The instrument TREFF is a measuring device for the characterisation of neutron optics’ elements. It consists of a reflektometer with a large area detector. There is the possibility for polarization analysis, even an option to perform 3D-polarisation analysis in transmission through the sample (neutron depolarization measurement).

The lower part (100x29mm²) of the neutron guide NL5-S (170x29mm²) is reflected by a PG crystal in the first monochromator bunker. In this way a wavelength of 4,75Å is selected. After passing through a cooled Be filter (to suppress the ^λ/₂ reflections) the neutron beam reaches the second monochromator crystal. The reflection turns it in a direction parallel to the original beam line. The following 2m long beam tube houses a polarising device (optional), a spin flipper, and 2 pairs of apertures to define the horizontal width. For a length of 1.6m between the apertures the beam tube is evacuated.

The apertures allow a maximum neutron beam cross section of 40x10mm². Using the polarisation analysis the beam cross section is reduced to 20x2mm².

Consequently the neutrons reach the sample position, a rotation table with 6 degrees of freedom to adjust the sample. The flux density of the monochromatic and unpolarized beam at the sample position is approximately 3x10^{5 neutrons}/_cm²s.

The detector arm (1.6m long), which is rotatable around the sample position, carries a tube filled with He containing another spin flipper and the detector. The detector is a position sensitive 2D scintillator with a resolution of < 1,5mm and a diameter of 80mm.

For the construction of TREFF substantial components of the HADAS reflektometer were used, which was operated in the neutron guide hall of the FRJ-2 in Jülich.