X-ray study of M -shell ionization of heavy atoms by 8.0-35.2-MeV Oq+ ions: The role of the multiple-ionization effects

The $M$-shell ionization in high-$Z$ atoms by ${\text{O}}^{q+}$ ions have been studied systematically in the energy range of 8.0\char21{}35.2 MeV in order to verify the available theoretical approaches describing the $M$-shell ionization by charged particles in asymmetric collisions. The measured $M$ x-ray spectra were analyzed taking into account the effects of x-ray line shifting and broadening caused by the multiple ionization in the $M$ and $N$ shells. The $M$-subshell ionization cross sections, derived by using the $M$-shell decay rates modified for the multiple ionization effects, have been compared with the theoretical predictions based on the plane-wave Born approximation (PWBA), the semiclassical approximation (SCA), and the binary-encounter approximation (BEA). In the PWBA approach two theoretical calculations were considered: the energy-loss Coulomb deflection perturbed stationary state relativistic (ECPSSR) theory and its recent modification called the energy-loss Coulomb deflection united and separated atoms relativistic (ECUSAR) theory, which corrects a description of the electron binding effect to account for the united and separated atoms (USA) electron binding energy limits. In the SCA calculations performed with relativistic hydrogenic wave functions the binding effect was included in the limiting cases of separated-atom (SA) and united-atom (UA) limits. The measured $M$-subshell ionization cross sections are the best reproduced by the SCA-UA calculations, with exception of the ${M}_{2,3}(3p)$-subshell cross sections which are strongly enhanced and cannot be reproduced by the discussed calculations.