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We present the results obtained from systematic studies of positron creation for a series of heavy-collision systems, with united charge Z,=Z I+Z 2 ranging from Z,= 164 (Pb+Pb) to Z,,= 184 (U+U) at bombarding energies close to the Coulomb barrier, using the Orangefl-spectrometer at GSI. For each collision system studied, the dominating continuous distributions due to quasiatomic and nuclear positron emission are determined accurately. This is essential in obtaining the characteristics of the still unexplained monoenergetic positron lines which appear in the energy range between 200 keV and 400 keV. Our results are compared with coupled-channels calculations for quasi-atomic positron creation. The latter describe quite well the global features of the measured spectra, but overestimate systematically their absolute values. From the comparison, a common normalization factor of about 0.75 can be established for the calculated spectra. In particular, the dependence on Z,, of the measured emission probabilities was found to follow a power law (ocZ,~95-+~), in fair agreement with the theoretical prediction.

Following a suggestion of Bang et al. we deduce that the total energy transfer associated with the narrow positron fines observed in heavy-ion collisions is given approximately by the sum of the rest masses of the positron and tfie accompanied electron plus their kinetic energies. Since the same behaviour has been established previously for the production process of the dynamically induced positrons, our observation represents a strong hint that the origin of the e + -lines may also be connected with the strong Coulomb field occuring in heavy-ion collisions.Recent high-resolution measurements on positron production carried out by the ORANGE collaboration at GSI revealed in the overcritical system U+U as well as in three subcritical systems [1] multiple e § -structures which, together with the correlated e+e -events at several sharp sum energies found by the EPOS collaboration [2], may suggest a common line source with internal modes of excitation. It was, in particular, found that the energies and widths of the lines are independent of the combined nuclear charge Z=, while their production cross sections increase strongly with Z~ (o~ Z~ 2:L~ ) [1]. Furthermore, it seems to be unlikely that ordinary nuclear processes are involved in the line production mechanism [1,2]. These new experimental facts are also not reconcilable with the originally suggested mechanism of spontaneous positron creation [3], and indicate the formation of a neutral system decaying into an e+e -pair [2]. The latter, however, has to be examined rigorously by further experiments. In the meantime, various theoretical models referring to this phenomenon have been developed: e.g. formation of a new neutral particle [4], magnetically bound e+e -resonances [5], formation of a poly-positronium complex [6] etc., but none of them seems to describe the entire experimental findings satisfactorily. So the origin as well as the production mechanism of the e + -lines still remain an exciting enigma.Bang et al. " [7] pointed out recently that a simple scaling relation for the dependence of the e + -line production probability P(o) on the heavy-ion scattering angle should be applyable, if the formation of the positron lines is related to the time-varying strong Coulomb field occuring in heavy-ion collisions. This was discussed previously for the continuous positron spectra [8]:The distance of closest approach Ro is related to the c.m. scattering angle 0 by Ro(O) = a[1 + 1~sin(O~2)], with the distance of closest approach in a head-on collision 2a = Z1Z~e~/Er .... and E~.,n. denoting the centre-ofmass incident energy. The minimum momentum transfer qo is given by qo = AE/(hv~), with AE being the total energy transfer, and voo the ion velocity at infinity. It should be stressed that this picture is valid for any single-step or multi-step excitation process induced by electromagnetic interaction and does nbt depend on the details of the states involved [7].For symmetric or nearly symmetric collision systems the ejectiles and recoils cannot be separated,...

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