The s-process or the slow neutron capture process is a nucleosynthesis process taking place at relatively low neutron densities in stars. It runs along the valley of beta stability since the neutron capture rate is much slower compared to the beta decay rate. The s-process occurs mainly during core helium burning and shell carbon burning phase in massive stars and during thermally pulsing helium burning phase in asymptotic giant-branch stars. The potential stellar neutron source for the s-process is associated with &alpha-capture reactions on light nuclei. The capture-reaction rates provide the reaction flow for the build-up of 22Ne neutron source during the helium-burning phase in these stars.
The low energy 26Mg resonances at stellar energies below 800 keV are predicted to have a critical influence on the &alpha-capture rates on 22Ne. Some of these resonances may also correspond to pronounced alpha cluster structure near the &alpha-threshold. However, these resonances have remained elusive during direct alpha capture measurements owing to the high Coulomb barrier and background from cosmic rays and beam induced reactions. Hence, in the present work, &alpha-inelastic scattering and &alpha-transfer measurements have been performed to probe the level structure of 26Mg nucleus in order to determine the 22Ne+&alpha-capture rates.
Both experiments have been performed using the high-resolution Grand Raiden Spectrometer at the Research Center for Nuclear Physics (RCNP), Osaka, Japan. For the &alpha-inelastic scattering measurement, a self-supporting solid 26Mg target was used and for the &alpha-transfer study via the (6Li,d) reaction, 22Ne gas enclosed in a gas cell with Aramid windows was used. The reaction products were momentum analysed by the spectrometer and detected at the focal plane equipped with two multi-wire drift chambers and two plastic-scintillation detectors. The focal plane detection system provided information on the position, the angle, the time of flight and the energy of the particles enabling the reconstruction of the kinematics at the target.
The focal plane energy calibration allowed for the study of 26Mg levels from Ex = 7.69 - 12.06 MeV in the (&alpha,&alpha’) measurement and Ex = 7.36 - 11.32 MeV in the (6Li,d) measurement. Six levels (Ex = 10717 (9) keV , 10822 (10) keV, 10951 (21) keV, 11085 (8) keV, 11167 (8) keV and 11317 (18) keV) were observed above the &alpha-threshold in the region of interest (10.61 - 11.32 MeV). The Ex = 10717 keV had a negligible contribution to the &alpha-capture rates. The Ex = 10951, 11167 and 11317 keV exhibited pronounced alpha-cluster structure and hence, dominated the &alpha-capture rates. The Ex = 11167 keV had the most appreciable impact on the (&alpha,&gamma) rate increasing it by 2 orders of magnitude above Longland et al. and Bisterzo et al. rates and by a factor of 3 above NACRE rate. Hence, the recommended 22Ne(&alpha,n) + 22Ne(&alpha,&gamma) rates, from the present work, strongly favour the reduction of s-process over-abundances associated with massive stars as well as AGB stars of intermediate initial mass. Also, the uncertainty range corresponding to the present rates suggest the need for a more refined measurement of the associated resonance parameters.