Monoenergetic positrons and correlated electrons from superheavy nuclear collisions

The origin of the narrow peaks in the energy distribution of positrons produced in superheavy ion-atom collisions has been experimentally investigated using the EPOS spectrometer at GSI Darmstadt. The author systematically studied the dependence of monoenergetic positron emission on the combined charge of the colliding nuclei Zu, and measured the velocity of the system emitting the peaks by their laboratory Doppler broadening. Motivated by the results of these studies, additional experiments designed to search for monoenergetic electrons emitted in coincidence with the positron peaks were performed. In the first series of measurements, narrow positron peaks between 300 and 400 keV were detected in each of five supercritical (U + Cm, Th + Cm, U + U, Th + U, and Th + Th) and one subcritical (Th + Ta) collision systems studied. The structures were produced with similar cross section (∼10 μ b/sr) in quasi-elastic collisions which differed slightly from Rutherford scattering. They cannot be attributed to nuclear conversion processes. The positron peak widths (each ∼75 keV) indicate that the emitting source lives for ≥10-20s, and has a mean laboratory velocity of ∼0.05c, consistent with the center-of-mass velocity. Spontaneous positron emission from the decay of the QED vacuum is unable to explain the independence of the peak energies on Zu