Restricted Neutron Transmission through a Cylindrical Guide Tube

The transmission and the number of reflections of successful neutron trajectories through a straight cylindrical tube with a coaxial aperture at its entrance have been calculated, assuming that a beam with a conical angular divergence is incident along the axis of the cylinder. The calculation models neutron transmission measurements through tubes that have been defined by a cadmium aperture placed at the entrance of the tube. The addition of an aperture not only lowers the acceptance, but also reduces the average number of reflections relative to unrestricted transmission. The aperture prevents neutrons having trajectories with tight spirals down the tube from entering near the periphery of the tube. This model might also be useful for understanding the reduced transmission through a tube with slightly crimped edges or through imperfectly cut glass polycapillaries.