Energy dependence of double giant resonances in pion double charge exchange

The energy dependence of the giant dipole resonance built on the isobaric analog state was measured in pion-induced double charge exchange on C13 at incident pion energies of 140-295 MeV. The cross section increases with energy. The excitation function has a typical non-spin-flip signature, expected from simple considerations and verified by sequential model calculations. Three narrower previously reported low-lying states in O13 (Ex=3.1, 4.5, and 6.1 MeV) exhibit a spin-flip signature and may correspond to the giant Gamow-Teller strength built on the isobaric analog state.