Which type of radiation cannot be produced at tube potentials of less than 70 keV?

Characteristic radiation is produced when high-speed electrons collide with the anode of an x-ray tube and dislodge inner-shell electrons from the target atoms. This process requires a certain minimum tube potential to provide sufficient energy for electrons to not only overcome the binding energy of the inner-shell electrons but also to produce radiation in the characteristic range.

When the tube potential is below 70 keV, there isn't enough energy to generate characteristic radiation. Instead, the primary radiation produced at lower tube potentials comprises Bremsstrahlung radiation, which results from the deceleration of electrons as they pass close to the nucleus of target atoms. Bremsstrahlung can be produced at a variety of energies, including those below 70 keV.

In contrast, gamma rays are a different form of radiation emitted from radioactive materials and are not produced in x-ray tubes, hence their production isn't affected by the kilovolt settings of x-ray machines. So, while both Bremsstrahlung and gamma rays can appear under various conditions, characteristic radiation specifically requires a tube potential of at least 70 keV to be effectively generated.