Th. Rubehn

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 and Dynamical Instabilities in the Multifragment Emission Process

A new, sensitive method allows one to search for the enhancement of events with nearly equal-sized fragments as predicted by theoretical calculations based on volume or surface instabilities. Simulations have been performed to investigate the sensitivity of the procedure. Experimentally, charge correlations of intermediate mass fragments emitted from heavy ion reactions at intermediate energies have been studied. No evidence for a preferred breakup into equal-sized fragments has been found. {copyright} {ital 1996 The American Physical Society.}

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.

Scaling laws in 3He induced nuclear fission.

Nuclear Science Division, Lawrence Berkeley National Laboratory, University of California, Berkeley, California 94720(February 8, 2008)Fission excitation functions of compound nuclei in a massregion where shell effects are expected to be very strong areshown to scale exactly according to the transition state pre-diction once these shell effects are accounted for. The factthat no deviations from the transition state method havebeen observed within the experimentally investigated exci-tation energy regime allows one to assign an upper limit forthe transient time of 10

Comment on 'Circumstantial Evidence for Critical Behavior in Peripheral Au+Au Collisions at 35 MeV/nucleon.'

A Comment on the Letter by P.F. Mastinu {ital et al., } Phys.Rev.Lett.{bold 76}, 2646 (1996). {copyright} {ital 1997} {ital The American Physical Society}

Characterization of nuclear physics targets using Rutherford backscattering and particle induced X-ray emission

Abstract Rutherford backscattering and particle induced X-ray emission have been utilized to precisely characterize targets used in nuclear fission experiments. The method allows for a fast and non-destructive determination of target thickness, homogeneity and element composition.

Probing the Quantum-Mechanical Equivalent-Photon Spectrum for Electromagnetic Dissociation of Relativistic Uranium Projectiles

Electromagnetic fission cross sections for the reactions U + (Be, C, Al, Cu, In, Au, U) at E/A = 0.6 and 1.0 GeV are compared to theoretical calculations using recently proposed quantum-mechanical equivalent-photon spectra. In contrast to semiclassical calculations, systematically lower cross sections are obtained that cannot reproduce the experimental results. Since electromagnetic fission cross sections are virtually independent of the excitation of the double giant dipole resonance (DGDR), this conclusion is not influenced by the strength of the DGDR. {copyright} {ital 1997} {ital The American Physical Society}

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.

Precise shell effects and barriers from fission probabilities of neighboring isotopes

Fission excitation functions have been measured and analyzed for a chain of neighboring compound nuclei, from 207Po to 212Po. We present a new analysis which provides an accurate description of the fission barriers and ground state shell effects. Estimates of the fusion cross section are also obtained. The improved accuracy achieved in this analysis may lead to a future detailed exploration of the saddle mass surface.