Systematics of cross sections for target K-vacancy production in heavy ion collisions

Abstract

Cross sections for K-shell ionization by heavy ions have been determined from the measurements of target K x-ray yields. The measurements were performed with Ar, Kr, and Xe ions at energies from 2.5 to 25 MeV/amu and self-supported metallic foil targets of Al, Ti, Cu, Zr, Ag, Sm, and Ta. The x-ray yields were measured with a Si(Li) detector, while the projectile ions were counted in coincidence with the x-rays using a plastic scintillation detector. In addition, the amount of secondary K-shell ionization and the degree of simultaneous L-shell ionization in primary K-shell ionizing collisions were assessed by performing high-resolution x-ray measurements on targets of Al, Ti, V, Co, and Cu with a curved crystal spectrometer. The results of the high resolution measurements revealed that the apparent average Lshell spectator vacancy fraction at the time of Kα x-ray emission, L p , may be represented by a universal function of the Geometrical Model’s parameter X for Z2 = 17 - 32. Multiple-vacancy Kα fluorescence yields and corrections for K-shell ionization by secondary processes were determined with the aid of the high resolution spectra for the targets Al, Ti, and Cu. Fluorescence yields for the other targets were determined using an extrapolation procedure. The resulting K-vacancy production cross sections for 2.5 to 6 MeV/amu projectiles were compared with a limited amount of available experimental data and shown to be in relatively good agreement. The ECPSSR predictions for all the targets except Al agreed reasonably well with experimental cross sections for Ar projectiles. The experimental cross sections for Kvacancy production in Al, Ti, Cu, Zr, and Ag were greatly deviated from the ECPSSR predictions. The cross sections for Kr on Sm and Ta were in good agreement with theory. The scaling properties of the Kα x-ray production cross sections were examined and a semiempirical “universal” curve was deduced that reproduces the measured cross sections to within ±30% on average. The relationship between the Kα x-ray production cross sections and the geometrical model’s universal variable also was examined.