Nuclear transport phenomena in the reactions tin-112 + calcium-48 and tin-112 + calcium-40 at E/A = 35 MeV.

摘要

An exclusive study has been performed of the reactions 112Sn + 48,40Ca at E/A = 35MeV. Probabilities and emission patterns of associated light particles (neutrons and protons), as well as the charge distributions of projectile-like fragments (PLF), are used to probe nuclear transport phenomena in the Fermi-energy domain. Correlations between PLF deflection angle and dissipated energy, the reaction yields, and PLF Z distributions are interpreted in terms of the reaction dynamics. The emission of nonequilibrium particles is compared to the theoretical models, and their role in limiting the energy dissipation is explored.; The PLF deflection functions demonstrate dissipative orbiting for both reactions. Calculations with the nucleon exchange model (NEM) qualitatively reproduce experimental PLF angular distributions and average correlations between atomic number and kinetic energy. The neutrons and protons emitted in both reactions stem dominantly from the statistical decay of primary reaction fragments. Their yields illustrate the relaxation of excitation energy and mass-to-charge density, although no degree of freedom is completely equilibrated. Distinct components of non-equilibrium particles with a rather hard energy spectra are observed. Up to 25% of the dissipated entrance-channel energy can be carried away by such particles, limiting thermal excitation of the system.; Yields and mission patterns of the non-equilibrium neutrons and protons reveal approximate symmetry and random emission in the nucleon-nucleon rest frame. However, the flow of these non-equilibrium particles is more neutron rich than either target or projectiles, indicating the importance of the nuclear surface. With increasing energy loss, the neutron-to-proton multiplicity ratio also shows a gradual evolution towards equalization, however, a full equilibration (mixing) is never achieved. The observed deviation from overall chemical equilibrium is very significant for the 112Sn + 48Ca reaction. The overall yields, energy spectra, and angular distributions predicted by Fermi-jet model calculations are not realistic. Predicted backward-angle jets are not observed in the experiment, while experimental multiplicities of non-equilibrium neutrons are significantly underestimated by the Fermi Jet model.