Properties of the giant dipole resonance built on the isobaric analog state

Abstract

Resonances were observed in pion-induced double charge exchange (at ${T}_{\ensuremath{\pi}}$=292 MeV) on $^{13}\mathrm{C}$, $^{59}\mathrm{Co}$, $^{93}\mathrm{Nb}$, $^{138}\mathrm{Ba}$, and $^{197}\mathrm{Au}$ in the continuum at excitation energies of 8.4, 27.5, 33.2, 38.7, and 44.2 MeV, respectively (Q=-27.4, -32.3, -35.8, -38.4, and -46.0 MeV). An angular distribution was measured for the resonance on $^{93}\mathrm{Nb}$ and observed to have a dipole shape. The measured cross sections and the angular distribution agree well with a simple sequential two-step calculation in which the intermediate states arise from single charge exchange to the isobaric analog state and to the giant dipole resonance. Based on their excitation energies, angular distribution for $^{93}\mathrm{Nb}$, and cross sections we identify the resonances as the giant dipoles built on the isobaric analog states. The new data together with the recent observation of the resonance in three other nuclei indicate that the excitation energies, widths, and cross sections for these resonances have a simple mass dependence.