Collisional ionization of helium by 250 eV electrons

For single ionization of helium by electron impact the energies and scattering angles of the scattered and ejected electrons have been measured in a coincident experiment at primary energies E0 between 30 and 80 eV. Two types of collision are found, those in which the momentum transfer to the ion produced is small (binary encounter) and those in which the momentum transfer is large (recoil encounter). The maximum of the coincidence rate measured due to collisions of the binary encounter type is located in forward direction. Its angular position is varying with the collision parameters, whereas its angular half width is approximately 60 degrees for all data obtained. The recoil collisions cause a maximum of the coincidence cross section at about 160 degrees with a half width larger than 90 degrees . The relative intensity of the binary encounter peak and the recoil peak as well as the total number of ionizing transitions depend strongly on the energies and angles of the two outgoing electrons. Binary peak and recoil peak do not have a common symmetry axis, probably due to the interaction between the two ionization electrons.