Conceptual design of the time-of-flight backscattering spectrometer, MIRACLES, at the European Spallation Source.

In this work, we present the conceptual design of the backscattering time-of-flight spectrometer MIRACLES approved for construction at the long-pulse European Spallation Source (ESS). MIRACLES's unparalleled combination of variable resolution, high flux, extended energy, and momentum transfer (0.2-6 angstrom(-1)) ranges will open new avenues for neutron backscattering spectroscopy. Its remarkable flexibility can be attributed to 3 key elements: the long-pulse time structure and low repetition rate of the ESS neutron source, the chopper cascade that tailors the moderator pulse in the primary part of the spectrometer, and the bent Si(111) analyzer crystals arranged in a near-backscattering geometry in the secondary part of the spectrometer. Analytical calculations combined with instrument Monte-Carlo simulations show that the instrument will provide a variable elastic energy resolution, delta((h) over bar omega), between 2 and 32 mu eV, when using a wavelength of lambda approximate to 6.267 angstrom (Si(111)-reflection), with an energy transfer range, (h) over bar omega, centered at the elastic line from -600 to +600 mu eV. In addition, when selecting lambda approximate to 2.08 angstrom (i.e., the Si(333)-reflection), delta((h) over bar omega) can be relaxed to 300 mu eV and (h) over bar omega from about 10 meV in energy gain to ca -40 meV in energy loss. Finally, the dynamic wavelength range of MIRACLES, approximately 1.8 angstrom, can be shifted within the interval of 2-20 angstrom to allow the measurement of low-energy inelastic excitations. Published by AIP Publishing.