This paper is a review of low-energy positron interactions with atoms and molecules. Processes of interest include elastic scattering, electronic and vibrational excitation, ionization, positronium formation and annihilation. An overview is presented of the currently available theoretical and experimental techniques to study these phenomena, including the use of trap-based positron beam sources to study collision processes with improved energy resolution. State-resolved measurements of electronic and vibrational excitation cross sections and measurement of annihilation rates in atoms and molecules as a function of incident positron energy are discussed. Where data are available, comparisons are made with analogous electron scattering cross sections. Resonance phenomena, common in electron scattering, appear to be less common in positron scattering. Possible exceptions include the sharp onsets of positron-impact electronic and vibrational excitation of selected molecules. Recent energy-resolved studies of positron annihilation in hydrocarbons containing more than a few carbon atoms provide direct evidence that vibrational Feshbach resonances underpin the anomalously large annihilation rates observed for many polyatomic species. We discuss open questions regarding this process in larger molecules, as well as positron annihilation in smaller molecules where the theoretical picture is less clear.

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Low-energy positron interactions with atoms and molecules

Detailed measurements of the total ionization cross-sections for Ne, Ar, Kr and Xe by positron impact are presented in the energy range from threshold up to ~1 keV. From these and other available data, the cross-sections for positronium formation are extracted. Comparison is made with other results where available. Structure in the positronium formation cross-section has been found which appears to be associated with positronium formation in excited states. Upper and lower limits on the contribution from these are estimated.

Total positron-impact ionization and positronium formation from the noble gases

Recent progress in the study of positronpositron and positroniumpositronium induced ionizationionization is reviewed within the context of the inert atoms. The focus is on experimental techniques and results, which comprise both integral and differential cross-sections.differential cross-sectionsintegrated cross-sections Measurements for positronium formationpositronium formation and direct ionizationdirect ionization by positron impact, accompanied by singly- or multiply-charged ions, are compared and discussed. First data, integral and differential, on the fragmentationfragmentation of positronium in collision with heliumhelium and xenonxenon are also presented. Comparisons with theories and other projectiles are made where possible and future prospects are considered.

Ionizing Collisions by Positrons and Positronium Impact on the Inert Atoms

When biological matter is irradiated by charged particles, a wide variety of interactions occur, which lead to a deep modification of the cellular environment. To understand the fine structure of the microscopic distribution of energy deposits, Monte Carlo event-by-event simulations are particularly suitable. However, the development of these track-structure codes needs accurate interaction cross sections for all the electronic processes: ionization, excitation, positronium formation and even elastic scattering. Under these conditions, we have recently developed a Monte Carlo code for positrons in water, the latter being commonly used to simulate the biological medium. All the processes are studied in detail via theoretical differential and total cross-section calculations performed by using partial wave methods. Comparisons with existing theoretical and experimental data in terms of stopping powers, mean energy transfers and ranges show very good agreements. Moreover, thanks to the theoretical description of positronium formation, we have access, for the first time, to the complete kinematics of the electron capture process. Then, the present Monte Carlo code is able to describe the detailed positronium history, which will provide useful information for medical imaging (like positron emission tomography) where improvements are needed to define with the best accuracy the tumoural volumes.

Positron follow-up in liquid water: I. A new Monte Carlo track-structure code.

We review recent cross section results for low-energy positron scattering from atomic targets. A comparison of the latest measurements and calculations for positron collisions with the noble gases and a brief update of the newest studies on other atoms is presented. In particular, we provide an overview of the cross sections for elastic scattering, positronium formation, direct and total ionisation, as well as total scattering, at energies typically between about 0.1 and a few hundred eV. We discuss the differences in the current experimental data sets and compare those results to the available theoretical models. Recommended data sets for the total cross section are also reported for each noble gas. A summary of the recent developments in the scattering from other atoms, such as atomic hydrogen, the alkali and alkaline-earth metals, and two-electron systems is finally provided.

Recent positron-atom cross section measurements and calculations

Single ionization of xenon and double ionization of helium, neon and xenon by positron impact have been studied at energies from threshold to ~1 keV The total single ionization cross section of xenon is seen to behave as expected In particular, single ionization with positronium formation is a strong channel at low energies For double ionization we found that positronium formation is strongly suppressed and makes no contribution to the total cross section in the light noble gases, while some evidence for this channel is observed in xenon

Ionization of helium, neon and xenon by positron impact

Total single and double ionization cross sections for argon and krypton by positron impact have been measured for projectile energies ranging from threshold to ~1 keV. The single ionization cross sections are seen to behave as expected, showing large contributions from positronium formation at low energies. In the double ionization cross sections there seems to be a lack of positronium formation at threshold, while a clear contribution from the process is seen at intermediate energies.

Ionization of argon and krypton by positron impact

The positron induced absolute single and double ionisation cross sections for the noble gases Ar, Kr and Xe have been determined in the energy region from below threshold to ∼1000 eV. With the use of an admixture of He to the target gas normalisation was performed by comparison to known single ionisation cross sections for He. Special interest has been taken in the energy gap of 6.8 eV below the direct double ionisation threshold energy, where only double ionisation with positronium formation is possible. This has been done to study the significance of double ionisation with Ps formation for the heavy noble gases.

K A Nielsen

Papers

Ionization of helium, neon and xenon by positron impact

Ionization of argon and krypton by positron impact

Ionisation of noble gases by positron impact