Molecular resonance and highly deformed fission fragments in28Si+28Si

Abstract

A high-resolution measurement of fragment-fragment-$\ensuremath{\gamma}$ triple coincidence events in the symmetric and near-symmetric mass exit channels from the ${}^{28}\mathrm{Si}{+}^{28}\mathrm{Si}$ reaction has been undertaken using the EUROGAM Phase II $\ensuremath{\gamma}$-ray spectrometer. The bombarding energy of ${E}_{\mathrm{lab}}{(}^{28}\mathrm{Si})=111.6 \mathrm{MeV}$ has been selected to populate the conjectured ${J}^{\ensuremath{\pi}}{=38}^{+}$ quasimolecular resonance in the ${}^{56}\mathrm{Ni}$ dinuclear system. In the ${}^{28}\mathrm{Si}{+}^{28}\mathrm{Si}$ symmetric mass exit channel, the resonance behavior is clearly verified at the chosen energy. The population of highly excited states in the ${}^{24}\mathrm{Mg},$ ${}^{28}\mathrm{Si},$ and ${}^{32}\mathrm{S}$ nuclei is discussed within a statistical fusion-fission model. Evidence is presented for selective population of states in the ${}^{28}\mathrm{Si}$ fragments arising from the symmetric fission of the ${}^{56}\mathrm{Ni}$ compound nucleus. The enhanced population of the ${K}^{\ensuremath{\pi}}{=3}_{1}^{\ensuremath{-}}$ band of the ${}^{28}\mathrm{Si}$ nucleus, indicative of an oblate deformed shape, suggests that the oblate configuration plays a significant role in the resonant process. Fragment angular distributions for the elastic and low-lying inelastic channels as well as $\ensuremath{\gamma}$-ray angular correlations for the mutual inelastic channel ${(2}^{+}{,2}^{+})$ indicate that the spin orientations of the outgoing fragments are perpendicular to the orbital angular momentum. This unexpected result, which is different from the alignment found for the resonance structures in the ${}^{24}\mathrm{Mg}{+}^{24}\mathrm{Mg}$ and ${}^{12}\mathrm{C}\mathrm{}{+}^{12}\mathrm{C}$ systems, suggests a situation where two oblate ${}^{28}\mathrm{Si}$ nuclei interact in an equator-to-equator stable molecular configuration. A discussion concerning the spin alignment and spin disalignment for different reactions such as ${}^{12}\mathrm{C}\mathrm{}{+}^{12}\mathrm{C},$ ${}^{24}\mathrm{Mg}{+}^{24}\mathrm{Mg},$ and ${}^{28}\mathrm{Si}{+}^{28}\mathrm{Si}$ is presented.