  It has long been recognized that observation of gamma rays originating from
nuclear deexcitation can be exploited to identify neutral-current
neutrino-nucleus interactions in water-Cherenkov detectors. We report the
results of a calculation of the neutrino- and antineutrino-induced gamma-ray
production cross section for oxygen target. Our analysis is focused on the
kinematical region of neutrino energy larger than ~200 MeV, in which
single-nucleon knockout is known to be the dominant reaction mechanism. The
numerical results have been obtained using for the first time a realistic model
of the target spectral function, extensively tested against electron-nucleus
scattering data. We find that at neutrino energy 600 MeV the fraction of
neutral-current interactions leading to emission of gamma-rays of energy larger
than 6 MeV is ~41%, and that the contribution of the p_3/2 state is
overwhelming.
