Peculiarities of Nuclear Fusion in Synthesis of Superheavy Elements

The small probabilities of synthesis of new superheavy elements at GSI (Darmstadt, Germany), Joint Institute for Nuclear Research (Dubna, Russia), and RIKEN (Wako, Japan) during the last decade stimulate the experimental and theoretical studies of the nuclear reaction mechanism. 1– 4 In preparation of these experiments, the main aim is to reach maximum cross sections of the yield of evaporation residues (ER) as a result of the de-excitation of the heated compound nucleus which is formed in complete fusion of the projectile and target nuclei. Because the ER excitation function in the synthesis of superheavy elements has very narrow width for "cold fusion" reactions (5–10 MeV) with 208 Pb and 209 Bi targets 5 and the width of the "hot fusion" reactions with 48 Ca projectile on actinide ta

Positron scattering on atoms and molecules

An overview is given of recent progress in the calculation of positron scattering on atoms and molecules using the convergent close-coupling method. Particular emphasis is given to those cases where positronium formation is one of the reaction channels, as well as the importance of demonstrating convergence with increasing orbital angular momentum of the bases used. Targets considered are atomic hydrogen, lithium, and molecular hydrogen

Angular anisotropy of the fusion-fission and quasifission fragments

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.Comment: 17 pages 8 figures. Submitted to Euro. Phys. Jour.

A Self-Consistent Model for Positronium Formation from Helium Atoms

The differential and total cross sections for electron capture by positrons from helium atoms are calculated using a first-order distorted wave theory satisfying the Coulomb boundary conditions. In this formalism a parametric potential is used to describe the electron screening in a consistent and realistic manner. The present procedure is self consistent because (i) it satisfies the correct boundary conditions and post-prior symmetry, and (ii) the potential and the electron binding energies appearing in the transition amplitude are consistent with the wave functions describing the collision system. The results are compared with the other theories and with the available experimental measurements. At the considered range of collision energies, the results agree reasonably well with recent experiments and theories. [Note: This paper will be published on volume 42 of the Brazilian Journal of Physics

Convergent close-coupling method for positron scattering from noble gases

We present the convergent close-coupling formulation for positron scattering from noble gases (Ne, Ar, Kr and Xe) within the single-center approximation. Target functions are described in a model of six p-electrons above an inert Hartree–Fock core with only one-electron excitations from the outer p6 shell allowed. Target states have been obtained using a Sturmian (Laguerre) basis in order to model coupling to ionization and positronium (Ps) formation channels. Such an approach is unable to yield explicit Ps-formation cross sections, but is valid below this threshold and above the ionization threshold. The present calculations are found to show good agreement with recent measurements

Antihydrogen formation in low-energy antiproton collisions with excited-state positronium atoms

© 2018, Springer Nature Switzerland AG. The convergent close-coupling method is used to obtain cross sections for antihydrogen formation in low-energy antiproton collisions with positronium (Ps) atoms in specified initial excited states with principal quantum numbers ni= 5. The threshold behaviour as a function of the Ps kinetic energy, E, is consistent with the 1/E law expected from threshold theory for all initial states. We find that the increase in the cross sections is muted above ni= 3 and that here their scaling is roughly consistent with ni2, rather than the classically expected increase as ni4

Convergent close coupling calculations for positron-magnesium scattering

While a two-centre convergent close-coupling approach to positron-magnesium scattering is developed, a single-centre method has been used to calculate total cross sections up to incident energies of 100 eV. The results agree very well with the measurements of Stein et al. [1] for positron energies above the ionisation threshold (7.6 eV). Similar accuracy is expected for energies below the positronium formation threshold (0.8 eV) where presently there are no experimental data to compare to. In this energy region we find a large p-wave resonance at 0.17 eV. Similar resonance behaviour was found in calculations by Mitroy and Bromley [2] at an energy of 0.1 eV

Configuration space method to calculate rearrangement matrix elements

A new method has been developed for calculating rearrangement matrix elements within the two-centre convergent close-coupling theory and applied to Ps formation in positron–atom collisions. The method utilises partial wave expansion of the Ps wavefunction that separates dependencies on coordinates of electron and positron. Resulting expressions contain slowly converging series which have been accelerated with use of the Euler–Wijngaarden transformation. The integrals in the method are free of the Coulomb singularity that appears in previously used momentum-space methods and therefore more straightforward to apply to complex and charged targets. The results are compared with the existing benchmark calculations for positron scattering on H and He+ targets

Convergence study of the close-coupling approach to positron-helium collisions

Recently the non-relativistic convergent close-coupling method has been extended into the relativistic domain [1]. When applied to electron impact collision processes for highly charged hydrogen-like ions, the RCCC method that utilizes relativistic kinematics and the Coulomb interaction in the absence of QED Breit and Mller corrections is suitable for target ions with atomic number Z up to Z 30. For Z larger than 30, such QED corrections become significant and must be included in the calculations. We have modified the RCCC computer code accordingly and report the results for Z = 100 selected excitation cross sections and for U91+ ionization cross sections which were found to be in good agreement with previous calculations of Fontes et al.[2]