Nuclear Physics 

The study of nuclear physics led to the realization that in order to understand the behaviour of atomic nuclei it was necessary to postulate the existence of elementary particles other than the proton (p) and the electron (e). In particular, as we have seen, the existence of the neutron (n) was established. Further, to account for the nuclear force, quite new types of particle had to be introduced pions (π^+,-,0) and other mesons. Then, in order to understand the beta decay process, a further particle was postulated the neutrino (v_e) and the corresponding antineutrino (ῡ_e). Over the years very detailed and painstaking investigation of nuclear processes and properties have given vital information about the interactions involving these new particles. For example, the study of beta decay showed that the interaction responsible for this process violated one of the fundamental symmetries believed to govern all physical processes. This symmetry (referred to technically as parity conservation) postulated that the mirror image of any basic physical process could also occur in the real world. Currently, studies of beta decay are, among other things, aimed at clarifying the important issue of whether or not the neutrino has a finite mass. Turning to nuclear forces, a great deal of information about their detailed nature has come from studying the way in which nucleons are scattered from each other using accelerators and also from the properties of very simple nuclei containing two or three nucleons only. In the case of three nucleon nuclei it has also emerged that three-body forces exist, which are such that the force between two nucleons is altered when a third nucleon is nearby. Returning to the tennis analogy to pion exchange, it is rather like the interaction between two tennis players when a third starts to interfere. In this ‘exchange’ context, an additional magnetic effect has also been observed in simple nuclei. Since a meson such as the pion can carry electric charge, there is a resultant electric current as it is exchanged between nucleons which, in turn, produces a magnetic field. This contributes to the magnetic properties of a nucleus (called a magnetic exchange effect). The above are just a few examples of the ways in which nuclear physics is able to throw light on fundamental elementary particle processes. Work of this kind continues as well as studies of what might be called ‘exotic’ nuclei, for example nuclei which have an excessive number of neutrons compared with the number of protons and which, as a result, have a ‘halo’ of neutrons extending well beyond the potential well confining the nucleons. Study of such nuclei can give information about ‘neutron matter’, which can help with our understanding of neutron stars. Nuclear physics continues to be an exciting part of physics.