G. Rudolf

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.

Mass drift in reactions between a heavy and a light nucleus

Abstract The dynamics of heavy-ion collisions is studied experimentally for metastable composite systems with interaction times between the collision time of quasi- and deep-inelastic processes and the capture time of fast-fission and compound-nucleus-fission reactions. Detailed angular, total kinetic energy and mass distributions are measured for reaction products with masses between the target and projectile masses including the symmetric masses. The mass drift is observed as a function of total kinetic energy and scattering angle.

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.

Angular dependence of the products in the 86Kr + 166Er collision

Abstract The reaction 86 Kr + 166 Er is studied at a bombarding energy of 8.2 MeV/u. The results obtained in a one-particle inclusive measurement using a large-area position-sensitive ionisation chamber are studied in detail with respect to the experimental observables: nuclear charge Z , c.m. scattering angle θ and total kinetic energy loss TKEL. It is investigated which quantity is the most relevant in order to follow the evolution of dissipative phenomena. First and second moments of the angular distributions ( θ 0 and Γ θ ) are extracted as a function of energy loss using an unfolding procedure for the positive and negative scattering angles. A linear correlation between θ 0 and σ 2 z is evidenced. Using simplifying assumptions an upper limit of 310 mb is estimated for a long-living symmetric fragmentation component.

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.

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.