High-resolution study of levels in the astrophysically important nucleus ${}^{26}\mathrm{Mg}$ and resulting updated level assignments

Background: The ${}^{22}\mathrm{Ne}(\alpha ,n){}^{25}\mathrm{Mg}$ reaction is an important source of neutrons for the $s$-process. Direct measurement of this reaction and the competing ${}^{22}\mathrm{Ne}(\alpha ,\gamma ){}^{26}\mathrm{Mg}$ reaction are challenging due to the gaseous nature of both reactants, the low cross section and the experimental challenges of detecting neutrons and high-energy $\gamma$ rays. Detailed knowledge of the resonance properties enables the rates to be constrained for $s$-process models.

Purpose: Previous experimental studies have demonstrated a lack of agreement in both the number and excitation energy of levels in ${}^{26}\mathrm{Mg}$. To try to resolve the disagreement between different experiments, proton and deuteron inelastic scattering from ${}^{26}\mathrm{Mg}$ have been used to identify excited states.

Method: Proton and deuteron beams from the tandem accelerator at the Maier-Leibnitz Laboratorium at Garching, Munich, were incident upon enriched ${}^{26}\mathrm{MgO}$ targets. Scattered particles were momentum-analyzed in the Q3D magnetic spectrograph and detected at the focal plane.

Results: Reassignments of states around $E_x=10.8\text{–}10.83$ MeV in ${}^{26}\mathrm{Mg}$ suggested in previous works have been confirmed. In addition, new states in ${}^{26}\mathrm{Mg}$ have been observed, two below and two above the neutron threshold. Up to six additional states above the neutron threshold may have been observed compared to experimental studies of neutron reactions on ${}^{25}\mathrm{Mg}$, but some or all of these states may be due to ${}^{24}\mathrm{Mg}$ contamination in the target. Finally, inconsistencies between measured resonance strengths and some previously accepted $J^{\pi }$ assignments of excited ${}^{26}\mathrm{Mg}$ states have been noted.

Conclusion: There are still a large number of nuclear properties in ${}^{26}\mathrm{Mg}$ that have yet to be determined and levels that are, at present, not included in calculations of the reaction rates. In addition, some inconsistencies between existing nuclear data exist that must be resolved in order for the reaction rates to be properly calculated.

Figure 1

Excitation-energy spectra for ${}^{26}\mathrm{Mg}$. See the figure for details of each spectrum. Vertical black lines denote a state that is observed at multiple angles; green dashed lines denote a contaminant peak. Black diamonds mark the ${}^{16}\mathrm{O}$ contaminant peaks. The solid red line is the fit.

Figure 2

Excitation-energy spectra for ${}^{26}\mathrm{Mg}$. See the figure for details of each spectrum. Vertical black lines denote a state that is observed at multiple angles; green dashed lines denote a contaminant peak. The solid red line is the fit.

Figure 3

Comparison of ${}^{28}\mathrm{Si}(p,p^{\prime }){}^{28}\mathrm{Si}$ (blue) and ${}^{28}\mathrm{Si}(d,d^{\prime }){}^{28}\mathrm{Si}$ (red) spectra at 40 degrees. The suppression of the $T=1$, 10.883- and 10.900-MeV states (diamonds) in ${}^{28}\mathrm{Si}(d,d^{\prime }$) is clear. The peak at 11.173 MeV, which only appears in the ($p,p^{\prime }$) spectrum, is due to ${}^{16}\mathrm{O}$ contamination. The broad state visible between 10.7 and 10.8 MeV in the ${}^{28}\mathrm{Si}(d,d^{\prime }){}^{28}\mathrm{Si}$ spectrum is due to ${}^{16}\mathrm{O}$ contamination.