THE POSITRON PEAK PUZZLE - RECENT RESULTS FROM APEX

Results are presented from a new experiment, APEX, designed to study t he previously reported sharp Lines in sum-energy spectra of positrons and electrons produced in collisions of very heavy ions. Data have bee n collected for U-238 + Ta-181 and U-238 + Th-232. No evidence is foun d for narrow structures similar to those previously reported. For the specific case of the isolated decay of a neutral particle of mass 1.4- 2.1 MeV/c(2) the upper limits on cross sections obtained are significa ntly less than previously reported. Data are also presented for intern al pair conversion in (206)pb. These results are used to set limits fo r the possible contribution to the pair yield of a 1780 keV transition in U-238 observed in heavy-ion gamma-ray coincidence measurements.