Adric Jones

Forward angle scattering effects in the measurement of total cross sections for positron scattering

Measurements of total scattering by positron impact have typically excluded a significant portion of the forward scattering angles of the differential cross section. This paper demonstrates the effect that this can have on measurements of the total cross section. We show that much of the apparent disagreement between experimental measurements of positron scattering from atoms and molecules may be explained by this excluded angular range. It is shown that this same effect may also lead to an anomalous energy dependence of some cross sections.

Total and positronium formation cross sections for positron scattering from H2O and HCOOH

Total and positronium formation cross sections have been measured for positron scattering from H2O and HCOOH using a positron beam with an energy resolution of 60 meV (full-width at half-maximum (FWHM)). The energy range covered is 0.5–60 eV, including an investigation of the behavior of the onset of the positronium formation channel using measurements with a 50 meV energy step, the result of which shows no evidence of any channel coupling effects or scattering resonances for either molecule.

Low-energy positron interactions with xenon

Low-energy interactions of positrons with xenon have been studied both experimentally and theoretically. The experimental measurements were carried out using a trap-based positron beam with an energy resolution of ?80?meV, while the theoretical calculations were carried out using the convergent close-coupling method and the relativistic optical potential approach. Absolute values of the grand total, positronium formation and grand total minus positronium formation cross sections are presented over the energy range of 1?60?eV. Elastic differential cross sections (DCS), for selected energies, are also presented both below and above the positronium formation threshold. Fine energy-step measurements of the positronium formation cross section over the energy range of 4.4?8.4?eV, and measurements of the elastic DCS at the energies of 5.33 and 6.64?eV, have been carried out to investigate the ionization threshold regions corresponding to the 2P3/2 and 2P1/2 states of the Xe+ ion. The present results are compared with both experimental and theoretical values from the literature where available.

Positron binding to alcohol molecules

Results are presented for positron binding to a selection of molecules containing the hydroxyl functional group. These molecules, which span in the range of carbon atoms from 1 (methanol) to 4 (1-butanol), have moderate permanent dipole moments ranging from about 1.4 to 2.4?D. The dependence of the binding energy on the magnitude of the molecular dipole polarizability and static dipole moment is studied. An effect that appears to be due to the localization of the bound positron is discussed.

Polarization dependence of n=2 positronium transition rates to Stark-split n=30 levels via crossed-beam spectroscopy

We produce Rydberg Ps by a two-step laser excitation from and from to states of principal quantum level that are Stark split by a motionally induced electric field. Our measurements are largely free of first-order Doppler shifts such that we are able to investigate the impact of laser polarization on the population of the closely spaced Stark levels. We find a variation in the distribution that is primarily dependent on the IR laser polarization with respect to the direction of the motionally induced electric field. With the IR light polarized parallel to the electric field F, the ratio of excitation probability to the levels of maximal Stark splitting compared to that of excitation to the states of minimal Stark splitting is found to be 3.37 ± 0.51, whereas with the IR light polarized perpendicular to F, the excitation ratio is 0.87 ± 0.64. Our results agree with those of Wall et al (2015 Phys. Rev. Lett. 114 173001) obtained with n = 11 and will be useful in the preparation of high-n states of Ps for a variety of experiments, including measuring the interaction of Ps with gravity, in precision time-of-flight (TOF) energy spectroscopy, and precision optical spectroscopy of Ps.

Low energy positron interactions - trapping, transport and scattering

Recent experiments, theory and modelling of positron interactions with atoms and molecules are discussed. The first half of the paper is devoted to binary collisions between positrons and crossed beams of atoms or molecules (in this case neon) and the second half deals with ensembles of non-interacting positrons, otherwise known as swarms which are transported through the background gas. We review the recent results on measurements of the cross sections based on obtained from collisional positron traps and subsequent calculations of transport properties of positron swarms that may be used to model thermalization experiments, collisional traps and possible applications of positrons in materials science and biomedicine. It was found that kinetic phenomena occur in positron transport that are mainly the result of the positronium (Ps) formation which has a larger cross section than elastic scattering in most gases and at the same time is a non-conservative process. Most importantly negative differential conductivity (NDC) occurs only for the bulk drift velocity while it does not exist for the flux property, a phenomenon that has not been observed for electrons.

High Resolution Positron Interactions

A high-resolution (ΔE ~ 60 meV FWHM) positron beam apparatus is briefly described and it has been used to measure total scattering, elastic scattering, positronium (Ps) formation, and inelastic scattering cross sections from a number of rare gas atoms. In this paper we present a selection of these results for helium, neon and argon and where possible, compare with previous measurements and contemporary theory. A particular search has been made for resonance and channel coupling effects at, or near, the opening of inelastic channels.

Low energy lepton scattering: recent results for electron and positron interactions

This report will review several new experimental developments for low energy (1-50 eV) scattering of both electrons and positrons from atoms and molecules. Experiments include an apparatus for studies of scattering from unstable molecular radicals, a position- and time-sensitive time-of-flight spectrometer for studies of near threshold electronic excitation of atoms and molecules and a trap-based positron scattering experiment for measurements of low energy positron scattering from rare gas atoms. Where possible, comparisons are also made with other experimental determinations and with contemporary scattering theory.

Low energy positron scattering from helium

Positron scattering from helium has been historically limited to experiments with energy resolution of 0.5 eV or worse. A new apparatus has been developed that allows the investigation of scattering processes with an energy resolution as good as 30 meV, allowing new experimental insight into positron helium interactions. High resolution measurements of low energy scattering from helium will be presented, with a particular emphasis on the development of benchmark cross sections, the investigation of threshold behaviour and the search for positron scattering resonances.

A Search for Wigner Cusps and Resonances in Positron Scattering by Atoms and Molecules

Detailed elastic cross-section measurements in the vicinity of the positronium (Ps) formation threshold for He, Ne, Ar, Kr and Xe show a cusp-like feature, made visible by a small and localised depression of between (2 to 15%) of the cross-section value. Other near-threshold structures such as scattering resonances have been demonstrated in close-coupling and convergent-close-coupling calculations for helium. This investigation is undertaken to explicitly search for the presence or otherwise of scattering structures using a high efficiency (~ 1 % statistical uncertainty) and high resolution (< 70 meV) transmission based apparatus.