Entrance channel limit on the fusion of 28Si with 12C at high energy.

Abstract

Cross sections for the fusion of $^{28}\mathrm{C}$ have been measured at center of mass bombarding energies of 39, 43.5, 48, and 54 MeV. Velocity and energy spectra of reaction products having charge Z\ensuremath{\ge}12 and mass number A\ensuremath{\ge}24 were studied at several angles between ${\mathrm{theta}}_{\mathrm{lab}=2\mathrm{\ifmmode^\circ\else\textdegree\fi{}}}$ and 13\ifmmode^\circ\else\textdegree\fi{}. Separation of fusion from other processes was made by comparison of kinematic calculations to measured spectra. The extracted critical angular momentum (${l}_{\mathrm{cr}{}^{\mathrm{fus})}}$ of 22\ensuremath{\Elzxh} indicates the existence of an entrance-channel-imposed limit on the high energy fusion cross section. More symmetric systems forming the same compound nucleus, $^{40}\mathrm{Ca}$, have considerably larger limiting angular momenta. This result is consistent with the previous observation of a maximum angular momentum of 22\ensuremath{\Elzxh} (relative to the entrance channel) for the orbiting dinuclear complex found in studies of back-angle deeply inelastic scattering of $^{28}\mathrm{Si}$ and $^{12}\mathrm{C}$.