Electron Flux Spectra and Radiation Yields in Solids Irradiated by Beta Rays Invited Paper

During the last few years a program has been under way at Oak Ridge National Laboratory to measure the electron flux in an irradiated medium. A typical source consists of several curies of beta radioactivity uniformly dispersed in a metal, the whole approximating a homogeneous, uniformly and isotropically irradiated medium. The electron flux escaping from a cavity within the medium is measured over the energy range from about 50 keV down to 1 eV. At the higher energies the flux observed is due entirely to the beta rays from the source, whereas below 10 keV the flux rises by as many as five orders of magnitude, this rise being due to the generation of secondary, tertiary, quarternary and higher generations of electrons. The measurements are in approximate agreement with the Spencer-Fano theory of electron slowing down above ten keV, but tend to indicate that the theory underestimates the flux below this energy by as much as a factor of four. Theoretical ionization and plasmon cross sections for aluminum have been combined with our flux data to calculate the probability of occurrence of a given type of event from a beta ray from a copper-64 or gold-198 source. Calculations show that most K-ionizations (~3) are caused by the primary beta ray whereas L-shell ionizations and plasmon excitations tend to come from the secondary electrons (~300 for the former and ~20,000 for the latter).