G. Laricchia

Ps beam production and scattering from gaseous targets

We have investigated the conversion of a positron beam into an ortho-Pa beam via charge exchange in gaseous H-2, He and Ar at energies up to 120 eV and gas pressures up to 14 mu mHg. H-2 has been found to be the most efficient converter at energies up to 90 eV. At higher energies, Ar has been found to be the best. Total cross sections of ortho-Ps scattering from H-2 and He have also been measured. A comparison with available theories suggests that electron exchange at lower energies and target inelastic collisions at higher energies are playing a major role in the collisions.

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.

Single and double ionization of neon, krypton and xenon by positron impact

Measurements of the single (σi+) and the ratio of double to single (R2) ionization cross sections for positron impact on neon, krypton and xenon are presented from near threshold to 1000 eV. By multiplying the R2 values of each target by the appropriate σi+, new data for the corresponding double (σi2+) ionization cross sections have also been obtained. The ratio between positron and electron σi+ maxima is generally seen to be lower for higher target atomic number, as expected from the static interaction between the projectile and undistorted target. Comparison with available data suggests that Ps formation accompanied by the production of singly or doubly charged ions is a significant channel in positron-atom scattering.

Positron impact ionization of atomic hydrogen

Ionization cross sections for positrons impacting on atomic hydrogen have been measured for kinetic energies in the range 15-700 eV. This has been done in a crossed-beam geometry where a magnetically guided positron beam intersects a hydrogen gas jet emanating from a radio frequency discharge tube. Electron impact ionization cross sections were also measured with the same apparatus thus facilitating comparison with, and normalization to, published results. The positron-atomic hydrogen results are found to be significantly lower than those obtained by Spicher et al. (1990).

Positron impact phenomena

Atomic collision experiments using low-energy positron beams have been actively pursued for more than two decades. With greater understanding of the basic physics governing the production of these beams, advances in technology have promoted studies beyond measurements of total collision cross sections. Such work forms the basis of this review and includes topics from elastic scattering, positronium formation and the associated production of positronium beams, excitation, ionisation and attachment.

ANTIHYDROGEN SYNTHESIS BY THE REACTION OF ANTIPROTONS WITH EXCITED-STATE POSITRONIUM ATOMS

Aspects of the possible reactions of trapped antiprotons with excited state positronium atoms to form antihydrogen are discussed. Conditions are identified whereby the antihydrogen produced may be suitable for capture in a neutral trap. A discussion is given of possible use of antihydrogen to test the quantization of electric charge involving precision comparisons of hydrogen and antihydrogen (Rydberg constants), and proton and antiproton cyclotron frequencies.

Antihydrogen Production by Positronium-Antiproton Collisions in an Ion Trap

A method for producing antihydrogen by the + Ps → + e- reaction is described. Included are the calculated capture cross sections, and short descriptions of the electrostatic slow positron beam, positronium-formation, and the antiproton ion trap to be used for antihydrogen production. With present available technology, collimated monoenergetic beams with an energy of a few keV and an intensity in the order of one per second can be produced by this method. Possible enhancements of this rate are discussed.

Experimentation with thin single crystal W foils as slow positron transmission mode moderators

Results are reported on the performance of single crystal W foils as positron moderators. A simple annealing technique has been developed which gives efficiencies comparable to those obtained by in situ heating under high vacuum conditions.

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.

The effect of laser annealing of thin W(100) films on positron transmission reemission properties

Results on e+ transmission reemission properties are presented for three single‐crystal W(100) films of thicknesses 1000, 2000, and 3100 A for incident e+ energies of 1.4–16 keV. The films were first cleaned with a 10‐s laser pulse, then annealed in O2 (10−6 Torr), and finally heated in a vacuum (10−9 Torr). Thereafter the films showed good reemission properties. The maximum transmitted yields of slow e+ were 38% (1000 A), 27% (2000 A), and 17% (3100 A) at incident e+ energies of 4, 5.2, and 7.2 keV, respectively. The energy distributions of the reemitted e+ were characterized by a full width half maximum of 0.15–0.2 eV located at 2.8 eV containing 60%–70% of the e+ superimposed onto a nearly uniform distribution covering the energy interval 0–2.8 eV.