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Gamma radiation photon energies are mostly within the range 1.2 keV (1.9 x to 1.2 GeV (1.9 x 10~10J) at the high end. These energy values give wavelengths from about 1 nm (1 x 10-9 m) to about 1 fm (1 x 10~15 m). The corresponding fre­ quency values are from about 0.3 EHz (0.3 x 1018 Hz) up to about 0.3 YHz (0.3 x 1024 Hz). Since there is no known upper energy limit, wavelength values vary some­ what from one book to the next. One reason for the stress on energies is that photon energies can be measured. Wavelengths are rarely measured, and then only if there is a need to verify that particles are photons and not something else. The lower energy end, up to a few MeV (a few percent of the overall energy range), overlaps with the X-ray segment, but X-rays do not come from nuclear relaxation processes.