Monochromator
BT8 has three monochromators : Cu(220), Cu(200) and a double focusing
Si(400). Because the (011) zone axis of the latter is perpendicular to the
plane of rotation, all permitted silicon reflections of the type 
and 
can be accessed by simply rotating the monochromator. However, due to
their intensity only Si(400), Si(311) and Si(511) are of interest (in this
order). For strain measurements, the optimal monochromator settings for each
sample depend on the d-spacing of interest, the desired 2θ value and the
intrinsic line width of the sample reflection (hkl). Besides the wavelength,
the horizontal curvature of the silicon monochromator is the other adjustable
parameter which controls the Figure of Merit as the achievable accuracy of the
peak position per unit of counting time. If resolution is more important then
the monochromator curvature can be optimized down to a FWHM (Δd/d) of 3x10-3.
Data collection times for strain measurements depend very much on the material.
For example, to measure the Fe(211) or the Fe(011) peak with Δd/d=1x10-4
accuracy requires 4 min for a 1 mm3 sampling volume at
2θ=90º at 5 mm depth in reflection geometry. This assumes no
intrinsic peak broadening and no preferred orientation. For aluminum the
counting time increases to more than 25 min. Routinely, measurements are
conducted with an accuracy of Δd/d=5 10-5 which requires four
times the counting time. The silicon monochromator with its (400) and (311)
reflections is superior to both copper crystals in all three parameters of
interest: resolution, Figure of Merit and intensity.
