Time-of-Flight Electron Velocity Spectrometer

A velocity selector is described which is designed for measurement of the total collision cross section of electrons in the energy region below 1 V. The system determines the distributions of times of flight of single electrons over a measured path at selected pressures of a specimen gas. The electrons are generated by repetitive illumination of a photoemitter by a spark gap, pass over a shielded drift path, enter an accelerating field, and impinge on an electron multiplier. Electronic equipment and data reduction for analysis of time-of-flight and amplitude distributions of electron counts are described; data illustrative of performance are presented, including typical transmission measurements in helium and argon which extend to 150 mV.