Target excitation and angular momentum transfer in reactions of E/A=11.9 MeV 28Si with 181Ta from 4 pi charged particle, neutron, and gamma -ray multiplicity measurements.

Abstract

The conversion of kinetic energy into target-like fragment excitation and orbital angular momentum into fragment spin has been studied in the reaction 333 MeV $^{28}\mathrm{Si}$ with $^{181}\mathrm{Ta}$. The light charged particles were detected in a small, highly segmented, 4\ensuremath{\pi} phoswich detector system placed in the spin spectrometer, a 4\ensuremath{\pi} NaI array which served as a neutron and gamma detector. Multiplicities of light charged particles and neutrons detected in coincidence with projectile-like fragments indicate that the excitation energy of the target-like fragment increases as the kinetic energy of the projectile-like fragment decreases through the quasielastic region and tends toward saturation as the kinetic energy of the projectile-like fragment approaches the kinetic energy corresponding to complete damping. Measurement of the \ensuremath{\gamma}-ray multiplicity in coincidence with the projectile-like fragments indicates that the angular momentum transferred to the target-like fragment increases with decreasing mass of the projectile-like fragment for the quasielastic energy region in contrast to the energy region corresponding to completely damped processes where the angular momentum of the target-like fragment decreases with increasing mass loss from the projectile. The influence of preequilibrium processes on both excitation energy and angular momentum transfer to the target-like fragment is discussed. These data present further evidence that l waves below the entrance-channel critical angular momentum for fusion must contribute to the nonfusing reaction channels.