Inelastic neutron scattering is currently the only technique available to probe low-energy collective excitations in materials with sub-meV resolution. The technique has been instrumental in making key discoveries in a broad range of materials, such as superconductors, multiferroic materials and quantum magnets. However, the applicability of the technique is limited by the inherently weak signals produced in a measurement. When either magnetic moments, samples or merely the scattering cross sections become very small, experiments quickly become infeasible. In addition, since mapping of significant portions of (Q, ω)-space can take up to one week, parametric studies of entire dispersion relations as a function of temperature, pressure and magnetic field are restricted to the most forgiving samples. In overcoming these limitations, improving the efficiency of triple-axis spectrometers (TAS) has therefore been a long-standing goal in the neutron spectroscopy community, thereby easing the constraints on feasibility. This is done by replacing the conventional TAS backend with multiplexing analyser/detector systems, designed to improve efficiency by increasing the number of simultaneous measured data points and the total scattering spatial angle covered by the analyser setup.
In this seminar, I will discuss some of the various implemented multiplexing concepts at neutron facilities, starting from the relatively compact and flexible back ends conceived in the early 2000s, then focusing on the much larger, more elaborate and more expensive setups of today (BAMBUS, CAMEA, MACS) and briefly go through how multiplexing is used in conjunction with a time-of-flight front as done on the BIFROST spectrometer at the ESS, yet to be completed. There are many choices to be made in designing a multiplexing backend, and I will go through some of the central dilemmas, in terms of analyser coverage, resolution and cost. A key recent development is the prismatic analyser concept, where small samples and small detectors can work in unison with a large analyser mosaic to drastically increase the energy resolution of a backend without sacrificing count rate. Spurions and background are always key concerns, and I will present some of the lessons learned mainly from the MultiFLEXX and CAMEA projects. Finally, I will wrap up by giving my take on the future role of multiplexing backends in neutron spectroscopy.