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

The emission of a 1.809-MeV γ-ray from the first excited state of ²⁶Mg, followed by β-decay of ²⁶Al in its ground state (denoted as ²⁶Al^g.s.), has been identified by several gamma-ray telescopes. To resolve the controversy over the possible sources of the observational 1.809-MeV γ-rays, one needs accurate knowledge of the production rate of ²⁶Al^g.s.. The ²⁵Al(p, γ)²⁶Si 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 ²⁶Si above the proton threshold (QPγ = 5.518 MeV). Illiadis et al. suggested that the ²⁵1Al(p,γ) ²⁶Si reaction should be dominated by a 3⁺ unnatural parity state at Ex = 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 ²⁸Si(⁴He,⁶He)²⁶Si 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 ²⁶Si. The 7018-keV level, which was only observed by Bardayan et al. with the ²⁸Si(p, t)²⁶Si reaction, was confirmed in our measurement. Among the candidates of unnatural parity states at Ex - 5678, 5916, and 5945 keV, the 5918-keV state was only observed within the error of the excitation energy. A new state at Ex = 6101 keV was also identified.