Positron production in heavy-ion collisions

Abstract

The ATLAS Positron Experiment APEX was built to study positron emission in collisions between very heavy ions. Narrow peaks were observed in such collisions at GSI, Darmstadt in the spectra of positrons and in the sum-energy spectra of electron-positron coincidences. APEX is a second-generation experiment which was specifically designed to look for the coincidence events and measure the opening angle between electrons and positrons. The first beam-induced positrons were detected using APEX in March 1993, and since then three additional runs were carried out. The first results for the collision system {sup 238}U + {sup 181}Ta show no evidence for sharp peaks in the electron-positron sum-energy spectrum. The current emphasis in this work is to obtain a complete understanding of the APEX apparatus. The atomic group is studying events involving coincidences between heavy ions and electrons. Since APEX measures the laboratory angles and energies of both electrons and heavy ions, it is possible to make an event-by-event Doppler correction of the electron spectra. These Doppler-corrected spectra show a number of lines which are attributed to conversion electrons which are emitted when a nuclear excited state decays by ejecting an inner-shell electron. The study of these spectra provide an important confirmationmore » of the proper functioning of APEX. We are particularly concerned with the atomic physics aspects of this process. In order to understand the electron spectra, it is necessary to account for the change in binding energy of the inner-shell electrons as a function of ionic charge. We are utilizing the GRASP relativistic atomic structure program to calculate the binding energies. This information, together with the measured gamma-ray energies, allows us to calculate the expected energies of the conversion electrons which we can then compare with the observed Doppler-corrected conversion electron energies.« less