You must make clear of what are the two major types of eletrons that give the signal for image. Backscattered electrons and Secondary electrons. Backscattered electrons are essentially primary beam electrons that go through elastic collisions with sample's electrons and come out with high energy. While secondary electrons are from sample's electrons due to inelastic scattering of the primary electrons, their enery is low and defined by less than 50eV.
Then you need to know the penetration "tear drop", at what depth and fraction of the volume different kind of signal comes out. SE, BSE and X-Ray. SE1 and SE2 normally from 5~10nm, BSE could be deep to 50nm. While X-Ray could be from almost the whole penetration tear drop except a thin rim (microns of depth). So I should say only a very small fraction of electrons knocked out their orbits generating X-ray becomes SE1 and SE2.