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In 1970s, Basbas et al.(Phys. Rev. A 1973, 7, 983; Phys. Rev. A 1978, 17, 1655) rose the possibility that PWBA ionisation crosssections could be expressed in terms of a universal ionisation cross-section. Starting from ECPSSR ionisation cross-sections and reproducing Basbas approach, a universal curve can be displayed against the reduced ion velocity, but data are too widespread to allow for a proper polynomial fit. In the present work, Basbas approach was revised and plotted against a rescaled, reduced ion velocity, eliminating most of the data spread and generating in practice a universal ionisation cross-section curve. In this revised approach, the screening parameter, θ n,o , assumes a major role in reaching the universal behaviour. Polynomial approximations are then made to the universal curves for incident H + and He 2+ ions. Cross-sections were then calculated using the polynomial approximations for target elements with atomic number from 11 to 92 and for ion beam energies from 50 keV to 10 MeV, for K-and L-shell ionisation cross-sections and are compared to experimental data from literature. Finally, X-ray yields were simulated for various thick target samples using these polynomial approximations and the outcomes are compared to data obtained at the ITN PIXE set-up.