Results of measurements of an environment neutron background at BNO INR RAS objects with the helium proportional counter

A method of measurements of the environmental neutron background at the Baksan Neutrino Observatory of the INR RAS are described. Measurements were done by using of a proportional counter filled with mixture of Ar(2 at)+$^3$He(4 at). The results obtained at the surface and the underground laboratory of the BNO INR RAS are presented. It is shown that a neutron background in the underground laboratory at the 4900 m w.e. depth is decreased by $\sim 260$ times without any special shield in a comparison with the Earth surface. A neutron flux density in the 5-1323.5~cm air height region is constant within the determination error and equal to $(7.1\pm0.1_{\rm{stat}}\pm0.3_{\rm{syst}})\times10^{-3}$ s$^{-1}\cdot$cm$^{-2}$.