Magnetic field-free measurements of the total cross section for positrons scattering from helium and krypton

An electrostatic beam has been used to perform scattering measurements with an angular-discrimination of less than or similar to 2 degrees. The total cross sections of positrons scattering from helium and krypton have been determined in the energy range (10-300) eV. This work was initially stimulated by the investigations of Nagumo et al (2011 J. Phys. Soc. Japan 80 064301), the first positron field-free measurements performed with a similarly high resolution, which found significant discrepancies at low energies with most other experiments and theories. The present results show good agreement with theories and several other measurements, even those characterized by a much poorer angular discrimination, implying a small contribution from particles elastically scattered at forward angles, as theoretically predicted for He but not for Kr.