Latest documents http://publikationen.stub.uni-frankfurt.de/index/index/ Tue, 07 Nov 2006 10:10:05 +0200 Tue, 07 Nov 2006 10:10:05 +0200 http://publikationen.stub.uni-frankfurt.de/frontdoor/index/index/docId/2669 Using relativistic Green’s-function techniques we examined single-electron excitations from the occupied Dirac sea in the presence of strong external fields. The energies of these excited states are determined taking into account the electron-electron interaction. We also evaluate relativistic transition strengths incorporating retardation, which represents a direct measure of correlation effects. The shifts in excitation energies are computed to be lower than 0.5%, while the correlated transition strengths never deviate by more than 10% from their bare values. A major conclusion is that we found no evidence for collectivity in the electron-positron field around heavy and superheavy nuclei. D. C. Ionescu; Walter Greiner; Berndt Müller; Gerhard Soff article http://publikationen.stub.uni-frankfurt.de/frontdoor/index/index/docId/2669 Tue, 11 Jul 2006 10:10:05 +0200 http://publikationen.stub.uni-frankfurt.de/frontdoor/index/index/docId/2430 The energy shift of the 1s1/2 state in 92238U due to virtual excitation of nuclear rotational modes is shown to be a considerable correction for atomic high-precision experiments. In contrast to this, nuclear polarization effects are of minor importance for Lamb-shift studies in 82208Pb. Günter Plunien; Berndt Müller; Walter Greiner; Gerhard Soff article http://publikationen.stub.uni-frankfurt.de/frontdoor/index/index/docId/2430 Fri, 07 Jul 2006 15:24:31 +0200 http://publikationen.stub.uni-frankfurt.de/frontdoor/index/index/docId/2427 The magnetic dipole scattering of neutrinos by the electrostatic potentials of single atoms as well as crystals is investigated. It is shown that scattering by a rigid cubic lattice can amplify the neutrino-atom cross section by a factor of N1/3, N being the number of scatterers. However, comparing the results with typical weak-interaction cross sections, the effect seems to be not observable in experiment. Jürgen Augustin; Berndt Müller; Walter Greiner article http://publikationen.stub.uni-frankfurt.de/frontdoor/index/index/docId/2427 Fri, 07 Jul 2006 12:24:55 +0200 http://publikationen.stub.uni-frankfurt.de/frontdoor/index/index/docId/2425 We consider the contribution of nuclear polarization to the Lamb shift of K- and L-shell electrons in heavy atoms and quasiatoms. Our formal approach is based on the concept of effective photon propagators with nuclear-polarization insertions treating effects of nuclear polarization on the same footing as usual QED radiative corrections. We explicitly derive the modification of the photon propagator for various collective nuclear excitations and calculate the corresponding effective self-energy shift perturbatively. The energy shift of the 1s1/2 state in 92238U due to virtual excitation of nuclear rotational states is shown to be a considerable correction for atomic high-precision experiments. In contrast to this, nuclear-polarization effects are of minor importance for Lamb-shift studies in 82208Pb. Günter Plunien; Berndt Müller; Walter Greiner; Gerhard Soff article http://publikationen.stub.uni-frankfurt.de/frontdoor/index/index/docId/2425 Fri, 07 Jul 2006 12:12:30 +0200 http://publikationen.stub.uni-frankfurt.de/frontdoor/index/index/docId/2471 Excitations of the atomic shell in heavy-ion collisions are influenced by the presence of a nuclear reaction. In the present Rapid Communication we point out the equivalence between a semiclassical description based on the nuclear autocorrelation function with an earlier model which employs a distribution of reaction times f(T). For the example of U+U collisions, results of coupled-channel calculations for positron creation and K-hole excitations are discussed for two schematic reaction models. Joachim Reinhardt; Berndt Müller; Walter Greiner; U. Müller article http://publikationen.stub.uni-frankfurt.de/frontdoor/index/index/docId/2471 Fri, 30 Jun 2006 12:59:44 +0200 http://publikationen.stub.uni-frankfurt.de/frontdoor/index/index/docId/2476 The energy shift of K electrons in heavy atoms due to the self-energy correction has been calculated. This process is treated to all orders in Zα, where Z denotes the nuclear charge. For the superheavy system Z=170, where the K-shell binding energy reaches the pair-production threshold (E1sb∼2mc2), a shift of +11.0 keV is found. This shift is almost cancelled by the vacuum polarization, leaving a negligible effect for all quantum-electrodynamical corrections of order α but all orders of Zα. Gerhard Soff; Paul Schlüter; Berndt Müller; Walter Greiner article http://publikationen.stub.uni-frankfurt.de/frontdoor/index/index/docId/2476 Fri, 30 Jun 2006 12:44:18 +0200 http://publikationen.stub.uni-frankfurt.de/frontdoor/index/index/docId/2481 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. Joachim Reinhardt; Berndt Müller; Walter Greiner article http://publikationen.stub.uni-frankfurt.de/frontdoor/index/index/docId/2481 Fri, 30 Jun 2006 11:30:58 +0200 http://publikationen.stub.uni-frankfurt.de/frontdoor/index/index/docId/2480 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. Gerd Schmidt; Walter Greiner; Ulrich Heinz; Berndt Müller article http://publikationen.stub.uni-frankfurt.de/frontdoor/index/index/docId/2480 Fri, 30 Jun 2006 11:21:52 +0200 http://publikationen.stub.uni-frankfurt.de/frontdoor/index/index/docId/2478 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. Ulrich Heinz; Walter Greiner; Berndt Müller article http://publikationen.stub.uni-frankfurt.de/frontdoor/index/index/docId/2478 Fri, 30 Jun 2006 11:07:06 +0200 http://publikationen.stub.uni-frankfurt.de/frontdoor/index/index/docId/2487 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. Jürgen Hofmann; Berndt Müller; Walter Greiner article http://publikationen.stub.uni-frankfurt.de/frontdoor/index/index/docId/2487 Fri, 30 Jun 2006 10:00:03 +0200 http://publikationen.stub.uni-frankfurt.de/frontdoor/index/index/docId/2488 We show that a uniformly accelerated observer experiences a "thermal" flux of Dirac particles in the ordinary Minkowski vacuum. Michael Soffel; Berndt Müller; Walter Greiner article http://publikationen.stub.uni-frankfurt.de/frontdoor/index/index/docId/2488 Fri, 30 Jun 2006 09:55:54 +0200 http://publikationen.stub.uni-frankfurt.de/frontdoor/index/index/docId/2496 This paper reports calculations of the influence of a reaction time T>10-21 s in deep-inelastic Xe-Pb collisions on the energy spectrum of δ electrons ejected in the same collision. It is shown that the lifetime of the superheavy composite system causes pronounced oscillations of width ε=h/T in the electron distribution, which survive the inclusion of multistep excitations and the folding with a lifetime distribution function. This effect may serve as an atomic clock for deep-inelastic collisions. Gerhard Soff; Joachim Reinhardt; Berndt Müller; Walter Greiner article http://publikationen.stub.uni-frankfurt.de/frontdoor/index/index/docId/2496 Wed, 28 Jun 2006 11:57:22 +0200 http://publikationen.stub.uni-frankfurt.de/frontdoor/index/index/docId/2502 The direct (Coulomb) formation of electron vacancies in the 1sσ state of superheavy quasimolecules is investigated for the first time. Its dependence on the impact parameter, projectile energy, and its contribution from excitations into the continum and higher bound states are determined. Wilfried Betz; Gerhard Soff; Berndt Müller; Walter Greiner article http://publikationen.stub.uni-frankfurt.de/frontdoor/index/index/docId/2502 Wed, 28 Jun 2006 11:27:19 +0200 http://publikationen.stub.uni-frankfurt.de/frontdoor/index/index/docId/2501 Binding energies and wave functions of inner-shell electronic states in superheavy quasimolecules with (Zp+Zt)α>1 are calculated. Ionization during a collision of very heavy ions is investigated within a molecular basis generated by the solutions of the two-center Dirac equation. Transitions to vacant bound states as well as direct excitation to the continuum are taken into account. We present theoretical values for the ionization probability as a function of impact parameter, bombarding energy, and combined nuclear charge. Our computed results are compared with recent experimental data. It is suggested that relativistic binding energies of electrons in superheavy quasimolecules can be determined experimentally via the impact-parameter dependence of ionization and the anisotropy of quasimolecular radiation. Gerhard Soff; Walter Greiner; Wilfried Betz; Berndt Müller article http://publikationen.stub.uni-frankfurt.de/frontdoor/index/index/docId/2501 Wed, 28 Jun 2006 11:12:35 +0200 http://publikationen.stub.uni-frankfurt.de/frontdoor/index/index/docId/2509 The theory of direct electron-positron pair production in the collision of heavy ions is formulated in the framework of the quasimolecular model. The pair production process acquires a collective nature for (Z1+Z2)α>1 and can be understood as the shakeoff of the strong vacuum polarization cloud formed in the quasimolecule. The total cross section is, e.g., 76 μb for Pb + Pb at Coulomb barrier energies. Gerhard Soff; Joachim Reinhardt; Berndt Müller; Walter Greiner article http://publikationen.stub.uni-frankfurt.de/frontdoor/index/index/docId/2509 Wed, 28 Jun 2006 10:20:10 +0200 http://publikationen.stub.uni-frankfurt.de/frontdoor/index/index/docId/2511 We show that information about quasimolecular electronic binding energies in transient atomic systems of Z=Z1+Z2 up to 184 can be obtained from three sources: (1) the impact-parameter dependence of the ionization probability; (2) the ionization probability in head-on collisions as a function of total nuclear charge Z; (3) the delta-electron spectrum in coincidence with K-vacancy formation in asymmetric collisions. Experiments are proposed and discussed. Gerhard Soff; Berndt Müller; Walter Greiner article http://publikationen.stub.uni-frankfurt.de/frontdoor/index/index/docId/2511 Wed, 28 Jun 2006 09:32:37 +0200 http://publikationen.stub.uni-frankfurt.de/frontdoor/index/index/docId/2523 The mechanisms of spontaneous and induced emission of radiation are derived from the Dirac equation in a rotating coordinate system. The molecular-orbital x-ray spectra exhibit a strong asymmetry with respect to the beam axis. The asymmetry peaks for the high-energy transitions, which can be used for spectroscopy of two-center orbitals. Berndt Müller; Walter Greiner article http://publikationen.stub.uni-frankfurt.de/frontdoor/index/index/docId/2523 Tue, 27 Jun 2006 14:48:56 +0200 http://publikationen.stub.uni-frankfurt.de/frontdoor/index/index/docId/2521 In critical or nearly critical heavy-ion collisions, induced as well as spontaneous energyless e-e+ pair creation result in the decay of the neutral vacuum. Induced transitions from the negative-energy continuum into a vacant molecular 1s level can occur even in the absence of diving and produce a substantial enhancement and broadening of the previously considered spontaneous positron spectrum. Total cross sections of 5 b have been calculated for U-U collisions. K. Smith; H. Peitz; Berndt Müller; Walter Greiner article http://publikationen.stub.uni-frankfurt.de/frontdoor/index/index/docId/2521 Tue, 27 Jun 2006 14:34:49 +0200 http://publikationen.stub.uni-frankfurt.de/frontdoor/index/index/docId/2527 The 1s bound state of superheavy atoms and molecules reaches a binding energy of -2mc2 at Z≈169. It is shown that the K shell is still localized in r space even beyond this critical proton number and that it has a width Γ (several keV large) which is a positron escape width for ionized K shells. The suggestion is made that this effect can be observed in the collision of very heavy ions (superheavy molecules) during the collision. Berndt Müller; Heinrich Peitz; Johann Rafelski; Walter Greiner article http://publikationen.stub.uni-frankfurt.de/frontdoor/index/index/docId/2527 Mon, 26 Jun 2006 13:25:54 +0200