Upper limit for theH1(n,γγ)H2cross section

A value of (- 0.8 ± 2.5) × ${10}^{-5\mathrm{}}$ has been measured for the ratio of double-photon to single-photon emission following neutron capture in hydrogen for $600 \mathrm{keV}<E_{\gamma }<1620\mathrm{} \mathrm{keV}$. The two Ge(Li) detectors used in the experiment subtended an angle of 85° at the ${\mathrm{H}}_2$O target and were shielded from each other to reduce the background from the cross registration of single $\gamma$ rays. The upper limit of the measured two-photon cross section (-3 ± 8 μb) is two orders of magnitude larger than the most recent theoretical predictions ($\approx 0.07$ μb) for this energy range.