Spin alignment and density matrix measurement in 28Si+12C orbiting reaction.

Abstract

Gamma-ray angular correlations have been measured for the damped reactions $^{12}\mathrm{C}$${(}^{28}$Si${,}^{12}$C${)}^{28}$Si between ${\mathrm{\ensuremath{\theta}}}_{\mathrm{c}.\mathrm{m}.}$=120\ifmmode^\circ\else\textdegree\fi{} and 160\ifmmode^\circ\else\textdegree\fi{} for ${\mathit{E}}_{\mathrm{c}.\mathrm{m}.}$=43.5 and 48 MeV. The spin alignments and density matrices have been deduced from the observed angular distribution of the gamma rays for $^{28}\mathrm{Si}$ states in -10Q0 and -16Q0 MeV for ${\mathit{E}}_{\mathrm{c}.\mathrm{m}.}$=43.5 and 48 MeV, respectively, and for $^{12}\mathrm{C}$(${2}_{1}^{+}$) state in strongly damped region Q-10 MeV. We find that the spin angular momenta of the nuclei are poorly aligned with respect to the normal to the reaction plane and the density matrices for the $^{12}\mathrm{C}$(${2}_{1}^{+}$) and $^{28}\mathrm{Si}$ states are almost diagonal with respect to the direction of motion of the outgoing particle. We also find qualitatively similar behavior of the spin alignments for $^{28}\mathrm{Si}$ states in strongly damped region. The measured density matrices and spin alignments are consistent with the picture of formation of a long-lived dinuclear complex undergoing orbiting, bending, and wriggling motions, but not with those obtained from statistical compound nucleus or sticking-model calculations.