Powder Diffraction - Spallation Neutron Source

Sample Displacement & Transparency
In Bragg-Brentano geometry, samples are ideally placed exactly at
rotation axis and all diffraction occurs from sample surface (highly
absorbing sample). Neither is commonly true.
Peak centers are shifted by
– Sample Displacement (SHFT), S s
– Sample transparency (TRNS), T s
DT = DT
+ S cosQ
+ T sin 2Q
' s s
displacement
-pRSs
=
36000
These corrections correlate very highly with the zero correction for 2,
ZERO. Do not refine this too.
Parallel-Beam instruments (neutron or synchrotron) are very tolerant of
displacement and transparency. Never refine SHFT or TRNS, but do
refine ZERO (correction to 2).
m
eff
- 9000
=
p RT
R is diffractometer radius
s
58