The structure of 23Al and astrophysical consequences

Motivated by existing nuclear astrophysics problems, the B-decay of the proton rich nucleus 23Al was studied for the first time with pure samples which were obtained by using the 1H(24Mg,23Al)2n reaction and the MARS recoil separator at Texas A&M University. B and B-y coincidence measurements were made with a fast tape-transport system, scintillator, BGO and HPGe y detectors. The experiment allowed us to measure absolute B branching ratios and to determine logft values for transitions to final states in 23Mg, including the isobaric analog state (IAS), and, therefore, to determine unambiguously the spin and parity of the 23Al ground state to be J^Pi = 5/2+. This work excludes the large increases in the radiative proton capture cross section for the reaction 22Mg(p, )23Al at astrophysical energies, which were implied by claims that the spin and parity of the 23Al ground state were J^Pi = 1/2+. More precise half life and mass determinations of 23Al were obtained from the experimental data. The logft for the Fermi transition to its isobaric analog state in 23Mg was also determined for the first time. This IAS and a state 16 keV below it were observed, well separated in the same experiment for the first time. The B-decay scheme of the proton rich nucleus 23Al was established. We can now solve a number of inconsistencies in the literature, exclude strong isospin mixing claimed before, and obtain a new determination of the resonance strength. The IAS and the state 16 keV below it are resonances in the 22Na(p, )23 Mg reaction at energies that are important in novae. This second state turns out to be the resonance that gives the most important contribution in the depletion of 22Na from novae. Both of the reactions of 22Mg(p, )23Al and 22Na(p, )23Mg have been suggested as possible candidates for diverting some of the flux in oxygen-neon novae explosions from the A=22 into the A=23 mass chain.