Search for Spontaneous Positron Production in Heavy-Ion Collisions

In collisions between heavy ions with heavy target atoms distances of closest approach deep within the atomic K-shell radii are achieved. When using projectile velocities which are slow compared to the orbital velocities of the inner-shell electrons, these electrons will adjust their orbitals during the course of such collisions, evolving for small internuclear separations into a quasiatomic system with a charge number Zua of the united atom (for an U+U-collision at 1.4 GeV projectile energy, for example, the distance of closest approach in a head-on collision is 17 fm while the K-shell radius1 of an atom with Zua =92+92=184 amounts to ~120 fm). In this manner transient, collision constructed quasimolecules are formed, and their formation provides a vehicle through which the behaviour of electrons in extremely large Coulomb fields may be studied.2