We report the observation of double isovector giant dipole resonances in (${\pi }^{+}$,${\pi }^{\mathrm{-}}$) and, for the first time, also in (${\pi }^{\mathrm{-}}$,${\pi }^{+}$) double-charge-exchange reactions at $T_{\pi }$=292 MeV. The resonances have been observed on nine targets covering a wide range of mass at Q values around ≊-50 MeV in (${\pi }^{+}$,${\pi }^{\mathrm{-}}$), but at significantly lower energy (Q=-33.8 MeV) in the inverse reaction on ${}_{}{}^{56}{}_{}{}^{}\mathrm{Fe}$. The centroid energies in both reactions are very close to the energies at which the Δ$T_z$=±2 double giant dipole ($J^{\pi }$${=0\mathrm{}}^{+}$,$2^{+}$,ΔT=2) states are expected to appear. Angular distributions have been measured for the (${\pi }^{+}$,${\pi }^{\mathrm{-}}$) resonances on ${}_{}{}^{40}{}_{}{}^{}\mathrm{Ca}$${,}^{56}$, Fe, and ${}_{}{}^{93}{}_{}{}^{}\mathrm{Nb}$ and each is observed to have a quadrupole shape. Based on their energies, angular distributions, cross sections, and the comparison between the (${\pi }^{+}$,${\pi }^{\mathrm{-}}$) and (${\pi }^{\mathrm{-}}$,${\pi }^{+}$) data on ${}_{}{}^{56}{}_{}{}^{}\mathrm{Fe}$, we identify the resonances as double giant dipole states (i.e. states arising from a charge-exchange dipole operator acting twice on the ground-state wave function). The data from the present and recent observations indicate that the newly discovered resonance is a general collective feature of all nuclei.

• Received 28 August 1989

DOI:https://doi.org/10.1103/PhysRevC.41.202