Electric-Monopole Transitions in Atomic Nuclei

Abstract

Low-energy electric-monopole or $E0$ transitions ($\ensuremath{\Delta}\mathrm{I}=0$, no) proceed solely by internal conversion, with zero units of angular momentum transferred to the ejected electron. Single gamma-ray emission of this multipole order is strictly forbidden. Electricmonopole pair production is possible for transition energies greater than $2m{c}^{2}$.It is pointed out that (1) the electric-monopole mode of de-excitation is available between any two equal-parity states of the same spin, zero or otherwise, (2) in such cases, $E0$ internal conversion may compete favorably with the paralleling magneticdipole and electric-quadrupole transitions in heavy nuclei, and (3) monopole matrix elements may be particularly useful in the study of nuclear structure.The relative and absolute conversion properties of electricmonopole transitions have been calculated relativistically, including the effects of the finite nuclear size and bound-state atomic screening. These results have been used to analyze the available experimental data on the 2+\ensuremath{\rightarrow}2+ transitions in ${\mathrm{Hg}}^{198}$, ${\mathrm{Pt}}^{196}$, and ${\mathrm{Pt}}^{192}$, and to determine upper limits for the monopole matrix elements. These upper limits appear appreciably smaller than the values for the matrix elements of well-known monopole transitions of the 0+\ensuremath{\rightarrow}0+ type.The possible significance of these results is considered with reference to current nuclear models.