Latest documents http://publikationen.stub.uni-frankfurt.de/index/index/ Fri, 14 Jul 2006 10:16:51 +0200 Fri, 14 Jul 2006 10:16:51 +0200 http://publikationen.stub.uni-frankfurt.de/frontdoor/index/index/docId/2384 A first testing ground for QED in the combined presence of a strong Coulomb field and a strong magnetic field is provided by the precise measurement of the hyperfine structure splitting of hydrogenlike 209Bi. We present a complete calculation of the one-loop self-energy correction to the first-order hyperfine interaction for various nuclear charges. In the low-Z regime we almost perfectly agree with the Z alpha expansion, but for medium and high Z there is a substantial deviation. Hans Persson; Stefan M. Schneider; Walter Greiner; Gerhard Soff; Ingvar Lindgren article http://publikationen.stub.uni-frankfurt.de/frontdoor/index/index/docId/2384 Fri, 14 Jul 2006 10:16:51 +0200 http://publikationen.stub.uni-frankfurt.de/frontdoor/index/index/docId/2407 A calculation of the vacuum-polarization contribution to the hyperfine splitting for hydrogenlike atoms is presented. The extended nuclear charge distribution is taken into account. For the experimentally interesting case 209Bi82+ we predict a delta-lambda- -1.6 nm shift for the transition wavelength of the ground-state hyperfine splitting. Stefan M. Schneider; Walter Greiner; Gerhard Soff article http://publikationen.stub.uni-frankfurt.de/frontdoor/index/index/docId/2407 Wed, 12 Jul 2006 10:17:26 +0200 http://publikationen.stub.uni-frankfurt.de/frontdoor/index/index/docId/2414 We discuss the multiplicity distribution of electron-positron pairs created in the strong electromagnetic fields of ultrarelativistic heavy-ion transits. Based on nonperturbative expressions for the N-pair creation amplitudes, the Poisson distribution is derived by neglecting interference terms. The source of unitarity violation is identified in the vacuum-to-vacuum amplitude, and a perturbative expression for the mean number of pairs is given. Christoph Best; Walter Greiner; Gerhard Soff article http://publikationen.stub.uni-frankfurt.de/frontdoor/index/index/docId/2414 Tue, 11 Jul 2006 10:52:41 +0200 http://publikationen.stub.uni-frankfurt.de/frontdoor/index/index/docId/2423 Ionization, pair creation, and electron excitations in relativistic heavy-ion collisions are investigated in the framework of the coupled-channel formalism. Collisions between heavy projectiles and Pb82+ are considered for various bombarding energies in the region E=500 up to 2000 MeV/u. Useful symmetry relations for the matrix elements are derived and the influence of gauge transformations onto the coupled-channel equations is explored. Klaus Rumrich; Gerhard Soff; Walter Greiner article http://publikationen.stub.uni-frankfurt.de/frontdoor/index/index/docId/2423 Tue, 11 Jul 2006 10:17:27 +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/2421 A method is presented to define unique continuum states for the two-center Dirac Hamiltonian. In the spherical limit these states become the familiar angular-momentum eigenstates of the radial Coulomb potential. The different states for a fixed total energy ‖E‖>m may be distinguished by considering the asymptotic spin-angular distribution of states with unique scattering phases. The first numerical solutions of the two-center Dirac equation for continuum states are presented. Karl-Heinz Wietschorke; Paul Schlüter; Gerhard Soff; Klaus Rumrich; Walter Greiner article http://publikationen.stub.uni-frankfurt.de/frontdoor/index/index/docId/2421 Fri, 07 Jul 2006 17:11:33 +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/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/2436 We evaluate the Delbrück scattering amplitude to all orders of the interaction with the external field of a nucleus employing nonperturbative electron Green's functions. The results are given analytically in form of a multipole expansion. Alexander Scherdin; Andreas Schäfer; Walter Greiner; Gerhard Soff article http://publikationen.stub.uni-frankfurt.de/frontdoor/index/index/docId/2436 Fri, 07 Jul 2006 12:04:09 +0200 http://publikationen.stub.uni-frankfurt.de/frontdoor/index/index/docId/2448 Conversion processes in light nuclei with transition energies above the e+, e- pair creation threshold are investigated within an analytical framework. In particular, we evaluate the ratio of electron transition probabilities from the negative energy continuum into the atomic K shell and into the positive energy continuum, respectively. The possible role of monoenergetic positron conversion with respect to the striking peak structures observed in e+ spectra from very heavy collision systems is examined. Paul Schlüter; Gerhard Soff; Walter Greiner article http://publikationen.stub.uni-frankfurt.de/frontdoor/index/index/docId/2448 Tue, 04 Jul 2006 11:07:31 +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/2479 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. Gerhard Soff; Joachim Reinhardt; Walter Greiner article http://publikationen.stub.uni-frankfurt.de/frontdoor/index/index/docId/2479 Fri, 30 Jun 2006 11:16:27 +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/2506 K- and L-shell ionization of 238U with Xe and U projectiles is investigated. Internal conversion following nuclear Coulomb excitation which is particularly important for deformed heavy nuclei is compared with direct ionization of inner-shell electrons in superheavy quasimolecules. Both processes exhibit different impact-parameter dependences. As a result of internal conversion, about 0.1-0.3 K holes per central collision are created. Gerhard Soff; Volker Oberacker; Walter Greiner article http://publikationen.stub.uni-frankfurt.de/frontdoor/index/index/docId/2506 Wed, 28 Jun 2006 10:47:09 +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/2510 Internal conversion of γ rays from Coulomb-excited nuclear levels cannot be neglected compared with the spontaneous and induced positron production in overcritical electric fields. It is shown that both processes are separable by their different distributions with respect to the ion angle and the positron energy. Volker Oberacker; Gerhard Soff; Walter Greiner article http://publikationen.stub.uni-frankfurt.de/frontdoor/index/index/docId/2510 Wed, 28 Jun 2006 09:40:00 +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/2519 In view of new high-precision experiments in atomic physics it seems necessary to reexamine nonlinear theories of electrodynamics. The precise calculation of electronic and muonic atomic energies has been used to determine the possible size of the upper limit Emax to the electric field strength, which has been assumed to be a parameter. This is opposed to Born's idea of a purely electromagnetic origin of the electron's mass which determines Emax. We find Emax≥1.7×1020 V/cm. Gerhard Soff; Johann Rafelski; Walter Greiner article http://publikationen.stub.uni-frankfurt.de/frontdoor/index/index/docId/2519 Tue, 27 Jun 2006 14:18:50 +0200