PHASESHIFT ANALYSIS OF EXPERIMENTAL ANGULAR-DISTRIBUTIONS OF ELECTRONS ELASTICALLY SCATTERED FROM HE, NE AND AR OVER THE RANGE 0.5 TO 20 EV

Absolute differential elastic cross sections have been measured for electron scattering by helium, neon and argon atoms over the incident electron energy range of 0.5 to 20 eV, and over the scattered electron angular range of 15 to 150 degrees . The experimental method uses crossed electron and modulated atom beams, 127 degrees electrostatic energy analysis of incident and scattered electrons, quadrupole mass analysis of the target gas and single-particle counting techniques. It is shown how the use of a chi-squared optimisation analysis leads to better knowledge of apparatus behaviour and of cross section determinations. Absolute cross section values have been established by the methods of (i) the parametrisation of the relative elastic angular distributions in terms of the first three phases at 22 energies and 27 angles within the above ranges, (ii) a phaseshift analysis around the 19.3 eV (2S) resonance in helium and (iii) the measurement of elastic scattering signal ratios in Ne and Ar at any energy and scattering angle (here 19.2 eV and 60 degrees ) for known relative source pressures.