Laue spots identification in Channel Cut Monochromators
"Channel cut crystals" are commonly used to monochromatize synchrotron radiation in the X-ray region. Due to the intense and continous nature of synchrotron radiation, one inconvenient of "channel cut crystals" is that the diffracted beam is accompanied by parallel beams of different energy that may overlap with the main diffracted beam. These beams are essentially the Laue divergent pattern of the first crystal that if intercepted by the 2nd crystal are redirected parallel to the incident beam direction. They become more numerous with increasing critical phonon energy and/or large Bragg angle.
| The figure shows the basic principle that produces the appearance of parallel diffracted beams in channel cut monochromators. Two diffracted beams are shown originated from two different (hkl) planes. |
Identyfing the Laue spots (or "Spurious" beams)
Kostroun [1] proposed a simple formulation to calculate the "spurious" beams for a given channel cut, that is, it is only neccesary to know the parameters of the monochromator (i.e., the gap and the angle respect to the incident beam if it is symmetric).
The following matlab programs use Kostroun's formulation, allowing you to calculate the Laue spots. It has been written for Si channel cut crystals. If you happen to have questions or some data we will be happy to test it (iperal@cells.es, juanhuix@cells.es ). You will find an example in the following file.
SpuriousReflections.m (main program)
setconstants.m (subroutine to set constants)
GetAtomicFormFactor.m (function to obtain fSi)
f0.dat( fSi table, it is used by GetAtomicFactor.m, please check if the path is correct in your computer)
check.m (functions that checks for allowed reflections of the Si crystal structure)
check_plot.m (functions that checks that multiples of the main reflected beam are not plotted)
weigth2size.m (function that calculates the spot size in the output figure depending on the weigth of the beam)
sisf.m (function that calculates the Fhkl2 of the Silicon crystal structure divided by fSi2)
[1] Vaclav O. Kostroun
Identification of "spurious" reflections in "channel-cut" monochromators"
Nuclear Instruments and Methods 172 (1980) 243-247
Laue spots and "monochromator glitches"
It should be mentioned that Laue spots are different from "monochromator glitches".
Crystal glitches are well known [2,3] and arise when two or more sets of Bragg planes in the monochromator crystal can simultaneously diffract X-rays at the same energy . This only happens at discrete energies (i .e . at particular angles of the monochromator crystal) and over a small range in energy. For this small energy range there is a decrease (sometimes an increase) in the diffracted intensity. You can find some experimental data on the Monochromator Crystal Glitch Library
[2] Z.U. Rek, G.S. Brown, and T. Troxel, in : EXAFS and Near Edge Structure 3
Eds. K.O . Hodgson, B. Hedman and J.E . Penner-Hahn (Springer, New York, 1984) p. 511.
[3] K.R Bauchspiess and E.D . Crozier, ibid ., p . 514 .
