Observation of second-order radiative transitions in nuclei

Second order nuclear radiative transitions have been observed in the decay of $^{137}$Ba from a $^{137}$Cs source. Such a decay is expected from second order perturbation theory at a level of 1/$ \alpha )^2$ , or about $10^-4$. Second order transitions have been observed before only in the special case of $0^+$ to $0^+$ transitions where there is no competing first order transition. In the present work, transitions, which proceed via unspecified virtual states in $^{137}$Ba and consist of various multipole mixtures, are reported. It is expected that these new data will stimulate further research into this long-neglected area of nuclear research.