The18Ne(α,p)21Na breakout reaction in x-ray bursts: Experimental determination of spin-parities forαresonances in22Mg via resonant elastic scattering of21Na+p

Abstract

The ${}^{18}$Ne($\ensuremath{\alpha}$,$p$)${}^{21}$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 ${\ensuremath{\theta}}_{\mathrm{c}.\mathrm{m}.}\ensuremath{\approx}175{}^{\ensuremath{\circ}}$ and 152${}^{\ensuremath{\circ}}$ by three $\ensuremath{\Delta}E$-$E$ 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 ${J}^{\ensuremath{\pi}}$ values for ten states above the $\ensuremath{\alpha}$ threshold in ${}^{22}$Mg have been experimentally determined in a single consistent measurement. We have made three new ${J}^{\ensuremath{\pi}}$ assignments and confirmed seven of the ten tentative assignments in the previous work. The ${}^{18}$Ne($\ensuremath{\alpha}$,$p$)${}^{21}$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($\ensuremath{\alpha}$,$p$)${}^{21}$Na rate significantly affects the peak nuclear energy generation rate and the onset temperature of this breakout reaction in these phenomena.