Refine
Year of publication
Document Type
- Article (31422) (remove)
Language
- English (16027)
- German (13387)
- Portuguese (696)
- French (387)
- Croatian (251)
- Spanish (250)
- Italian (134)
- Turkish (113)
- Multiple languages (36)
- Latin (35)
Has Fulltext
- yes (31422)
Keywords
- Deutsch (503)
- taxonomy (451)
- Literatur (299)
- new species (194)
- Hofmannsthal, Hugo von (185)
- Rezeption (178)
- Übersetzung (163)
- Filmmusik (155)
- Johann Wolfgang von Goethe (131)
- Vormärz (117)
Institute
- Medizin (5408)
- Physik (1958)
- Biowissenschaften (1152)
- Biochemie und Chemie (1113)
- Extern (1108)
- Gesellschaftswissenschaften (803)
- Frankfurt Institute for Advanced Studies (FIAS) (753)
- Geowissenschaften (593)
- Präsidium (453)
- Philosophie (448)
Two-center level diagrams for the neutron orbitals in the scattering of 16O on 25Mg and of 17O on 24Mg are calculated by using a deformed potential for 24,25Mg. Possible consequences of the nuclear Landau-Zener mechanism, namely the promotion of nucleons at avoided level crossings, and of the rotational coupling between crossing molecular single-particle orbitals are studied for inelastic excitation and neutron transfer. The important excitation and transfer processes, which are enhanced by the promotion process and the rotational coupling, are presented. NUCLEAR REACTIONS Heavy ion scattering, theory of nucleon transfer, molecular wave functions, asymmetric two center shell model, single particle excitation, deformed nuclei.
Collisions of very heavy ions at energies close to the Coulomb barrier are discussed as a unique tool to study the behavior of the electron-positron field in the presence of strong external electromagnetic fields. To calculate the excitation processes induced by the collision dynamics, a semiclassical model is employed and adapted to describe the field-theoretical many-particle system. An expansion in the adiabatic molecular basis is chosen. Energies and matrix elements are calculated using the monopole approximation. In a supercritical (Z1+Z2≳173) quasiatomic system the 1s level joins the antiparticle continuum and becomes a resonance, rendering the neutral vacuum state unstable. Several methods of treating the corresponding time-dependent problem are discussed. A projection-operator technique is introduced for a fully dynamical treatment of the resonance. Positron excitation rates in s1/2 and p1/2 states are obtained by numerical solution of the coupled-channel equations and are compared with results from first- plus second-order perturbation theory. Calculations are performed for subcritical and supercritical collisions of Pb-Pb, Pb-U, U-U, and U-Cf. Strong relativistic deformations of the wave functions and the growing contributions from inner-shell bound states lead to a very steep Z dependence of positron production. The results are compared with available data from experiments done at GSI. Correlations between electrons and positrons are briefly discussed.
We define a new scalar-tensor theory with an effective gravitational coupling constant depending on a scalar field. The coupling is such that the gravitational interaction decreases with the strength of the scalar field. We show that this is not sufficient to prevent the gravitational collapse of sufficiently massive dense objects.
We calculate the spin polarization of 1sσ vacancies and emitted δ electrons induced by the strong magnetic field (|Bmax|∼1016G) in collisions of very heavy ions (Z1+Z2=178). The electron excitations are determined by the solution of coupled-channel equations within the quasimolecular basis states including the vector potential. The formulation is extended to the many-electron case. Spin polarizations of the order of 5-10% for impact energies below the Coulomb barrier are predicted.
The origin and importance of electron-translation effects within a molecular description of electronic excitations in heavy-ion collisions is investigated. First, a fully consistent quantum-mechanical description of the scattering process is developed; the electrons are described by relativistic molecular orbitals, while the nuclear motion is approximated nonrelativistically. Leaving the quantum-mechanical level by using the semiclassical approximation for the nuclear motion, a set of coupled differential equations for the occupation amplitudes of the molecular orbitals is derived. In these coupled-channel equations the spurious asymptotic dynamical couplings are corrected for by additional matrix elements stemming from the electron translation. Hence, a molecular description of electronic excitations in heavy-ion scattering has been achieved, which is free from the spurious asymptotic couplings of the conventional perturbated stationary-state approach. The importance of electron-translation effects for continuum electrons and positrons is investigated. To this end an algorithm for the description of continuum electrons is proposed, which for the first time should allow for the calculation of angular distributions for δ electrons. Finally, the practical consequences of electron-translation effects are studied by calculating the corrected coupling matrix elements for the Pb-Cm system and comparing the corresponding K-vacancy probabilities with conventional calculations. We critically discuss conventional methods for cutting off the coupling matrix elements in coupled-channel calculations.
Different collective deformation coordinates for neutrons and protons are introduced to allow for both stretching and γ transitions consistent with experiments. The rotational actinide nuclei 234-238U and 232Th are successfully analyzed in this model. NUCLEAR STRUCTURE 232Th, 234-238U calculated B (E2) values, collective model.
Dynamics of Coulomb fission
(1980)
A general formalism is described for the treatment of Coulomb fission, within the framework of the semiquantal theory. We develop a model for the fission probabilities of levels excited in Coulomb excitation. This model contains penetration of the double-humped fission barrier, competition from gamma and neutron emission, and the spreading of the collective states into noncollective compound states. For 74184W + 92238U, the fission probability at θc.m.=180° is increased by a factor of 3.9, 3.3, and 2.0 at E/ECoul=0.77, 0.85, and 0.935, respectively, compared to the simplified sharp cutoff model used in earlier model calculations. The enhancement comes from barrier penetration. The damping of the fission probability due to spreading into noncollective compound states is small. Prompt Coulomb fission (near the distance of closest approach) is studied in a one-dimensional model. The results clearly imply that prompt fission is negligible. We have also studied the sudden approximation for collective rotational levels in connection with Coulomb fission. At high spins (I≈20), it leads to significant errors. Contrary to the basic assumption of the sudden approximation that the nuclear symmetry axis remains fixed during the collision, it is shown that Coulomb excitation results in a strong alignment of the nuclear symmetry axis perpendicular to the beam axis at small internuclear distances. NUCLEAR REACTIONS, FISSION Semiquantal theory of prompt and asymptotic Coulomb fission, study of double-humped barrier penetration, damping effects, neutron and γ emission. Calculated σ(Ep, θc.m.=180°).
Phase transitions in nuclear matter A method for the description of spin-isospin phase transitions in nuclear matter is developed. It allows a complete description of the pion condensation phase transition in the framework of the Landau-Migdal Fermi liquid theory. The equation of the order parameter is derived and the condensation energy is calculated. We study the influence of pion condensation on the nuclear equation of state and the temperature dependence of pion condensation. NUCLEAR STRUCTURE Description of pion-condensed ground state by Green's function technique.
In heavy ion collisions, the molecular single-particle motion may cause specific structures in the energy dependence of the cross sections which arise by the promotion of nucleons at level crossings according to the Landau-Zener excitation mechanism. In order to examine this effect in asymmetric heavy ion collisions, we have calculated level diagrams of the two-center shell model for the target projectile combinations 13C + 16O and 12C + 17O and analyzed with respect to inelastic excitation and neutron transfer. We select certain reactions as possible candidates for showing enhanced cross sections for nucleon excitation and transfer due to real and avoided level crossings near the Fermi level.