weapons-grade materials, such as UO$_2$ or PuO$_2$. There is also interest in this reaction for several other applications in nuclear physics, e.g. the production of $^{21}$Ne in Earth’s mantle. Neutron source calculations predict the expected neutron spectrum for specific nuclear source material. These calculations rely on nuclear data. In the energy range of the alpha-decay of actinide elements ($E_{alpha}=4-9$~MeV), little data are available for the o18an reaction, and it has therefore been highlighted as a focus for experimental efforts. Existing data for this reaction are limited, with large uncertainties, and no partial cross section or neutron angular distribution data. ODeSA - the ORNL Deuterated Spectroscopic Array - is an array of neutron detectors that was built and characterized for use with the spectrum unfolding method for neutron spectroscopy. This dissertation details the characterization of this array at the Edward's Accelerator Laboratory, and its subsequent deployment for measurements at the University of Notre Dame. The o18an reaction was measured over the range $2leq E_{alpha}leq 8$~MeV using the Sta. ANA accelerator, where primary reaction neutrons were measured with ODeSA and secondary gamma rays were detected using 2 high-purity germanium detectors. Partial cross sections were extracted for the first five excited states in $^{21}$Ne populated by the o18an reaction using the secondary gamma rays. The current results are in good agreement with previous measurements and highlight the inadequacies of available evaluations. Neutron partial differential cross sections were extracted for the ground and first excited state, and the combined yield for higher lying, closely-spaced excited states was also extracted. In addition, partial cross sections for the first and second excited states in inelastic alpha scattering on $^{18}$O were also extracted using secondary gamma rays. The partial cross sections deduced from the gamma rays in this dissertation were used to generate data libraries for use in neutron source calculations using the SCALE software. The performance of these new libraries was evaluated by comparing to existing neutron source measurements of $^{18}$O-enriched PuO$_2$ fuel matrices. A least squares analysis was used to quantify the agreement between the neutron source calculations and the source data. It was concluded that the incorporation of present partial cross section data into the data library improved the quality of the neutron source calculations. Looking to the future, full neutron differential cross section analysis up to $E_{alpha}=8$~MeV will be completed, in addition to angle integration of the neutron angular distributions to extract the partial cross sections. These neutron data can then also be incorporated into SCALE data libraries to further improve the performance of neutron source calculations.