On the Angular Distribution in Nuclear Reactions and Coincidence Measurements

Theorems concerning the general form of the angular distribution of products of nuclear reactions and distintegrations are derived. These theorems are based only on the invariance properties of the physical process under space rotation and under inversion. The following examples are studied in detail: (i) angular correlation between the electron and the neutrino in $\ensuremath{\beta}$-decay; (ii) angular correlation between a $\ensuremath{\beta}$-ray and a $\ensuremath{\gamma}$-ray emitted in succession by a nucleus; and (iii) angular correlation between two $\ensuremath{\gamma}$-rays emitted in succession by a nucleus.