Absolute triple- and double-differential cross sections for ionisation of helium by electron impact

Absolute (e, 2e) cross sections have been measured for electron impact ionisation of helium with a much higher accuracy than obtained before. In a coplanar symmetric geometry (theta = 450) both outgoing electrons - with equal velocities - were detected in coincidence. The resulting experimental triple differential cross sections for incoming electrons of 200-2800 eV contain an experimental error smaller than 20%. At the same time the absolute double differential cross sections have been determined at 450 to an accuracy of 10%. The data are compared with various theoretical results. (author)