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The energy absorbed by a substance exposed to an ionizing radiation is used partly in ionization, partly in optical excitation. Arguments based on atomic mechanics are presented, to show that, the greater the ionization potential, the greater is the share of the absorbed energy which is actually spent in ionization. This explains why the ionization potential of a substance has little influence upon the amount of energy that must be absorbed by that substance per each pair of ions actually produced.
U. Fano (1946) studied this question.