Interpretation of Range Measurements for Kilovolt Electrons in Solids

The maximum range of electrons, as distinguished from the extrapolated or practical range, has been measured in Al, Ni, Ag, and Au for electron energies between 1.0 and 10.0 kev. The technique utilized involves the detection of the onset of penetration through freely supported evaporated films by observation of the current collected in a $2\pi$ geometry. The range defined in this manner may be compared with the predictions of stopping theory, with little influence of scattering processes. The results show no significant $Z$ dependence of the exponent in the range-energy relation, in agreement with results at higher energies. Some indications exist for an increasing value of the absolute range with increasing $Z$ when expressed in mg/${\mathrm{cm}}^2$, in accordance with the theoretically predicted decrease in stopping power for heavier elements. The results are compared with recent measurements using the onset and decline of luminosity in a phosphor layer to define the range, which leads to a strong $Z$ dependence of the range-energy relation in the oppasite sense to that observed by absorption techniques. It is shown that the differences in results may be traced to the nature of the detecting technique. The luminescence detector is sensitive to the total energy carried by the penetrating electrons in excess of a certain critical value rather than to their number.