[Ifeffit] Double crystal monochromator

sankaran anantharaman sankaran.anantharaman at gmail.com
Fri Dec 4 18:21:52 CST 2009

Hi Matthew and Matt,

Thanks for your rapid replies and explanation describing the reason
for the difference between the rocking curve and the Darwin.
I indeed only have little bit of idea about DuMond diagrams and not
much and would have to seek further help to understand it better other
than that it is a transfer function and its application in a symmetric
Bragg reflection.

In the meantime, the figure attached as PDF file and caption given in
quotations is what I am trying to understand with the help of the text
in the book in addition to what Matthew explained.

I hope that over the weekend I am still confused but at a higher level.

Thanks once again.

Start quote"
Figure Caption:
Non-dispersive geometry (left): X-rays from a white source are
incident on two crystals aligned in the same orientation. The central
ray (full line) will be Bragg reflected by both the crystals and will
emerge parallel to the original ray. A ray incident at a higher angle
than that of the central ray will only be Bragg reflected if it has a
longer wavelength. The angle of incidence this ray makes with the
second crystal is the same as that it made with the first, and will be
Bragg reflected. The DuMond diagram in the lower part shows that a
scan of the second crystal has a width equal to the convolution of the
Darwin widths of the two crystals, independent of the incident angular
divergence. Dispersive geometry (right): A ray incident at a higher
angle than the central ray at the first crystal will be incident at a
lower angle at the second crystal . The second crystal must be rotated
by the amount 2(DELTAtheta)in for Bragg's law to be fulfilled. " End
quote. Jens Als-Nielsen

Universität Stuttgart
Institute of Physical Chemistry
Pfaffenwaldring 55
70569 Stuttgart

Tel.: +49 711/685-64463
Fax: +49 711/685-64443
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