An Etymological Dictionary of Astronomy and Astrophysics
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The short wavelength, high energy end of the → electromagnetic spectrum. Hard X-rays are typically those with energies greater than around 10 keV. The dividing line between hard and → soft X-rays is not well defined and can depend on the context.

→ hard; → X-rays.

X-ray photons with energies between about 0.1 to 10 keV. → hard X-rays.

→ soft; → X-rays.