The Backscattering of Beta-Particles from Gases

Abstract

During the course of experimental measurements in an ionization chamber (1, 2) to determine relative values of W, the average energy to produce an ion pair in various gases by (-particles, large discrepancies from the expected ionization current values were often observed when the active source took the form of a thin sheet at the center of a large flat electrode. The diminution of ionization current which occurred especially in certain gases, was at last traced to a phenomenon of scattering of the/ -particles from the gas itself back to the electrode plate. This resulted in a reduced ionization path for such /-particles and hence a reduction in the observed value of the ionization current. Since such an effect may necessitate very large corrections in absolute 3-ray ionization chamber measurements, and since it might under certain experimental conditions be easily overlooked, it seems worth while to give a summary of our experiments here. Relative ionization current measurements in different gases were carried out for thin /-particle sources of H3, Ni6, and C14. The active source was in each case deposited in a thin layer upon the central collecting electrode of an ionization chamber, shown schematically in Fig. 1. The collecting electrode varied greatly in size and shape throughout the course of the experiment. Since the range of the /-particles is much larger for C14 than for H3 and Ni63, it was found advantageous to use two similar ionization chambers of different size. For H3 and Ni63 the chamber was a brass cylinder of internal diameter 9.5 cm and height 7.0 cm, as is shown in Fig. 1. For C14, in order to avoid the use of excessive gas pressures, the chamber was made larger-of diameter 20.5 cm and height 21.0 cm. The general technique of making the ionization current measurements has already been described (1, 2). In brief, the central collecting electrode of the ionization chamber was connected to a vibrating-reed electrometer, which in turn fed into a Brown strip-chart recorder. As the potential of the insulated chamber electrode rose with the collection of ions, the needle of the Brown recorder drifted across the scale. This drift was opposed at frequent intervals by counterpotentials applied to