INCLUSIVE MEASUREMENTS OF ELECTROMAGNETIC DISSOCIATION OF AU-197 TARGETS

Cross sections for the electromagnetic dissociation of Au-197 targets by 1.7 GeV/nucleon Ne-20 and 1 GeV/nucleon Kr-86, Au-197, and Bi-209 beams have been measured by an activation method. We observe systematic discrepancies between the experimental data and a simple Weizsacker-Williams calculation: For the heaviest projectiles, the calculation overpredicts the ln cross sections, whereas the 2n and 3n cross sections are systematically underpredicted. The deviations can be significantly reduced, however, by calculations including isoscalar and isovector giant quadrupole excitations as well as the possibility to excite multiphonon states. The most important experimental result of the present study is the observation of large cross sections for three-neutron removal from Au-197 with Au-197 and Bi-209 projectiles which can only be explained by a dominant contribution from two-phonon giant dipole excitation. For an almost quantitative reproduction for all xn channels observed, we have to assume a higher excitation probability by roughly a factor of 2 for the double giant dipole resonance as compared to the harmonic approach.