Nicholas Kaloskamis

The production of positrons in heavy ion collisions has been of great interest during the past decade. The unexpected discovery of correlated sharp positron-electron peaks in the energy distributions has motivated further research. In two independent experiments, several peaks have been studied, but their origin still remains unclear. The limitations of these first studies prompted a new experiment (APEX) to be designed. It has been assembled at Argonne National Laboratory, where beams of very heavy ions at Coulomb barrier energies are available from the ATLAS accelerator.

This dissertation reports on the design and instrumentation of the APEX spectrometer, and illustrates its performance with the first data. A solenoid magnetic field is used for transporting leptons away from the target and focusing them on arrays of Si detectors. The 511 keV $\gamma$-rays emitted after the annihilation of the positron are detected in arrays of position-sensitive NaI(Tl) detectors, which were the focus of our construction effort at Yale, and are described in special detail. The scattered heavy ions are detected in an array of multi-wire proportional counters. A big advantage of APEX over prior studies is the full reconstruction of the lepton kinematics. Data from the $\rm\sp{238}U+{\sp{181}Ta}$ at 6.1 MeV/nucleon reaction are presented, from an experiment performed in August 1993. The data set is adequately large to demonstrate the ability for very clean positron identification. Emphasis is also given to the reconstruction of lepton angles which allow the extraction of the positron-electron angular distribution and the invariant mass spectrum. Definite conclusions about all peak production scenarios are however prohibited by the statistics, but neutral particle decay can be excluded for the $\rm\sp{238}U+{\sp{181}Ta}$ system.