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.
Collisional ‘heating’ of molecular rotations by positron impact: a computational analysis of the quantum dynamics
Low-energy collisional excitation of rotational levels by impact with slow positron beams is considered by carrying out a space-frame (SF) dynamical calculation at energies well below the positronium formation threshold. The close-coupled quantum equations are solved exactly, using a recently developed modified variable phase approximation (MVPA) and the interaction forces include static interaction exactly and a local, DFT modelling of correlation–polarisation effects. The results are employed to obtain heating and quenching efficiency coefficients for a series of linear molecules in the gas phase and at low collision energies.
Proposed beam diagnostics instrumentation for the LANSCE refurbishment project
Presently, the Los Alamos National Laboratory is in the process of planning a refurbishment of various subsystems within its Los Alamos Neutron Science Center accelerator facility. A part of this LANSCE facility refurbishment will include some replacement of and improvement to existing older beam-diagnostics instrumentation. While plans are still being discussed, some instrumentation that is under improvement or replacement consideration are beam phase and position measurements within the 805-MHz side-coupled cavity linac, slow wire profile measurements, typically known as wire scanners, and possibly additional installation of fast ionization-chamber loss monitors. This paper will briefly describe the requirements for these beam measurements, what we have done thus far to answer these requirements, and some of the technical issues related to the implementation of the instrumentation.