Calculation of the mean ionization potentials of the elements for stopping-power computations

The mean ionization potential Ī of an element of atomic number Z has been calculated on the basis that the velocity distribution among the orbital electrons of an atom is given by n(uₛ)=f(Z)uₛv₀, where n(uₛ) is the number of orbital electrons with velocity less than uₛ, v₀ is given by e², and $f(Z)$ assumes the values 0.28Z2/3 for Z≤45.5 and Z1/3 for Z≥45.5. The calculated values for the case of light elements are in fair agreement with the corresponding experimental values of the mean excitation potentials, but for the case of the heavier elements these are appreciably lower than the experimental values.