Secondary Effects in Ionization by Hard Gamma-Rays

The observations presented in this paper show that the ionization produced in a counting chamber by gamma-rays from a given source depends in a complicated way upon: (a) the nature (atomic) and thickness of the walls of the chamber; (b) the nature, amount, and distribution of matter in the neighborhood of the chamber. When the walls are sufficiently thick to stop the secondary beta-particles (the actual effective ionizing agencies) the secondary influences from the outside are greatly reduced, and the uncertainties involved among measurements with different thicknesses of the same wall material may be no more than a few percent. The results emphasize the effects of nuclear scattering of the secondary beta-particles. This action is effective in any material from which beta-rays may pass to the gas of the chamber, and consequently, the atomic properties of any such materials must be considered in interpreting ionization measurements. The following effects of nuclear scattering are discussed in their relation to ionization measurements: (1) equilibrium conditions between primary and secondary radiation with attention to the effects of "filtering" (2) reflection of the secondaries by the wall material; (3) the dependence of the "stopping power" for electrons, of the wall on the atomic number of the material. In the second part of the paper, the front and back wall effects are treated separately. Transition effects between different materials on the front wall are shown. These transition curves are discussed in connection with "stopping power," equilibrium conditions, and photoelectric conversion of the gamma-radiation. The bearing of these phenomena on measurements of gamma-radiation with different counting chambers is discussed.