Positron spectroscopy in elastic and dissipative heavy ion collisions

A new magnetic transport system, the Tori spectrometer, which is able to measure positrons and electrons simultaneously, has been used to study the dynamics of collisions between very heavy ions via their influence on the positron and δ-electron spectra. While these spectra obtained in coincidence to elastic scattering agree well with theory based on pure Rutherford trajectories, those observed in coincidence to dissipative collisions can only be described by calculations taking into account a time delay in the collision process. A quantitative analysis gives an estimation for the nuclear contact time in the U + U collision.