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

TL;DR

This study addresses the need for quasi-stellar neutron spectra to study s-process neutron captures by extending laboratory sources beyond $^{7}$Li$(p,n)^{7}$Be to the near-threshold $^{18}$O$(p,n)^{18}$F reaction. It introduces OxyGen, a Monte Carlo tool that models the neutron energy and angular distributions near threshold and integrates with Geant4 to simulate detector response and underpin Bayes unfolding of TOF data. Validation against PTB measurements and comparison with Heil et al. data show that OxyGen can reliably predict neutron spectra for planning and analyzing SARAF experiments with different proton energies, using a liquid $H_2^{18}O$ target. The work demonstrates a practical approach to extend the accessible $kT$ range for MACS studies, improving our ability to constrain s-process cross-sections in AGB stars and guiding targeted experimental programs at SARAF.

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 ($α$,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$25 keV. Expanding the range of reactions used for measuring neutron-induced s-process cross-sections can offer valuable insights into the s-process in AGB stars. One such reaction is 18O(p,n)18F, which can be used to mimic s-process cross-sections, as its neutron energy distribution is similar to the thermal flux distribution at the stellar environment where the 13C($α$,n)16O reaction (kT = 8 keV) takes place. Heil et al. showed that the neutron energy spectrum emitted from the 18O(p,n)18F reaction at proton energy of Ep = 2582 keV, close to the reaction threshold of 2574 keV, closely resembles a Maxwellian flux with kT$\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.

Figures (22)

• Figure 1: Experimental setup. A proton beam of $E_p=2580.5$ keV impinges on a thick Ta$_2^{18}$O$_5$ target with Ta backing. The proton beam induces the $^{18}$O$(p,n)$ reaction which serves as the neutron source. The neutrons are detected by a movable $^6$Li-glass detector, moving along a circular track at a distance of 20 cm. The proton beam is pulsed, and the neutron energy is determined by their time of flight (TOF). The experiment was repeated with the detector positioned at a 0° angle relative to the beam axis and up to 60° in 10° increments.
• Figure 2: A picture of the experimental setup, including the proton beam (A), $^{18}$O target with backing (B), movable Li-glass detector (C) and the circular track on which the detector was placed (D).
• Figure 3: This sketch outlines the data acquisition scheme employed in the experiment.
• Figure 4: The pulse height (PH) and time of flight (TOF) for $0^\circ$ are shown both as a 2D histogram and as projections onto each axis, with the counts displayed on a logarithmic scale (z-axis for panel A and y-axis for panels B-D). The 2D histogram of PH versus TOF is presented in panel A, the 1D projection onto PH is shown in panel B, and the 1D projection onto TOF in panel C. Panel D shows the TOF projection restricted to events with PH values between 60 and 110 channels.
• Figure 5: Daily TOF spectra measured at $0^\circ$ throughout the experiment, demonstrating the stability of the proton beam. The spectra were obtained after applying the PH cut of 60-110 channels, consistent with the extraction shown in Fig. \ref{['fig:rawTOFvsPH']}D.