Differential Elastic Scattering of Positrons from Argon Atoms at Low Energies

Abstract

We have measured relative cross sections in the angular range of 26° to 65° at 8.5 and 30 eV. Our 30-eV data show a monotonic increase with decreasing scattering angle, in disagreement with three theoretical calculations which predict a structure of the cross section in this range, but in qualitative agreement with a prediction of Joachain and Potvliege, derived from optical-potential calculations at and above 100 eV. At 8.5 eV, below all inelastic thresholds, our data agree very well with the theoretical results of McEachran and Stauffer as well as Nakanishi and Schrader but disagree with other calculations.PACS numbers: 34.90.+q Atomic-physics scattering experiments with positrons are performed in order to aid in the development of electron and positron scattering theory. Positron-atom scattering is an interesting test case for theory because of the absence of exchange with atomic electrons which complicates the theory of electron-atom scattering at low energies.Low-energy beams of monoenergetic positrons are obtained by moderation of fast positrons (from p decay or pair creation) in a metal and reacceleration of the positrons which emerge from the moderator with nearly thermal energies. Despite considerable improvements in source and moderator technology, positron experiments are still much more difficult than corresponding electron experiments because the available intensities are orders of magnitude smaller. For many years only totalscattering cross sections could be measured in gas-cell transmission experiments. Recently, however, more sophisticated partial-cross-section measurements have become feasible. 1,2 In a pioneering experiment, Coleman and McNutt 3 derived absolute differential elastic e + -Ar cross sections for forward-scattering angles from time-of-flight spectra of a gas-target transmission experiment with longitudinal magnetic guiding field. This method is restricted to energies below the first inelastic threshold because above it the time-of-flight analysis cannot unambiguously distinguish between loss of axial velocity due to wide-angle scattering and due to energy loss. More versatile but also more difficult are positron-atom crossed-beam experiments on differential scattering as have been pursued by the experimental groups at Wayne State University and Universitat Bielefeld for several years. The Wayne State group (Hyder et al.) published their first results on e+-Ar elastic scattering form 100 to 300 eV in 1986, 4 and later Kauppila and Stein reported measurements at lower energies. 5 We present here our first results, of which a part was recently reported. 6 Four theoretical groups have obtained results for low energies by means of different methods. McEachran and Stauffer employed an adiabatic polarized-orbital approximation without the need to fit to experimental data. The other three groups performed model potential calculations: The polarization potentials of Nakanishi and Schrader 8 and Datta et al. 9 contain one effective radius and an experimental value for the polarizability of...

Low-energy elastic and inelastic scattering of positrons from argon

“…However, overall the current data are consistent with previous experiments at all energies from 2-50 eV, with some small disagreement with Smith et al [5] at higher scattering angles for 5 and 8 eV. At the higher energies from 15-50 eV, [4]; and , Smith et al [5]. Please note that where the error bars are not plotted, the lower error bound extends below zero.…”

“…1(d) and 2(a)-2(c) for energies 10-30 eV. The dip is more dramatic at energies around the threshold and is echoed, especially, in the experimental data of Floeder et al [4] taken at 8.5 eV and Smith et al [5] taken at 5 and 8.7 eV. In the 15 and 20 eV data of Smith et al [5] the dip is still present, but appears less distinct relative to the lower energy measurements.…”

“…Previous relative experimental data for positron-Ar DCS are taken from Floeder et al [4] and Smith et al [5], where the original data were scaled to match theory available at the time, while DCS drawn from Coleman and McNutt [2] were scaled using absolute total scattering cross-section measurements from Coleman et al [3]. In order to compare with the present folded DCS, the shape of the unfolded CCC curve was used to estimate the effect of folding on the experimental data and the folded data were scaled to match the CCC calculation at 60 • for Floeder et al [4] and Smith et al [5]. The folded data from Coleman and McNutt [2] were rescaled, using total cross .…”

“…[3]. With one exception, other previous experimental DCS for positron-Ar scattering have been relative [4,5] and were typically scaled to match theoretical models at a specific angle, thus allowing comparison of the angular dependence only. Since these initial measurements, the development of the Surko buffer gas trap and highly sensitive manometers have enabled measurements of absolute DCS with improved angular resolution and statistical uncertainties.…”

Low-energy positron-argon elastic scattering at 30°

“…Although differential studies of positron collisions remain scarce (for recent reviews see, e.g., Laricchia and Charlton 1999a, b), results now exist for elastic scattering, Ps formation, transfer and direct ionization, including, in the latter case, a triply differential measurement (Kövér and Laricchia 1998). In the case of elastic scattering, as recently discussed by Kauppila et al (1996), the available results concern primarily the inert atoms (see also Coleman and McNutt 1979, Floeder et al 1988, Smith et al 1990, Gao et al 1998, Gilbert et al 1999. Together with theoretical calculations (McEachran et al 1979, 1986, Bartschat et al 1988, McAlinden and Walters 1994, Meyerhof and Laricchia 1997), these measurements have indicated the importance of coupling effects to the Ps formation channel.…”