Search for Doubly RadiativenpCapture

We have made a search for doubly radiative $\mathrm{np}$ capture $n+p\to {\gamma }_1+{\gamma }_2+{}_{}{}^2{}_{}{}^{}\mathrm{H}$. It has been suggested that this process might contribute to the total $\mathrm{np}$-capture cross section and thus explain the long-standing discrepancy in $\mathrm{np}$ capture (${\sigma }_{\mathrm{expt}}=334.2\mathrm{}\pm 0.5$ mb and ${\sigma }_{\mathrm{th}}=302.5\mathrm{}\pm 4.0$ mb). Previous $\mathrm{np}$-capture experiments have been insensitive to the energy distribution of the capture $\gamma$ rays. In this experiment we searched for two-photon events by looking for $\gamma$ rays in coincidence from a ${\mathrm{H}}_2$O target exposed to a beam of thermal neutrons using two 5×5-cm NaI detectors. Analysis of the coincidence spectra indicates two-photon $\mathrm{np}$-capture events have not been observed. An upper limit at the 3 standard deviation confidence level of ${\sigma }_{2\gamma }\le 1.0$ mb can be placed on the doubly radiative $\mathrm{np}$-capture cross section. Therefore the two-photon cross section does not explain the discrepancy between the theoretical and experimental $\mathrm{np}$-capture cross sections.