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Abstract. We present the results obtained from a series of e+e--coincidence measurements in heavy-ion collisions using the double-Orange fl-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 -~ 20 keV) e +e-lines were observed in the sum-energy spectra, with kinetic energies ranging from ~555 keV to ~810keV, 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 ~ 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 (] v• < 0.02c) PACS: 14.60.Cd; 25.70.Cd; 25.70.Ef lnroductionOver a decade ago, two experimental groups at GSI in Darmstadt, the ORANGE and the EPOS collaborations, have discovered the existence of a narrow (FWHM-70 keV), and unexpected, e + line in the positron spectra obtained from heavy-ion (HI) collisions near the Coulomb barrier [1,2]. The line is superimposed on continuous distributions, from quasi-atomic positron emission and Dedicated to Prof. B. Povh on the occasion of his 60th birthday from nuclear positron background, which can be determined quantitatively [3,4]. At the beginning the line was attributed to the process of spontaneous positron creation in the supercritical electric fields (that is Z, = Zproj + Ztarg > 173) of a long-lived di-nuclear complex formed in a collision with time delay [5]. Further, improved measurements of positron spectra, however, could resolve a series of narrow e + lines [6,7] which occur also in the so-called subcritical systems (Z, < 173), and, thus, exclude spontaneous positron emission as the origin of these lines.Subsequent e +e--coincidence experiments have shown that the e + lines correlate with e-lines of approximately equal energy, consistent with the assumption of a two-body decay of an emitter whose velocity distribution is small enough to give rise to a narrow line in the measured sum-energy spectra [8, 93. This has provoked the idea that a hitherto unknown neutral particle (mass~ 1.8 MeV/c2), decaying into e+e -pairs, might be involved, and a variety of theoretical models was proposed regarding its nature [10]. The situation became more complex since in various collision systems, investigated with higher experimental ...

We present new results obtained from a series of follow-up e + e − -coincidence measurements in heavy-ion collisions, utilizing an improved experimental set-up at the double-Orange β-spectrometer of GSI. The collision system 238 U + 181 Ta was reinvestigated in three independent runs at beam energies in the range (6.0−6.4)×A MeV and different target thicknesses, with the objective to reproduce a narrow sum-energy e + e − -line at ∼635 keV observed previously in this collision system. At improved statistical accuracy, the line could not be found in these new data. For the "fission" scenario, an upper limit (1σ) on its production probability per collision of 1.3×10 −8 can be set which has to be compared to the previously reported value of [4.9±0.8(stat.) ± 1.0(syst.)]×10 −7 . Based on the new results, a reanalysis of the old data shows that the continuous part of the spectrum at the line position is significantly higher than previously assumed, thus reducing the production probability of the line by a factor of two and its statistical significance to ≤3.4σ.Previous e + e − -coincidence measurements in heavy-ion collisions at the Coulomb barrier, performed at the UNILAC accelerator of GSI by the EPOS and ORANGE collaborations, have shown narrow e + e − -sum-energy lines with energies in the range 550-810 keV [1][2][3][4]. Their features were found to be complex, and did not fit into any conventional atomic and/or nuclear production process. In particular, the speculation that a hitherto unknown neutral particle (mass∼1.8 MeV/c 2 ), decaying into e + e − pairs, might be involved [1], has conclusively been ruled out by subsequent Bhabha-scattering experiments [5]. Thus, the origin of this phenomenon has remained a puzzle, without a satisfactory explanation until now.On the other hand, all the data reported previously were incomplete as far as a systematic dependence on the collision parameters and the lepton emission scenario is concerned, and were suffering from limited statistical significance. Even more, a consistent description of the line characteristics (i.e., energies, cross sections) could not be achieved by comparing the results of both groups. For instance, the cross sections of the lines reported by us for 238 U + 238 U and 238 U + 208 Pb collisions [2,4] were found to be an order of magnitude smaller than those quoted by the EPOS collaboration [1,3] and, in particular, a line at ∼760 keV, initially reported by EPOS for 238 U + 232 Th collisions [1], has not been observed in our experiments. From our investigations, the most distinct evidence exists for a sum-energy line at ∼635 keV observed in the collision system 238 U + 181 Ta by using a beam energy of 6.3×A MeV and 1000 µg/cm 2 thick 181 Ta target [4]. The line was seen with the so far highest statistical significance (6.5σ) by selecting e + e − -pairs in coincidence with two heavy ions (HI), whose kinematics is consistent with fission of the U after the collision. However, the opening-angle distribution of the e + e − -pairs associated with this line, ...

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