P. Kreutz

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 .

Charge correlations as a probe of nuclear disassembly

Abstract We have studied multi-fragment decays of Au projectiles after collisions with C, Al, Cu and Pb targets at a bombarding energy of 600 MeV/nucleon. We examine the correlations between the charges emitted in these reactions. These correlations are given as a function of the total charge in bound fragments, Z bound , at forward angles, which is a measure of the violence of the collision and can be related to the impact parameter. The charge distributions have been fitted by a power law and the extracted τ parameter exhibits a minimum as a function of Z bound . We observe a strong reduction in the maximum charge, Z max , of the event with decreasing Z bound . For those events where Z max is less than half Z bound , the relative sizes of the two largest charges within the event cover the full spectrum of possibilities. The charge-Dalitz plots indicate that the multi-fragmentation events are not an extension of symmetric fission reactions. The event-by-event charge moments are examined to measure the size of the charge fluctuations. All of the charge correlations are independent of the target when plotted as a function of Z bound . The results are compared to both nuclear statistical and percolation calculations. The model predictions differ from each other, establishing that the observables are sensitive to how the available phase space is populated. The sequential nuclear model predicts too asymmetric a decay, while the simultaneous model predicts too symmetric a break-up. The percolation model, which was adjusted to reproduce the size of Z max , correctly predicts the mean behaviour and the fluctuations of the lighter fragments.

Breakup conditions of projectile spectators from dynamical observables

Momenta and masses of heavy projectile fragments (Z greater than or equal to 8), produced in collisions of (197)AU With C, Al, Cu, and Pb targets at EIA = 600 MeV, were determined with the ALADIN magnetic spectrometer at SIS. Using this information, an analysis of kinematic correlations between the two and three heaviest projectile fragments in their rest frame was performed. The sensitivity of these correlations to the conditions at breakup was verified within the schematic sos model. For a quantitative investigation, the data were compared to calculations with statistical multifragmentation models and to classical three-body calculations. With classical trajectory calculations, where the charges and masses of the fragments are taken from a Monte Carlo sampling of the experimental events, the dynamical observables can be reproduced. The deduced breakup parameters, however, differ considerably from those assumed in the statistical multifragmentation models which describe the charge correlations. If, on the other hand, the analysis of kinematic and charge correlations is performed for events with two and three heavy fragments produced by statistical multifragmentation codes, good agreement with the data is found with the exception that the fluctuation widths of the intrinsic fragment energies are significantly underestimated. A new Version of the multifragmentation code MCFRAG was therefore used to investigate the potential role of angular momentum at the breakup stage. If a mean angular momentum of 0.75(h)over bar>/nucleon is added to the system, the energy fluctuations can be reproduced, but at the same time the charge partitions are modified and deviate from the data.

Multi-fragment events as a probe of nuclear disassembly

Abstract We review the recent results on the multi-fragment decay of heated nuclear systems that are formed in asymmetric heavy-ion collisions. Particular emphasis is placed on those observables that are sensitive to the fluctuations in the decaying system and their possible role in extracting the physics of phase transitions in nuclear systems.

QMD simulation of multifragment production in heavy ion collisions at E/A=600 MeV

Abstract With the ALADiN forward spectrometer the fragmentation of gold nuclei at 600 MeV per nucleon after interaction with carbon, aluminum, copper and lead targets has been investigated. The results are compared to Quantum Molecular Dynamics calculations using soft and hard equations of state.

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.

Disassembly of highly excited nuclei: From evaporation to vaporization

Abstract We have studied the fragmentation of Au projectiles interacting with targets of C, Al, Cu and Pb at an incident energy of E/A = 600 MeV. The experiment was performed with the ALADiN forward spectrometer at SIS. The employed inverse kinemantics allowed nearly complete detection of projectile fragments with charge Z ≥ 2. In addition, the number of light particles for each collision was measured. We compare different observables characterizing the event and investigate the impact parameter dependence of the fragmentation process. It is found that the correlation between the mean multiplicity of intermediate mass fragments 〈Mimf〉 and Zbound, the sum charges of all projectile fragments with Z≥2, is independent of the target nucleus. This universal behavior is consistent with an - at least partial - equilibration of the projectile fragment prior to its decay. Finally we compare to two classes of statistical models, the sequential decay code GEMINI and statistical multifragmentation models.

The rise and fall of multifragment production in 197Au + C, Al, and Cu reactions at E/A = 600 MeV

We have studied the transition from the evaporation to the vaporization regime in 197Au induced reactions on C, Al, and Cu targets at a beam energy of 600 MeV per nucleon. The experiment was performed with the ALADIN forward spectrometer at SIS. We find that with increasing violence of the collision the mean multiplicity of intermediate mass fragments first increases to a maximum ⋍ 3.5 and then decreases again. Calculations using the BUU model suggest that the fragmentation is governed by the energy Edep deposited into the Au nucleus and that the maximum of is reached around Edep = 8 MeV/nucleon.