A gamma ray is a high-energy photon, conventionally associated with a transition of an atomic nucleus, radioactive decay or a particle reaction. X-rays and gamma rays are both electromagnetic radiation; their energy ranges overlap, so the production mechanism can be more informative than an arbitrary energy boundary.
A nuclear energy level transition emits a gamma ray whose energy is approximately the level difference, with a correction for nuclear recoil. Detectors infer the incoming energy from interactions such as photoelectric absorption, Compton scattering and, above threshold, pair production. A measured gamma-ray spectrum can therefore identify nuclear transitions even when a detector does not absorb the entire photon energy in each event.
Absorption of a gamma ray into an excited nuclear level followed by re-emission. The absorber-frame photon energy must overlap the absorption line; nuclear recoil, Doppler broadening and the natural decay width affect resonance. This nuclear process differs from atomic resonance fluorescence used in quantum optics.
An electric quadrupole transition is a radiative transition of an atomic nucleus with electric multipole order two. The emitted photon carries two units of angular momentum and positive parity, so the atomic nucleus has unchanged parity.[ref]
NuDat 3 Glossary by National Nuclear Data Center. Multipolarity, E2 entry
Addition of angular momentum imposes . Thus and can be E2, whereas cannot emit one E2 photon. On the even-, nuclear rotational band, the adjacent downward transitions have ; that band pattern is more restrictive than the general E2 selection rule.
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Their energy is very high compared example to more common radiation such as visible spectrum, and there is a neat reason for that: it's because the strong force that binds nuclei is strong so transitions lead to large energy changes.
A decay scheme such as Figure "caesium-137 decay scheme" illustrates well how gamma radiation happens as a byproduct of radioactive decay due to the existence of nuclear isomer.
Gamma rays are pretty cool as they give us insight into the energy levels/different configurations of the nucleus.
They have also been used as early sources of high energy particles for particle physics experiments before the development of particle accelerators, serving a similar purpose to cosmic rays in those early days.
But gamma rays they were more convenient in some cases because you could more easily manage them inside a laboratory rather than have to go climb some bloody mountain or a balloon.
The positron for example was first observed on cosmic rays, but better confirmed in gamma ray experiments by Carl David Anderson.