The signal produced in neutrino observatories by the pair-annihilation neutrinos emitted from a 20 $M_{odot}$ pre-supernova star at the silicon burning phase is estimated. The spectrum of the neutrinos with an average energy $sim$2 MeV is calculated with the use of the Monte Carlo method. A few relevant reactions for neutrinos and anti-neutrinos in modern detectors are considered. The most promising results are from $bar{ u}_e + p longrightarrow n + e^{+}$ reaction. During the Si-burning phase we expect 1.27 neutrons/day/kiloton of water to be produced by neutrinos from a star located at a distance of 1 kpc. Small admixture of effective neutron-absorbers as e.g. NaCl or GdCl$_{3}$ makes these neutrons easily visible because of Cherenkov light produced by electrons which were hit by $sim$8 MeV photon cascade emitted by Cl or Gd nuclei. The estimated rate of neutron production for SNO and Super-Kamiokande is, respectively, 2.2 and 41 events per day for a star at 1 kpc. For future detectors UNO and Hyper-Kamiokande we expect 5.6 and 6.9 events per day even for a star 10 kpc away. This would make it possible to foresee a massive star death a few days before its core collapse. Importance of such a detection for theoretical astrophysics is discussed.
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