Probing astrophysically important states in the Mg 26 nucleus to study neutron sources for the s process

Background: The Ne22(α,n)Mg25 reaction is the dominant neutron source for the slow neutron capture process (s process) in massive stars, and contributes, together with C13(α,n)O16, to the production of neutrons for the s process in asymptotic giant branch (AGB) stars. However, the reaction is endothermic and competes directly with Ne22(α,γ)Mg26 radiative capture. The uncertainties for both reactions are large owing to the uncertainty in the level structure of Mg26 near the α and neutron separation energies. These uncertainties affect the s-process nucleosynthesis calculations in theoretical stellar models. Purpose: Indirect studies in the past have been successful in determining the energies and the γ-ray and neutron widths of the Mg26 states in the energy region of interest. But, the high Coulomb barrier hinders a direct measurement of the resonance strengths, which are determined by the α widths for these states. The goal of the present experiments is to identify the critical resonance states and to precisely measure the α widths by α-transfer techniques. Methods: The α-inelastic scattering and α-transfer measurements were performed on a solid Mg26 target and a Ne22 gas target, respectively, using the Grand Raiden Spectrometer at the Research Center for Nuclear Physics in Osaka, Japan. The (α,α′) measurements were performed at 0.45?, 4.1?, 8.6?, and 11.1? and the (Li6,d) measurements at 0? and 10?. The scattered α particles and deuterons were detected by the focal plane detection system consisting of multiwire drift chambers and plastic scintillators. The focal plane energy calibration allowed the study of Mg26 levels from Ex = 7.69-12.06 MeV in the (α,α′) measurement and Ex = 7.36-11.32 MeV in the (Li6,d) measurement. Results: Six levels (Ex = 10717, 10822, 10951, 11085, 11167, and 11317 keV) were observed above the α threshold in the region of interest (10.61-11.32 MeV). The α widths were calculated for these states from the experimental data. The results were used to determine the α-capture induced reaction rates. Conclusion: The energy range above the α threshold in Mg26 was investigated using a high resolution spectrometer. A number of states were observed for the first time in α-scattering and α-transfer reactions. The excitation energies and spin-parities were determined. Good agreement is observed for previously known levels in Mg26. From the observed resonance levels the Ex = 10717 keV state has a negligible contribution to the α-induced reaction rates. The rates are dominated in both reaction channels by the resonance contributions of the states at Ex = 10951, 11167, and 11317 keV. The Ex = 11167 keV state has the most appreciable impact on the (α,γ) rate and therefore plays an important role in the prediction of the neutron production in s-process environments.