Gamma Decay

Gamma decay is a type of radioactive decay that happens when an isotope is at a high energy level, usually represented with the letter "m" at the end of the name of the isotope (e.g. Uranium-239m). Normally, this is a consequence of a previous radioactive decay (either alpha or beta).

Since the isotope is at a high energy level, to reach a more stable (lower energy) level it needs to emit some energy (as energy is always conserved). This energy is emitted in the form of a highly energetic photon that is expelled from the nucleus. This is called gamma radiation. Since the mass of a photon is extremely low, the loss of mass in the nucleus is almost non-existent.

Gamma decay doesn't modify the atomic mass or the atomic number of the isotope, it simply takes it to a lower energy level.