Two-phonon giant resonances in sup 1 sup 3 sup 6 Xe, sup 2 sup 0 sup 8 Pb, and sup 2 sup 3 sup 8 U

The excitation of the double-phonon giant dipole resonance was observed in heavy projectile nuclei impinging on targets of high nuclear charge with energies of 500-700 MeV/nucleon. New experimental data are presented for sup 1 sup 3 sup 6 Xe and sup 2 sup 3 sup 8 U together with further analysis of earlier data on sup 2 sup 0 sup 8 Pb. Differential cross sections d sigma/dE sup * and d sigma/d theta for electromagnetic excitations were deduced. Depending on the isotope, cross sections appear to be enhanced in comparison to those expected from a purely harmonic nuclear dipole response. The cumulative effect of excitations of two-phonon states composed of one dipole and one quadrupole phonon, of predicted anharmoniticies in the double-phonon dipole response, and of damping of the dipole resonance during the collision may account for the discrepancy. In addition, decay properties of two-phonon resonances were studied and compared to that of a statistical decay.