The relativistic increase of the specific primary ionization in helium

Abstract

The relativistic increase of the specific primary ionization produced by electrons in a helium-alcohol mixture, has been determined from measurements of mean length made on photographs of post-expansion tracks in a Wilson cloud chamber. The results show a logarithmic increase of the relative ionization as a function ofp/mc fromp/mc∼5 up top/mc∼300 and there is some evidence of the density effect atp/mc∼678 where the primary ionization relative to minimum is 1.50±0.03. The extent of the logarithmic increase and the ratio of the plateau to the minimum ionization are significantly greater than those calculated for pure helium from the theory of Budini and Taffara. Measurements of the specific primary ionization by the technique of gap counting are shown to be uncorrelated with variations in the relative condensation efficiency (θ) in the range 0.2</θ</0.6. This enables precise measurements of ionization to be made which do not necessitate critical adjustments of expansion conditions in the cloud chamber.