Search for Doubly Radiative np Capture

We have made a search for doubly radiative $\mathrm{np}$ capture $n+p\ensuremath{\rightarrow}{\ensuremath{\gamma}}_{1}+{\ensuremath{\gamma}}_{2}+^{2}\mathrm{H}$. 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 (${\ensuremath{\sigma}}_{\mathrm{expt}}=334.2\ifmmode\pm\else\textpm\fi{}0.5$ mb and ${\ensuremath{\sigma}}_{\mathrm{th}}=302.5\ifmmode\pm\else\textpm\fi{}4.0$ mb). Previous $\mathrm{np}$-capture experiments have been insensitive to the energy distribution of the capture $\ensuremath{\gamma}$ rays. In this experiment we searched for two-photon events by looking for $\ensuremath{\gamma}$ rays in coincidence from a ${\mathrm{H}}_{2}$O target exposed to a beam of thermal neutrons using two 5\ifmmode\times\else\texttimes\fi{}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 ${\ensuremath{\sigma}}_{2\ensuremath{\gamma}}\ensuremath{\le}1.0$ 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.