Discipline: Technology and Engineering
Subcategory: Physics (not Nanoscience)
Session: 4
Ellen Leibowitz - Georgia Institute of Technology
The goal of this project was to design a dual-energy, double-bounce, high resolution x-ray monochromator for ultra-precise backscattering studies of the thermal expansion of Silicon Carbide-4 Hexagonal (SiC-4H) at low temperatures. SiC-4H is a technologically important polymorph of SiC, which can potentially be used as an efficient semiconductor.
The monochromator narrows the range of wavelengths coming from a source. Silicon crystals can be used as monochromator’s elements reflecting x-rays in near backscattering configuration [1]. Accurate design and polishing of the silicon crystals used in the monochromator are important in its construction.
Two Si backscattering Bragg reflections were chosen so that their d-spacings matched those of two independent backscattering reflections in SiC-4H. Specifically, the chosen backscattering reflections were 7 5 3 and 10 8 2 for Si, and the matching reflections for SiC-4H were 1 3 10 and 6 0 8 respectively.
Switching between the two backscattering energies required crystal rotations in the range of a few degrees. Fine tuning of the photon energies is accomplished by high-precision scanning of the temperature of the Si crystals [2-4]. The Si crystals were cut from a section of a Si boule with asymmetric reflection geometry [1] to increase the footprint of the incident x-ray beam on the crystal surface, which can help to reduce possible crystal deformations due to thermal x-ray loading [2]. Two rectangular 20x20x40 mm3 specimens were produced. Laue x-ray diffractometry was used to verify crystal orientation.The working crystal surfaces were polished using a developed multi-step procedure in order to reduce damage to the crystal lattice from cutting.
After polishing the surface, they had a mirror-like appearance except for some remaining cutting grooves on one of the crystals about ? length from the edge. The observed mirror-like appearance suggests efficient removal of crystal damage, which is important for achieving narrow energy bandwidth and high reflectivity of the backscattering reflections. Additional steps to improve the crystal quality could include chemical etching and further polishing [5].
Subsequently, my research mentor confirmed good quality of the surfaces using x-ray topography and successfully used the monochromator to produce the double-backscattered beams. In addition, several advances have been made with regard to the temperature control of the monochromator.
References:
[1]Y. Shvydko, X-ray optics: high-energy-resolution applications. Berlin: Springer, 2004.
[2] High energy resolution backscattering monochromator, European Synchrotron Radiation Facility (ESRF). [Online]. Available: http://www.esrf.eu/UsersAndScience/Experiments/DynExtrCond/ID28/BeamlineLayout/OH3/BackscatteringMonochromator [Accessed: 25-Jul-2017].
[3]Y.-W. Tsai, Y.-Y. Chang, Y.-H. Wu, K.-Y. Lee, S.-L. Liu, and S.-L. Chang, High-resolution interference-monochromator for hard X-rays, Optics Express, vol. 24, no. 26, p. 30360, Dec. 2016.
[4] Alexeev, P., Asadchikov, V., Bessas, D., Butashin, A., Deryabin, A., Dill, F., Ehnes, A., Herlitschke, M., Hermann, R., Jafari, A., Prokhorov, I., Roshchin, B., Rahlsberger, R., Schlage, K., Sergueev, I., Siemens, A. and Wille, H. (2016). The sapphire backscattering monochromator at the Dynamics beamline P01 of PETRA III. Hyperfine Interactions, 237(1).
[5] M. Wieczorek et al., Advances X-ray Analysis 52, 193 (2009)
Funder Acknowledgement(s): John Grazul is acknowledged for help with crystal polishing. John Kopsa is acknowledged for cutting and grinding the Si crystals. Polishing was done in CCMR facilities in Duffield Hall. This work is based upon research conducted at the Cornell High Energy Synchrotron Source (CHESS) which is supported by the National Science Foundation under DMR-1332208. Special thanks to the Cornell High Energy Synchrotron Source for supporting the Summer Undergraduate Research in Science and Engineering (SUNRiSE) program.
Faculty Advisor: Stanislav Stoupin, sas456@cornell.edu
Role: I designed the Si crystals to be cut at the appropriate angles and created a Solidworks drawing to instruct the machinist on how to cut the Si boule. I assisted in the polishing process. I also verified the crystallographic orientation using Laue x-ray diffractometry after the cutting and polishing process. I modelled the monochromator set-up in Solidworks to assist in future project set-up.