The Attenuation of Neutrons by Air Ducts in Shields

The attenuation of fast neutrons by long, thin air ducts in shields is considered from a simple phenomenological point of view. It is shown that the wall-scattered flux in a straight duct is small compared to the uncollided neutrons which travel directly in air from one end of the duct to the other. An expression for the attenuation due to a bend is derived and this is generalized to the case of several bends.