What is radioactive decay?

Radioactive decay is the spontaneous transformation of an unstable atomic nucleus into a more stable configuration, accompanied by the emission of radiation. This process occurs because the original nucleus has an imbalance of forces or an unfavourable neutron-to-proton ratio, making it energetically unstable.

Radioactive decay is the spontaneous transformation of an unstable nucleus into a more stable one, accompanied by the emission of radiation.

During decay, the nucleus releases energy in the form of particles (such as alpha or beta particles) and/or electromagnetic radiation (gamma photons). The transformation changes the composition of the nucleus, often producing a different element or isotope.

Radioactive decay is a random process at the level of individual atoms, but when large numbers of atoms are considered, it follows predictable statistical laws.

Understanding the physics

A nucleus becomes unstable when the balance between the strong nuclear force and electrostatic repulsion cannot be maintained. This may occur if there are too many or too few neutrons relative to protons, or if the nucleus is simply too large for the strong force to overcome proton–proton repulsion.

Decay occurs because moving to a new nuclear configuration results in a lower total energy state, typically one with a higher binding energy per nucleon. The difference in energy between the parent and daughter nuclei is released during the decay process.

There are several mechanisms by which a nucleus can decay. A neutron-rich nucleus may undergo beta minus decay, converting a neutron into a proton. A proton-rich nucleus may undergo positron emission or electron capture, converting a proton into a neutron. Very heavy nuclei may undergo alpha decay, emitting a helium nucleus. In some cases, the nucleus remains the same element but transitions from an excited state to a lower energy state by emitting a gamma photon, this process is known as isomeric transition.

Although it is impossible to predict when a specific atom will decay, the probability of decay for a large population of atoms is constant. This gives rise to exponential decay behaviour and predictable half-lives.

Where this matters clinically

All nuclear medicine imaging depends on radioactive decay. The emitted radiation (particularly gamma photons or positron annihilation photons) is what allows external detection by gamma cameras or PET scanners. The type of decay determines the imaging modality and influences radiation dose and image quality.