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 c.m. velocity. Particularly notable is an apparent independence of the peak energies on $Z_u$. These observations are discussed in the context of the spontaneous decay of the QED vacuum and other new potential sources of line positron spectra.

• Received 18 February 1985

DOI:https://doi.org/10.1103/PhysRevLett.54.1761