When an electron hits the target so hard on its inner shell, the target material will radiate an EM wave of energy, equal to the energy difference between the innermost and the next shell called K & L1 respectively.
Luckily for us, the air molecules in the atmosphere make it impossible for any electron to travel the whole path from emitter to collector without any collision, in fact there would be millions of such collisions and the electrons can never obtain very high speeds, enough to generate any radiation upon impact. My latest tests for radiation within lifters confirmed no radiation when lifters are powered in air.
The following calculations apply only in pure vacuum.
For aluminium (our foil target), K = 1.56 keV , and L1 = 0.118 keV
The energy difference between the two shells is 1.442 keV.
So, when a charge of sufficient energy hits the aluminium foil, an energy packet of 1.442 keV will be released.
Using E = hf
1.442E3 x e/h = f
This gives f = 3.486E17 Hz, which happens to be the so called 'soft X-ray' band.
Alternatively, knowing the atomic number for Al (Z=13), approximate values can be found. Energy released by an electron shift from L to K = (Z-1)2 * 10.2 eV = 1.46 keV, and f=E/h gives 3.55E17 Hz.