Neutron Scattering

Powder diffraction also may be discussed on the basis of the Ewald-construction . An ideal
polycrystalline sample is
characterised by a very large number of anbitrary oriented small
ciystallites . Therefore, for a powder only I H I is defmed without preferred orientation . In
Fig . 4 the corresponding sphere with radius 1111 = 1/dhkl is drawn around the origin of the
reciprocal lattice at 000 . For each Bragg-reflection the circle of intersection with the Ewaldsphere
yields a diffraction cone . All reflections with equal inteiplanar spacing are perfectly
superposed and cannot be separated.
7 . 3 Powder diffractometer
There are two principally différent powder diffraction techniques : thé angular-dispersive
ADP-method and thé energy-dispersive EDP-method, better known as time-of-flight method
in thé case of neutron diffraction .
In thé ADP measurement thé sample is irradiated by a
monochromatic beam (;, = const .) . To each dhki belongs a Bragg-angle 0yki.Most of thé
neutron powder diffractometers at
steady-state reactor sources work according to thé ADP
method (e . g . thé D2B-instrument at thé HFR/ILL in Grenoble) . The angular resolution of a
powder diffraction diagram depends on thé beam divergences before and behind thé
0 .6
4
r
D2B
2
Cornu, X-ray }
Soie lier ~._ ._
0 .0é
CHESS
Diffraction Angle {°)
Fig .
5 . Comparison of thé half-widths of powder lines for selected neutron powder
diffractometers : D2B at thé HFR/ILL in Grenoble(F), HRPD at NBSR in Lucas
Heights (USA), SEPD at ANL in Argonne (USA) - thé time-of-flight data of this
instrument with Ad/d z 1 .5-10-3 are converted in 20-values - and X-ray powder
diffractometers : conventional and synchrotron facilities (CHESS, USA) [1]
7-7