Radioactive decay is ubiquitous. It occurs everywhere on Earth and throughout the universe. The most common forms occur when an unstable nucleus spits out an alpha particle, consisting of two neutrons and two protons, a beta particle consisting of an energetic positron or electron, or a gamma ray, consisting of a high energy photon.
When these powerful forms of radiation pass through matter, they strip electrons from atoms and molecules, leaving a trail of charged particles in their wake. Detecting these charged particles is simply a matter of mopping them up with an electric field, in a device known as a Geiger counter.
This gives physicists a good indication of local levels of radiation. But they also have other techniques for characterizing the decays in more detail. For example, they can measure the amount of energy a decay particle deposits in a material by studying the light it produces as it passes through, or by the heat it generates.
