Electron-Positron-Paircreation in Heavy Ion Collisions

Abstract

Recent experiments to study e+-e- paircreation in heavy ion atom collisions at energies close to the Coulomb barrier are reviewed. For high combined charges of the collision system Zu = Z1 + Z2 one finds pairs produced by the strong time changing Coulomb field with cross sections rising proportional to Z16u. The characteristics of the e+ production line Zu and scattering angle dependence as well as their spectral distribution is well understood theoretically. Superimposed on the e+ continua, e+ lines were discovered with energies independent on Zu but with cross sections which rise with Z22u.The line energies are grouped around 250 and 340 keV for all systems with 164 < Zu < 184. For heavy systems (U-Th) and U-U) a third line at about 400 keV is seen.There is strong indication that these positron lines have a common source namely neutral particles produced in the high Coulomb fields, which decay into e+-e- pairs. This is indicated by e+-e- coincidence experiments which show evidence for energy and angle correlated e+-e- emission expected for a particle decay.