Angular correlations in nuclear reactions

Experiments involving the measurement of angular distributions and correlations of nuclear radiations fall into three groups: those involving simply conservation laws, studies of nuclear interactions and investigations of the properties of nuclear states. Elementary theoretical considerations and typical or important experimental results are presented with particular emphasis on the third group. When all the states involved are `pure' states with defined angular momentum and parity the correlations are always polynomials in even powers of cos θ only whose coefficients depend on the angular momenta of the states and the nature of the radiations. This condition of pure states is satisfied for all radioactive processes and generally for nuclear reactions at a resonance. Consequently it has been possible to deduce the angular momentum quantum number of a number of excited states from observed angular correlations between gamma-rays and alpha-rays, protons, and gamma-rays respectively. In non-resonant induced reactions a compound nucleus is frequently formed with so high an excitation that overlapping states of opposite parities contribute to the process. In this case odd powers of the cosine may appear and the angular distributions are not necessarily symmetrical about 90°. This is particularly true in reactions of the type (d, p) or (d, n) and, probably, (d, t) in which no true compound nucleus is formed at all. The distribution in these reactions is usually strongly forward.