Ionization of Argon, Neon and Helium by A, Ne and He Atoms

The ionization of argon, neon and helium under impact of A, Ne and He atoms was investigated by a method which eliminated errors due to secondary electrons. When argon, neon and helium were bombarded with their own atoms, ionization was found to set in at approximately 100 equivalent volts in each case. When any one of these three rare gases was bombarded with atoms of either of the other two, no ionization could be detected. If ionization had set in at 150 volts or less it should have been apparent.