The Probability ofKIonization of Nickel by Electrons as a Function of Their Energy

Thin targets of Ni were bombarded with electrons at 12 to 183 kv and intensities of the $K\alpha$ doublet were measured in arbitrary units. The results were converted to absolute cross sections for ionization by comparison with Smick and Kirkpatrick's absolute measurements at 70 kv. At any voltage $V=U{V}_K$, where $V_K$ is the $K$ ionization voltage, the cross section is well represented empirically as ${\Phi }_K=7.3\mathrm{}{\left(\frac{e}{V_K}\right)}^2{U}^{-0.837\mathrm{}}{log}_{10}U,$ with $e$ and $V_K$ both in electrostatic units. Burhop's theory is confirmed with accuracy probably well within the limits of error imposed on it by the Born approximation and neglect of relativity, exchange and other minor factors. The effect of relativity is found by comparison of the cross sections for Ni with ones for Ag, previously measured in this laboratory. Relativity increases the cross sections by moderate percentages, which increase with voltage. Deduction of these percentages yields data for a hypothetical non-relativistic element; and Burhop's non-relativistic theory fits this element best. Smith's cross sections for helium are compared with these non-relativistic cross sections and with those for real nickel. At low $U$'s the cross sections for helium are notably less than would be predicted by simple analogy with the other elements, presumably because of unusually great effects in helium, due to movement of the electron which remains in the atom.