The IN16B Millennium Project will keep ILL at the forefront of high energy resolution backscattering spectroscopy. This is possible with a technologically innovative Phase Space Transformation (PST) chopper, which prepares an optimum phase space volume for the monochromator by "compressing" a relatively large energy bandwidth, offered by a ballistic neutron guide and a selector, into a narrower energy band at the expense of an increased beam divergence. A large area focusing “perfect crystal monochromator” in backscattering cuts out an extremely narrow wavelength band and sends it to the sample. For spectroscopy this monochromator is moved by a fast linear motor Doppler drive along the beam direction. In the secondary spectrometer vacuum housing the beam is scattered from the sample and is analysed by nearly 15 m² of perfect crystals in backscattering. The analysed neutrons are sent to a position sensitive detector array, which records neutrons as a function of the monochromator speed.
The resolution and wavelength band can be changed by exchanging the complete analyser surface and/or the monochromator on the Doppler drive. For such a change the spectrometer has to rotate around the PST axis. Standard configuration is Si(111) with deformed crystals (~0.9 μeV), options are Si(111) with undeformed, polished crystals (~0.35 μeV) and Si(311) with deformed crystals (~3 μeV and access to large Q). Possible extensions for GaAs(002) (8 times better resolution than Si(111)) and for a time-of-flight-backscattering are foreseen.
Because of the large wavelength band and a high flux impinging on the PST (~40 cm from the detector), backscattering at the end of guide has a risk for higher background. A focusing deflector can be exchanged for a guide piece which allows to place the secondary spectrometer on a side position (low background option: “IN16 mode”).
The instrument will be unique in flux (more than 10 times faster than IN16) and flexible. It ensures the sub-micro-eV resolution range at simultaneous access to large Q-values and therefore is complementary to spallation source near-backscattering spectrometers. New science (thin films, surfaces, very high pressure, small samples, kinetic experiments...) and new experimental possibilities (GaAs with 8 time better resolution, He-polarisation filters, time-of-flight backscattering) come into reach.