Positron cross sections and transport in water

A detailed understanding of the behaviour of positrons and electrons as they pass through liquids is critical for a number of applications, from positron emission tomography and ion therapy to cosmic ray detectors and materials characterisation. In particular, transport in liquid water is vital for medical applications because of its similarity to human tissue, and is an area of continued research. This thesis presents newly measured experimental cross sections for positrons in water as well as several Monte Carlo simulation techniques aimed at improving our models of positron and electron transport. In particular, special efforts have been made to model the effects of elastic coherent scattering, which arise due to the position and velocity correlations between molecules in liquids and other dense media. The experimental scattering results include the first measurements of integral and differential elastic positron cross sections for water vapour, as well as detailed grand total and positronium formation cross sections for the same. Performed on the positron beamline apparatus at the Australian National University, this transmission experiment passed a high-resolution beam of positrons through a scattering cell containing water vapour. The parallel component of the energy of the positrons after scattering was analysed to determine the ratio between the scattered and unscattered portions of the beam, from which absolute total cross sections were calculated. The experiment further utilised a differentiated magnetic field to separate elastic scattering from the other scattering processes, and to distinguish between scattering angles in order to measure angle-differential cross sections. An original Monte Carlo track-structure simulation code has been written which aims to precisely model the transport behaviour of electrons and positrons in dilute gases, dense gases and liquids. This simulation incorporates several new features to improve its ability to model systems with high particle loss rates, varying electric fields and fully-differential ionisation interactions. Each feature has been rigorously tested against benchmark systems from the literature and, where necessary, against Boltzmann equation solutions of new benchmark systems. The simulation has also been applied to model elements of the positron trapping apparatus which is a critical component of the positron scattering experiment. The simulation's validity has been extended beyond dilute gases by including a treatment of the coherent elastic scattering that is caused by the structure of dense media. Following the theories of Van Hove, Cohen and Lekner, either a static or dynamic structure factor can be combined with gas-phase cross sections to form a modified scattering cross section that partially accounts for the temporal and spatial correlations of nearby molecules. The benchmarked Monte Carlo simulation techniques are then used to calculate transport profiles for positrons in liquid water, using the measured water cross sections. These profiles are estimates of the spatial distributions of positronium formation and energy deposition, from the positrons' emission until their first positronium formation event. Comparisons between simulations employing different cross section sets demonstrate the importance of a complete and accurate set of scattering cross sections for positrons in water

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

Total, elastic, and inelastic cross sections for positron and electron collisions with tetrahydrofuran

We present total, elastic, and inelastic cross sections for positron and electron scattering from tetrahydrofuran (THF) in the energy range between 1 and 5000 eV. Total cross sections (TCS), positronium formation cross sections, the summed inelastic integral cross sections (ICS) for electronic excitations and direct ionization, as well as elastic differential cross sections (DCS) at selected incident energies, have been measured for positron collisions with THF. The positron beam used to carry out these experiments had an energy resolution in the range 40–100 meV (full-width at half-maximum). We also present TCS results for positron and electron scattering from THF computed within the independent atom model using the screening corrected additivity rule approach. In addition, we calculated positron-impact elastic DCS and the sum over all inelastic ICS (except rotations and vibrations). While our integral and differential positron cross sections are the first of their kind, we compare our TCS with previous literature values for this species. We also provide a comparison between positron and electron-impact cross sections, in order to uncover any differences or similarities in the scattering dynamics with these two different projectiles

Stranger to Familiar: Wild Strepsirhines Manage Xenophobia by Playing

The power of play in limiting xenophobia is a well-known phenomenon in humans. Yet, the evidence in social animals remains meager. Here, we aim to determine whether play promotes social tolerance toward strangers in one of the most basal group of primates, the strepsirhines. We observed two groups of wild lemurs (Propithecus verreauxi, Verreaux's sifaka) during the mating season. Data were also collected on nine visiting, outgroup males. We compared the distribution of play, grooming, and aggressive interactions across three conditions: OUT (resident/outgroup interactions), IN (resident/resident interactions in presence of outgroups) and BL-IN (baseline of resident/resident interactions in absence of outgroups). Play frequency between males was higher in OUT than in IN and BL-IN conditions; whereas, grooming was more frequent in IN than in OUT and BL-IN conditions. Aggression rates between resident and outgroup males were significantly higher than those between residents. However, aggressions between resident and outgroup males significantly decreased after the first play session and became comparable with resident-resident aggression levels. The presence of strangers in a well-established group implies the onset of novel social circumstances, which sifaka males cope with by two different tactics: grooming with ingroup males and playing with outgroup ones. The grooming peak, concurrently with the visit of outgroups, probably represents a social shield adopted by resident males to make their pre-existing affiliation more evident to the stranger “audience”. Being mostly restricted to unfamiliar males, adult play in sifaka appears to have a role in managing new social situations more than in maintaining old relationships. In particular, our results indicate not only that play is the interface between strangers but also that it has a specific function in reducing xenophobia. In conclusion, play appears to be an ice-breaker mechanism in the critical process that “upgrades” an individual from stranger to familiar

A multi-term solution of the space-time Boltzmann equation for electrons in gases and liquids

In this study we have developed a full multi-term space–time solution of Boltzmann’s equation for electron transport in gases and liquids. A Green’s function formalism is used that enables flexible adaptation to various experimental systems. The spatio-temporal evolution of electrons in liquids in the non-hydrodynamic regime is benchmarked for a model Percus–Yevick (PY) liquid against an independent Monte Carlo simulation, and then applied to liquid argon. The temporal evolution of Franck–Hertz oscillations in configuration and energy space are observed for the model liquid with large differences apparent when compared to the dilute gas case, for both the velocity distribution function components and the transport quantities. The packing density in the PY liquid is shown to influence both the magnitude and wavelength of Franck– Hertz oscillations of the steady-state Townsend (SST) simulation. Transport properties are calculated from the non-hydrodynamic theory in the long time limit under SST conditions which are benchmarked against hydrodynamic transport coefficients. Finally, the spatio-temporal relaxation of low-energy electrons in liquid argon was investigated, with striking differences evident in the spatio-temporal development of the velocity distribution function components between the uncorrelated gas and true liquid approximations, due largely to the presence of a Ramsauer minimum in the former and not in the latter

Simulations of pulses in a buffer gas positron trap

In this study we simulate positron transport properties for various configurations of the gases and electric fields used in the Australian Positron Beamline Facility positron trap, which is based on the Surko buffer-gas trap. In an attempt to further improve the time and energy resolution of the trap and thus the associated scattering experiments, we apply a Monte-Carlo simulation procedure to a variety of possible configurations of the dumping stage of the trap

Monte Carlo study of coherent scattering effects of low-energy charged particle transport in Percus-Yevick liquids

We generalize a simple Monte Carlo (MC) model for dilute gases to consider the transport behavior of positrons and electrons in Percus-Yevick model liquids under highly nonequilibrium conditions, accounting rigorously for coherent scattering processes. The procedure extends an existing technique [Wojcik and Tachiya, Chem. Phys. Lett. 363, 381 (2002)CHPLBC0009-261410.1016/S0009-2614(02)01177-6], using the static structure factor to account for the altered anisotropy of coherent scattering in structured material. We identify the effects of the approximation used in the original method, and we develop a modified method that does not require that approximation. We also present an enhanced MC technique that has been designed to improve the accuracy and flexibility of simulations in spatially varying electric fields. All of the results are found to be in excellent agreement with an independent multiterm Boltzmann equation solution, providing benchmarks for future transport models in liquids and structured systems

Low-energy positron and electron scattering from tetrahydrofuran and 3-hydroxy-tetrahydrofuran

We present new cross section results from a joint experimental and theoretical investigation into low-energy positron and electron scattering from two targets of biological interest, namely tetrahydrofuran and 3-hydroxy-tetrahydrofuran. We compare and discuss the total, elastic and inelastic cross sections for these species in the light of potential positron and electron-induced damage in biomolecular systems