Positrons from 1.4-GeV Uranium-Atom Collisions

Abstract

A large fraction of the number of positrons observed in collisions of $^{238}\mathrm{U}$ + $^{238}\mathrm{U}$, $^{238}\mathrm{U}$ + $^{208}\mathrm{Pb}$, and $^{208}\mathrm{Pb}$ + $^{208}\mathrm{Pb}$ at relative velocities $\frac{v}{c}=0.11$ and for distance of closest approach ${R}_{min}<40$ fm can be assigned to pair creation of nonnuclear origin, induced by the time-varying Coulomb fields of the projectile and target charges. An order-of-magnitude increase in the positron yield is observed by increasing the united-atom charge from 164 to 184.