Nuclear Physics: the study of the protons and neutrons at the centre of an atom and the interactions that hold them together in a space just a few femtometres (10-15 metres) across. Example nuclear reactions include radioactive decay, fission, the break-up of a nucleus, and fusion, the merging of nuclei. Discoveries in nuclear physics have led to applications in many fields. This includes nuclear power, nuclear weapons, nuclear medicine and magnetic resonance imaging, industrial and agricultural isotopes, ion implantation in materials engineering, and radiocarbon dating in geology and archaeology.