Energy Dependence of Reactions at Thresholds

As a development of Wigner's work on the subject, the behavior of nuclear reactions at thresholds is discussed by employing the $\mathcal{R}$-matrix formalism with special attention to the effect of a nuclear reaction threshold in a particular channel on the energy dependence in other channels. In addition to the characteristic cusp-like behavior for $L=0\mathrm{}$, $Z_1{Z}_2=0\mathrm{}$ it is found that for this case it is also possible to have another type of plot of cross section versus energy. The second type differs from the first by a relative change of sign of the extra effect on one side of the threshold.