ACCEPTANCE DIAGRAM ANALYSIS OF THE PERFORMANCE OF MULTIDISK NEUTRON VELOCITY SELECTORS

Transmission and absorption properties of helical multidisk neutron velocity selectors can be calculated using the method of acceptance diagrams. In this approach neutron trajectories are characterized by a transverse spatial coordinate and by a coordinate that is linearly related to wavelength and angle relative to the selector axis. It is then possible to classify neutron trajectories according to the number of absorbing sectors that they cross within the selector. In this way we can determine the extent to which a given selector is successful in eliminating neutrons that get through the selector other than via one of the intended helical channels. We describe the design algorithm for selectors such as those in use at the National Institute of Standards and Technology, and we discuss the properties of a selector built according to this algorithm. We also consider what happens when the direction of the neutron beam is reversed. The problem of optimizing the design of a multidisk velocity selector is briefly discussed.