Radiative Electron Capture by Channeled Oxygen Ions

Abstract

Recently Schnopper et al. [1] reported a new feature in the x-ray spectra observed when solid targets were bombarded with energetic, highly-stripped heavy ions. They observed a broad x-ray band well above the highest energy characteristic x-ray line from the projectile. This new feature was identified as radiative electron capture (REG). The phenomenon of REC becomes most probable when the incident heavy ions are completely stripped, and no outer shell electrons are available to fill K-shell vacancies. Then a free or weakly bound target electron can be captured directly into the K-shell of the moving ion, emitting a photon. The energy of a REC photon resulting from the capture of a free electron by an ion of charge Z, mass M, and energy E, is given by the expression:

$${{\text{E}}_{\text{p}}} = {{\text{Z}}^{2\,}}\,{\text{Ry}}\,\,{\text{ + }}\,\frac{{{{\text{M}}_{\text{e}}}}}{{\text{M}}}\,\,{\text{E}}$$

(1)

where Ry is the Rydberg energy and M_e the electron mass. The first term is the binding energy of an electron in the hydrogen-like ion and the second the energy of an electron moving at a velocity equivalent to that of the ion. As noted by Schnopper [1],the actual width of the observed REC line is affected by a number of things. In measurements utilizing heavy ion beams incident on amorphous solids, the heavy ions will have a distribution of charge states inside the solids and each charge state can result in a REC photon of a different energy as can be seen from eq. (1).

Research sponsored by the U. S. Atomic Energy Commission under contract with Union Carbide Corporation.