EQUILIBRIUM AND NONEQUILIBRIUM COMPLEX FRAGMENT EMISSION IN 50-100 MEV/U LA-139+C-12 REACTIONS

Complex fragment emission (Z > 2) has been studied in the reactions of 50, 80, and 100 MeV/u La-139 + C-12. Charge, angle, and energy distributions were measured inclusively and in coincidence with other complex fragments, and were used to extract source rapidities, velocity distributions, and cross sections. The binary signature of the coincidence events and the sharpness of the velocity distributions illustrate the primarily 2-body nature of these reactions. Calculations based on statistical compound nucleus decay have been compared with the experimental data. The emission velocities, angular distributions, and absolute cross sections of fragments of 20 less-than-or-equal-to Z less-than-or-equal-to 35 at 50 MeV/u, 19 less-than-or-equal-to Z less-than-or-equal-to 28 at 80 MeV/u, and 17 less-than-or-equal-to Z less-than-or-equal-to 21 at 100 MeV/u are consistent with the binary decay of compound nuclei formed in incomplete fusion reactions in which the La-139 projectile picks up about one-half of the C-12 target. At 80 and 100 MeV/u, statistical model calculations are also able to reproduce the isotropic portion of the cross section for lighter and heavier fragments. However, a significant fraction of the total cross section for these fragments is due to non-equilibrium emission. Although the emission process is still mainly binary, and the relative velocity between the fragments is determined by their mutual Coulomb repulsion, the anisotropic angular distributions and the magnitudes of the absolute yields are incompatible with standard compound-nucleus statistical decay.