The ThALES instrument is situated at the end of the cold neutron guide H53 at a distance of 19.0 m from the horizontal cold source of ILL high-flux reactor. It comprises the primary spectrometer, with the incident beam casemates and the monochromator shielding, and the secondary spectrometer, with the sample table and the analyzer-detector assemblies. The sample table, analyzer and detector move around the monochromator shielding drum within the experimental zone.

To achieve an unprecedented sensitivity in studies of weak signals, the primary spectrometer of ThALES will offer high incident monochromatic flux at sample position, free of contamination by high-order reflections and parasitic scattering from the monochromator and its environment.

To this end a velocity selector (2), acting as a broadband filter suppressing neutrons with wavelength of λ/2, λ/3 etc. will be placed at the exit of the neutron guide in the casemate. The incident beam casemate as well as the monochromator will be enclosed in a massive shielding consisting of a sandwich of lead (300 mm) and high-density polyethylene (HDPE, 600 mm), optimized to provide not only an efficient biological protection, but also to minimize background counts in the experimental data. The choice of shielding material is dictated by the requirement of a nonmagnetic environment in experiment area.

For experiments requiring a high momentum resolution the monochromator curvature can be reduced both in the horizontal and in the vertical plane (down to flat positions) and the elliptically tapered guide end-section can be replaced by a straight m = 1 guide section to provide an incoming beam-divergence smaller 60’. To this end a three-stage lifting system is foreseen in the casemate, with the third position being reserved for an eventual future pre-polarizing guide section.