USING BRAGG OPTICS FOR HIGH RESOLUTION NEUTRON POWDER DIFFRACTION. V.Wagner*, P.Luká, P.Mikula, J. Aroun and M.Vrána; *Physikalisch-Technische Bundesanstalt, 38116 Braunschweig, Germany Nuclear Physics Institute, 25068 Rez near Prague, Czech Republic

The performances of three set-ups for high-resolution powder diffraction, which are based on Bragg diffraction optics with bent perfect Si or Ge crystals, are discussed. By focusing in real and momentum space considerably higher luminosity and resolution are obtained than in conventional diffractometers using plane crystals and Soller collimators. Under optimum conditions, a FWHM of the individual powder lines of about 3x10^-3 rad on 2[[theta]]-scale (or even less) can be achieved - though in a limited range of scattering vector Q around 30 nm^-1. Substantial advantages of the presented two and three (with an analyzer) axes arrangements arise in the case of small sample dimensions (less than 5 mm diameter) [1], high density of powder reflections in the investigated Q-range or in the case of a peak profile analysis [2]. The useful range (Q is mainly determined by the choice of both the lattice planes and the curvatures of the monochromator and analyzer. It can be adapted to the experimental requirements by using different reflections and the easy control of the bending radii of the crystals.

Each of the three focusing configurations can be implemented in conventional diffractometers, those in combination with a 1D position-sensitive detector being be most promising. Further improvements in the resolution and luminosity are possible by using thinner bent crystals (better resolution), sandwich-type monochromator (higher luminosity), and/or doubly-bent designs for simultaneous vertical and horizontal focusing [3].

[1] N.Niimura et al., Physica B, 213&214 (1995) 786.

[2] P.Klimanek et al., J.de Physique, Coll. C7, 3 (1993) 2143.

[3] V.Wagner et al., Nucl. Instr. & Meth. in Phys. Res., A338 (1993) 53.