We performed a nuclear resonance fluorescence experiment to determine the energy and quantum numbers of excited states in ${}^{26}\mathrm{Mg}$. Spin-parity ambiguities of excited states in ${}^{26}\mathrm{Mg}$, the compound nucleus for the $s$-process neutron source ${}^{22}\mathrm{Ne}$($\alpha ,n$)${}^{25}\mathrm{Mg}$, result in large uncertainties in the reaction rates. The present work uses the monoenergetic $\gamma$-ray beam from the High-Intensity $\gamma$-ray Source to probe states in the excitation energy range of $E_x=10.8$ to $11.4$ MeV. Five excited states were observed and unambiguous quantum numbers were assigned at $E_x=10\hspace{0.5em}573.3(8)$ keV ($J^{\pi }=1^{-}$), $E_x=10\hspace{0.5em}647.3(8)$ keV ($J^{\pi }=1^{+}$), $E_x=10\hspace{0.5em}805.7(7)$ keV ($J^{\pi }=1^{-}$), $E_x=10\hspace{0.5em}949.1(8)$ keV ($J^{\pi }=1^{-}$), and $E_x=11\hspace{0.5em}153.5(10)$ keV ($J^{\pi }=1^{+}$). The two natural parity states, located between the $\alpha$-particle and neutron thresholds, are expected to significantly influence the rate of the competing ${}^{22}\mathrm{Ne}$($\alpha ,\gamma$)${}^{26}\mathrm{Mg}$ reaction. An important finding of this work is that the $E_x=11\hspace{0.5em}154$ keV level has unnatural parity, contrary to previous results, and thus does not contribute to the ${}^{22}\mathrm{Ne}$$+\mathrm{\alpha }$ reaction rates.

• Received 4 August 2009

DOI:https://doi.org/10.1103/PhysRevC.80.055803