Methods of synchrotron radiation monochromatization (research review)

OBRABOTKAMETALLOV MATERIAL SCIENCE Vol. 26 No. 3 2024 at the SRS can be treated as a kind of “powerful microscope”. The spectral range of synchrotron radiation energies is very wide (from 10 eV (and less) to 100 keV (and more)). At the same time, different methods of research realized at SR sources require using photons of different energies (or wavelengths). Accordingly, researchers need to select from a wide spectrum of radiation that part of it which is the most important for the technique. In most cases, the range of X-ray radiation (XR) should be highlighted from the broad spectrum of SR. The wavelengths are comparable to the size of atoms, which allows researchers to analyze the atomic crystalline structure of solids, the near-order of liquids and amorphous objects. Radiation corresponding to the X-ray range of electromagnetic waves is characterized by energy values ranging from 1 to 100 keV. A “white” beam (i.e., radiation with a wide range of wavelengths) emerges from the insertion devices of the storage ring in which a stream of elementary particles, usually electrons or positrons, circulates. However, for most experiments, there should be a beam with a “narrower” range of parameters necessary to solve the problems set by a researcher. In most cases, monochromatic radiation is used at beamlines, and its formation is provided by special devices called monochromators. Those located at beamline together with slits, filters, focusing systems form radiation with the required characteristics. From a technical point of view, monochromators are one of the most complex and high-tech devices for beamline. The production of monochromators belongs to the critical technologies that ensure the effective use of SR for studying the structure of materials. As a rule, the main unit of a monochromator is a pair of crystals that allow to extract from the whole SR spectrum a diffracted beam corresponding to a narrow band of wavelengths and to direct it to a specimen. Passing through the monochromator, the incoming beam which includes the entire spectrum of radiation generated by the insertion device is converted into monochromatic or “pink” radiation. These types of radiation differ from each other by the degree of monochromaticity which is understood as the ratio Δλ/λ, where λ and Δλ are the peak value of the wavelength and spectral width of the radiation that has passed through the monochromator, respectively. Synchrotron radiation corresponding to the ratio Δλ/λ = 10-4÷10-3 is called monochromatic [1]. To solve some problems, “pink” radiation is also used, its degree of monochromaticity is Δλ/λ = 10-2÷10-1 [2]. When conducting experiments with a “white” beam, a monochromator is not necessary. Thus, for example, the Laue method assumes the exposure of a fixed single crystal to “white” (continuous) radiation. The presence of a wide range of wavelengths in the X-ray spectrum makes it possible to fulfill the Wolf — Bragg condition, i.e., to manifest the effect of diffraction of X-rays. If it is a question of conducting experiments by methods associated with using “pink” and monochromatic radiation, various types of monochromators are used; its features are discussed in this paper. The principle of monochromators operating is based on diffraction of X-rays. The features of diffraction on crystals were described by Bragg and Wolf in 1913. Based on the condition now called the Wolf – Bragg law, “white” radiation hitting a crystal can be decomposed into beams characterized by a narrow band of wavelengths. Depending on the technique and the objectives, different wavelength ranges may be required for experiments. According to the Wolf – Bragg law, extracting a given wavelength (and thus photon energy) requires a certain angle of incidence of radiation on the crystal, which is controlled by the goniometer, one of the most important mechanisms of a monochromator. In addition to the goniometer, which allows a monochromator to be adjusted to different energy levels, the device includes such elements as vacuum pumps, a cooling system and sensors that ensure the operation of all devices. While designing and subsequent operating a monochromator, it is important to have a quantitative understanding of the intensity and brightness of the beam, which are directly dependent on crystal positioning, technical errors and deviations specific to the instrument. In addition, it is important to understand the properties of the radiation source (in the context of this paper, the source is understood as rotating magnets or insertion devices located on the storage ring of the synchrotron). Fig. 1 shows the schematic diagram of a specialized SRS [3]. It consists of such elements as an electron gun based on the thermoelectron emission effect (1), a linear accelerator of electron (linac) (2), a booster (3), bending magnets (4), radiofrequency resonators (5), insertion devices (undulators, wigglers) (6), a beamline (7), a front-end (8), an optical hutch – the first room with optical devices (9), and an experimental hutch (10). The electron gun (1) emits electrons and leads it to the linear accelerator (2), where the particles are accelerated according to the resonance principle, passing through the gaps of a high-frequency electric

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