Simultaneous electron capture and bound-free pair production in relativistic collisions of heavy nuclei with atoms

Abstract

We consider a two-step process that may occur in relativistic ion-atom collisions. In this process, within a single ion-atom collision, an atomic electron is captured by the ion via the radiative and/or nonradiative capture channels, while the colliding nuclei of the atom and ion simultaneously produce an electron-positron pair with the electron created in a bound state of the ion. As a result, the ion picks up two electrons and a positron is emitted. We calculate the total cross section for this process in collisions of incident $1--4\text{ }\text{GeV}/\text{u}$ ${\text{U}}^{92+}$ projectiles with various atomic targets and discuss its dependence on the collision energy and target atomic number. We also analyze whether this process can compete with electron recombination proceeding via the excitation of the negative-continuum states, in which an atomic electron incident on a bare nucleus is captured into a bound state of the ion by transferring the energy excess to a negative-continuum electron with the latter undergoing a transition into a bound state. According to our estimates, the electron recombination dominates the two-step process in collisions with light and intermediately heavy atoms $({Z}_{a}\ensuremath{\lesssim}50)$, while in collisions with heavy atoms the latter leads to much larger cross sections.