FOCUSING TECHNIQUES FOR HIGH-RESOLUTION
NEUTRON POWDER DIFFRACTOMETRY: BRAGG DIFFRACTION ANGLE-ANALYSIS
METHOD
P. Mikula1, J.
aroun1, M. Vrána1,
P. Luká1 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:
- The beam of monochromatic neutrons selected by the bent
monochromator from the white spectrum is focused on the
sample (focusing in real space).
- 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.
- 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).
- 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.
- P. Mikula, M. Vrána, P. Luká, J.
aroun and V. Wagner, Proc. of EPDIC IV,
Materials Science Forum, 228-231 (1996)
269-274.
- P. Mikula, P. Luká and M. Vrána,
Physica B, 234-236 (1997) 1058-1060.
- P. Mikula, M. Vrána, P. Luká, J.
aroun, P. Strunz, H.J. Ullrich and V. Wagner, Proc.
of ICRS-5 held in Linkping - Sweden, June 16-18, 1997, in
print.