A pulsed positronium beam using a positron buffer gas trap.

We describe the operation of a pulsed positronium beam based on a two-stage buffer gas positron trap (BGT) or a Surko trap. The BGT allows the areal density and temporal spread of the positron beam to be tailored. This tailored positron beam is used to form a positronium beam via charge exchange with an atomic or molecular gas.

Positron scattering from pyridine.

We present a range of cross section measurements for the low-energy scattering of positrons from pyridine, for incident positron energies of less than 20 eV, as well as the independent atom model with the screening corrected additivity rule including interference effects calculation, of positron scattering from pyridine, with dipole rotational excitations accounted for using the Born approximation. Comparisons are made between the experimental measurements and theoretical calculations. For the positronium formation cross section, we also compare with results from a recent empirical model.

Near-Threshold Ionization of Argon by Positron Impact.

The direct single-ionization cross section for Ar by positron impact has been measured in the region above the first ionization threshold. These measurements are compared to semiclassical calculations which give rise to a power law variation of the cross section in the threshold region. The experimental results appear to be in disagreement with extensions to the Wannier theory applied to positron impact ionization, with a smaller exponent than that calculated by most previous works.

Total cross sections for electron scattering by 1-propanol at impact energies in the range 40-500 eV.

Absolute total cross section (TCS) measurements for electron scattering from 1-propanol molecules are reported for impact energies from 40 to 500 eV. These measurements were obtained using a new apparatus developed at Juiz de Fora Federal University-Brazil, which is based on the measurement of the attenuation of a collimated electron beam through a gas cell containing the molecules to be studied at a given pressure. Besides these experimental measurements, we have also calculated TCS using the Independent-Atom Model with Screening Corrected Additivity Rule and Interference (IAM-SCAR+I) approach with the level of agreement between them being typically found to be very good.

Positron kinetics in an idealized PET environment.

The kinetic theory of non-relativistic positrons in an idealized positron emission tomography PET environment is developed by solving the Boltzmann equation, allowing for coherent and incoherent elastic, inelastic, ionizing and annihilating collisions through positronium formation. An analytic expression is obtained for the positronium formation rate, as a function of distance from a spherical source, in terms of the solutions of the general kinetic eigenvalue problem. Numerical estimates of the positron range - a fundamental limitation on the accuracy of PET, are given for positrons in a model of liquid water, a surrogate for human tissue.

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.

Positron interactions with water-total elastic, total inelastic, and elastic differential cross section measurements.

Utilising a high-resolution, trap-based positron beam, we have measured both elastic and inelastic scattering of positrons from water vapour. The measurements comprise differential elastic, total elastic, and total inelastic (not including positronium formation) absolute cross sections. The energy range investigated is from 1 eV to 60 eV.

Low energy positron interactions with uracil--total scattering, positronium formation, and differential elastic scattering cross sections.

Measurements of the grand total and total positronium formation cross sections for positron scattering from uracil have been performed for energies between 1 and 180 eV, using a trap-based beam apparatus. Angular, quasi-elastic differential cross section measurements at 1, 3, 5, 10, and 20 eV are also presented and discussed. These measurements are compared to existing experimental results and theoretical calculations, including our own calculations using a variant of the independent atom approach.

Cross sections for positron impact with 2,2,4-trimethylpentane.

2,2,4-Trimethylpentane (C8H18), a hydrocarbon produced all over the world on a large scale in the processing of crude oil, has long been known and used in the energy sector. It has also recently attracted the attention of the radiation physics and chemistry community, owing to its applications in medical imaging techniques. Charged-particle interactions with this species unfortunately remain mostly unknown.

A joint theoretical and experimental study for elastic electron scattering from 1,4-dioxane.

We present results of measurements and calculations of elastic electron scattering from 1,4-dioxane in the energy range of 0-1000 eV. Absolute differential and integral elastic cross sections have been measured using a crossed electron-molecule beam spectrometer and the relative flow technique, at four energies in the 10-30 eV range and for scattered electrons in the angular range 20°-129°. The measured cross sections are compared with results from R-matrix computations, at the static exchange plus polarization level, calculated at energies between 0-20 eV, and with calculations employing the independent atom model with the screening corrected additivity rule (IAM-SCAR).

Current prospects on Low Energy Particle Track Simulation for biomedical applications.

The Low Energy Particle Track Simulation code is a radiation interaction simulation tool specifically designed to describe electron and positron interactions below 10 keV at a molecular level. Relying on carefully selected, preferentially experimental input parameters that account for all expected scattering processes, it provides detailed results about all collisional events undergone by an incident radiation particle during its slowdown until thermalisation. Here, we give an up-to-date description of its input data sources and selection procedure and summarise the current contents of the resulting database.

Experimental and theoretical cross sections for positron collisions with 3-hydroxy-tetrahydrofuran.

Cross section results from a joint experimental and theoretical investigation into positron scattering from 3-hydroxy-tetrahydrofuran (3H-THF) are presented. Total and positronium (Ps) formation cross sections have been measured from 1 to 190 eV using the positron beamline at the Australian National University, which has an energy resolution between 60 and 100 meV. The total cross section (TCS) and the elastic and total inelastic integral cross sections in the energy range between 1 and 1000 eV have been computed within the Independent Atom Model using the Screening Corrected Additivity Rule approach.

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).

A comprehensive and comparative study of elastic electron scattering from OCS and CS2 in the energy region from 1.2 to 200 eV.

We report absolute differential cross sections (DCSs) for elastic electron scattering from OCS (carbonyl sulphide) and CS(2) (carbon disulphide) in the impact energy range of 1.2-200 eV and for scattering angles from 10° to 150°. Above 10 eV, the angular distributions are found to agree quite well with our present calculations using two semi-phenomenological theoretical approaches.

Low-energy electron and positron transport in gases and soft-condensed systems of biological relevance.

We present a study of electron and positron transport in water in both the gaseous and liquid states using a Boltzmann equation analysis and a Monte-Carlo simulation technique. We assess the importance of coherent scattering processes when considering transport of electrons/positrons in dense gases and liquids. We highlight the importance of electron and positron swarm studies and experiments as a test of the accuracy and completeness of cross-sections, as well as a technique for benchmarking Monte-Carlo simulations.

Electron scattering from pyrazine: elastic differential and integral cross sections.

We report on new measurements for elastic electron scattering from pyrazine. Absolute differential cross sections (DCSs) at seven discrete energies in the range 3-50 eV, and over the scattered electron angular range 10°-129°, were determined using a crossed electron-molecular beam spectrometer in conjunction with the well-established relative flow technique. Integral elastic cross sections were subsequently derived from those DCS data at each energy.

Differential elastic electron scattering cross sections for CCl4 by 1.5-100 eV energy electron impact.

We report absolute elastic differential, integral and momentum transfer cross sections for electron interactions with CCl(4). The incident electron energy range is 1.5-100 eV, and the scattered electron angular range for the differential measurements varies from 15°-130°.

Modelling low energy electron and positron tracks for biomedical applications.

Purpose: To incorporate the effects of low energy electrons and positrons into radiation interaction models.

Materials And Methods: The simulation method proposed here was based on experimental and theoretical cross section data and energy loss spectra we have previously derived. After a summary of the main techniques used to obtain reliable input data, the basis of a Low Energy Particle Track Simulation (LEPTS) procedure was established.

Observation of threshold effects in positron scattering from the noble gases.

Channel coupling is a phenomenon that has been investigated for many scattering processes, and is responsible for the formation of cusps or steps in the cross sections for open scattering channels at, or near, the onset of a new scattering channel. It has long been speculated that the opening of the positronium formation channel may lead to the formation of such cusp features in the elastic positron scattering cross section. In this work, elastic scattering of positrons has been measured in the region of the positronium formation threshold for the noble gases He-Xe.

Substitution effects in elastic electron collisions with CH3X (X=F, Cl, Br, I) molecules.

We report absolute elastic differential, integral, and momentum transfer cross sections for electron interactions with the series of molecules CH(3)X (X=F, Cl, Br, I). The incident electron energy range is 50-200 eV, while the scattered electron angular range for the differential measurements is 15 degrees-150 degrees. In all cases the absolute scale of the differential cross sections was set using the relative flow method with helium as the reference species.

Metastable helium: a new determination of the longest atomic excited-state lifetime.

Exited atoms may relax to the ground state by radiative decay, a process which is usually very fast (of order nanoseconds). However, quantum-mechanical selection rules can prevent such rapid decay, in which case these "metastable" states can have lifetimes of order seconds or longer. In this Letter, we determine experimentally the lifetime of the longest-lived neutral atomic state-the first excited state of helium (the 2(3)S1 metastable state)-to the highest accuracy yet measured.

A positron trap and beam apparatus for atomic and molecular scattering experiments.

An instrument has been designed and constructed to provide new insights into fundamental, low energy positron scattering processes. The design is based on the Surko trap system and produces a pulsed positron beam with an energy resolution of as good as 54 meV. The design and operation of the apparatus is explained, while the first experimental results from this apparatus have been demonstrated in recent publications.

Angle-resolving time-of-flight electron spectrometer for near-threshold precision measurements of differential cross sections of electron-impact excitation of atoms and molecules.

This article presents a new type of low-energy crossed-beam electron spectrometer for measuring angular differential cross sections of electron-impact excitation of atomic and molecular targets. Designed for investigations at energies close to excitation thresholds, the spectrometer combines a pulsed electron beam with the time-of-flight technique to distinguish between scattering channels. A large-area, position-sensitive detector is used to offset the low average scattering rate resulting from the pulsing duty cycle, without sacrificing angular resolution.

Mirror design for two-dimensional magneto-optic lenses and compressors.

A novel mirror arrangement that enables large interaction lengths between atomic beams and laser fields by use of a small amount of laser power is presented. Its application to focusing and compression of neutral atomic beams is discussed.

Absolute electron scattering cross sections for the CF2 radical.

Using a crossed electron-molecular beam experiment, featuring a skimmed nozzle beam with pyrolytic radical production, absolute elastic cross sections for electron scattering from the CF2 molecule have been measured. A new technique for placing measured cross sections on an absolute scale is used for molecular beams produced as skimmed supersonic jets. Absolute differential cross sections for CF2 are reported for incident electron energies of 30-50 eV and over an angular range of 20-135 deg.