The Relative Stopping Powers of Pure Gases to That of Air

Abstract

Preliminary reports giving some experimentally determined stopping powers of pure gases relative to air have been published (1, 2). These were published prior to completion of the experimental work. It is the purpose of this paper to present complete experimental results for the gases H2, He, N2, 02, Ne, A, Kr, and Xe using the /-ray spectra emitted by thick sources of S35, p32, and Y90 and to compare these with theoretically computed values. The experimental method and the apparatus used have been completely described in an earlier report (2). Theoretical values were computed in the following manner. First, it must be realized that the electron spectrum which is being considered is not that emitted by the radioactive material being utilized. The spectrum which is of interest is one which includes the primary spectrum emitted but which has been degraded by the passage of the electrons through the material (polystyrene) and secondary electrons generated in the material and in the gas. Since the experimental conditions met all requirements of the Bragg-Gray relationship (3, 4), the electron spectrum is the same in the solid electrode as it is in each gas studied. On the basis of the computed spectrum, the energy delivered per gram of each gas was then calculated. The relative stopping power per gram of gas to that of a gram of air was then obtained by taking the ratio of the energy delivered to a gram of each of these gases to that delivered to a gram of air.