The 18Ne(a,p)21Na breakout reaction in x-ray bursts: experimental determination of spin-parities for alpha resonances in 22Mg via resonant elastic scattering of 21Na+p

Abstract

The $^{18}$Ne($\alpha$,$p$)$^{21}$Na reaction provides a pathway for breakout from the hot CNO cycles to the $rp$-process in type I x-ray bursts. To better determine this astrophysical reaction rate, the resonance parameters 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 ion beam was produced at the CNS Radioactive Ion Beam Separator and bombarded an 8.8 mg/cm$^2$ thick polyethylene target. The recoiled protons were measured at scattering angles of $\theta_{c.m.}$$\approx 175 {^\circ}$ and 152${^\circ}$ by three $\Delta E$-$E$ silicon telescopes. The excitation function was obtained with a thick-target method over energies $E_x$($^{22}$Mg)=5.5--9.2 MeV. The resonance parameters have been determined through an $R$-matrix analysis. For the first time, the $J^{\pi}$ values for ten states above the alpha threshold in $^{22}$Mg have been experimentally determined in a single consistent measurement. We have made three new $J^{\pi}$ assignments and confirmed seven of the ten tentative assignments in the previous work. The $^{18}$Ne($\alpha$,$p$)$^{21}$Na reaction rate has been recalculated, and the astrophysical impact of our new rate has been investigated through one-zone postprocessing x-ray burst calculations. We find that the $^{18}$Ne($\alpha$,$p$)$^{21}$Na rate significantly affects the peak nuclear energy generation rate and the onset temperature of this breakout reaction in these phenomena.