Yes. I now notice the 511 KeV which is the energy of an electron.In operating scintillation gamma ray detectors, certain gamma ray standards are used for calibration:In the case of 22Na the photon is actually produced by annihilation of a positron, so the energy can be calculated from the mass of the electron which is accurately known.
How are these energy standards determined?
I think evan_au may be wrong that the line energy is calculated based on the difference in mass in nuclear reactions. Gamma decay involves no mass changes; it comes from difference between the excited nuclear states going back to the ground states emitting a gamma ray photon.