We report results from new positron–argon total cross-section (TCS) measurements. Agreement with the corresponding recent data of Jones et al (2011 Phys. Rev. A 83 032701) is found to be very good, except at the lowest energies of common measurement. Excellent qualitative agreement is also found between our measurements and an improved convergent close-coupling (CCC) calculation which was undertaken as a part of this study. This level of accord between our experimental and theoretical TCSs has enabled us to determine an experimental scattering length ( a ) of a = −4.9 ± 0.7 au for the positron–argon system. That value is in excellent agreement with the relativistic polarized orbital optical potential approach result of Jones et al ( a = −4.7 au) and our CCC result of a = −4.3 au.
We propose a method to convert measured positronium annihilation rates in gaseous xenon into total and differential cross sections of positronium-xenon collisions in an ultralow-energy region of less than 80 meV where their experimental determinations as functions of the positronium kinetic energy are extremely difficult. This method makes it possible to determine not only the
s-wave collisional parameters but also the
p-wave and
d-wave parameters. We have found a small positive value of the scattering length, ${A}_{0}=2.06\ifmmode\pm\else\textpm\fi{}0.10\phantom{\rule{0.16em}{0ex}}{a}_{0}$, which indicates that the positronium-xenon interaction in this energy region is repulsive and suggests that it is dominated by the scattering amplitude of the positron rather than that of the electron. An extrapolation of the analytical result into the experimentally inaccessible energy regions from 80 meV to 1.0 eV indicates that there should not be a Ramsauer-Townsend minimum but rather a peak in the total cross section at an energy of approximately 0.4 eV.
Recent results for positron scattering from noble gases over an energy range from 0.5 to 60eV are presented. Measurements include the grand total ( σ GT ), Ps formation ( σ Ps ) and Grand total – Ps formation ( (σ GT -P s ) cross sections. Some preliminary DCS results will also be presented. Work on a formulation of modified effective range theory (MERT) is being undertaken to determine the value of the scattering length [1] which may be useful for identifying a bound state. Plans for experiments on metal atoms will be outlined.
Total cross sections for the scattering of low-energy electrons and positrons by atomic beryllium in the energy range below the first inelastic thresholds are calculated. A Ramsauer–Townsend minimum is seen in the electron scattering cross sections, while no such effect is found in the case of positron scattering. A minimum total cross section of 0.016 a 0 2 at 0.0029 eV is observed for the electron case. In the limit of zero energy, the cross sections yield a scattering length of −0.61 a 0 for electron and a 0 for positron scattering.
Positron cooling and annihilation in room temperature noble gases is simulated using accurate scattering and annihilation cross sections calculated with many-body theory, enabling the first simultaneous probing of the energy dependence of the scattering and annihilation cross sections. A strikingly small fraction of positrons is shown to survive to thermalization: ∼0.1 in He, ∼0 in Ne, ∼0.15 in Ar, ∼0.05 in Kr, and ∼0.01 in Xe. For Xe, the time-varying annihilation rate Z[over ¯]_{eff}(τ) is shown to be highly sensitive to the depletion of the momentum distribution due to annihilation, conclusively explaining the long-standing discrepancy between gas-cell and trap-based measurements. Overall, the use of the accurate atomic data gives Z[over ¯]_{eff}(τ) in close agreement with experiment for all noble gases except Ne, the experiment for which is proffered to have suffered from incomplete knowledge of the fraction of positrons surviving to thermalization and/or the presence of impurities.
The scattering of orthopositronium from He, Ne, Ar,
Li+, Na+, and
K+ is investigated with the fixed-core stochastic variational method. The scattering length for $\mathrm{Ps}\ensuremath{-}\mathrm{He}$ scattering was
1.57a0, a value consistent with the threshold cross sections derived from three positron lifetime experiments. The scattering lengths for the $\mathrm{P}\mathrm{s}\ensuremath{-}\mathrm{N}\mathrm{e}$ and $\mathrm{P}\mathrm{s}\ensuremath{-}\mathrm{A}\mathrm{r}$ systems were
1.55a0 and
1.79a0, respectively. That there was only a 15% variation among the scattering lengths is compatible with the experiment of Coleman et al. [J. Phys. B 27, 981 (1994)], who found the low-energy cross sections for He, Ne, and Ar to be about the same size. The scattering lengths for Ps scattering from
Li+, Na+, and
K+ were
12.9a0, 28.5a0, and $\ensuremath{-}{1.9a}_{0}.$ The relatively small magnitude of the ${\mathrm{P}\mathrm{s}\ensuremath{-}\mathrm{K}}^{+}$ scattering length strongly supports previous suggestions that the ${\mathrm{P}\mathrm{s}\ensuremath{-}\mathrm{K}}^{+}$ system does not support a bound state. The annihilation parameter
1Zeff has also been computed as part of the analysis. The present values of
1Zeff are about 2.5--3 times smaller than the accepted experimental values since short-range electron-positron correlations were not taken into consideration when the annihilation matrix element was evaluated.