Double giant dipole resonance in the (-,+) reaction

The double isovector giant dipole state has been observed in the (-,+)Tx=+2 double-charge-exchange reaction on C13, Al27, Ca40, Fe56, Co59, and Nb93. The resonances observed in the (-,+) reaction are closely related via Coulomb displacement energy and isospin symmetry to the resonances measured in the inverse (+,-) reaction on the same nuclei. The new observations provide a direct determination of the upper isospin component of the double giant dipole state, which is generally very weak in the (+,-) reaction. The comparison between the double dipole in the two double-charge-exchange modes gives valuable information on the isospin splitting and the Pauli-blocking effects for isotensor transitions.