  We propose to exploit the angular distribution of the positrons emitted in
the inverse beta decay to extract a possible antineutrino signal from the
Superkamiokande background. From the statistics collected in just 101.9 days
one obtains a model independent upper bound on the antineutrino flux (for
energy greater than 8.3 MeV) Phi < 9*10^4 cm^-2 s^-1 at the 95% C.L. By
assuming the same energy spectrum as for the 8B neutrinos, the 95% C.L. bound
is Phi < 6*10^4 cm^-2 s^-1. Within three years of data taking, the sensitivity
to neutrino-antineutrino transition probability will reach the 1% level, thus
providing a stringent test of hybrid oscillation models.
