Loss of Electrons from Collecting Electrode in High-Energy Photon Beams Investigated with a Double Extrapolation Chamber

Abstract

In the course of taking readings on surface dose (1) with the high-energy photons of cobalt and betatron radiations, it was found that the measurement with positive saturation potential on a parallel-plate ionization chamber was considerably greater than that with negative. A similar effect has been reported (2) for cobalt. Failla (3), in 1937, called attention to the problem. In the present case, the difference between the two measurements becomes less as the thickness of the upper electrode is increased, vanishing at the equilibrium thickness for cobalt. With the betatron there is a slight reversal at this point, the reading with negative potential on the upper electrode becoming slightly greater than with positive. A double extrapolation chamber was designed in which the lower plate was made of a conducting plastic which could be increased in thickness, while the upper plate was increased in the usual way. For a thin upper electrode, the difference between positive and negative readings became greater ...