Isobaric Spin in Nuclear Physics

field of low energy physics, the term low energy meaning that no consideration is given explicitly to elementary particles other tha n neutrons and protons. Associated phenomena such as beta and gamma deca y are only briefly discussed as we wish to empha size the most recent developments which for the most part involve only nuclear particles. Hist orically, isobaric spin in nucle ar physics has developed in a spasmodic manner. The introduction of isobaric spin into nuclear physics by Heisenberg (1) was taken up in more detail by Wigner (2) and others (3) in the thir ties, but it was more than a decade before detailed experime nts were carried out by Wil kinson et al. (4). Much of this work was sti mul ated by the app earance of selection rules for electric dipole transitions (5) and nuclear reactions (6), particularly those involving deu terons and alpha particles. The existence of selection rules for beta decay was postula ted somewhat earlier by Wigner (7). The major empha sis in this era (1950-1960) was on light nuclei, simply because the isobaric spin formalism was expected to be inappropriate for heavy nuclei because of the la rge Coulomb interactions involved. In 1961 the important experiments of Anderson et al. (8) sho wed beyond any doubt that isobaric spin concepts were important in heavy nuclei and it is the major develop ments following these experiments with which this review is particularly concerned. Much of the earlier work is covered in an excellent review article by MacDonald (9) . A general treatment of the sym­ metry properties involved in the isobaric spin formalism has been given by Wick (1 0) and the relation of isobaric spin to the energy le vels of nuc lei amply discu ssed by Inglis (11). The reader is referred to these articles for a detailed discussion of the early work on isobaric spin.