Observation of Two-Photon Decay inn−pCapture

Coincident $\gamma$ rays of total energy 2.22 MeV, the deuteron binding energy, were observed following the capture of subthermal neutrons by an ${\mathrm{H}}_2$O target. The differential cross section, assuming a $\frac{1}{v}$ neutron energy dependence, increases from about 0.2 μb/keV at equal $\gamma$ energies to about 1.0μb/keV for a $\gamma$ energy of 1600 keV. The branching ratio for emission of two $\gamma$ rays, both in the $\gamma$-ray energy range from 600 to 1620 keV, to single 2.22-MeV $\gamma$ rays was found to be (1.05±0.16)×${10}^{-3\mathrm{}}$.