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Multi-fragment decays of 129 Xe, 197 Au, and 238 U projectiles in collisions with Be, C, Al, Cu, In, Au, and U targets at energies between E/A = 400 MeV and 1000 MeV have been studied with the ALADIN forward-spectrometer at SIS. By adding an array of 84 SiCsI(Tl) 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.

Multifragmentation has been measured for I97 Au+ 197 Au collisions at El A =100, 250, and 400 MeV. The mean fragment multiplicity increases monotonically with the charged particle multiplicity at El A =100 MeV, but decreases for central collisions with incident energy, consistent with the onset of nuclear vaporization. Molecular dynamics calculations follow some trends but underpredict the observed fragment multiplicities. Including the statistical decay of excited residues improves the agreement for peripheral collisions but worsens it for central collisions. PACS numbers: 25.70.Pq, 25.70.GhHighly excited systems can be formed during energetic nucleus-nucleus collisions, which expand due to thermal pressure [1,2] or via dynamical compression-decompression cycles [3]. For systems which expand to low densities where bulk nuclear matter is thermodynamically unstable, the growth of density fluctuations may favor multifragment disintegrations [4][5][6], and such disintegrations have been observed [7][8][9][10][11][12]. While definitive interpretations are premature, calculations predict that the onset of multifragmentation and the transition from multifragmentation into vaporization may be sensitive to the low density equation of state [2,13] and the liquid-gas phase transition of nuclear matter [14][15][16][17].The incident energy dependence of multifragmentation has been recently explored for 36 Ar+ 197 Au collisions between £7. 4=35 and 110 MeV [7]. These investigations reveal large fragment multiplicities for central collisions, which increase monotonically with incident energy. Over a broader range of incident energies, however, calculations predict a maximum in the fragment multiplicity for central collisions at El A ~ 100 MeV [18], and decreasing multiplicities thereafter, consistent with the onset of nuclear vaporization [4,5].

Breakup temperatures were deduced from double ratios of isotope yields for target spectators produced in the reaction 197 The good agreement with the breakup temperatures measured previously for projectile spectators at an incident energy of 600 MeV per nucleon confirms the universality established for the spectator decay at relativistic bombarding energies. The measured temperatures also agree with the breakup temperatures predicted by the statistical multifragmentation model. For these calculations a relation between the initial excitation energy and mass was derived which gives good simultaneous agreement for the fragment charge correlations.The energy spectra of light charged particles, measured at θ lab = 150• , exhibit Maxwellian shapes with inverse slope parameters much higher than the breakup temperatures. The statistical multifragmentation model, because Coulomb repulsion and sequential decay processes are included, yields light-particle spectra with inverse slope parameters higher than the breakup temperatures but considerably below the measured values. The systematic behavior of the differences suggests that they are caused by light-charged-particle emission prior to the final breakup stage. Keywords:197 Au projectiles and targets, E/A = 600 and 1000 MeV; measured fragment cross sections, isotopic yield ratios; deduced breakup temperatures, pre-breakup emission; analysis using quantum statistical and statistical multifragmentation models.

Breakup temperatures in central collisions of 197 Au 1 197 Au at bombarding energies E͞A 50 to 200 MeV were determined with two methods. Isotope temperatures, deduced from double ratios of hydrogen, helium, and lithium isotopic yields, increase monotonically with bombarding energy from 5 to 12 MeV, in qualitative agreement with a scenario of chemical freeze-out after adiabatic expansion. Excited-state temperatures, derived from yield ratios of states in 4 He, 5,6 Li, and 8 Be, are about 5 MeV, independent of the projectile energy, and seem to reflect the internal temperature of fragments at their final separation from the system. [ S0031-9007(98) . The temperatures were derived from double ratios of helium and lithium isotopic yields while the excitation energies were obtained by adding up the kinetic energies of the product nuclei and the mass excess of the observed partition with respect to the ground state of the reconstructed spectator nucleus. The double-bended shape of the caloric curve and its similarity to predictions of microscopic statistical models [2][3][4], has stimulated considerable experimental and theoretical activities. In particular, the second rise of the temperature to values exceeding 10 MeV at high excitation energies has initiated the discussion of whether nuclear temperatures of this magnitude can be measured reliably (see , and references given in these recent papers) and whether this observation may indeed be linked to a transition towards the vapor phase [7,8]. Obviously, a well-founded understanding of the significance of the employed temperature observables [9] is indispensable when searching for signals of the predicted liquid-gas phase transition in nuclear matter.Here, we present the results of temperature measurements for central collisions of 197 Au 1 197 Au at incident energies E͞A 50 to 200 MeV. These collisions are characterized by a collective radial flow of light particles and fragments which, over the covered energy range, evolves as a dynamical phenomenon closely connected to the initial stages of the reaction [10]. Global equilibrium is clearly not achieved. If local equilibrium is reached, the associated temperatures should reflect the adiabatic cooling of the rapidly expanding system. Two temperature observables were used simultaneously. Isotope temperatures were deduced from double ratios of isotopic yields [11] and excited-state temperatures were derived from the correlated yields of lightparticle coincidences [9,12,13]. It will become evident from the diverging results that this represents more than a methodical test and that the two types of thermometers are sensitive to different stages of the fragment formation and emission.Beams of 197 Au with E͞A 50, 100, 150, and 200 MeV, provided by the heavy-ion synchrotron SIS, were directed onto targets of 75 mg͞cm 2 areal density. Two multidetector hodoscopes, consisting of 96 and of 64 Si-CsI(Tl) telescopes in closely packed geometries, were placed on opposite sides with respect to the beam axis. Four high-resolution te...