Isospin Symmetry violation in mirror E1 transitions: Coherent   contributions from the Giant Isovector Monopole Resonance in 67As - 67Se
  The assumption of an exact isospin symmetry would imply equal strengths for mirror E1 transitions (at least, in the long-wavelength limit). Actually, large violations of this symmetry rule have been indicated by a number of experimental results, the last of which is the 67As - 67Se doublet investigated at GAMMASPHERE. Here, we examine in detail various possible origins of the observed asymmetry. The coherent effect of Coulomb-induced mixing with the high-lying Giant Isovector Monopole Resonance is proposed as the most probable process to produce a large asymmetry in the E1 transitions, with comparatively small effect on the other properties of the parent and daughter levels. 
P. G. Bizzeti, G. de Angelis, S. M. Lenzi, R. Orlandi
nucl-th nucl-ex