Latest documents http://publikationen.stub.uni-frankfurt.de/index/index/ Fri, 30 Jun 2006 12:59:44 +0200 Fri, 30 Jun 2006 12:59:44 +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/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/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/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