aripekka
commented
1 year ago Hi,
What you observe is a real phenomenon. Bending causes a strain gradient normal to the crystal planes which causes broadening of the reflectivity curve because at different depths of the crystal the beam sees a different spacing of the crystal lattice. However as you move the beam away from the normal incidence, the beam also sees the orientation of the crystal planes differently at different depths.
At the magic angle cot^2 theta = n*nu, where nu = 1 for cylindrical bending and 2 for spherical, these effects cancel and you effectively regain the Darwin width of the perfect crystal. If you increase the Bragg angle further you should see the reflectivity curve broaden again but now the tail flips to the other side of the main peak.
The equation above can be found e.g. in Roberto Verbeni et al. "Multiple-element spectrometer for non-resonant inelastic X-ray spectroscopy for electronic excitations" Journal of Synchrotron Radiation (2009) as Eq. (3) but unfortunately its derivation not explained there. It doesn't look exactly the same as what you wrote in your question (and at the moment of writing I don't know why) but in my understanding it should be related to the same phenomenon.
On Tue, 20 Jun 2023, 11.52 Konstantin Klementiev, @.***> wrote:
Hello,
I have an observation that I don't know how to interpret.
I calculate Bragg reflectivity of a 0.3 mm flat Si 660 crystal at 12000 eV, I get a narrow reflectivity curve. I define a 1m meridional radius and get a very broad curve. Now I set a 1m sagittal radius and the narrow width is restored (!), just shifted. This behavior looks wrong to me.
In strain_term we get sin(theta)2nu/(1-nu) - cos(theta)^2/sin(theta) that terns zero at some angle. Why is it so? Is the derivation correct only close to 90 degrees?
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kklmn
Hello,
I have an observation that I don't know how to interpret.
I calculate Bragg reflectivity of a 0.3 mm flat Si 660 crystal at 12000 eV, I get a narrow reflectivity curve. I define a 1m meridional radius and get a very broad curve. Now I set a 1m sagittal radius and the narrow width is restored (!), just shifted. This behavior looks wrong to me.
In strain_term we get sin(theta)2nu/(1-nu) - cos(theta)^2/sin(theta) that terns zero at some angle. Why is it so? Is the derivation correct only close to 90 degrees?