Physics beyond PIXE

The paper reveals geometrical effects that govern the accuracy of PIXE measurements: distinction between thin and thick targets, misorientation of the sample, surface roughness and contribution of the invisible matrix. It shows an importance of the mean X-ray production distance and its application in differential measurements. It further reviews the data base needed for thick target calculations and the importance of developing more accurate theoretical models based on the semiclassical picture and coupled channel approach. An example of transitions in a perturbed harmonic oscillator demonstrates that the united atom limit does not exist at adiabatic collisions and that the cross section likely exhibits an oscillatory behaviour.