The use of neutron optical devices on beam-hole experiments on beam-hole experiments

Abstract

A survey is given on the applicability of neutron optical devices as an experimental tool for beam hole experiments at lowenergies, namely: plane or bent neutron reflecting mirrors, and straight or curved neutron reflecting beam tubes in which neutrons can be guided by multiple reflections. These can be used in the following way: 1. (1) Bent mirrors can be applied for small angle scattering experiments if a slit-shaped beam is available. They allow investigations at smaller angles than without a mirror. Further, the background is greatly reduced by the mirror arrangement. 2. (2) A straight neutron conducting tube gives a gain in slow neutron intensity of G ≅ γ c 2 gW . γ c is the critical angle on the tube wall (of the order of 1·5. 10 −2 for 10 Å neutrons) and Ω is the geometrical solid angle transmitted through the tube. 3. (3) A bent total reflecting tube allows the conduction of slow neutrons to a place far distant from the core and away from the fast neutron beam. The reduction of intensity by these long tubes is rather small. The transmitted solid angle is of the order of 4 γ c 2 , independent of tube length. By means of this arrangement experiments can be placed in a region with extremely low background, e.g. outside the main reactor building. Further, the number of experiments can be increased by gain in space close to the shielding wall of the reactor. The obtainable intensities are compared with more conventional means of the separation of slow neutrons from the background beam (diffraction crystals or filters). Two special applications, for small angle scattering and for capture γ-ray experiments, are discussed in more detail.