Positron creation in crossed-beam collisions of high-energy, fully stripped heavy ions is investigated within the coupled-channel formalism. In comparison with fixed-target collisions of highly stripped heavy-ion projectiles positron production probabilities are enhanced by more than one order of magnitude. The increase results from the possibility to excite electrons from the negative energy continuum into all bound states. The positron spectrum is shifted towards higher energies because of the absence of electron screening. Rutherford scattering as well as nuclear collisions with time delay are investigated. We also discuss the filling of empty bound states by electrons from pair-production processes.

During collisions of heavy nuclei with a combined charge Z> 160 the electronic 1s-state is deeply bound due to the strong Coulomb field, for Z> 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 created K-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 of E0-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.

Atomic excitations are used to obtain information on the Course of a nuclear reaction. Employing a semiclassical picture we calculate the emission of S electrons and positrons in deep inelastic nuclear reactions for the example of U + U collisions incorporating nuclear trajectories resulting from two different nuclear friction models. The emission spectra exhibit characteristic deviations from those expected for elastic Coulomb scattering. The theoretical probabilities are compared with reCent experimental data by Backe et al. A simple model is used to estimate the influence of a threebody breakup of the compound system upon atomic excitations. I. THE DESCRIPTION OF ELECTRONIC EXCITATIONSIn collisions of very heavy ions superheavy quasiatomic Systems are created, where the electrons experience for a period of time T-10-=' s the combined nuclear charge Z = Z p + Z T of the projectile and the target nucleus. The resulting enormous binding energies and the highmomentum components of the wave functions necessitate a relativistic treatment of the dynamical behavior of the electrons. Under these conditions excitation Drocesses of electrons and positrons are appropriately described in a semiclassical picture based on the time-dependent twoCenter Dirac equation

The energy distribution of positrons emitted in quasimolecular collisions of Uranium on Uranium and Uranium on Curium has been measured by several groups. Peak structures in the positron spectra were observed. We discuss the possibility that these structures originate from internal conversion processes following nuclear Coulomb excitation or transfer reactions. Consequences for the nuclear photon spectra and the 3-ray distribution are pointed out and experimental procedures for an unambiguous determination of the significance of conversion processes are discussed.The spectra of emitted positrons produced in quasimolecular collisions of very heavy ions were subject of widespread experimental investigations [1][2][3][4][5][6][7][8][9][10][11][12] during the last years. Of particular interest are encounters with combined nuclear charges of Z 1 +Z2>173 and bombarding energies close to the nuclear Coulomb barrier (El, b-~ 5.9 MeV/u). In these colliding systems supercritical electric fields are created for a short period of ~10 -21 s, which may lead to spontaneous positron production provided a partial depletion of the K-shell has occured on the incoming path of the trajectory. A more profound discussion of this peculiar process is presented in [13] and in the references therein. The energy distribution of positrons measured [2][3][4][5][6][7][8] in 92U-92 U and 92U-96Cm collisions for particular kinematical conditions displays a distinct peak structure at a kinetic positron energy of E~in-~0.63-~320keV (natural units h=c=m=l are used) with a width of about 0.14_~70keV. Depending on the ion scattering angle and on the projectile energy further, but less pronounced structures appear at higher positron energies. As an example we relate our following inferences to the positron distribution detected [2] in U+U collisions at Elau~--5.73 MeV/u and for ion scattering angles between 38 ~ and 52 ~ . Two different explanations for the physical origin of these structures were discussed. According to the suggestion of J. Reinhardt et al. [13][14][15][16] a small fraction of about 5.10 .3 of the scattered ions within the given experimental angular window originate not from Rutherford collisions but from "sticking" reactions. In these reactions a giant nuclear system with Z= 184 is assumed to be formed with a lifetime that is slightly smaller than the characteristic time scale for spontaneous positron creation of T~10 -19 s. As the calculations in [14] have shown this causes a striking "spontaneous peak" in the positron spectra in fair agreement with the measured data [1][2][3][4][5][6][7][8]. The experimental verification of these ideas, indeed, would prove for the first time the spontaneous decay of the vacuum and simultaneously the existence of superheavy nuclear quasimolecules and thus would have far-reaching consequences. Alternatively, it was argued that internal pair creation following nuclear Coulomb excitation or transfer reactions might be responsible for peaks in the energy distribution of created positrons (cf. also [17,

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