C. K. Kwan

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.

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.

Positron differential quasielastic scattering by molecules

Abstract An overview of relative differential cross section (DCS) measurements in the range 30–135° for quasielastic scattering (elastic scattering plus rotational and vibrational excitations) of 4–200 eV positrons by N 2 , O 2 , CO, CO 2 , N 2 O and CH 4 , along with recent measurements for C 2 H 2 and S 6 is reported. For N 2 , O 2 , CO and CH 4 well-defined structure is found in the DCSs below the positronium (Ps) formation thresholds, which becomes washed out as the energy is increased above the Ps formation threshold. N 2 O and CO 2 show the least evidence of structure below (and above) the Ps threshold, while SF 6 exhibits noticeable structure not only below, but also above the Ps threshold. These observations indicate that the Ps formation channel may have an effect on the elastic scattering channel, as does the degree of symmetry of the target system, which is consistent with earlier elastic DCS measurements for the inert gas atoms.

Measurements of total and positronium-formation cross sections for the scattering of positrons by alkali atoms

We present a review of our recent measurements of total cross sections (QTs) for the scattering of positrons by Na, K, and Rb, and positronium-formation cross sections (QPss) for Na and K. For our total cross section measurements, a beam transmission technique has been used. For ourQPs measurements, our approach involves setting upper and lower limits onQPs using a combination of (1) measuring the transmission of the positron beam with the angular discrimination of the apparatus made as poor as possible, and (2) measuring the 511 keV annihilation gamma rays in coincidence produced by the decay of para-Ps formed in the scattering cell. Comparison with theoretical calculations shows that our measuredQTs andQPss for Na and K agree reasonably well with a close coupling approximation (CCA) calculation which takes into account the formation of Ps in then=1 andn=2 states. In the 3–10 eV energy range, this calculation predicts a peak in theQTs andQPss for K which also appears in our measurements. The absence of such a peak in our measuredQTs andQPss (preliminary) for Na in this energy range is also consistent with the same theory. Comparisons with five-state CCA calculations ofQT which do not take Ps-formation into account also show good agreement with our positron-Na, K, and RbQT measurements for energies above 20 eV, but show dramatic departures from our measurements below 10 eV for K and Rb.

Measurements of positron–molecule differential scattering cross-sections

Abstract Relative differential cross-sections (DCSs) have been measured for the quasi-elastic scattering (elastic scattering plus rotational and vibrational excitations) of positrons from CH 4 , N 2 , O 2 , CO, CO 2 , N 2 O, C 2 H 2 , and SF 6 molecules in a crossed-beam experiment. The energy range extends from below the positronium (Ps) formation threshold of each molecule to well above the Ps formation threshold and the angular range extends from 30° to 135°. For CH 4 , N 2 , CO, O 2 , and SF 6 the DCS exhibits structure (a clear minimum and maximum) at low energies that tends to wash out as the energy is increased above the Ps formation threshold. In contrast, for N 2 O, CO 2 , and C 2 H 2 we find no pronounced structure below the Ps formation threshold. These comparisons suggest that the degree of spherical symmetry of the target may be correlated with the presence of the structure.

Evidence for resonance and absorption effects in positron-atom differential elastic scattering

This review focuses on recent positron-atom elastic differential cross section (DCS) measurements which reveal evidence that interference effects may be occurring between the elastic and inelastic scattering channels. These effects appear as (1) absorption effects which are observable in relative elastic DCSs by the “washing out” of structure in the DCS versus angle as the positron energy is increased above the positronium formation threshold, and (2) a new type of resonance-like structures at intermediate energies (well above the normal inelastic thresholds) that have been observed in absolute elastic DCS measurements when plotted at a fixed scattering angle versus energy. A discussion is provided of possibly relevant information on positron-atom scattering that also indicates that coupling is occurring between various scattering channels.

Measurements of positronium formation cross sections for positron-ar and-K scattering

We have made preliminary measurements of positronium (Ps) formation cross sections for 9 to 452 eV positrons scattered from Ar atoms and for 1 to 20 eV positrons scattered from K atoms. Our experimental approach involves setting lower and upper limits on Ps formation cross sections using a combination of (1) the detection of the coincidences of 511 keV annihilation gamma rays produced by the decay of para-Ps and by the interaction of ortho-Ps with the wall of the scattering cell in which the Ps is formed, and (2) the determination of scattering cross sections associated with the measured transmission of the positron beam through the gas in our scattering cell with the angular discrimination of our apparatus deliberately made as poor as possible. The constraints placed by these lower and upper limits are used to check for consistency with prior experimental and theoretical results for Ar and to provide the first measurements of Ps formation cross sections for K, which are compared with available theoretical results.

Measurements of positronium formation cross sections for positron–potassium atom scattering

We report preliminary measurements of positronium (Ps) formation cross sections for 1–20 eV positron–potassium atom collisions. Our results are compared with prior theoretical results.

Search for resonance‐like structure in the total scattering of positrons by argon and krypton

Total scattering cross sections (QT’s) have been measured for positrons scattered by argon and krypton using a beam transmission technique. We are specifically searching for resonance‐like structure in the QT’s.