Polarisation potentials for positron-molecule collisions: positron-H, scattering
Model polarisation potentials for use in calculating positron-molecule cross sections are often based on approximations in which terms that depend on the sign of the charge of the projectile are neglected. Thus, these potentials do not fully distinguish between electron and positron scattering. The validity of using such potentials for positron- H2 collisions is investigated by performing scattering calculations using electron- and positron-H, model polarisation potentials. Results show that, because of differences in the way electrons and positrons distort the target charge cloud, sign-dependent terms in the adiabatic potential can significantly influence the total cross section. Low-energy cross sections are also shown to be rather sensitive to how non-adiabatic polarisation effects are taken into account. Using a cut-off fully adiabatic positron-H, polarisation potential, theoretical total cross sections in reasonably good agreement with recent measurements are obtained.
Non-adiabaticity and other aspects of polarisation in positron-molecule scattering
The authors have investigated various aspects of polarisation/correlation effects at low energies for the prototypical positron-N2 system. First, they have found that, like positron-H2 cross sections, those for this system are highly sensitive to terms in the (adiabatic) polarisation potential that explicitly depend on the sign of the charge of the projectile. Insight into the nature of these sign-dependent distortions is afforded by three-dimensional graphs of polarised N2 density functions. Second, they have explored and characterised nonadiabatic (correlation) effects in this system using a simple two-parameter model, they have found that the onset of these effects in the near-target region in positron scattering is much more gradual than in electron scattering. By including in scattering calculations adiabatic and non-adiabatic effects, they obtain theoretical positron-N2 cross sections in good agreement with recent experimental data.
Investigation of scatter radiation of non-circular X-ray scan trajectories
Modern interventional X-ray devices are highly maneuverable and allow complex 3D imaging trajectories. New scan trajectories enable not only optimized object sampling but also introduce new approaches for dynamic 3D imaging. In this paper, we investigated the scatter radiation of two different scan trajectories. The scatter radiation was sampled by a rail setup with dosimeters at defined positions. After data analysis, contour maps were generated which show for four exemplary heights the scatter radiation distribution. The alternative scan trajectory showed smaller dose exposures at heights correlated to upper torso and head areas of a model physician. Keywords—Image acquisition, alternative scan trajectory, circular tomosynthesis, scatter radiation distribution