β decay of proton-rich nucleus ${}^{23}\mathrm{Al}$ and astrophysical consequences

We present the first study of the β 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&M University, collected on a fast tape-transport system, and moved to a shielded location where β and β-γ coincidence measurements were made. We deduced β 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 $J^{\pi }=5/2^{+}$. We discuss how this excludes the large increases in the radiative proton capture cross section for the reaction ${}^{22}\mathrm{Mg}(p,\gamma ){}^{23}\mathrm{Al}$ at astrophysical energies, which were implied by claims that the spin and parity is $J^{\pi }=1/2^{+}$. 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 ${}^{22}\mathrm{Na}$($p,\gamma$)${}^{23}\mathrm{Mg}$ reaction at energies important in novae. The reactions ${}^{22}\mathrm{Mg}(p,\gamma ){}^{23}\mathrm{Al}$ and ${}^{22}\mathrm{Na}(p,\gamma ){}^{23}\mathrm{Mg}$ 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.

Figure 1

Astrophysical $S$ factor for the ${}^{22}\mathrm{Mg}$($p,\gamma$)${}^{23}\mathrm{Al}$ reaction, calculated assuming $J^{\pi }=5/2^{+}$ (dashed line), or $J^{\pi }=1/2^{+}$ (orbital inversion, full line) for the g.s. of ${}^{23}\mathrm{Al}$.

Figure 2

The gamma-ray spectrum in coincidence with betas from ${}^{23}\mathrm{Al}$ decay. (a) The energy range E${}_{\gamma }=0\text{−}4$ MeV (notice the log scale). (b) The energy range E${}_{\gamma }=4\text{−}9$ MeV (linear scale). SE and DE denote single-escape and double-escape, respectively.

Figure 3

The relevant part of the decay scheme of ${}^{23}\mathrm{Al}$.