Z. Phys. A346, 153-168 (1993) ZEITSCHRIFT FOR PHYSIK A 9 Springer-Verlag 1993 Investigations of correlated e + e- emission in heavy-ion collisions near the Coulomb barrier I. Koenig, E. Berdermann, F. Bosch, P. Kienle, W. Koenig, C. Kozhuharov, A. Schr6ter, H. Tsertos Gesellschaft fiir Schwerionenforschung (GSI), Planckstrasse 1, W-6100 Darmstadt, Germany Received: 6 August 1992/Revised version: 29 December 1992 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 col- lision systems U + U, U + Pb, and U+ Ta were investig- ated 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 con- tinuous 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 un- equally shared between positrons and electrons, thus be- ing 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 1. lnroduction Over 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 deter- mined 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 col- lision with time delay [5]. Further, improved measure- ments 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 spontan- eous positron emission as the origin of these lines. Subsequent e +e--coincidence experiments have shown that the e + lines correlate with e- lines of approx- imately equal energy, consistent with the assumption of a two-body decay of an emitter whose velocity distribu- tion 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 pro- posed regarding its nature [10]. The situation became more complex since in various collision systems, investig- ated with higher experimental sensitivity, several e+e -- sum-energy lines were found [-9, 11,123, exhibiting a rather anomalous pattern of angular and energy correla- tions [11, 12]. Their features do not fit into any conven- tional atomic and/or nuclear process, nor does the ini- tially suggested simple two-body scenario offer a viable explanation for the major part of these data any longer. Thus, the origin of this phenomenon remains a puzzle, in particular, since unknown elementary or composite neu- tral free particles in this mass range are conclusively ruled out meanwhile [13]. On the other hand, the appearance of this phenomenon only in heavy-ion collisions so far im- plies that its origin is closely connected with either the strong Coulomb fields, which occur in such collisions, or with the heavy nuclei themselves. In this paper we describe a set of experimental invest- igations of the momentum and angular correlation of e+e - pairs emitted in collisions of U ions at incident energies slightly below and above the Coulomb barrier with U, Pb, and Ta targets. The experiments were per- formed at the UNILAC accelerator of GSI using the