S. Armitage

Electron-like scattering of positronium.

The atom formed from an electron and positron interacts with molecules almost as if it were a free electron. Positronium (Ps), a hydrogen-like atom composed of an electron and its antimatter partner, the positron, is formed in considerable quantities whenever positrons interact with matter. It has unexpectedly been found to scatter from a wide variety of atoms and molecules in a way very similar to that of a bare electron moving at the same velocity, despite Ps being neutral and twice the mass.

Ionizing Collisions by Positrons and Positronium Impact on the Inert Atoms

Abstract 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.

Production of collimated positronium from molecular nitrogen

The efficiency for conversion of a positron beam into a positronium (Ps) beam by charge exchange from molecular nitrogen has been investigated in the range of Ps kinetic energy, EPs = 30–250 eV. Measurements have also been made for the production efficiency of collimated Ps from molecular hydrogen in order to make a direct comparison with previous work. The results show that while H2 remains the best converter at lower energies, N2 becomes more efficient from 90 eV upwards. Using N2 as a production gas for collimated Ps, the accessible energy range for Ps scattering is increased by at least 100 eV. A measure of the differential Ps formation cross-section from N2, as well as indirect estimates of Ps–N2 total cross-sections, have also been obtained.

Positronium- and positron-H2O total cross sections

The total cross sections for positron and positronium scattering from H2O molecules have been measured for incident energies between 7 and 417 eV, and 10 and 100 eV, respectively. The experimental system has been characterized with respect to its angular acceptance of both scattered positrons and positronium in order to correct the data for forward-scattering errors once differential cross sections become available. The present data are compared with previous results for electron and positron total cross sections.

Positronium scattering from atoms and molecules

Recent progress in the generation of monoenergetic positronium beams and the study of positronium interactions with simple atoms and molecules are reviewed. The current status on measurements of total cross-sections is summarized and experimental results are compared with available theories. New developments towards the first experimental determination of a positronium-impact partial cross-section are outlined.

Positron and positronium interactions with atoms and molecules

Abstract Recent advances in the field of positron-induced ionization and positronium collisions are reviewed. Major outstanding problems are highlighted.