Neutrino-induced reactions on 18O and implications of 18O mixture in water Cherenkov detectors on supernova neutrino events

Neutrino-nucleus reaction cross sections on ¹⁸O are evaluated by shell-model calculations and compared with those on ¹⁶O. Important contributions from Gamow-Teller transitions are noticed for ¹⁸O (${\nu }_e$, e⁻) ¹⁸F in contrary to the case for ¹⁶O, where spin-dipole transitions are dominant contributions. Calculated cross sections for ¹⁸O (${\nu }_e$, e⁻) ¹⁸F are shown to be larger than for ¹⁶O at low neutrino energies below 20 MeV in natural water with the 0.205% admixture of ¹⁸O due to the lower threshold energy (1.66 MeV) for ¹⁸O than that for ¹⁶O (15.42 MeV). The resulting electron spectra, that is, the cross sections as functions of emitted electron energy $T_e$, are also shown to be quite different, reflecting the different threshold energies. The electron spectra from (${\nu }_e$, e⁻) reactions on ¹⁸O and ¹⁶O in water Cherenkov detectors for supernova neutrino detection are investigated for both the cases with and without the neutrino oscillation and compared with those of the neutrino-electron scattering. It has been shown that the contribution from ¹⁸O (0.205% mixture) enhances the rates from ¹⁶O by 60% for the case without the oscillation and by 20-30% for the case with the oscillation below $T_e$ =20 MeV. For the case with the neutrino oscillation, the event rates for ¹⁸O and ¹⁶O become comparable to those of the neutrino-electron scattering. However, their rates at low energy ($T_e<$ 20 MeV) are much lower than those of the neutrino-electron scattering, which is important for the pointing accuracy to the supernova direction.