Selecting neutrons in curved selection channels: φi is the angle of incidence-reflection of the neutron to the surface at i–1-reflection; φ2 ≤ φs; φ1 is the angle to the surface of the selection element for the primary entrance of the neutron n; ∆φ = φ1 – φs; hs is the thickness of the near-wall layer of the selected flux; R (x) is the radius of curvature of the selection element surface.