Universality of spectator fragmentation at relativistic bombarding energies

Mul ti-fragment decays of ¹²⁹Xe, ¹⁹⁷Au and ²³⁸U projectiles in collisions with Be, C, Al, Cu, In, Au and U targets at energies between E/A = 400 and 1000 MeV have been studied with the ALADIN forward-spectrometer at SIS. By adding an array of 84 Si-CsI(T1) telescopes the solid-angle coverage of the setup was extended to θ_lab = 16°. This permitted the complete detection of fragments from the projectile-spectator source. The dominant feature of the systematic set of data is the Z_bound universality that is obeyed by the fragment multiplicities and correlations. These observables are invariant with respect to the entrance channel if plotted as a function of Z_bound, where Z_bound is the sum of the atomic numbers Z_i of all projectile fragments with Z_i ≥ 2. No significant dependence on the bombarding energy nor on the target mass is observed. The dependence of the fragment multiplicity on the projectile mass follows a linear scaling law. The reasons for and the limits of the observed universality of spectator fragmentation are explored within the realm of the available data and with model studies. It is found that the universal properties should persist up to much higher bombarding energies than explored in this work and that they are consistent with universal features exhibited by the intranuclear cascade and statistical multifragmentation models.