Probing Astrophysically Important States In The Mg-26 Nucleus To Study Neutron Sources For The S Process

Background: The Ne-22(alpha,n)Mg-25 reaction is the dominant neutron source for the slow neutron capture process (s process) in massive stars, and contributes, together with C-13(alpha,n)O-16, to the production of neutrons for the s process in asymptotic giant branch (AGB) stars. However, the reaction is endothermic and competes directly with Ne-22(alpha,gamma)Mg-26 radiative capture. The uncertainties for both reactions are large owing to the uncertainty in the level structure of Mg-26 near the a 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. gamma-ray and neutron widths of the Mg-26 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 alpha widths for these states. The goal of the present experiments is to identify the critical resonance states and to precisely measure the a widths by alpha-transfer techniques.Methods: The alpha-inelastic scattering and alpha-transfer measurements were performed on a solid Mg-26 target and a Ne-22 gas target, respectively, using the Grand Raiden Spectrometer at the Research Center for Nuclear Physics in Osaka, Japan. The (alpha,alpha(vertical bar)) measurements were performed at 0.45 degrees, 4.1 degrees, 8.6 degrees, and 11.1 degrees and the (Li-6, d) measurements at 0(degrees) and 10 degrees. The scattered alpha 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 Mg-26 levels from E-x = 7.69-12.06 MeV in the (alpha,alpha(vertical bar)) measurement and E-x = 7.36-11.32 MeV in the (Li-6, d) measurement.Results: Six levels (E-x = 10717, 10822, 10951, 11085, 11167, and 11317 keV) were observed above the alpha threshold in the region of interest (10.61-11.32 MeV). The a widths were calculated for these states from the experimental data. The results were used to determine the alpha-capture induced reaction rates.Conclusion: The energy range above the a threshold in Mg-26 was investigated using a high resolution spectrometer. A number of states were observed for the first time in alpha-scattering and alpha-transfer reactions. The excitation energies and spin-parities were determined. Good agreement is observed for previously known levels in Mg-26. From the observed resonance levels the E-x = 10717 keVstate has a negligible contribution to the alpha-induced reaction rates. The rates are dominated in both reaction channels by the resonance contributions of the states at E-x = 10951, 11167, and 11317 keV. The E-x = 11167 keV state has the most appreciable impact on the (alpha,gamma) rate and therefore plays an important role in the prediction of the neutron production in s- process environments.