1ST OBSERVATION OF THE COULOMB-EXCITED DOUBLE GIANT-DIPOLE RESONANCE IN PB-208 VIA DOUBLE-GAMMA 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 Bi-209 on Pb-208. 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 Pb-208 target. The sum energy of coincident gamma-gamma 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-gamma decay channel.