The electric monopole transition in nuclei

The emission of a γ-ray in the transition between two nuclear states which both have zero spin and even parity (0+) is forbidden since the photon must carry away one unit of angular momentum. The transition may proceed, however, by the internal conversion process through the interaction of an atomic electron with the charge distribution within the confines of the nuclear volume. Such transitions (0+ → 0+) have been of limited interest since in those cases only the monopole component is possible and the corresponding E0 matrix element is relatively simple. Of greater interest are the Jπ → Jπ (J ≠ 0) transitions which exhibit significant E0 strength. Until recently, few transitions of this type were observed (particularly in odd-mass nuclei) and E0 was not expected to compete significantly with M1 and E2. A major change in this picture has occurred as a result of studies of nuclear shape coexistence where a significant enhancement of the E0 strength has been observed in the Jπ → Jπ transitions between the shape coexisting configurations. A discussion of the E0 process in nuclei and its role in the study of nuclear structure will be presented