Different representations of continuum in the positron-hydrogen scattering problem

A two-centre convergent close coupling method is applied to positron-impact ionisation of hydrogen. Different ways of distributing the pseudostates representing the continuum are investigated. It is found that calculations of the grand total and total ionisation cross sections are independent of the distribution of continuum pseudostates

Surface-integral formulation of scattering theory

We formulate scattering theory in the framework of a surface-integral approach utilizing analytically known asymptotic forms of the two-body and three-body scattering wavefunctions. This formulation is valid for both short-range and long-range Coulombic interactions. New general definitions for the potential scattering amplitude are presented. For the Coulombic potentials, the generalized amplitude gives the physical on-shell amplitude without recourse to a renormalization procedure. New post and prior forms for the Coulomb three-body breakup amplitude are derived. This resolves the problem of the inability of the conventional scattering theory to define the post form of the breakup amplitude for charged particles. The new definitions can be written as surface integrals convenient for practical calculations. The surface-integral representations are extended to amplitudes of direct and rearrangement scattering processes taking place in an arbitrary three-body system. General definitions for the wave operators are given that unify the currently used channel-dependent definitions

Two-center convergent close-coupling calculations for positron-lithium collisions

We report on two-center convergent close-coupling calculations of positron-lithium collisions. The target is treated as one active electron interacting with an inert ion core. The positronium formation channels are taken into account explicitly utilizing both negative- and positive-energy Laguerre-based states. A large number of channels and high partial waves are used to ensure the convergence of the cross sections. We find the Ramsauer-Townsend minimum in total and elastic cross sections at an impact energy E of about 0.0016 eV. As found previously forH and He, the contributions to the breakup cross section from both the Li and the Ps centers become the same as the threshold is approached

Antihydrogen Formation via Antiproton Scattering with Excited Positronium

Use of CCC method to calculate for the first time very accurate cross sections for Hbar formation in antiiproton-Ps collisions close to threshold for a numebr of excited Ps states. Discovery of novel 1/E behaviour for the cross ections near thresold for excited states

Indirect methods in nuclear astrophysics

We discuss recent developments in indirect methods used in nuclear astrophysics to determine the capture cross sections and subsequent rates of various stellar burning processes, when it is difficult to perform the corresponding direct measurements. We discuss in brief, the basic concepts of Asymptotic Normalization Coefficients, the Trojan Horse Method, the Coulomb Dissociation Method, (d,p), and charge-exchange reactions

Accurate stopping power calculations for antiprotons and protons

© Published under licence by IOP Publishing Ltd. The convergent close-coupling method is applied to calculate antiproton and proton stopping cross sections for atomic and molecular targets. Excellent agreement with experimental measurements is obtained for antiprotons in helium while unexpectedly large disagreement is found for the hydrogen molecule, which is inconsistent with very good agreement between our ionisation cross section and the experiment

Trojan Horse as an indirect technique in nuclear astrophysics. Resonance reactions

The Trojan Horse method is a powerful indirect technique that provides information to determine astrophysical factors for binary rearrangement processes $x + A \to b + B$ at astrophysically relevant energies by measuring the cross section for the Trojan Horse reaction $a + A \to y+ b + B$ in quasi-free kinematics. We present the theory of the Trojan Horse method for resonant binary subreactions based on the half-off-energy-shell R matrix approach which takes into account the off-energy-shell effects and initial and final state interactions.Comment: 6 pages and 1 figur

Internal consistency in positron-hydrogen-scattering calculations

Internal consistency in a close-coupling approach to positron-hydrogen scattering is investigated with a particular focus on the potential overlap between the atomic and positronium continua. We present results for total, total ionization, and 1s positronium-formation cross sections for projectile energies up to 100 eV. We show that, irrespective of whether the continuum is treated by one center, or the other, or both, the same cross sections are generally obtained. This is true only if sufficiently large orbital angular momentum is taken in the close-coupling expansion. Furthermore, unitarity of the close-coupling approach ensures convergence of the physically observable cross sections even if the individual components are not convergent

Astrophysical 3He(α,γ)7Be and 3H(α,γ)7Li direct capture reactions in a potential-model approach

The astrophysical 3He(α,γ)7Be and 3H(α,γ)7Li direct capture processes are studied in the framework of the two-body model with potentials of a simple Gaussian form, which describe correctly the phase shifts in the s, p, d, and f waves, as well as the binding energy and the asymptotic normalization constant of the ground p3/2 and the first excited p1/2 bound states. It is shown that the E1transition from the initial s wave to the final p waves is strongly dominant in both capture reactions. On this basis the s-wave potential parameters are adjusted to reproduce the new data of the LUNA Collaboration around 100 keV and the newest data at the Gamov peak estimated with the help of the observed neutrino fluxes from the sun, S34(23+6−5keV)=0.548±0.054 keV b for the astrophysical Sfactor of the capture process 3He(α,γ)7Be. The resulting model describes well the astrophysical Sfactor in the low-energy big-bang nucleosynthesis region of 180–400 keV; however, it has a tendency to underestimate the data above 0.5 MeV. The energy dependence of the S factor is mostly consistent with the data and the results of the no-core shell model with continuum, but substantially different from the fermionic molecular dynamics model predictions. Two-body potentials, adjusted for the properties of the 7Be nucleus, 3He+α elastic scattering data, and the astrophysical S factor of the 3He(α,γ)7Bedirect capture reaction, are able to reproduce the properties of the 7Li nucleus, the binding energies of the ground 3/2− and first excited 1/2− states, and phase shifts of the 3H+α elastic scattering in partial waves. Most importantly, these potential models can successfully describe both absolute value and energy dependence of the existing experimental data for the mirror astrophysical 3H(α,γ)7Licapture reaction without any additional adjustment of the parameters