Natural X-ray are photons emanating from deep layers of atoms whose energy is characteristic of the emitter atom. These energies grow as the square of electric charge of the nucleus  and range from a few eV for hydrogen to about 90 keV for uranium. The higher the energy is  (and shorter the wavelength), the more penetrating the X-ray is.
More energetic and penetrating X-rays, can be produced from beams of accelerated electrons. Their energy range, up to tens of MeV, overlaps that gamma rays, and their label as gamma or X becomes a matter of taste. The range of X-radiations is widely used in contemporary astrophysics, medicine, crystallography.
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