No abstract

The " internal conversion coefficient " of a given y-ray is definedf as the probability th a t the y-ray will be absorbed by one of the planetary electrons of the atom. If we denote by a the internal conversion coefficient, and by A the probability per unit tim e of the emission of a y-ray by the nucleus (the Einstein A coefficient), then the number of electrons ejected per unit time is Aa, and the number of quanta escaping unabsorbed is A(1 -a). The quantity actually measured is the ratio of these two, namely a/(l -a).Experim ental values of a have been obtained by Ellis and Aston for eight of the y-rays of Radium C, and three for Radium B. For Radium C the lines measured lie between 6 and 22 X 105 electron v o lts ; 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 X 105 electron volts, a is much bigger, of order of magnitude 0*2.The problem of internal conversion has been treated theoretically by Miss Swirles,J Casimir § and Hulme.|| These authors assume th a t the excited nucleus radiates the field of a dipole, namely,Az -B _|_ complex conjugate f A* = Ay = 0, J where q -The probability per unit time of the ejection of an electron with energy Av is then calculated, using the methods employed in the treatm ent of the photo electric effect; the internal conversion coefficient is obtained by dividing by 16tc2B2v/3/m ?, the number of quanta per unit time radiated by the field (1.1).

In the first part of this paper we discuss the radiation from a single charged particle moving in an arbitrary central field of force and obeying Dirac's equation. We consider the electric quadripole and magnetic dipole radiation as well as the electric dipole. We derive the selection rules for the magnetic dipole radiation and collect together for reference the corresponding selection rules for the electric dipole and quadripole radiations. In the second part we discuss the relative intensities of the various types of radiation, treating in detail the cases where the selection rules for magnetic dipole and electric quadripole are simultaneously satisfied. Finally we show that these results have an important bearing on the theory of internal conversion of γ-rays. The internal conversion of soft γ-rays occurs with such high probability that the theory is unable to account for the experimental results unless it is assumed that the radiation is largely magnetic dipole in character. On the other hand, Fisk and Taylor (loc. cit.) were unable to account for the presence of magnetic dipole radiation in appreciable amounts. We show that this is due to the fact that, of the two possible transitions (a and e of § 2) in which both magnetic dipole and electric quadripole radiation can be emitted, Fisk and Taylor considered only the second. In the case of the second, corresponding to a transition between two distinct terms, we show that Fisk and Taylor were correct in predicting a negligible amount of magnetic dipole radiation, but in the case of the first, corresponding to a transition between two levels of one multiplet term, we find that there is indeed a high percentage of magnetic dipole radiation.

No abstract

No abstract