Alpha-Particle Scattering and Production of an Unnatural-Parity State inSi28

Alpha-particle scattering from ${\mathrm{Si}}^{28}$ has been studied at six bombarding energies between 16.2 and 27.0 MeV. Angular distributions for scattering from the ground and four excited states are presented at each energy, and total cross sections are obtained from these differential cross sections. Because the unnatural-parity excited state at 6.27 MeV cannot be excited by alpha-particle scattering under direct-interaction conditions, special attention was paid to measuring the angular distribution of the cross section for production of this state. The angular distribution itself does not provide any unambiguous indication of the reaction mechanism responsible for production of the 6.27-MeV state; identification of the mechanism will require a full theoretical analysis of these data.