Characteristics of cobalt-60 penumbral regions

Ionization chambers with wall materials of various electron densities have been employed to investigate the penumbral region of a cobalt-60 beam of side length 10 cm. Measurements were made in air with chambers of various inside diameters. The homogeneous walls of the ionization chambers were 0.5 g/cm2 in thickness. Extrapolation techniques to zero inside diameter and to infinite density were used, respectively, to eliminate the effect of the inside diameter of the chamber and to determine the penumbral photon-fluence distribution. The method also quantitates both the penumbral primary-dose profiles in water-equivalent media and the degradation of these profiles in media of low electron density.