Penetration of Low-Energy Positrons through Thin Metallic Foils

The penetration of positrons (0.544-MeV maximum) from a ${\mathrm{Na}}^{22}$ source through absorber foils of A1, Cu, Sn, and Au has been measured in vacuum (${10}^{-3\mathrm{}}$ torr). It was found that the transmitted-positron intensity decreases exponentially with the thickness of the absorber over different ranges of transmission for different absorbers. Measurements on A1 foils were performed at atmospheric pressure and vacuum to determine the effect of the air path on the mass-absorption coefficient. It was found that the onset of the exponential absorption occurs at a higher relative transmission value when the experiment is performed in air. However, there is no significant effect on the slope of the straight-line portion of the graph which is proportional to the value of the mass-absorption coefficient. At transmitted-positron intensities below 4%, the $Z$ dependence can be described by $\frac{t}{A}=K{Z}^{-\frac{4}{3}}$, where $t$ is the thickness in mg/${\mathrm{cm}}^2$, $A$ is the mass of the atom, and $K$ is a constant.