PHYS01200804001 Sohrab Abbas - Homi Bhabha National Institute

k
b
k
O
H
. ni
sin( B
S)
. (18)
. n sin( )
i
B
S
is negative in the Bragg case and positive in the Laue case. The diffracted wave exits the incidence
surface in the Bragg case, but propagates inside the crystal in the Laue case. In the reciprocal space
(Fig.3), n i lies between the directions QQ and QR in the Bragg, and between QQ and QQ in the
Laue case. For a given incidence angle in the Bragg case, two tie points, in general, are excited
either on the - or on the β-branch. In the Laue case on the other hand, one tie point each on the
and β branches is excited.
Fig.4 Asymmetric (a) Bragg and (b) Laue configurations of neutron incidence on a single crystal.
2.2.2 Bragg Case
In the Bragg configuration, incidence and exit surfaces are identical. Thus, for incidence
represented by point R in Fig.3, the exit wave vector k H is obtained, in accordance with the
boundary condition I), by producing TB, to intersect the sphere of diffraction CLL of radius k O
about H at the point C, say. Then k H = CH. The wave vector of the external diffracted wave thus
20