A 2D Acceptance Diagram Description of Neutron Primary Spectrometer Beams

Many types of neutron spectrometer use a conventional primary spectrometer consisting of some collimator, a crystal monochromator and a second collimator. Conventional resolution descriptions use instrument parameter values to deduce the beam character and thence the instrument transmission and resolution. This article solves the inverse problem of choosing beam elements to deliver some desired beam character and shows that there are many possible choices of elements to deliver any given beam character. Dealing with this multiplicity seems to be a central issue in the search for optimal instrument designs especially if using numerical methods. The particular approach adopted here is to extend the 2D "Acceptance Diagram" view of the in-scattering-plane component of primary spectrometer beams to include horizontally curved monochromators and a variety of collimator types (beamtubes, guides, Soller collimators and radial Soller collimators). This visual approach clarifies the effect of primary spectrometer variables on the sample position beam and suggests a novel mechanically simple primary spectrometer design offering great flexibility coupled with maximised transmission.