Resonant scattering of Na-22 + p studied by the thick-target inverse-kinematic method

Abstract

Background: In presolar low-density graphite grains, an extraordinarily large Ne-22/Ne-20 ratio or even nearly pure Ne-22 is found, pointing to the condensation of radioactive Na-22 in grains. Supernovae and neon-rich novae are the main events that produce Na-22 via the explosive hydrogen burning process. The Na-22(p,gamma)Mg-23 reaction is one of the key reactions that influences the Na-22 abundance in ejecta. Purpose: The present work aims to explore the proton resonant states in Mg-23 relevant to the astrophysical Na-22(p,gamma)Mg-23 reaction. The determined Mg-23 resonant parameters can be used to evaluate the Na-22(p,gamma)Mg-23 reaction rate. Method: A low-energy Na-22 radioactive ion beam is produced via the H-1(Ne-22, Na-22)n reaction, and used to measure the experimental excitation function of the Na-22 + p resonant scattering with a conventional thick-target inverse kinematic method. R-matrix analysis is applied to deduce the Mg-23 resonance parameters from the experimental excitation function. Results: Three proton resonance states in Mg-23 are observed. Spins/parities and the proton partial widths are determined. The deduced excitation energies agree with the compiled values. Conclusions: The new spin and parity assignments allow us to perform a shell-model calculation of the. widths of the Mg-23 resonant states for the evaluation of the Na-22(p,gamma) Mg-23 astrophysical reaction rate. The two s-wave resonant states established in this work at 8.793 and 8.916 MeV in Mg-23, respectively, increase the total reaction rate by about 5% at a temperature greater than 2 GK.