Neutron flux measurement using activated radioactive isotopes at the Baksan underground scintillation telescope

Preliminary results of a neutron background measurement at the Baksan underground scintillation telescope (BUST) are presented. The external planes of the BUST are fully covered with standard scintillation detectors shielding the internal planes and suppressing thus background events due to cosmogenic and local radioactivity. The shielded internal planes were used as target for the neutron flux registration. The experimental method is based on the delayed coincidences between signals from any of the BUST counters. It is assumed that the first signal is due to inelastic interaction of a neutron with the organic scintillator, while the second signal comes from the decay of an unstable radioactive isotope formed when the fast neutron interacts with the ¹²C nuclei. Using the Monte-Carlo method (GEANT4) we also simulated propagation of neutrons through a layer of scintillator. The experimentally found muon induced neutron flux is j =1.3^+0.7_-0.3 ×10^-10cm^-2s^-1 for neutron energies E ≥ 22MeV, which is in a qualitative agreement with similar measurements of other underground laboratories as well as with predictions of the GEANT4.