T. S. Stein

Total-cross-section measurements for positrons and electrons colliding with H 2 , N 2 , and C O 2

Total scattering cross sections have been measured in the same apparatus for positrons and electrons colliding with H/sub 2/, N/sub 2/, and CO/sub 2/ using a beam transmission technique. The projectile impact energies range from 1--500 eV for e/sup +/-H/sub 2/, 2--500 eV for e/sup -/-H/sub 2/, 0.5--750 eV for e/sup +/-N/sub 2/, 2.2--700 eV for e/sup -/-N/sub 2/, 0.5--60 eV for e/sup +/-CO/sub 2/, and 2--50 eV for e/sup -/-CO/sub 2/. The onset of positronium formation is clearly seen by an abrupt rise in the total cross sections for positrons colliding with each of the molecules at the respective positronium-formation thresholds. The positron measurements are compared with the electron measurements at intermediate energies for H/sub 2/ and N/sub 2/. This comparison reveals a merging of the cross sections for H/sub 2/ at energies above 200 eV, while for N/sub 2/ the electron results remain higher than the positron results at all energies. Estimates are made of potential experimental errors, as well as the experimental resolution for discrimination against projectiles scattered at small forward angles.

Total-cross-section measurements for positrons and electrons colliding with H/sub 2/, N/sub 2/, and CO/sub 2/

Total scattering cross sections have been measured in the same apparatus for positrons and electrons colliding with H/sub 2/, N/sub 2/, and CO/sub 2/ using a beam transmission technique. The projectile impact energies range from 1--500 eV for e/sup +/-H/sub 2/, 2--500 eV for e/sup -/-H/sub 2/, 0.5--750 eV for e/sup +/-N/sub 2/, 2.2--700 eV for e/sup -/-N/sub 2/, 0.5--60 eV for e/sup +/-CO/sub 2/, and 2--50 eV for e/sup -/-CO/sub 2/. The onset of positronium formation is clearly seen by an abrupt rise in the total cross sections for positrons colliding with each of the molecules at the respective positronium-formation thresholds. The positron measurements are compared with the electron measurements at intermediate energies for H/sub 2/ and N/sub 2/. This comparison reveals a merging of the cross sections for H/sub 2/ at energies above 200 eV, while for N/sub 2/ the electron results remain higher than the positron results at all energies. Estimates are made of potential experimental errors, as well as the experimental resolution for discrimination against projectiles scattered at small forward angles.

Comparisons of Positron And Electron Scattering By Gases

Publisher Summary This chapter reviews many of the interesting comparisons, primarily experimental measurements that can be made between positron and electron scattering by the same atoms and molecules with the hope that these comparisons may help to stimulate a better overall understanding of electron scattering processes. The chapter explains most of the experimental results. The e+ and e– cross section measurements of a given group have been made in the same experimental system with the same technique. The relative comparisons between e+ and e– measurements will be more meaningful than the actual quantitative values because many of the potential experimental errors would be the same for e+ and e– measurements, and as a result not affect the relative comparisons. An important motivation for investigating the scattering of positrons by atoms and molecules provides a better understanding of the scattering of electrons by atoms and molecules, the latter being of importance in many different fields of science and technology such as plasma physics, laser development, gaseous electronics, astrophysics, and aeronomy. As positrons differ from electrons only by the signs of their electric charge, comparison measurements of the scattering of positrons and electrons by the same atoms and molecules can reveal interesting similarities and differences that arise from the basic interactions contributing to e+ ,– scattering.

Production of a monochromatic, low energy positron beam using the 11B(p,n)11C reaction

A new technique has been developed to produce an intense positron source by making use of the 11B(p,n)11C reaction. Positrons with well‐defined low energies have been observed emerging from the irradiated boron target itself without any additional moderating materials. A description of the apparatus used to produce the 11C and to detect the low energy positrons is given.

Transmission experiment for measuring total positron–atom collision cross sections in a curved, axial magnetic field

We describe and analyze a transmission experiment developed for measuring total scattering cross sections for low‐energy poistrons (0.3–250 eV) colliding with gases in a region containing a curved, axial magnetic field. It is determined that spiralling does not appreciably increase the beam path length through the curved, axial magnetic field in the gas scattering region. An overall test of the experimental apparatus and technique is made by measuring total cross sections for electrons colliding with helium and argon atoms in the energy range from 1.5 to 30 eV, where it is found that the present measurements are within 15% of the most reliable prior measurements.

Measurements of positron scattering by hydrogen, alkali metal, and other atoms

Abstract Recent developments in measurements of total and positronium (Ps) formation cross sections for positrons (in the energy range of 1–300 eV) scattered by hydrogen, alkali metal, and other atoms are reviewed. Measurements and calculations of total and Ps formation cross section (QPss) for positrons scattered by hydrogen atoms are in very good agreement, and for the most part there is also good agreement for sodium, potassium, and rubidium atoms, but there is a puzzling discrepancy between measured and recently calculated QPss for sodium. Preliminary measurements of QPss for Mg show a very rapid rise to a large maximum value less than 2 eV above the Ps formation threshold energy (0.8 eV) which may be related to the proximity of that threshold to zero energy. It appears that structure observed in e+–Ar and Kr QPs measurements may be related to capture of inner-subshell electrons.

Positron—Gas Scattering Experiments

Publisher Summary Positron–atom scattering experiments involve interactions of antimatter with matter and can help provide a better understanding of the scattering of electrons by atoms and molecules. The scattering of electrons is a subject of great importance to many different fields of science and technology such as plasma physics, laser development, gaseous electronics, astrophysics, and studies of the earths upper atmosphere. Comparisons between e + -atom (molecule) and e – -atom (molecule) scattering reveal some interesting differences and similarities. The static interaction is attractive for electrons and repulsive for positrons, while the polarization interaction is attractive for both projectiles. The exchange interaction contributes to e – scattering but does not play a role in e + scattering. The combined effect of the static and polarization interactions is that they add to each other in e – scattering, whereas there is a tendency toward cancellation in e + scattering. There are four objectives of this chapter: (1) to point out the most significant developments in the first decade of e + -gas scattering experiments, (2) to search for some consistent patterns in the experimental results in cases where several different groups have investigated the same collision processes, (3) to present some puzzling questions raised by the new generation of experiments that go beyond Q T measurements, and (4) to indicate some experimental areas of e + -gas scattering that would be interesting and feasible to investigate in the near future.

Near-thermal energy width of moderated high energy positrons

Abstract More than 10 −7 of the positrons emitted by 11 C (produced by proton bombardment of boron) emerge from the room-temperature boron target with a low energy (less than 1 eV) and a measured energy width of (0.100+0.010) eV.

Evidence for channel-coupling effects in positron scattering by sodium and potassium atoms

In this letter we present measurements of total cross sections for 1-10 eV positrons scattered from sodium (Na) atoms. Comparisons of these measurements and other recent measurements of total and positronium (Ps) formation cross sections for positron-Na and potassium (K) scattering by our group with the close-coupling approximation calculations by Hewitt et al. (1993) provide rather clear evidence that coupling effects between Ps formation and other scattering channels are playing a very important role in positron scattering by Na and K.

POSITRON-MOLECULE SCATTERING EXPERIMENTS

Abstract We have extended our measurements of the relative elastic differential cross sections (DCSs) and positronium formation cross sections (QPss) for positron scattering to include molecules of varying complexity and symmetry. Diffraction effects observed in our earlier e+–Ar and e+–CH4 DCS measurements [Phys. Rev. A 55 (1997) 4244] are shown to have disappeared for a linear tri-atomic molecule, CO2. In contrast to the structureless, bell-shaped Ps-formation curves of H and H2 [Phys. Rev. A 55 (1997) 361], the Ps-formation cross section curves for CH4, CO2, and SF6 show structure which may be associated with electron transfer of inner orbital electrons of the molecule.