STRUCTURE OF AL-24 FOR THE STELLAR REACTION-RATE OF MG-23 (P,GAMMA) AL-24

The nuclear levels of Al-24 were studied by the Mg-24(He-3,t) Al-24 reaction at 60 MeV. Several new states were identified including a level at 2.787 MeV just above the proton threshold in Al-24, and considerable level shifts were observed for most levels around the threshold. The spin-parity assignments were also made for many states including the threshold states by the angular distribution measurement. This clarifies the spin-parity and the excitation energy of the critical levels assumed before for the stellar reaction rate calculation of Mg-23 (p, gamma) Al-24. A large increase of the reaction rate is predicted, based on the present experimental result, at the temperature region of T9 = 0.2-0.8 which would be crucial for the break-out process of the NeNa cycle to the MgAl cycle. This result suggests that the onset temperature of Mg-23(p, gamma) Al-24 should be lowered roughly by 10%. Application of the (He-3, t) reaction at 60 MeV for spin assignment is also discussed.