On Directional Correlation of Successive Quanta

A theoretical investigation shows that there should be a correlation between the directions of propagation of the quanta emitted in two successive transitions of a single radiating system. This correlation is described by a function $W\left(\theta \right)$ which gives the relative probability that the second quantum will be emitted at an angle $\theta$ with the first; $W$ is determined by the angular momenta of the three levels involved in the two transitions and by the multipole order of the radiation emitted in these transitions. The explicit forms of $W$ for all angular momenta and for dipole and quadrupole radiation are given; experimental determination of $W$ in any given case should limit these factors to a small number of possibilities. This has particular interest as a means of investigating the nuclear energy levels involved in $\gamma$-radiation; here $W$ should be observable by measuring the variation with $\theta$ of gamma-gamma coincidence counting rates.