X-rays are used to study the atomic and microstructure properties of matter. Such studies are conducted with special accelerator complexes called synchrotrons. A synchrotron source generates powerful electromagnetic radiation with a wavelength equal to fractions of a nanometer. Some X-rays are reflected from the atomic planes of a crystal and some go through the crystal plane that plays the role of a beam-splitter (or the so-called semitransparent mirror). If the radiation passes through monochromators-optical devices that consist of two or more ideal crystals - its optimal exit wavelength can be regulated. The parameters of electromagnetic radiation depend on the material that the optical element is made of. By improving the properties of optical devices one can increase the quality and efficiency of X-ray research methods and use modern scientific unique megascience facility to their full potential.
Most modern-day X-ray optical elements are based on silicon and germanium crystals. However, they get heated under the X-ray radiation from a synchrotron source, and high temperatures cause their crystal lattice to change leading to the distortion of the reflected beam. Optical elements made of artificial diamonds provide better beam quality, as their coefficient of thermal expansion and thermal conductivity are higher than in silicon elements. However, lab-grown diamonds contain not only carbon but also nitrogen. This inconsistency creates tension in the crystal and leads to uneven distances between the atoms. The cut of a crystal mainly depends on its internal structure, and the distribution of growth sectors (the areas that are formed when layers of substance grow on top of each other) correlates with the placement of nitrogen atoms. On the borders of these growth sectors, stress fields are formed. When a crystal is grown artificially, it is extremely difficult to control nitrogen level and distribution. Therefore, historically, the quality of plates made of nitrogen-bearing diamonds had been considered low for them to be used in optical elements. A team from BFU, together with their foreign colleagues, managed to disproof this belief and to obtain plates with sufficient defectless areas.
The team used BARS, a unique device for the manufacture of ultrahard materials, to grow two synthetic diamond crystals at 1,500°? and under the pressure of over 50 thousand atmospheres. The obtained crystals had almost perfect atomic grids. Then, small bits were chipped off from the crystals, and thin plates were made from them. First, their quality was assessed using X-ray examination, and after that, the plates were studied using the high-resolution diffractometry method on a synchrotron source. After scanning the plates, the team obtained high-resolution rocking curves--the charts that helped them evaluate the structural perfection of the crystals.
"The deflection angle of a crystal towards radiation changes depending on the energy of the incoming beam and the plane that it reflects from. This angle is called the Bragg angle. We incline a crystal at this angle, reflected radiation hits a detector, and then we start rocking it. The rocking curve that we get shows the correlation between the intensity of the reflected radiation and the deflection angle of the crystal. Then we compare the rocking curve with a pre-calculated theoretical curve of a perfect crystal," said Anatoly Snigirev, the head of the International Science and Research Center "Coherent X-ray Optics for Megascience facilities", BFU.
Having analyzed the charts, the team concluded, that although the crystal plates had many imperfections along the edges, there were large clear areas in their centers that accounted for over 50% of the total plate. Given that the defects usually become visible during the cutting and polishing of diamonds, the potential use of nitrogen-bearing diamonds in X-ray optics depends on improving these processes. Diamond crystals are needed for manufacturing of different optical elements, such as monochromators, beam-splitters, interferometers, and refractive lenses.
The study was carried out jointly with colleagues from the V.S. Sobolev Institute of Geology and Mineralogy SB RAS (Russia, Novosibirsk) and the German Electron Synchrotron DESY (Germany, Hamburg).
We are grateful to Nataliya Klimova, a scientific consultant and a junior researcher at the International Science and Research Center "Coherent X-ray Optics for Megascience facilities", BFU, for her assistance in preparing this article.