Joachim Reinhardt

Quantum electrodynamics of strong fields

Up to now, our considerations in this book have mainly treated the behaviour of electrons (or positrons) under the influence of weak perturbations. In this chapter we want to deal with phenomena that occur in the presence of strong electromagnetic fields.

The decay of the vacuum in the field of superheavy nuclear systems

In the strong Coulomb field of a nucleus or quasimolecule withZ≳172 a change of the QED vacuum has been predicted, signalled by the spontaneous emission of positrons if holes in theK-shell are available. The dynamical semiclassical theory of positron excitation in heavy ion scattering is presented and extended to collisions with nuclear contact. Interference patterns in the energy spectrum of the emitted positrons and, for sufficiently long reaction timeT, the emerging of a characteristic line at the position of the 1sσ-resonance is predicted. The position and width of the 1sσ-resonance is calculated in dependence on the nuclear configuration, the effect of electron screening is taken into account. Recent experimental indications [1, 2] for structures in the spectra of positrons emitted in U-U collisions at energies close to the Coulomb barrier are discussed in terms of spontaneous positron creation. The observed spectra could be explained under the assumption of long lasting nuclear reactions (T≳4·10−20 s) with a cross section in the millibarn region. Various consequences of this interpretation are discussed.

Systematic Investigations of Binding Energies of Inner-Shell Electrons in Superheavy Quasimolecules

Electronic binding energies in superheavy quasimolecules are calculated using the monopole approximation, finite size and screening effects are included. The validity of the monopole approximation is discussed. A phenomenological description of the binding energy as a function of the total charge (Z1 + Z2) and the two-center separation R is given. It is shown, that the 1sσ-ionization rate does not depend on the projectile or target charge, but only on the total charge of the superheavy quasimolecule.

Positrons from supercritical fields of giant nuclear systems

During collisions of heavy nuclei with a combined chargeZ≳160 the electronic 1s-state is deeply bound due to the strong Coulomb field, forZ≧173 it even enters as a resonance the lower continuum of the Dirac-Hamiltonian. In pure Rutherford scattering no qualitative indication for the filling of a dynamically createdK-hole by the spontaneous positron creation process is predicted, but the study of heavy-ion collisions with nuclear time delay due to the attractive nuclear force promises clear signatures for the decay of the vacuum. Emphasis is laid also on the quantitative influence of the electron-electron interaction and ofE0-transitions in the giant nuclear system on positron emission, the latter treated in a classical approximation. We compare our results with recent experimental data of two different groups at GSI, Darmstadt.

Is there evidence for the production of a new particle in heavy-ion collisions?

The possibility of the production of a new neutral elementary particle in heavy-ion collisions is discussed.

Coulomb effects on electromagnetic pair production in ultrarelativistic heavy-ion collisions

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

Coupled channel analysis of inner-shell vacancy formation in superheavy quasimolecules

We calculate the ionization probability of the quasimolecular 1sσ-state in collisions of very heavy ions with (Z1+Z2) α≳1. Multistep excitation processes between bound and continuum states are investigated. Due to the multistep processes the number of created 1sσ-vacancies is typically increased by a factor 2–5 over that obtained within time-dependent perturbation theory (i.e. the one-step processes). Some implications of the many-electron problem and the Pauli principle are discussed. The δ-electron distribution is compared with recent experimental data.

Nuclear bremsstrahlung and electron-positron pair creation in heavy-ion collisions

Nuclear bremsstrahlung is investigated as a background effect in the X-ray-spectroscopy of superheavy electronic molecules. The intensity ratio between electric dipole and quadrupole radiation as function of frequency is given. The total cross section for electron-positron pair creation due to conversion of bremsstrahlungs-photons was found to beσ=3.8·10−8b for the54132Xe-92238U system andσ=5.8·10−8b for the92238U−92238U system at the Coulomb barrier. This demonstrates that bremsstrahlung pair creation can be neglected compared with the induced positron production in overcritical quasimolecules.

Low energy e+e- scattering

Electron-positron scattering provides a model-independent test for the existence of neutral particles coupling to the e+e- field. The authors review the effects of possible particle resonances X0 in the energy range between threshold and a few MeV on electron-positron scattering. The presentation focuses on theoretical aspects, but results of precision measurements of Bhabha scattering and e+e- annihilation in flight are also briefly surveyed.

Delta electrons: An atomic clock for deep inelastic collisions

We calculate the δ-electron distribution resulting from heavy ion collisions with projectile energies above the Coulomb barrier. It is shown that the life time (Δτ≳10−21s) of superheavy composite systems causes pronounced oscillations in the electron spectrum. The width of the oscillations is found to be ΔE=h/Δτ. This effect can be used to measure nuclear sticking times quite accurately.