  The principle of operation, design, registration system and main
characteristics of a fast neutrons 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 neutrons/(cm^2 s^1) and lower with the
neutron energy above 700 keV. The detection efficiency of fast neutrons with
simultaneous energy measurement is within 0.02-0.06. The neutron spectrometer
was designed taking into account requirements for minimizing the gamma-ray
background. It is a calorimeter based on a pure liquid organic
scintillator-thermalizer with helium proportional counters as capturers of
thermalized neutrons distributed uniformly over the volume. Low-level signal
electronics for the spectrometer were designed with signal-to-noise ratio
optimization and full pulse shape analysis required for efficient rejection of
background events. A data acquisition and processing system is based on fast
(100 MHz) two-channel PC/AT interfaced digital oscilloscope. The acquisition
software was written in the C programming language.
