Tracks in Fission-Fragment-Bombarded Lead Iodide

Abstract

The nature of fission-fragment tracks in crystals of lead iodide is sensitively dependent on the thickness of the crystal as well as on the energy, mass, and direction of the incident particle. As the thickness of the specimen is increased the white contrast tracks seen in the electron microscope become discontinuous, and with diffraction contrast they change to tracks that are dark on a lighter ground. At even greater thicknesses no tracks are visible, but only a random array of inclined dislocation loops. Tracks formed from fission fragments arising from thick pieces of uranium foil show branching that is interpreted as due to the registration of knocked-on atoms. Calculations of the rate of energy loss for fragments in lead iodide, made on the basis of a treatment first given by Bohr, are in qualitative agreement with the observations.