Ionization at relativistic energies and polarization effects

Abstract

A theory of the polarization effects in the energy loss of a charged particle is given, in which the mechanism of the loss is different from that of the Fermi theory. This theory leads to a distribution of the loss between ionization-excitation and emission of Cerenkov radiation which differs considerably from that of the theories on Fermi’s lines, although the stopping power is not essentially changed, being somewhat less than in those theories. This treatment leads to an increase of the direct ionization at impact parameters larger than atomic dimensions after the relativistic minimum, the increase being large in the case of gases. The Fermi saturation effect of the loss at distances larger than atomic dimensions does also exist in the present theory, although due to a different mechanism. The relativistic increase of the loss in distant interactions is due largely to an increase of the radii of action for ionization and excitation (Bohr-Williams mechanism), but there is also a contribution of the Cerenkov radiation (Fermi mechanism). The saturation arises from a limitation of the increase of the radii of action due to the polarization of the medium and to the saturation of the emission of Cerenkov radiation. This theory leads to a modification of the formula of Frank and Tamm for the rate of emission of Cerenkov radiation, the emission of high energy Cerenkov radiation being considerably reduced.