FOCUS is a time and space focusing time of flight spectrometer for cold neutrons located at the end of the curved guide RNR 11. By means of a vertically converging neutron guide the size of the white beam is reduced and then chopped by a pre-selector disc chopper.
A horizontally and vertically focusing monochromator (size 18cm × 18cm) with variable curvature in both directions focuses the beam through a Fermi-chopper on the sample (w × h ≤ 3.5cm × 5cm). The theta angle of the monochromator can be varied continously from 17.5 to 70 degree. For incident wavelengths larger than 4 Å a Be-filter is used together with the first chopper to suppress higher order contamination.
The disc chopper also acts as anti-overlap chopper. The scattered neutrons pass a 2.5 m flight distance through a radial collimator and an argon filled flight box before being detected in three banks of totally 375 3He counter tubes of rectangular shape. The scattering angle covers a range from +10 to +130 degree. A 2D-multidetector in the angular range between -1.4 to -24 degree was installed recently.
The distances neutron-guide - monochromator and monochromator - sample are both variable such that the spectrometer can be operated either in time focusing (TF) or monochromatic focusing (MF) mode, hence being suited for quasi- elastic or inelastic scattering, respectively. At the sample position space is reserved for standard cryostats, furnaces, magnets, pressure cells etc.
FOCUS was built in cooperation with the group of Prof. Hempelmann at Saarland University, DE, (BMBF-project 03-HE4SA2-2, BMBF-project 03-HE7SAA).