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

TL;DR

This paper investigates the 18O(p,n)18F reaction as a neutron source that mimics stellar s-process conditions, developing simulations to predict neutron spectra for future nuclear astrophysics experiments.

Contribution

It introduces the OxyGen computational tool for predicting neutron spectra from the 18O(p,n)18F reaction, aiding experimental planning at SARAF.

Findings

OxyGen accurately predicts neutron energy spectra.

Simulations match experimental results.

The method supports future s-process studies.

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.