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Triple differential cross sections far the single ionization of helium by 400eV. 250 eV and 150 eV electrons have been measured in the range of momentum transfer from 0.35 to 1.1 atomic units. The large number of measurements covers most of the dynamical situations under which electron impact ionization for a given impad energy can occur. The measurements therefore serve as a rather complete set of data far comparison with theory.Several second-order theories have been tested. A second Born approximation with an improved wavefunction for the low-energy ejected electron is most suitable 10 describe the process.