Revised decay properties of the key 93-keV resonance in the ${}^{25}\mathrm{Mg}(p,\gamma )$ reaction and its influence on the MgAl cycle in astrophysical environments

The $\gamma$-decay properties of an excited state in ${}^{26}\mathrm{Al}$ at 6398.3(8) keV have been reexamined using the ${}^{11}\mathrm{B}+{}^{16}\mathrm{O}$ fusion-evaporation reaction. This level represents a key 93.1(8)-keV resonance in the ${}^{25}\mathrm{Mg}+p$ system and its relative branching to the ${}^{26}\mathrm{Al}$ ground state, $f_0$, has been determined to be $0.76\pm 0.03$ (stat.) $\pm 0.10$ (syst.). This is a significantly higher value than the most recent evaluation and implies a considerable increase in the production of cosmic $\gamma$ rays from ${}^{26}\mathrm{Al}$ radioactivity.

Figure 1

(a) $\gamma \text{−}\gamma$ coincidence spectrum with a gate placed on the 1012-keV transition from the $2^{+}$, 2069-keV level in ${}^{26}\mathrm{Al}$. (b) $\gamma \text{−}\gamma$ coincidence spectrum with a gate placed on the 1342-keV transition from the $2^{+}$, 1759-keV level in ${}^{26}\mathrm{Al}$. (c) $\gamma \text{−}\gamma \text{−}\gamma$ spectrum with gates placed on the 2743- and 417-keV transitions showing coincidences with the $2^{+}$, 3160-keV level in ${}^{26}\mathrm{Al}$. (d) $\gamma \text{−}\gamma \text{−}\gamma$ spectrum with gates placed on the 476- and 1012-keV transitions showing coincidences with the $3^{+}$, 2545-keV level in ${}^{26}\mathrm{Al}$.

Figure 2

Observed angular distributions of (a) the 4330-keV transition and (b) 3239-keV $\gamma$ decay from the 93-keV resonance in the ${}^{25}\mathrm{Mg}+p$ system. The measured angular distributions are consistent with a $2^{-}$ spin-parity assignment for the 93-keV resonant state.

Figure 3

Partial level scheme of ${}^{26}\mathrm{Al}$. The transition energies in color refer to: (red) primary decays of the 6398-keV level; (blue) Gating transitions in the present study; (green) side-feeding (contaminant) transitions. The colored arrows mark the key transitions impacting the present revision of the ground state branching ratio: notice that the contaminant 4329-keV transition (green), shown here in Fig. 1, has almost the same energy as the 4328-keV primary transition (red). A significant population of the 6874-keV level will therefore contaminate the peak near 4328 keV that is observed in the coincidence spectrum gated on the 1012-keV transition (blue), as can be seen in Fig. 3(a) of Ref. [18].