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- Energie (4)
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- Cluster description of cold (neutronless) alpha ternary fission of 252Cf (1999)
- A coplanar three body cluster model (two deformed fragments and an alpha particle) similar to the model used for the description of cold binary fission was employed for the description of cold (neutronless) alpha accompanied fission of 252Cf. No preformation factors were considered. The three body potential was computed with the help of a double folding potential generated by the M3Y-NN effective interaction and realistic fragment ground state deformations. From the minimum action principle, the alpha particle trajectory equations, the corresponding ternary barriers, and an approximate WKB expression for the barrier penetrability are obtained. The relative cold ternary yields were calculated as the ratio of the penetrability of a given ternary fragmentation and the sum of the penetrabilities of all possible cold ternary fragmentations. Different scenarios were considered depending on the trajectories of the fragments. It was shown that two regions of cold fragmentation exist, a deformed one corresponding to large fragment deformations and a spherical one around 132Sn, similarly to the case of the cold binary fission of 252Cf. We have shown that for the scenario corresponding to the Lagrange point, where all forces acting on the alpha particle are in equilibrium, the cold alpha ternary yields of 252Cf are strongly correlated with the cold binary yields of the daughter nucleus 248Cm into the same heavy fragments. For all other scenarios only the spherical splittings are favored. We concluded that due to the present available experimental data on cold alpha ternary yields only the Lagrange scenario could describe the cold alpha ternary fission of 252Cf.

- Coulomb effects on electromagnetic pair production in ultrarelativistic heavy-ion collisions (1999)
- We calculate the asymptotic high-energy amplitude for electrons scattering at one ion, as well as at two colliding ions, by means of perturbation theory. We show that the interaction with one ion eikonalizes and that the interaction with two ions causally decouples. We are able to put previous results on perturbative grounds and propose further applications for the obtained rules for interactions on the light cone. We discuss the implications of the eikonal amplitude on the pair production probability in ultrarelativistic peripheral heavy-ion collisions. In this context the Weizsäcker-Williams method is shown to be exact in the ultrarelativistic limit, irrespective of the produced particles’ mass. A new equivalent single-photon distribution is derived, which correctly accounts for Coulomb distortions. The impact on single-photon induced processes is discussed.

- Shell structure of superheavy nuclei in self-consistent mean-field models (1999)
- We study the extrapolation of nuclear shell structure to the region of superheavy nuclei in self-consistent mean-field models—the Skyrme-Hartree-Fock approach and the relativistic mean-field model—using a large number of parametrizations which give similar results for stable nuclei but differ in detail. Results obtained with the folded-Yukawa potential which is widely used in macroscopic-macroscopic models are shown for comparison. We focus on differences in the isospin dependence of the spin-orbit interaction and the effective mass between the models and their influence on single-particle spectra. The predictive power of the mean-field models concerning single-particle spectra is discussed for the examples of 208Pb and the spin-orbit splittings of selected neutron and proton levels in 16O, 132Sn, and 208Pb. While all relativistic models give a reasonable description of spin-orbit splittings, all Skyrme interactions show a wrong trend with mass number. The spin-orbit splitting of heavy nuclei might be overestimated by 40%–80%, which exposes a fundamental deficiency of the current nonrelativistic models. In most cases the occurrence of spherical shell closures is found to be nucleon-number dependent. Spherical doubly magic superheavy nuclei are found at 184298114, 172292120, or 184310126 depending on the parametrization. The Z=114 proton shell closure, which is related to a large spin-orbit splitting of proton 2f states, is predicted only by forces which by far overestimate the proton spin-orbit splitting in 208Pb. The Z=120 and N=172 shell closures predicted by the relativistic models and some Skyrme interactions are found to be related to a central depression of the nuclear density distribution. This effect cannot appear in macroscopic-microscopic models or semiclassical approaches like the extended Thomas-Fermi-Strutinski integral approach which have a limited freedom for the density distribution only. In summary, our findings give a strong argument for 172292120 to be the next spherical doubly magic superheavy nucleus.

- Kaon effective mass and energy from a novel chiral SU(3) symmetric Lagrangian (1999)
- A new chiral SU(3) Lagrangian is proposed to describe the properties of kaons and antikaons in the nuclear medium, the ground state of dense matter and the kaon-nuclear interactions consistently. The saturation properties of nuclear matter are reproduced as well as the results of the Dirac-Brückner theory. After taking into account the coupling between the omega meson and the kaon, we obtain similar results for the e ective kaon and antikaon energies as calculated in the one-boson-exchange model while in our model the parameters of the kaon-nuclear interactions are constrained by the SU(3) chiral symmetry. PACS number(s): 14.40.Aq, 12.39.Fe, 21.30.Fe

- Freeze out in hydrodynamical models (1999)
- We study the effects of strict conservation laws and the problem of negative contributions to final momentum distribution during the freeze out through 3-dimensional hypersurfaces with space-like normal. We study some suggested solutions for this problem, and demonstrate it on one example. PACS: 24.10.Nz, 25.75.-q

- Rho meson broadening in hot and dense hadronic matter (1999)
- The modification of the width of rho mesons due to in-medium decays and collisions is evaluated. The decay width is calculated from the imaginary part of the one-loop selfenergy at finite temperature. The collision width is related to the cross sections of the rho + pion and the rho + nucleon reactions. A calculation based on an e ective Lagrangian shows the importance of including the direct pho pi - > pho pi scattering which is dominated by the a1 exchange. A large broadening of the spectral function is found, accompanied by a strength suppression at the pole. http://www.arxiv.org/abs/nucl-th/9812059

- Equilibrium and nonequilibrium effects in nucleus nucleus collisions (1999)
- Abstract: Local thermal and chemical equilibration is studied for central AqA collisions at 10.7 160 AGeV in the Ultrarelativis- . tic Quantum Molecular Dynamics model UrQMD . The UrQMD model exhibits strong deviations from local equilibrium at the high density hadron string phase formed during the early stage of the collision. Equilibration of the hadron resonance matter is established in the central cell of volume Vs125 fm3 at later stages, tG10 fmrc, of the resulting quasi-isentropic expansion. The thermodynamical functions in the cell and their time evolution are presented. Deviations of the UrQMD quasi-equilibrium state from the statistical mechanics equilibrium are found. They increase with energy per baryon and lead to a strong enhancement of the pion number density as compared to statistical mechanics estimates at SPS energies. PACS: 25.75.-q; 24.10.Lx; 24.10.Pa; 64.30.qt

- Critical review of quark gluon plasma signatures (1999)
- Noneequilibrium models (three-fluid hydrodynamics and UrQMD) use to discuss the uniqueness of often proposed experimental signatures for quark matter formation in relativistic heavy ion collisions. It is demonstrated that these two models - although they do treat the most interesting early phase of the collisions quite differently(thermalizing QGP vs. coherent color fields with virtual particles) - both yields a reasonable agreement with a large variety of the available heavy ion data.

- Dissociation of expanding c anti-c states in heavy ion collisions (1999)
- We study J/psi suppression in AB collisions assuming that the charmonium states evolve from small, color transparent configurations. Their interaction with nucleons and nonequilibrated, secondary hadrons is simulated using the microscopic model UrQMD. The Drell-Yan lepton pair yield and the J/psi Drell-Yan ratio are calculated as a function of the neutral transverse energy in Pb+Pb collisions at 160 GeV and found to be in reasonable agreement with existing data.

- Relativistic quantum transport theory of hadronic matter : the coupled nucleon, Delta, and pion system (1999)
- We derive the relativistic quantum transport equation for the pion distribution function based on an effective Lagrangian of the QHD-II model. The closed-time-path Green s function technique and the semiclassical, quasiparticle, and Born approximations are employed in the derivation. Both the mean field and collision term are derived from the same Lagrangian and presented analytically. The dynamical equation for the pions is consistent with that for the nucleons and Delta's which we developed before. Thus, we obtain a relativistic transport model which describes the hadronic matter with N,Delta, and pi degrees of freedom simultaneously. Within this approach, we investigate the medium effects on the pion dispersion relation as well as the pion absorption and pion production channels in cold nuclear matter. In contrast to the results of the nonrelativistic model, the pion dispersion relation becomes harder at low momenta and softer at high momenta as compared to the free one, which is mainly caused by the relativistic kinetics. The theoretically predicted free piN->Delta cross section is in agreement with the experimental data. Medium effects on the piN->Delta cross section and momentum-dependent Delta-decay width are shown to be substantial. PACS-numbers: 24.10.Jv, 13.75.Cs, 21.65.1f, 25.75.2q