INVESTIGATIONS OF CORRELATED E+E- EMISSION IN HEAVY-ION COLLISIONS NEAR THE COULOMB BARRIER

We present the results obtained from a series of e+e--coincidence measurements in heavy-ion collisions using the double-Orange beta-spectrometer at GSI. The collision systems U + U, U + Pb, and U + Ta were investigated at bombarding energies close to and slightly above (U+Ta) the Coulomb barrier. For all systems studied, very narrow (FWHM congruent-to 20 keV) e+e- lines were observed in the sum-energy spectra, with kinetic energies ranging from -555 kev to -810 kev, superimposed on a continuous distribution mainly due to uncorrelated e+e- emission. Particularly in the U + Ta system, a pronounced sum-energy line appears at approximately 634 kev, predominantly in deep-inelastic collisions. In some cases (e.g. U + Pb) the line characteristics is consistent with a two-body decay mode of an emitter which moves with the c.m. velocity of the colliding ions. However, other lines, and in particular the 634 kev line (U + Ta), exhibit a rather isotropical opening-angle distribution whereas their energy is unequally shared between positrons and electrons, thus being in clear disagreement with this scenario. In general, the data preclude an emission from the separated (moving) nuclei, and, in the latter cases, provide evidence that the e+e--pair decay occurs in the vicinity of the Coulomb field of a third heavy (positively charged) partner having only a small transverse velocity (\nu(perpendicular-to)\ < 0.02c)