Level structure of ²¹Mg and the ²⁰Na(p, γ)²¹Mg stellar reaction rate

The nuclear level structure of ²¹Mg has been studied by the ²⁴Mg(³He, ⁶He)²¹Mg reaction at 74 MeV. Angular distributions of the three-nucleon transfer reaction (³He, ⁶He) are measured for the first time, and successfully analyzed with exact finite-range distorted-wave-Born-approximation calculations, giving clear transferred angular momentum assignments. More than 20 states have been identified with excitation energy and spin-parity determinations, including a possible s-wave resonance just above the proton threshold. One of the s-wave resonances assumed in the previous stellar reaction rate estimates is found to be a bound state. The stellar reaction rate of the ²⁰Na(p, γ)²¹Mg process is estimated using the experimental data. The results predict the ignition of the proton radiative-capture process at T = 1×10⁸K under typical nova conditions (hydrogen density, X_Hρ{variant} = 5×10⁵ g/cm³). This temperature happens to be in agreement with the previous theoretical estimates, but the new reaction rate is a few orders of magnitude smaller than the previous predictions for T {reversed tilde equals} 2-10×10⁸K. The results also suggest that the nucleosynthesis flow of the rapid-proton process will run up to ²¹Mg immediately after breakout from the hot-CNO cycle.