Abstract :
Effective electromagnetic transition operators pertinent to the model space in the shell model are formulated for many-j-shell configurations and in isospin formalism within the framework of the first-order perturbation theory. The correlated operators thus obtained consist of one-body and two-body transition operators, the former of which is considered to be the origin of effective charges, while the latter cannot be incorporated into the idea of effective charges. Electromagnetic transitions in the 1f7/2-shell nuclei are studied systematically by using the effective transition operators so generated. It is found that the two-body contributions are very important not only in the higher-multipole transitions but also in the E2 transitions, to the extent that the concept of effective charge breaks down, and they are indispensable in achieving reasonable agreement with experiment.
