Nuclear decay (Radioactive decay) occurs when an unstable atom losses energy by emitting ionizing radiation. Radioactive decay is a random process at the level of single atoms, in that, according to quantum theory, it is impossible to predict when a particular atom will decay. Gamma decay is one of many types of radioactive decay.
Gamma decay is a type of radioactive decay in which a gamma ray is emitted from an atomic nucleus. In gamma decay a nucleus changes from a higher energy state to a lower energy state through the emission of electromagnetic radiation (photon). The number of protons (and neutrons) in the nucleus does not change in this process, so the daughter and parent nuclei are the same chemical element.

Key features of gamma rays are summarized in following few points:

  • Gamma rays are high-energy photons (about 10 000 times as much energy as the visible photons), the same photons as the photons forming the visible range of the electromagnetic spectrum – light.
  • Photons have no mass and no electrical charge, therefore they cannot directly ionize matter, neither gamma rays.
  • Despite this fact, gamma rays ionize matter via indirect ionization.
  • Although a large number of possible interactions are known, there are three key interaction mechanisms  with matter.
  • Gamma rays travel at the speed of light and they can travel thousands of meters in air before spending their energy.
  • Since the gamma radiation is very penetrating matter, it must be shielded by very dense materials, such as lead or uranium.
  • The distinction between X-rays and gamma rays is not so simple and has changed in recent decades.  According to the currently valid definition, X-rays are emitted by electrons outside the nucleus, while gamma rays are emitted by the nucleus.
  • Gamma rays frequently accompany the emission of alpha and beta radiation.