Radioactivity
Radioactivity was gradually understood in terms of the disintegration of atomic nuclei. The three types of radiation are alpha decay, beta decay and gamma decay. Alpha radiation occurs when an unstable nucleus breaks down so as to eject a fast-moving nucleus of helium, which consists of two protons and two neutrons. Beta radiation occurs when an unstable nucleus breaks down so that a neutron in the nucleus splits into a proton, which stays in the nucleus, and an electron (or positron, which is identical to an electron but with a positive charge) is ejected from the nucleus at very high velocity. After alpha or beta decay, the remaining nucleus is very often left in an ‘excited’ state from which it ‘relaxes’ with the emission of a gamma (or X-ray) photon. This is the origin of gamma radiation.
