Doubly radiativenpcapture

The theory of the reaction $n+p\to d+\gamma +\gamma$ is reviewed with emphasis on the dominant, or ($E1,E1\mathrm{}$), mode. The differences between the length and the gradient electric dipole operators are studied in detail and the two-photon cross section (${\sigma }_{2\gamma }$) is calculated using both operators. The length operator yields a very reliable value of ${\sigma }_{2\gamma }=0.118\mathrm{}$ μb (± 1%). The result obtained with the gradient operator is ∼20% smaller and has large uncertainties. Results for other two-photon modes are also presented.