The $^{18}$O$(p,n)^{18}$F Reaction as a Quasi-stellar Neutron Source

Abstract

The s-process in AGB stars produces elements with atomic mass numbers $A\gtrsim60$ through successive neutron captures and beta decays. In stellar environments where the s-process occurs, neutrons quickly thermalize, adopting a Maxwell-Boltzmann energy distribution determined by the local temperature independent of their production via ($\alpha$,n) reactions. Laboratory experiments reproduce this Maxwell-Boltzmann neutron energy spectrum using (p, n) reactions. Specifically, the 7Li(p,n)7Be reaction is commonly employed to measure s-process cross-sections at kT$\approx$25 keV. Expanding the range of reactions used for measuring neutron-induced s-process cross-sections can offer valuable insights into the s-process in AGB stars. One such reaction is 18O(p,n)18F, which can be used to mimic s-process cross-sections, as its neutron energy distribution is similar to the thermal flux distribution at the stellar environment where the 13C($\alpha$,n)16O reaction (kT = 8 keV) takes place. Heil et al. showed that the neutron energy spectrum emitted from the 18O(p,n)18F reaction at proton energy of Ep = 2582 keV, close to the reaction threshold of 2574 keV, closely resembles a Maxwellian flux with kT$\approx$5 keV. This experiment was repeated at PTB and a computational tool, OxyGen, was created to calculate the expected neutron energy spectrum and angular distribution at a planned liquid water-based 18O target at SARAF. OxyGen was incorporated into Geant4 transport simulations, which were compared to experimental results. Overall, the findings of this thesis demonstrate that the OxyGen simulation provides a reliable prediction of the neutron energy spectrum for the 18O(p,n)18F reaction and can be effectively used to plan and analyze future experiments at SARAF for different proton energies.