Cross sections involved in the calculation of the Fe Kα line EW. Solid lines: cross sections (in units of barn per ambient H-atom) for producing the 6.4 keV line by the impact of fast electrons (left panel) and protons (right panel), assuming solar metallicity. Dashed lines: differential cross section (in barn per H-atom per keV) for producing 6.4 keV X-rays by bremsstrahlung of fast electrons (left panel) and inverse bremsstrahlung from fast protons (right panel), in a medium composed of H and He with H/He = 0.1. The ratio of these two cross sections gives the EW of the 6.4 keV line (in keV) for a mono-energetic beam of accelerated particles.