Evidence for superdeformed shape isomeric states in 28Si at excitations above 40 MeV through observations of selective particle decays of 16O+12C resonances in 8Be and alpha channels.

Abstract

Excitation functions of the reactions $^{12}\mathrm{C}$${(}^{16}$O${,}^{8}$Be${)}^{20}$Ne and $^{12}\mathrm{C}$${(}^{16}$O${,}^{4}$He${)}^{24}$Mg leading to several states have been measured in the range ${\mathit{E}}_{\mathrm{c}.\mathrm{m}.}$=25.7--38.6 MeV. From cross channel correlation analysis two intermediate resonance structures at ${\mathit{E}}_{\mathrm{c}.\mathrm{m}.}$=26.9 and 29.5 MeV corresponding to excitation energies of 43.7 and 46.2 MeV in $^{28}\mathrm{Si}$ have been identified. Angular distribution measurements for $^{12}\mathrm{C}$${(}^{16}$O${,}^{4}$He${)}^{24}$${\mathrm{Mg}}_{\mathrm{g}.\mathrm{s}.}$ have also been made at the lower resonance at ${\mathit{E}}_{\mathrm{c}.\mathrm{m}.}$=26.9 MeV which enable spin assignment of ${\mathit{J}}^{\mathrm{\ensuremath{\pi}}}$=${14}^{+}$ to this resonance. Striking observation of preferential $^{8}\mathrm{Be}$ decay of both the resonances to the 8-particle--4-hole states in $^{20}\mathrm{Ne}$ is interpreted as evidence for the resonances to be highly deformed shape isomeric configurations in $^{28}\mathrm{Si}$ expected from deformed shell-model calculations.