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

We calculate the ionization probability of the quasimolecular 1sσ-state in collisions of very heavy ions with (Z
1+Z
2) 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.

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

An analytical formula for the excitation probability of the quasimolecular 1sσ orbital as a function of the impact parameterb is derived for collision systems withα(Z1+Z2)≳1. This formula describes well all existing experimental data for those collision systems except for the heaviest system Pb+Cm (Z1+Z2=178) at impact parametersb≲40 fm. It is discussed in which way energies of the quasimolecular 1sσ orbital can be extracted from experimental 1sσ vacancy production data.

The quasimolecular 1sσ orbital in collision systemsα(Z1+Z2)≳1, its excitation and spectroscopy

The impact parameter dependence of the 1sσ excitation probability for 47 MeV/u Xe + Au collisions has been measured for impact parameters between 40 and 200 fm A determination of the total 1sσ excitation cross section from the data gives (10,8 ± 2)b in good agreement with results of an independent measurement

Measurement of shape and absolute value of 1sσ excitation probability in 4.7 MeV/u Xe+Au collisions

Using particle x-ray coincidence techniques, the probability to produceK-shell vacancies in 4.7 MeV/u Pb+Z2 (42≦Z2≦92) collisions for impact parametersb<100 fm was measured. For these impact parameters, the 1sσ excitation probability is greater than 1 σ 10−2 and the 2p1/2σ excitation probability is greater than 1 σ 10−1. Binding energies of the 1sσ state derived from the measured probabilities agree well with theoretical calculations.

Measurements of the probability for 1 sσ excitation in very heavy ion-atom-collisions and determination of the binding energy of the 1 σ molecular orbital

Using particle x-ray coincidence techniques, the probability forK shell ionization has been measured absolutely at two impact parameters for collisions of 5.8 MeV/amu208Pb on Ag and Au. In the asymmetric collision system Pb-Ag, the 1sσ excitation probability of the PbK shell is 2.2% at 25 fm impact parameter, and an exponential probability distribution falls off too quickly to account for the measured total cross section. For the symmetric system Pb-Au, the latter conclusion is also made for the 2pσ excitation probability although in this case, the probability is much larger being 29% at 42 fm.

An absolute measurement of the probability for 1 s σ and 2 p σ excitation of the lead K shell

The charge state dependence ofK andL x-ray production has been studied for 1.4 MeV/amu Cu+q (16≦q≦23) collisions with Kr and Xe gas targets. For q≧20 an almost linear increase in theK x-ray emission of Cu and Kr, and in theL x-ray emission of Xe is observed. The experimental results are compared to existing theoretical models based on the quasimolecular picture.

A. Warczak

Papers

An absolute measurement of the probability for 1 s σ and 2 p σ excitation of the lead K shell

Charge state dependence of characteristic x-ray emission under single collision conditions for 1.4 MeV/amu Cu ions