The Measurement of Natural Alpha-Particles Ejected from Solids

Exact expressions for the ionization due to the emission of alpha-particles from radioactive solids are set up and the results of their graphical integration are given. The total ionization per ${\mathrm{cm}}^2$ of surface, both with and without absorbers; the variation of ionization with height above the radioactive surface; the ionization from thin films; the ionization from substances containing all members of the U, Th and Ac series, both with and without absorbers; and the counting rate for alpha-particle counting chambers, are given in equations accompanied by appropriate graphs and tables. The older approximate theories break down for actual alpha-particles, particularly those of short range. If $q$ is ion pairs per ${\mathrm{cm}}^2$ per sec. formed above the surface of a radioactive solid; $N$, the number of alpha-particles per sec. per ${\mathrm{cm}}^3$ emitted in the solid; $k$, ion pairs per alpha-particle of range $R$ in air; and $R^{\prime }$, the alpha-particle range in the solid: then $q=\epsilon \mathrm{Nk}{R}^{\prime }$, and the numerical coefficient $\epsilon$ has values between 0.114 and 0.150 for the alpha-particles from all known radioactive substances.