Beta decay of proton-rich nucleus Al-23 and astrophysical consequences

Abstract

We present the first study of the beta decay of Al-23 undertaken with pure samples. The study was motivated by nuclear astrophysics questions. Pure samples of Al-23 were obtained from the momentum achromat recoil separator (MARS) of Texas AandM University, collected on a fast tape-transport system, and moved to a shielded location where beta and beta-gamma coincidence measurements were made. We deduced beta branching ratios and log ft values for transitions to states in Mg-23, and from them determined unambiguously the spin and parity of the Al-23 ground state to be J(pi)=5/2(+). We discuss how this excludes the large increases in the radiative proton capture cross section for the reaction Mg-22(p,gamma)Al-23 at astrophysical energies, which were implied by claims that the spin and parity is J(pi)=1/2(+). The log ft for the Fermi transition to its isobaric analog state (IAS) in Mg-23 is also determined for the first time. This IAS and a state 16 keV below it are observed, well separated in the same experiment for the first time. 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. Both states are resonances in the Na-22(p,gamma)Mg-23 reaction at energies important in novae. The reactions Mg-22(p,gamma)Al-23 and Na-22(p,gamma)Mg-23 have both 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.