First Study of Neutron Tagging with a Water Cherenkov Detector

Abstract: A first study of neutron tagging is conducted in Super--Kamiokande, a 50,000-ton water Cherenkov detector. The tagging efficiencies of thermal neutrons are evaluated in a 0.2 % GdCl${3}$-water solution and pure water. They are determined to be, respectively, 66.7 % for events above 3 MeV and 20 % with corresponding background probabilities of 2 $\times$ 10$^{-4}$ and 3 $\times$ 10$^{-2}$. This newly developed technique may enable water Cherenkov detectors to identify $\bar \nu{e}$'s geological or astrophysical sources as well as those produced by commercial reactors via the delayed coincidence scheme.