Gammetta

Meet Gammetta, a nucleus with energy to spare who lets it go as a flash of gamma light.

After an alpha or beta decay, the new nucleus is often left in an excited state, with its protons and neutrons arranged at higher energy than they need. It settles into its ground state by emitting a gamma-ray photon, much as an excited atom gives out visible light, but with roughly a million times more energy per photon. Paul Villard discovered this very penetrating radiation in 1900, and Rutherford later named it gamma rays.

Nothing about the nucleus's make-up changes: A and Z stay the same, so it is the same isotope with less energy. Cobalt-60, for example, beta-decays to an excited nickel-60, which then gives out two gamma rays of about 1.17 and 1.33 MeV.

Gamma rays sterilise medical equipment and food, treat cancers in radiotherapy, and technetium-99m, which emits 140 keV gamma rays, is widely used in medical imaging.

Gamma rays are hard to stop. Unlike alpha particles they pass through paper and skin, so shielding uses thick lead or concrete, and even then the intensity falls gradually rather than to zero.