As discussed, we have two kinds of radiation mechanisms radiating within the x-ray & gamma ray band. The resulting radiation will be similar to the above diagram. The characteristic radiation peaks will be added over a background noise floor, which is due to the Bremsstrahlung mechanism. The wavelengths of the characteristic lines are independent of the pd - they are just characteristic of the metal target. The background radiation cuts off sharply at A or B as the wavelengths diminish down to a certain point at which maximum radiation frequency occurs. This maximum limit for the radiation frequency will depend only upon the voltage across the electrodes, and is independent of the metal target characteristics.
hf(max) = eV
Conversely, if we need to know the minimum pd required to reach a particular maximum radiation frequency we use:
So, suppose we need to reach the Gamma radiation band at 3E19 Hz, then
V(min) = 3E19 x 6.62e-34/ 1.6e-19
V(min) = 124 kV
Note that when operating pulsed dc supplies, sharp spikes on the secondary might reach higher hv values than the nominal output, and the duration of these spikes is significant at such frequencies.