Positron – an alternative probe in electron scattering

Grzegorz Karwasz
Physics Department, University of Trento


First scattering experiments were performed by Paul Lenard in 1895,
before the “official” discovery of electron by J.J. Thompson.
The idea, expressed also by A. Einstein in 1905 was to find “real” dimensions of atoms.
Non-monotonic behaviour of cross sections vs. energy was found by Ramsauer and Townsend in 1921,
before the “matter wave” hypothesis by de Broglie.

Positron scattering measurements date from early seventies –
Kauppila and Stein claimed Ramsauer- Townsend minima for all noble gases.
Recently, high-quality beams (UCL London, San Diego) allowed to discover new phenomena like
resonances near vibrational threshold in molecules and measure
partial cross sections with a precision higher than that for electrons.

With a new, electrostatic beam in Trento we measured benzene, aniline, cyclohexane, argon, nitrogen, helium.
First three of them show a rise of cross sections at low energies, indicating scattering on a polarization potential,
like in modified-effective range theory (O’Malley, 1963).

In helium, virtual positronium seems to be formed  below the “real” positronium threshold,
showing-up with a resonant-like signal similar to the one observed in electron scattering ny J. Schulz (1964).

In argon, nitrogen (but also hydrogen, carbon dioxide and many others) we claim not a Ramsauer minimum,
but a flat energy-dependence of cross sections, like in a classical model of  rigid-sphere scattering.

Do positrons see “real” dimension of atoms?