CHAPTER V - RADIATION

Abstract

Publisher Summary The interaction of electrons with an electromagnetic field can be treated by means of perturbation theory. This is because the electromagnetic interaction is comparatively weak, as is shown by the smallness of the corresponding dimensionless coupling constant, that is, the fine-structure constant. For applications to systems of strongly interacting particles, including nuclei, a semiphenomenological theory is used, in which the transition currents appear as empirically determined quantities subject only to the conditions of space–time symmetry and to the equation of continuity. The transition probability under the action of a perturbation is given by the perturbation theory. The angular distribution of the photons is obtained by summation over all polarizations of the photon and the secondary nucleus. The averaging with respect to polarizations is done by substituting the density matrices of the unpolarized states. The laws of conservation of angular momentum and parity imply certain selection rules that restrict the possible changes in the state of the radiating system.