I expect that the manner of generation of the radiation determines which type of polarization is present.

Physicists consider that the “elementary” photon is circularly-polarized. I’ve never seen a discussion of why, but I suspect it stems from the fact that the Poynting vector (the magnitude of which measures the magnitude of the energy transmitted, and the direction of which determines the direction of transmission of the energy) has a magnitude that does not vary with time only for circularly-polarized radiation; for plane-polarized radiation, the magnitude of the Poynting vector oscillates sinusoidally with time around a mean value that is the constant value that the otherwise identical circularly-polarized radiation would have. Physicists like the idea that electromagnetic radiation has a characteristic energy value determined by its frequency; the circularly-polarized version behaves like this.

Of course, it’s something of a moot point. The reason we give Röntgen credit is that he realised the significance of his discovery and shared it with the world, unlike the rather secretive Tesla.

My understanding is that the choice of what is called an “X-ray” or a “gamma ray” is a more or less arbitrary distinction based on their historical origins. It makes sense, though, that there is more to a photon than just its average energy, for instance bandwidth. There’s probably a blog post worth writing in there about the choice of names…

“X-ray photons have energies which range from 120 eV (electron volts) to 120 keV (thousand electron volts)”

I guess you mean that tubes of this design can only produce X-rays up to 120 keV. For diagnostic radiology tubes have a maximum around 150keV (for reasons of usefulness, contrast decreases at higher energies due to the change from photoelectric to Compton interactions). For superficial X-ray therapy, there are “orthovoltage” machines that go up to around 500keV.

Once you start to use linear accelerators, it is common to use 15MV X-ray beams (15MeV max energy, modal energy around 5MeV). This is currently common around the world for treating prostate cancer, for example.

Once you look into astronomy, X-ray photon energies can, of course, get even wilder.

The reason I think it is worth noting this is that it is a common misconception that some sort of useful distinction between types of photons (gamma, X-ray, …) can be made based on energy. When usually it is the mode of production that is the most important difference.

I’ll reveal a bit more about my textbook in a couple of months… until I get the first draft done, I don’t want to talk about it too much!

But an unfortunate side effect is that the non-specialist public (who may have had one or two science courses in college, but never went back afterwards to look up all these experiments) are largely unaware of how much experimental evidence there is for the things in science textbooks.

Perhaps it would help if if introductory science classes concentrated more on this kind of experimental evidence, and less on teaching the distilled end results without any supporting information. Maybe a “ten greatest experiments in science” approach, instead of the “ten greatest ideas in science”?

P.S. I’m interested to hear more about your textbook… what will the subjet be?