First observation of the Coulomb-excited double giant dipole resonance inPb208via double-γ decay

The photon decay of the relativistic Coulomb excitation of the single and double giant dipole resonance (GDR) in the target has been observed in the system 1A GeV ${}_{}{}^{209}{}_{}{}^{}\mathrm{Bi}$ on ${}_{}{}^{208}{}_{}{}^{}\mathrm{Pb}$. For peripheral events which are dominated by relativistic Coulomb excitation, a large Lorentzian structure in the photon energy spectrum is peaked at 13.3±0.1 MeV with a width of 4.1±0.1 MeV, corresponding to the single GDR in the ${}_{}{}^{208}{}_{}{}^{}\mathrm{Pb}$ target. The sum energy of coincident γ-γ pairs shows a broad feature at 25.6±0.9 MeV with a Lorentzian width of 5.8±1.1 MeV, which we assign to the double GDR observed via the two-γ decay channel.