K-shell vacancy production and sharing in (0.2-1.75) MeV/A Fe, Co, Cu + Cr collisions

In slow, near-symmetric collisions, a large enhancement of K-shell ionization cross sections over the predictions of the Coulomb excitation theory is explained in the electron promotion model. The dominant process of K-shell vacancy production is the ionization of the 2pσ molecular orbital (MO) at small inter-nuclear distances, followed by sharing of the primary vacancy among the K-shells of the partners. Beams of 56 Fe, 59 Co and 63 Cu obtained at Bucharest FN-tandem accelerator were directed onto a self-supported 52 Cr target of 1 mg/cm2 thickness. X-rays were detected by a Ge detector placed at 90 angle relative to the beam axis, while the elastically scattered projectiles were registered with a scintillation detector placed at 10.5 angle with respect to the beam. Efficiency and energy calibration have been done. Here we report K-shell ionization cross sections and mean vacancy sharing probabilities for the near-symmetric collision systems Fe, Co and Cu + Cr at 0.2-1.75 MeV/u energy. The cross section values have been corrected for multiple ionization in the outer (L, M) shells by using the energy and yield shift method. Mean ionization probabilities per electron have been estimated. The present results show that an exponential dependence of the vacancy sharing in function of reciprocal velocity is less fulfilled at higher (>1.5 MeV/u) energies. Concerning the multiple ionization data, there is the interesting result of lower multiple ionization probabilities for the Cr M-shell, which are comparable with those for the Cr L-shell. This could be qualitatively explained by a rapid partially filling of the M-shell vacancies within the solid target. Comparison of the experimental data with model calculations are presented. (authors)