The penetration effect in electric dipole internal conversion

Nuclear-structure-dependent contributions to the internal-conversion process are considered in conjunction with highly retarded electric dipole transitions. Formulas for a theory of anomalous internal conversion for electric transitions are given. These formulas are applied to explain the strong E1 anomalies for L conversion found experimentally by Asaro, Stephens, Hollander and Perlman. Two different types of anomalous matrix elements occur, one associated with the nuclear charge, the other with the nuclear current. It is found that the contribution associated with charge is negligible. In the current terms we distinguish two parts, the convection current and spin current, which are associated with different selection rules and generally both are important. From the empirical data of the L I and L II anomaly the sign of the ratio of nuclear matrix elements which gives the dominant contribution to the anomalous conversion amplitude is deduced. The anomalous nuclear matrix elements are evaluated for the single-particle model. Effects of pair correlation are discussed. The agreement with experiments is satisfactory.