A. Wörner

Universality of spectator fragmentation at relativistic bombarding energies

Abstract Multi-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(Tl) telescopes the solid-angle coverage of the setup was extended to θlab = 16°. This permitted the complete detection of fragments from the projectile-spectator source. The dominant feature of the systematic set of data is the Zbound 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 Zbound, where Zbound is the sum of the atomic numbers Zi of all projectile fragments with Zi ⩾ 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 of spectators in relativistic heavy ion reactions

Abstract Using the ALADIN forward-spectrometer at SIS we have studied multi-fragment decays of 197 Au projectiles after collisions with C, Al, Cu and Pb targets at a bombarding energy of E A = 600 MeV . The new data presented in this work comprise the measured cross sections of multifragment processes, the N Z ratios of the produced fragments, and the differential distributions of fragment multiplicities and of observables built on the charge correlations within the event. The 197 Au+Cu data are analyzed with the statistical multifragmentation model. It is shown that all observables can be simultaneously reproduced on an absolute scale, apart from an overall normalization constant which relates the number of model events to the measured cross section. A continuous distribution of excited residual nuclei, used as input for the calculations, was derived from the criterion of a best fit to the data. It exhibits a correlation between decreasing mass number A and increasing excitation energy E x A and a saturation of the excitation energy at E x A ≈ 8 MeV .

Thermal and chemical freeze-out in spectator fragmentation

Isotope temperatures from double ratios of hydrogen, helium, lithium, beryllium, and carbon isotopic yields, and excited-state temperatures from yield ratios of particle-unstable resonances in {sup 4}He, {sup 5}Li, and {sup 8}Be, were determined for spectator fragmentation, following collisions of {sup 197}Au with targets ranging from C to Au at incident energies of 600 and 1000 MeV per nucleon. A deviation of the isotopic from the excited-state temperatures is observed which coincides with the transition from residue formation to multi-fragment production, suggesting a chemical freeze-out prior to thermal freeze-out in bulk disintegrations.

Present status of the caloric curve of nuclei

Abstract Spectator decay was studied for the system Au + Au at an energy of 1000 A·MeV and the decay of the interaction region at energies between 50 and 200 A·MeV. In both cases temperatures were derived from several double-ratios of neighboring isotopes and from the population of excited states in 5 Li and 4 He. Agreement was found among the different isotope temperatures and also among the two excited state temperatures. The comparison of isotope and excited state temperatures, however, reveals large differences, which cannot be explained by feeding corrections. At incident energies between 600 and 1000 A·MeV the energy spectra of fragments and also neutrons of the decaying projectile spectator were measured. Whereas the slope parameters of the energy spectra and mean energies for fragments with Z ≥ 2 are independent of the incident energy, a strong dependence is found for the lightest particles, so that preequilibrium contributions to the spectator decay should be taken into account.

Multifragmentation and flow: peripheral vs. central collisions

Abstract Multifragment decays of heavy nuclei have been studied at the ALADIN spectrometer system at beam energies between 100 and 1000 MeV per nucleon. The observed fragment distributions signal a universality of spectator decays at bombarding energies E A ≥ 400 MeV . The role of the radial flow for the fragmentation process is explored by comparing fragment distributions measured for Au+Au collisions at E A = 100 MeV in central collisions and at E A = 1000 MeV in more peripheral reactions. At both energies the maximum multiplicity of intermediate mass fragments (IMFs) normalized to the size of the decaying system is about one IMF per 30 nucleons but the element distributions show significant differences. Within a coalescence picture the suppression of heavy fragments in central collisions at E A = 100 MeV may be related to a reduction of the density in momentum space which is caused by a large collective expansion velocity component.