Alpha, Beta and Gamma Radioactivity

Some atomic nuclei are unstable: too many protons, too many neutrons, or too much energy. To become stable, they transform spontaneously by emitting radiation. This is radioactivity, discovered by Becquerel in 1896, then studied by Marie and Pierre Curie. The phenomenon is random (we cannot predict when a given nucleus will decay) and insensitive to temperature or pressure.

There are three kinds of radiation, from most to least ionizing, and above all from least to most penetrating:

   type      emitted                  penetration
   -------   ----------------------   ------------------------------
   alpha α   helium nucleus (2p+2n)   stopped by a sheet of paper
   beta β-   a fast electron          stopped by an aluminum plate
   gamma γ   a very energetic photon  stopped only by thick lead

   α  --|paper|
   β  -------|aluminum|
   γ  ------------------------|lead|

Alpha radiation is heavy and strongly charged: very ionizing, but not very penetrating (skin stops it). Beta radiation is an electron: more penetrating. Gamma radiation is a very high-energy electromagnetic wave, emitted when an excited nucleus de-excites: the most penetrating, it requires thick shielding.

This hierarchy explains the precautions taken: an alpha source is dangerous mainly if it is ingested, whereas a gamma source is dangerous even at a distance. Understanding what is emitted is the first step toward writing what happens inside the nucleus.