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 extemal 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 (2, + Z, 2 173) 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 projectionOperator technique is introduced for a fully dynamical treatment of the resonance. Positron excitation rates in s„, andp,,] states are obtained by numencal 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.

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Theory of positron production in heavy-ion collisions

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.

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

Pair creation by internal conversion

Electron excitations in superheavy quasimolecules

We recall how nearly half a century ago the proposal was made to explore the structure of the quantum vacuum using slow heavy-ion collisions. Pursuing this topic we review the foundational concept of spontaneous vacuum decay accompanied by observable positron emission in heavy-ion collisions and describe the related theoretical developments in strong fields QED.

Probing QED Vacuum with Heavy Ions

Positron emission in PbPb and PbU collisions

We show that emission of high-energy electrons and photons in nuclear collisions at intermediate energies is sensitive to the space-time evolution of the reaction. The electron and photon spectra measure related but complementary quantities connected with the nuclear charge distribution. We show that at 60 MeV/u beam energy production of 50 MeV electrons is predicted to measurable probability.

G. Soff

Papers

Positron emission in PbPb and PbU collisions

Electromagnetic probes of nuclear collisions at intermediate energy