Thermal and mechanical joints to cryo-cooled silicon monochromatorcrystals Page: 3 of 6
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Figure 3. Scaled dimensions of the channel cut monochromator. The top and bottom diffracting surfaces are slightly
angled by 0.28 degrees to reduce exit beam motion 
crystal (Figure 3) is such that it requires light clamping to hold the lower part of the crystal against the
cooled copper bracket. Light clamping prevents distortion of the crystal and is achieved by means of copper
beryllium spring clips. The nickel-plated copper and silicon surfaces were wetted by applying the eutectic
and scraping with a soft wire brush, then they were lightly clamped together at room temperature. It was
noted that the base of the crystal was not flat but was bowed convex by ~5m. We initially did not change
this, arguing that the thicker interface region would help spread out and reduce the strain in the crystal that
will inevitably be present due to the different thermal expansion coefficients of the various materials
involved.. On cool-down, the eutectic interface performed satisfactorily for a few months of operation but
then began to deteriorate as thermocouples attached to the crystal indicated that with beam irradiation the
crystal was failing to cool down satisfactorily. The joint took about a year of operation to fail totally - at
the end the joint had very high thermal resistance such that the crystal operated at > 200C with beam on.
Temperature cycling the cryo-head to re-melt the eutectic had little effect on the performance. During the
slow failure of the eutectic joint, an off line program was launched to determine a reliable thermal junction.
A photograph of the current final monochromator assembly that shows the spring clips and various thermal
joints is shown in Figure 4.
Figure 4. Photograph of the current channel cut monochromator showing hold down clips, aperture thermaljoints.
3. TESTING OF THE CRYO-JOINTS
The test setup was an evacuated assembly identical to the monochromator in the beamline with the
exception that the channel cut monochromator was replaced by a simple block of silicon with a resistor
strapped to it through which current could be passed to simulate a 15 watt power load. Thermocouples were
placed at strategic points and monitored. Typical cool down plots are shown in Figure 5. Figure 5a shows
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MacDowell, A.; Fakra, S. & Morrison, G. Thermal and mechanical joints to cryo-cooled silicon monochromatorcrystals, article, July 14, 2006; Berkeley, California. (https://digital.library.unt.edu/ark:/67531/metadc893362/m1/3/: accessed March 26, 2019), University of North Texas Libraries, Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.