[mcxtrace-users] improved perfect crystal model

[Mendenhall, Marcus]

My group at NIST does extensive work with high-quality single-crystal optics to make xray spectrometers.  Using the "PerfectCrystal.comp" component turned up numerous computational errors and shortcomings in the model.

I have taken the code from it, and both fixed the errors in it, and extended it to be more useful in a practical context.  This code should still look quite familiar to someone who has read the original PerfectCrystal.comp model.

The issues fixed are:

1) rays with non-zero kx were reflected incorrectly
2) complex arithmetic in DarwinReflectivity had errors
3) atomic motion not handled at all, so rocking curve width was not right.

The extensions are:
1) now fully handles polarization.  If ray comes in with Ex and Ey correctly defined, the outgoing polarization is a correctly computed from these. If Ex and Ey are zero on entrance, a new, random, polarization is generated, and then properly reflected.  If multiple bounces are used, the polarization propagates correctly.
2) added a crystal_type field which allows one to select diamond, bcc, or fcc cubic structures, or to explicitly provide the phase factors from which to compute form factors
3) added a field for the Debye-Waller temperature factor, and scaling of structure factors for the atomic motion.  Rocking curves now agree in high detail both with XOP and with our own experimental data.

I have spent considerable effort validating the entire result.  I think this is a good representation of a flat, perfect crystal under realistic conditions.


Anyone who has been using the PerfectCrystal.comp object, I would appreciate it if you would take a hard look at this and let me know if you see issues.  This should eventually probably turn up in the official McXtrace distribution.

For the future:  I will extend this to handle bent crystals, to that Johansson monochromators can be modeled.

Marcus Mendenhall

Materials Measurement Science Division
National Institute of Standards and Technology
100 Bureau Dr. stop 8370 (217/B115)
Gaithersburg MD 20899 USA
phone: +1(301) 975-8631

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