FOCUSING TECHNIQUES FOR HIGH-RESOLUTION NEUTRON POWDER DIFFRACTOMETRY: BRAGG DIFFRACTION ANGLE-ANALYSIS METHOD

P. Mikula¹, J. Šaroun¹, M. Vrána¹, P. Lukáš¹ and V. Wagner²

¹Nuclear Physics Institute, 250 68 Rez near Prague, Czech Republic ²Physikalisch-Technische Bundesanstalt, Bundesallee 100, 38116 Braunschweig, Germany E-mail: mikula@ujf.cas.cz

Keywords: Neutron diffraction, powder diffractometry, Bragg-diffraction optics

Recently, focusing techniques have become widely used for solution of special structural tasks requiring a high Dd/d - resolution (d is the lattice spacing) in neutron diffractometry [1,2]. Even though that an unconventional high-resolution is achieved in a limited range of the momentum transfer, their successful employment is supported by the facts, that a focusing neutron diffractometer operates with „open" beams without Soller collimators resulting in its high luminosity as well as with samples of small dimensions (the sample width is usually less than 5 mm) in combination with a position sensitive detector (PSD). Two principally different diffractometer performances are used in NPI Rez for measurements on polycrystalline samples:

• two axis diffractometer using a quasiparallel beam diffracted by the sample and PSD,
• three axis diffractometer with a cylindrically bent perfect crystal-analyzer set in a special asymmetric diffraction geometry in combination with PSD.

A common features of these instruments are:

1. The beam of monochromatic neutrons selected by the bent monochromator from the white spectrum is focused on the sample (focusing in real space).
2. The efffective mosaicity of the monochromator is much smaller than the angular divergency of the „opened" beam, there is a strong l-q correlation in the incoming and outgoing beams with respect to the samle and this correlation can be easily manipulated.
3. Depending on the monochromator bending radius, the beam diffracted by the sample is either (quasi)parallel or suitably divergent for a further analysis by the optimally bent analyzer (focusing in momentum space).
4. Omitting the spatial resolution of PSD, there are only small blurring effects contributing to the instrumental resolution which come from the effective mosaicities of the bent perfect crystals and the width of the sample and which can be easily estimated [3].

Some excellent properties of our powder diffractometers installed at the medium-power research reactor LVR-15 which are dedicated to strain/stress measurements will be demonstrated.

1. P. Mikula, M. Vrána, P. Lukáš, J. Šaroun and V. Wagner, Proc. of EPDIC IV, Materials Science Forum, 228-231 (1996) 269-274.
2. P. Mikula, P. Lukáš and M. Vrána, Physica B, 234-236 (1997) 1058-1060.
3. P. Mikula, M. Vrána, P. Lukáš, J. Šaroun, P. Strunz, H.J. Ullrich and V. Wagner, Proc. of ICRS-5 held in Linkping - Sweden, June 16-18, 1997, in print.