Since the energy loss of a slow heavy particle is due predominantly to recoiling atoms, ionization by secondary heavy particles contributes a large fraction of the total ionization resulting from a slow heavy particle that is stopped in a gas. If the secondary heavy particle ionization efficiency is low, the over-all efficiency for the production of ion pairs is greatly reduced for low energies of the primary particle. The secondary heavy particle ionization efficiency satisfies an integro-differential equation in which the ratio of electronic to atomic stopping cross sections plays a critical role. Data by Madsen on the ionization by recoiling particles in alpha-decay are used to set limits on the ionization by slow argon particles in argon, from which it is possible to make crude estimates of the energy of fission fragments which fails to give rise to ionization at the usual rate. In argon, ionization defects of the order of several Mev are to be expected.

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