Improving the transmission and flexibility of neutron primary spectrometers

Many types of neutron spectrometer use a conventional primary spectrometer consisting of some collimator, a crystal monochromator and a second collimator. This article develops the description of the in-scattering plane component of the beam produced by such primary spectrometers using a graphical approach, 2D "Acceptance Diagrams", for horizontally curved monochromators and a variety of collimator types (beamtubes, guides, conventional and radial Soller collimators). This visual approach clarifies the effect of primary spectrometer variables on the sample position beam. Conventional resolution descriptions use instrument parameter values to deduce the beam character produced at the sample and thence the full instrument transmission and resolution. This article solves the inverse problem of how to choose beam elements to deliver some desired beam character at the sample and shows that there are usually many equivalent choices. Dealing with this multiplicity seems to be a central issue in the search for optimal instrument designs, particularly if using numerical methods. The approach adopted here suggests a novel, mechanically simple primary spectrometer design offering great flexibility coupled with maximised transmission.