Electron Loss Cross Sections for Helium Atoms Passing Through Gases

Neutral helium atoms having kinetic energies in the range 100 to 450 kev are passed through a gas cell in which there is an intense magnetic field. When a few microns pressure of gas is admitted to the cell, an attenuation of the transmitted beam takes place, due to collisions in which the helium atoms become ionized, and removed from the unidirectional beam by the magnetic field. The method will not distinguish between collisions in which one or both helium electrons are lost. The sum of the cross sections for the two processes is measured, per atom of gas traversed. The cross sections rise in a rather uniform manner as energy increases in this range. In units of 10^-17 cm² per atom of gas traversed, the values at 100 and at 450 kev are: for air, 30.2±{}0.8 and 43.9±{}1.5; for hydrogen, 5.8±{}0.6 and 12.5±{}0.8; and for helium, 11.6±{}0.6 and 16.8±{}0.4.