Average Energy Expended in Producing Ion Pairs—S35Absolute Value for Air

Abstract

Application of the Bragg-Gray principle to a small air gap in a homogeneous medium with a uniform concentration of beta-ray-emitting material yields the relation: (1) where W is the average energy expended by a beta ray in the production of an ion pair in air, E the energy emitted by the beta rays per gram of wall per second, N the number of ion pairs formed per second per gram of air, and Sss is the relative stopping power per gram of the medium with respect to air. A parallel-plate extrapolation type ionization chamber may be used to determine W from Eq. 1 if the radioactive material is uniformly distributed in the walls of the chamber. If the material is distributed in only one of the two chamber walls, as is expected, the ionization current is exactly halved, so that W may be measured under this condition. We have measured W by this method, using an aqueous solution of Na2S35O4 as one electrode of the extrapolation chamber. The solution thickness was greater than the maximum beta-ray range to insure that the energy emitted and absorbed per unit mass was the same. The energy emission rate was determined in a microcalorimeter (1) by Dr. W. Mann of the National Bureau of Standards, to whom we are greatly indebted. The other, collecting, electrode was a guard-ring type aluminum plate. Since this electrode is not atomically equivalent to water, experiments were performed to determine the magnitude of the change in ionization current produced by substitution of walls of different atomic composition (2). The substitution of aluminum for water results in an ionization current increase by a factor of 1.147 for the beta rays of S35. The spacing was changed by varying the height of liquid. For electrode spacings of 0.6 mm. or less, the current per gram of air was found to be constant. The value of Ssa used was calculated from experimental data of the relative stopping power of hydrogen and oxygen to air for S35 beta rays (3). The value of W obtained is 33.5 ev, but it must be considered tentative, pending the experimental determination of small corrections due to the presence of water vapor in the air gap of the chamber.