Monday, January 21, 2013

1301.4283 (J. J. He et al.)

Key $^{18}$Ne($α$,$p$)$^{21}$Na breakout reaction in x-ray bursts: first experimental determination of spin-parities for $α$ resonances in $^{22}$Mg via resonant elastic scattering of $^{21}$Na+$p$    [PDF]

J. J. He, L. Y. Zhang, A. Parikh, S. W. Xu, H. Yamaguchi, D. Kahl, S. Kubono, J. Hu, P. Ma, S. Z. Chen, Y. Wakabayashi, B. H. Sun, H. W. Wang, W. D. Tian, R. F. Chen, B. Guo, T. Hashimoto, Y. Togano, S. Hayakawa, T. Teranishi, N. Iwasa, T. Yamada, T. Komatsubara
The $^{18}$Ne($\alpha$,$p$)$^{21}$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. To 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 a 8.8 mg/cm$^2$ thick polyethylene target. The recoiled protons were measured at scattering angles of $\theta_{c.m.}$$\approx175{^\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, we have experimentally determined the $J^{\pi}$ values for ten states above the $\alpha$ threshold in $^{22}$Mg. 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. Our new rate significantly affects the peak nuclear energy generation rate and the onset temperature of this breakout reaction in these phenomena.
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