Photoexcitation of astrophysically important states in ²⁶Mg

We performed a nuclear resonance fluorescence experiment to determine the energy and quantum numbers of excited states in ²⁶Mg. Spin-parity ambiguities of excited states in ²⁶Mg, the compound nucleus for the s-process neutron source ²²Ne(α,n)²⁵Mg, result in large uncertainties in the reaction rates. The present work uses the monoenergetic γ-ray beam from the High-Intensity γ-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=10 573.3(8) keV (J^π=1^-), E_x=10 647.3(8) keV (J^π=1⁺), E_x=10 805.7(7) keV (J^π=1^-), E_x=10 949.1(8) keV (J^π=1^-), and E_x=11 153.5(10) keV (J^π=1⁺). The two natural parity states, located between the α-particle and neutron thresholds, are expected to significantly influence the rate of the competing ²²Ne(α,γ)²⁶Mg reaction. An important finding of this work is that the E_x=11 154 keV level has unnatural parity, contrary to previous results, and thus does not contribute to the ²²Ne+α reaction rates.