Bremsstrahlung radiation of a Proton

Question

I often hear Bremsstrahlung being discussed in relation to electrons, for example, x-ray generation. Although I was reading a review on Proton Therapy and when discussing the influence of the various interactions within the body it quotes

proton Bremsstrahlung is theoretically possible, but at therapeutic proton beam energies this effect is negligible

Therapeutic energies are of the order of a 100 or so MeV, but it is unclear to me why electrons produce Bremsstrahlung radiation at far lower energies (KeV).

I suspect the answer may lie in the application, electrons are being fired at a dense material and hence slow down faster than protons would do through tissue which is relatively sparse.

Answer 1

No, it is a feature of the particle involved and nothing (directly) to do with the target.

Bremsstrahlung is due to the radiation produced when charges are accelerated - braking counts as 'acceleration' - and the effect actually depends on the acceleration squared. .

This braking is produced by electrostatic forces between the charge of the particle and the charges of the target nuclei/electrons. These forces are pretty much the same for proton and for electron beams. But the electrons gets a bigger acceleration, by a factor $m_p/m_e=1836$ (plus relativistic effects). So the effect is bigger for electrons by a factor of several million.