 Solution

A Positron signal is correct for the reactor's antineutrinos, through inverse beta decay,A true electron neutrino charged current interaction instead produces an electron, for example ${\nu }_e+n\to e^{-}+p$. This corrects the source's loose use of “neutrino” for the reactor beam.

After slowing, the Positron annihilates with an electron, generally producing two $511$ keV photons. The prompt visible energy includes both the Positron kinetic energy and annihilation energy; neglecting the small neutron recoil,A delayed neutron capture supplies a powerful coincidence tag. Detecting only two annihilation photons discards useful prompt energy and does not uniquely identify the charged current event.