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 Ex=10.8 to 11.4 MeV. Five excited states were observed and unambiguous quantum numbers were assigned at Ex=10 573.3(8) keV (Jπ=1-), Ex=10 647.3(8) keV (Jπ=1+), Ex=10 805.7(7) keV (Jπ=1-), Ex=10 949.1(8) keV (Jπ=1-), and Ex=11 153.5(10) keV (Jπ=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 Ex=11 154 keV level has unnatural parity, contrary to previous results, and thus does not contribute to the 22Ne+α reaction rates.