Astrophysically important Si-26 states studied with the Si-28(He-4, He-6)Si-26 reaction

Abstract

The emission of a 1.809-MeV gamma-ray from the first excited state of Mg-26, followed by beta-decay of Al-26 in its ground state (denoted as Al-26(g,s,)), has been identified by several gamma-ray telescopes. To resolve the controversy over the possible sources of the observational 1.809-MeV gamma-rays, one needs accurate knowledge of the production rate of Al-26(g.s.). The 25Al(p,gamma)Si-26 reaction is one of the most important astrophysical reactions to be investigated, but its rate is uncertain due to the lack of level information on Si-26 above the proton threshold (Q(p gamma) = 5.518 MeV). Illiadis et al. suggested that the Al-25(p, gamma)Si-26 reaction should be dominated by a 3(+) unnatural parity state at E-x = 5970 keV. Recent studies proposed several states as candidates for the 3(+) states. However, the spin-parity assignments for these states are still uncertain. In the present work, we measured the Si-28(He-4, He-6)(26) Si reaction at 120 MeV to confirm the unnatural parity states just above the proton threshold. The measurement was performed with the high-resolution particle analyzer at the Center for Nuclear Study, University of Tokyo. We observed a total of 22 excited states in Si-26. The 7018-keV level, which was only observed by Bardayan et al. with the Si-28(p, t)Si-26 reaction, was confirmed in our measurement. Among the candidates of unnatural parity states at E-x = 5678, 5916, and 5945 keV, the 5918-keV state was only observed within the error of the excitation energy. A new state at E-x = 6101 keV was also identified.