## 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 3 10^{-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=1 10^{-4} accuracy requires 4 min for a 1 mm^{3 }
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. For practical purposes, the silicon monochromator with its (400) and (311)
reflections are superior to both copper crystals in all three parameters of interest: resolution, Figure of Merit and
intensity.

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Ancillary Equipment

The Residual Stress Measurement Program

Last modified 03-April-2003 by website owner: NCNR (attn: Thomas Gnaeupel-Herold)