Fully Determined (e,3e) Experiments for the Double Ionization of Helium

Angular distributions of the two ejected electrons following electron impact double ionization of helium have been measured using a multicoincidence multiangle $(e,3e)$ spectrometer at an incident energy $\sim 5.5\mathrm{keV}$ and equal outgoing energies $E_b{=E}_c=10\mathrm{}\mathrm{eV}$. The measured distributions are similar to photodouble ionization $(h\nu ,2e)$ ones, but with evidence of additional nondipolar contributions which tend to fill up the characteristic node at the mutual angle ${\theta }_{\mathrm{bc}}=180\mathrm{}°$. They are also in satisfactory agreement with recent $(e,3e)$ calculations.