β decay of proton-rich nucleusAl23and astrophysical consequences

Abstract

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