Yale University School of Medicine, New Haven, Connecticut.

The response and wall correction factors for various ionization chambers in a cobalt-60 gamma-ray field have been calculated using a Monte Carlo photon-electron transport code. Among the chamber parameters studied are chamber wall material and its thickness, central electrode material and its dimensions, and the shape and size of the sensitive volume. The calculations show that the response and wall correction factors are sensitive to the shape and volume of the ionization chamber, but relatively independent of the choice of material for the chamber wall and electrode when these are compared on the basis of electron density. Data are presented for cylindrical, plane-parallel, and spherical ionization chambers constructed from carbon, magnesium, aluminum, water, Lucite, polystyrene, and ICRU muscle, as well as for a number of commercially available ionization chambers.