Inclusive measurements of electromagnetic dissociation ofAu197targets

Cross sections for the electromagnetic dissociation of ${}_{}{}^{197}{}_{}{}^{}\mathrm{Au}$ targets by 1.7 GeV/nucleon ${}_{}{}^{20}{}_{}{}^{}\mathrm{Ne}$ and 1 GeV/nucleon ${}_{}{}^{86}{}_{}{}^{}\mathrm{Kr}$, ${}_{}{}^{197}{}_{}{}^{}\mathrm{Au}$, and ${}_{}{}^{209}{}_{}{}^{}\mathrm{Bi}$ beams have been measured by an activation method. We observe systematic discrepancies between the experimental data and a simple Weizsäcker-Williams calculation: For the heaviest projectiles, the calculation overpredicts the 1n 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 ${}_{}{}^{197}{}_{}{}^{}\mathrm{Au}$ with ${}_{}{}^{197}{}_{}{}^{}\mathrm{Au}$ and ${}_{}{}^{209}{}_{}{}^{}\mathrm{Bi}$ 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.