Absolute triple differential cross sections for high-energy electron impact ionisation of helium

The triple differential cross sections for electron impact ionisation of helium are investigated in a coplanar coincidence experiment (e, 2e reaction) for a primary electron energy near 8 keV and ejected electron energies of 100 and 337 eV. The data are made absolute by integrating over all ejection directions (assuming cylindrical symmetry of the ejected electron distribution) and comparing with the corresponding doubly differential cross section. The latter is also measured in a separate experiment and is made absolute by use of the Bethe sum rule. The overall accuracy of this calibration procedure is estimated to be better than 10%. As in previous low-energy measurements, two lobes are found in the angular distribution of the ejected electron. The data are compared with different theoretical models. At large momentum transfer where the recoil lobe almost completely disappears and the collision may be considered as a binary electron-electron process, the 'corrected' impulse approximation reproduces fairly well the experiment.