Gamma ray or \(\gamma \) ray is a particularly pervasive electromagnetic radiation. It arises from the decay of atomic nuclei of many naturally occurring or artificially produced radioactive nuclides.
Although the particles, which are emitted on alpha and beta decay usually are very energetic they can not penetrate matter as well as the uncharged photons gamma ray.
In general any electromagnetic radiation with quantum energies above about 200 \( keV \) (that is more energetic than X-rays) are called gamma ray, irrespective of the nature of their creation. In this sense, the term is usually used in astronomy, where the genesis of the radiation is usually not known. But also in radiation protection, the term is used when the type of radiation for a specific task is not relevant.
A nucleus usually is in an excited state after a alpha or beta decay. During the transition to a less highly excited state or the ground state it releases energy in the form of gamma ray.
Therefore the gamma ray can only have certain energies corresponding to the energy difference between the two states have.
Instead of \(\alpha \) or \(\beta \) decay, the excited state can also originate from neutron capture or other nuclear reactions or through the previous absorption of a photon from gamma ray.
Gamma radiation penetrates matter a lot easier than alpha or beta particles. To protect yourself you several meters thick lead walls depending on the intensity of the radiation.