(e,2e) cross sections for ionization of helium in coplanar symmetric geometry

Distorted-wave Born approximation (DWBA) calculations are reported for coplanar symmetric ionization of He(1s2) at energies of 100, 150 and 200 eV. Comparison is made with very recent experimental data and good agreement is found particularly with regard to the presence of a dip-bump structure at the larger angles. This structure is at variance with earlier experimental and theoretical work using the second Born approximation. There is a serious conflict between recent absolute measurements at 200 eV and an earlier absolute determination of the 200 eV cross section at an angle of 450. The former is supported by DWBA while the latter is supported by the factorized distorted-wave impulse approximation (DWIA). This normalization problem poses a substantial challenge to both theory and experiment. (author)