D. N. Delis

Equilibrium and Non-equilibrium Complex Fragment Emission in 50-100 MeV/u 139La + 12C Reactions

Abstract Complex fragment emission ( Z > 2) has been studied in the reactions of 50, 80, and 100 MeV/u 139 La + 12 C. 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 ⩽ Z ⩽ 35 at 50 MeV/u, 19 ⩽ Z ⩽ 28 at 80 MeV/u, and 17 ⩽ Z ⩽ 21 at 100 MeV/u are consistent with the binary decay of compound nuclei formed in incomplete fusion reactions in which the 139 La projectile picks up about one-half of the 12 C 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.

Mass asymmetric fission barriers for 75Br

Fragments with atomic numbers covering nearly the entire range of the mass-asymmetry coordinate (4 < Z < 27) were observed from the 5.0, 6.2, 6.9, 8.0, 10.2 and 12.7 MeV/A 63Cu + 12C reactions. Energy spectra and angular distributions show the presence of projectile-like and target-like components along with an isotropic component. The isotropic component appears as a Coulomb ring in the invariant cross-section plots indicating the presence of a binary compound nucleus decay which is confirmed by the coincidence data. Excitation functions were constructed for each Z value and a nearly complete set of mass-asymmetric barriers has been extracted for 75Br. There is excellent agreement between the experimentally determined barriers and the finite-range model predictions.

Complex fragment production and multifragmentation in 139La-induced reactions at 35, 40, 45 and 55 MeV/u

Abstract Complex fragment emission ( Z >3) has been studied in the reactions of 35, 40, 45 and 55 MeV/u 139 La+X. Charge, angular, and energy distributions were measured inclusively and in coincidence with other complex fragments, and were used to extract source rapidities, velocity distributions, and cross sectins. Multifragment events increase with both bombarding energy and entrance-channel mass asymmetry. The excitation functions for multifragment events rise strongly with excitation energy. These excitation functions are independent of the target-projectile combination and bombarding energy suggesting, the formation of an intermediate nuclear system, whose decay properties depend mainly on its excitation energy and angular momentum.

Multifragmentation: Surface instabilities or statistical decay?

Boltzmann-Nordheim-Vlasov calculations show multifragmentation that seems to originate from surface instabilities. These instabilities are traced to a sheet instability caused by the proximity interaction. Experimental data, on the other hand, suggest that multifragmentation may be dominated by phase space.

A complete ridge-line potential for complex fragment emission

Cross sections were measured for fragments (4<Z<27) from the 5.0,6.2,6.9,8.0,10.2 and 12.7 MeV/N63Cu+12C reactions. Excitation functions were constructed for each Z value, and a nearly complete set of mass-asymmetric barriers has been obtained for75Br. There is excellent agreement between the experimentally determined barriers and the finite-range model calculations, while there is strong disagreement with the liquid-drop model calculations.