  The secondary $\gamma$ rays emitted following a nuclear reaction are often
relatively straightforward to detect experimentally. Despite the large volume
of such data, a practical formalism for describing these $\gamma$ rays in terms
of partial-wave $T$-matrix elements has never been given. The partial-wave
formalism is applicable when $R$-matrix methods are used to describe the
reaction in question. This paper supplies the needed framework, and it is
demonstrated by the application to the ${}^{15}{\rm
N}(p,\alpha_1\gamma){}^{12}{\rm C}$ reaction.
