  The SNO+ experiment collected data as a low-threshold water Cherenkov
detector from September 2017 to July 2019. Measurements of the 2.2-MeV $\gamma$
produced by neutron capture on hydrogen have been made using an Am-Be
calibration source, for which a large fraction of emitted neutrons are produced
simultaneously with a 4.4-MeV $\gamma$. Analysis of the delayed coincidence
between the 4.4-MeV $\gamma$ and the 2.2-MeV capture $\gamma$ revealed a
neutron detection efficiency that is centered around 50% and varies at the
level of 1% across the inner region of the detector, which to our knowledge is
the highest efficiency achieved among pure water Cherenkov detectors. In
addition, the neutron capture time constant was measured and converted to a
thermal neutron-proton capture cross section of $336.3^{+1.2}_{-1.5}$ mb.
