No abstract

A two-centre close-coupling method that includes positronium (Ps) formation channels has been developed for positron scattering on molecular hydrogen. Calculations are performed within the fixed-nuclei approximation. Results for the grand total, Ps-formation and direct ionization cross sections are presented. In general, good agreement with experiment is seen in all the calculated cross sections.

The convergent close-coupling method with two-centre expansions has been developed to calculate positron scattering on helium. The method utilizes a multiconfigurational description of helium wavefunctions. Positronium formation is taken into account explicitly as electron capture into the positronium states. Direct-scattering, positronium-formation and breakup cross sections are calculated at all energies of practical relevance. Convergence in the calculated cross sections is demonstrated by increasing the basis size and orbital quantum number of the included states for each of the centres. Better agreement with experimental data is found when a multiconfigurational description is used for the helium wavefunctions in comparison with the recent frozen-core results.

The anisotropy in the angular distribution of the fusion-fission and quasifission fragments for the 16 O+ 238 U, 19 F+ 208 Pb and 32 S+ 208 Pb reactions is studied by analyzing the angular momentum distributions of the dinuclear system and compound nucleus which are formed after capture and complete fusion, respectively. The orientation angles of axial symmetry axes of colliding nuclei to the beam direction are taken into account for the calculation of the variance of the projection of the total spin onto the fission axis. It is shown that the deviation of the experimental angular anisotropy from the statistical model picture is connected with the contribution of the quasifission fragments which is dominant in the 32 S+ 208 Pb reaction. Enhancement of anisotropy at low energies in the 16 O+ 238 U reaction is connected with quasifission of the dinuclear system having low temperature and effective moment of inertia. PACS: 25.70.Jj-Fusion and fusion-fission reactions; 25.70.Lm-Strongly damped collisions; 25.85.Ge-Charged-particle-induced fission, capture, fusion, quasifission, angular anisotropy of fragments.

A two-centre convergent close-coupling method, which includes the positronium-formation channel, has been developed for positron scattering on helium. Convergent, pseudoresonance-free cross sections have been obtained. This is only possible if complete expansions are used on both the positronium and helium centres. The method is valid for all projectile energies and all transitions, including breakup, which is associated with excitation of positive-energy helium and positronium pseudostates. Generally, good agreement between calculated cross sections and available experimental data has been found across all incident energies.