High-sensitive spectrometer of fast neutrons and the results of fast neutron background flux measurements at the Gallium–Germanium Solar Neutrino Experiment

The principle of operation, design, registration system and main characteristics of a fast neutron spectrometer are described. The spectrometer is intended for direct measurements of ultra low fluxes of fast neutrons. It is sensitive to neutron fluxes of 10−7 cm−2 s−1 and lower. The detection efficiency of fast neutrons with simultaneous energy measurement was determined from Monte-Carlo simulation to be equal to 0.11±0.01. The background counting rate in the detector corresponds to a neutron flux of (6.5±2.1)×10−7 cm−2 s−1 in the range 1.0–11.0 MeV. The natural neutron flux from the surrounding mine rock at the depth of 4600 m of water equivalent was measured to be (7.3±2.4)×10−7 cm−2 s−1 in the range 1.0–11.0 MeV. The flux of fast neutrons in the SAGE main room was measured to be <2.3×10−7 cm−2 s−1 in 1.0–11.0 MeV energy range.