Investigation of thermonuclear ${}^{18}$Ne($\hat{I}\pm$,$p$)${}^{21}$Na rate via resonant elastic scattering of ${}^{21}$Na+$p$

Abstract

The ${}^{18}$Ne($\hat{I}\pm$,$p$)${}^{21}$Na reaction is thought to be one of the key breakout reactions from the hot CNO cycles to the rp-process in type I x-ray bursts. In this work, the resonant properties of the compound nucleus ${}^{22}$Mg have been investigated by measuring the resonant elastic scattering of ${}^{21}$Na+$p$. An 89 MeV ${}^{21}$Na radioactive beam delivered from the CNS Radioactive Ion Beam Separator bombarded an 8.8 mg/cm${}^2$ thick polyethylene (CH${}_2$)${}_n$ target. The ${}^{21}$Na beam intensity was about 2$\times$10${}^5$ pps, with a purity of about 70% on target. The recoiled protons were measured at the center-of-mass scattering angles of $\hat{I}{¸}_{c.m.}$$\approx$175.2${}^{\circ }$, 152.2${}^{\circ }$, and 150.5${}^{\circ }$ by three sets of $\hat{I}”E$-$E$ telescopes, respectively. The excitation function was obtained with the thick-target method over energies $E_x$(${}^{22}$Mg)=5.5--9.2 MeV. In total, 23 states above the proton-threshold in ${}^{22}$Mg were observed, and their resonant parameters were determined via an $R$-matrix analysis of the excitation functions. We have made several new $J^{\ddot{I}}€$ assignments and confirmed some tentative assignments made in previous work. The thermonuclear ${}^{18}$Ne($\hat{I}\pm$,$p$)${}^{21}$Na rate has been recalculated based on our recommended spin-parity assignments. The astrophysical impact of our new rate has been investigated through one-zone postprocessing x-ray burst calculations. We find that the ${}^{18}$Ne($\hat{I}\pm$,$p$)${}^{21}$Na rate significantly affects the peak nuclear energy generation rate, reaction fluxes, as well as the onset temperature of this breakout reaction in these astrophysical phenomena.