The “internal conversion coefficient” of a given γ-ray is defined as the probability that the γ-ray will be absorbed by one of the planetary electrons of the atom. If we denote by α the internal conversion coefficient, and by A the probability per unit time of the emission of a γ-ray by the nucleus (the Einstein A coefficient), then the number of electrons ejected per unit time is Aα, and the number of quanta escaping unabsorbed is A(1 — α). The quantity actually measured is the ratio of these two, namely α/(l — α). Experimental values of α have been obtained by Ellis and Aston for eight of the γ-rays of Radium C, and three for Radium B. For Radium C the lines measured lie between 6 and 22 x 10 5 electron volts; the internal conversion coefficients lie between 0.006 and 0.001, and do not vary smoothly with the frequency. For three lines of Radium B of energy in the neighbourhood of 3 Xx10 5 electron volts, α is much bigger, of order of magnitude 0.2.

A theory of the internal conversion of γ-rays. (1932). Proceedings of the Royal Society of London. Series A, Containing Papers of a Mathematical and Physical Character, 138(836), 665–695.