In operating scintillation gamma ray detectors, certain gamma ray standards are used for calibration:
Energy (KeV) Na-22, 511 Mn-54, 835 Co-57, 122
How are these energies determined.
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Besides the szintillation gamma-ray detectors there are the semiconductor-based gamma-ray detectors. These semiconductor-based detectors (and especially the germanium-based one) provide a much better energy resolution in comparison to the szintillation detectors.
Another approach relies on using Germanium detectors - a crystal of hyperpure germanium that produces pulses proportional to the captured photon energy; while more sensitive, it has to be cooled to a low temperature, requiring a bulky cryogenic apparatus.
Therefore you can calibrate a szintillation detector with the gamma energies measured with a semiconductor-based detector.
But this immediately raises the next question:
How can a semiconductor-based gamma-ray detector be calibrated?
For low-energy gamma-rays you can measure their
wavelength $\lambda$ from diffraction by crystals.
This is the same method as used for measuring X-ray wavelengths
(see also Bragg's law),
because low-energy gamma-rays and high-energy X-rays are essentially
the same thing.
From the wavelength $\lambda$ the energy $E$ of the gamma-photon
can be calculated by $E = h\nu = hc/\lambda$.
However, this method is applicable only if the wavelength is not much smaller than the atomic distances in the crystal (i.e. for low-energy gamma-rays), otherwise the diffraction angles would be unmeasurable small.