ABSTRACT
In beta decay a neutron–hole proton or proton–hole neutron excitation is generated. The distribution of the beta strength determines not only the ß half-lives, and the rates for ß-delayed neutron emission and fission, but also the shape of the emitted electron and neutrino spectra. The ability to calculate these quantities is of central importance to numerous applications in nuclear physics and neighbouring disciplines such as astrophysics and nuclear engineering. The chapter is concerned with the matrix elements for allowed transitions. It considers the simple structure of Fermi transitions. These may be calculated practically independently of the details of the nuclear structure. The principal characteristic of double beta decay matrix elements is their collectivity. In double beta decay, several or many shell transitions always contribute. The shell model of the atomic nucleus is based on the assumption that the individual nucleons move in a potential well in discrete energy orbitals, just like the electrons in the atomic shell.
