Inclusive measurements of electromagnetic dissociation of 197Au targets.

Cross sections for the electromagnetic dissociation of [sup 197]Au targets by 1.7 GeV/nucleon [sup 20]Ne and 1 GeV/nucleon [sup 86]Kr, [sup 197]Au, and [sup 209]Bi beams have been measured by an activation method. We observe systematic discrepancies between the experimental data and a simple Weizsaecker-Williams calculation: For the heaviest projectiles, the calculation overpredicts the 1[ital n] cross sections, whereas the 2[ital n] and 3[ital n] 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 [sup 197]Au with [sup 197]Au and [sup 209]Bi projectiles which can only be explained by a dominant contribution from two-phonon giant dipole excitation. For an almost quantitative reproduction for all [ital 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.