No abstract

Narrow positron peaks are observed in five supercritical collision systems with combined nuclear charge 180^ Z u ^ 188. The peaks do not originate from nuclear internal pair conversion and their production appears to occur in a narrow projectile-energy interval near the Coulomb barrier. The line shapes are consistent with emission by a source moving with the cm. velocity. Particularly notable is an apparent independence of the peak energies on Z w . These observations are discussed in the context of the spontaneous decay of the QED vacuum and other new potential sources of line positron spectra.