Experimental study ofβ-delayed proton decay ofAl23for nucleosynthesis in novae

Abstract

The $\ensuremath{\beta}$-delayed $\ensuremath{\gamma}$ and proton decay of $^{23}\mathrm{Al}$ has been studied with an alternative detector setup at the focal plane of the momentum achromat recoil separator MARS at Texas A University. We could detect protons down to an energy of 200 keV and determine the corresponding branching ratios. Contrary to results of previous $\ensuremath{\beta}$-decay studies, no strong proton intensity from the decay of the isobaric analog state (IAS) of the $^{23}\mathrm{Al}$ ground state at ${E}_{x}=7803$ keV in $^{23}\mathrm{Mg}$ was observed. Instead we assign the observed low-energy group ${E}_{p,\mathrm{c}.\mathrm{m}.}=206$ keV to the decay from a state that is 16 keV below the IAS. We measured both proton and gamma branches from the decay of this state at ${E}_{x}=7787$ keV in $^{23}\mathrm{Mg}$, which is a very rare case in the literature. Combining our data with its measured lifetime, we determine its resonance strength to be $\ensuremath{\omega}\ensuremath{\gamma}={1.4}_{\ensuremath{-}0.4}^{+0.5}$ meV. The value is in agreement with older direct measurements, but disagrees with a recent direct measurement. This state is the most important resonance for the radiative proton capture $^{22}\mathrm{Na}$($p,\ensuremath{\gamma}$)$^{23}\mathrm{Mg}$ in some astrophysical environments, such as novae.