Theory of the relativistic radiative electron capture incorporating effects of the internal conversion process.

Abstract

We have calculated cross sections of the radiative electron capture into a projectile K-shell orbit (K-REC) by using the relativistic impulse approximation, fully incorporating the internal conversion process, that is, ${e}^{+}$${e}^{\mathrm{\ensuremath{-}}}$ pair creation and successive ${e}^{+}$${e}^{\mathrm{\ensuremath{-}}}$ pair annihilation. The internal conversion process (ICP) is found to play a vital role with an increase of the projectile nuclear charge, while contributions to the ICP from an increase of the relative velocity are less pronounced for, especially, light-ion impacts. Furthermore, the K-REC photon is distributed according to a ${\mathrm{cos}}^{2}$(${\ensuremath{\theta}}_{L}$/2) rule with respect to a photon emission angle ${\ensuremath{\theta}}_{L}$ in the laboratory frame in the ultrarelativistic energy domain. This feature is quite different from the conventional ${\mathrm{sin}}^{2}$ ${\mathrm{\ensuremath{\theta}}}_{\mathrm{L}}$ dependence, which is valid at most up to an impact velocity below 0.9 of the velocity of light. Calculations are also carried out for the linear polarization correlation of emitted photons using the same approximation. A crossover feature, i.e., a sign inversion of the polarization correlation, comes out in the forward direction relative to 90\ifmmode^\circ\else\textdegree\fi{} with an increase in the impact velocity. This effect is also due to the relativistic effect of the REC.