No 8 (2023)

Polarized neutron reflectometry was used to study Dy/Gd superlattices with different ratios of Dy and Gd layer thicknesses: 1 : 1, 2 : 1, 3 : 1. It has been experimentally shown that the formation of helical magnetic ordering in Dy layers with a period incommensurate with the period of the superlattice appears as a magnetic superlattice reflection, which is forbidden for structural reasons at a ratio of the thicknesses of the Dy and Gd layers 1 : 1. Otherwise, the formation of helical magnetic ordering has little effect on the shape of the neutron reflectometry curves. Thus, the optimization of the structure of rare-earth superlattices for the neutron reflectometry experiment makes it possible to detect helical magnetic ordering in superlattices with a period incommensurate with the structural superlattice ordering.

Development of compact laboratory-scale neutron sources is of importance both for fundamental physical research and practical applications (for example, neutron radiography and spectroscopy). One of the most promising approaches to the development of such a source is the implementation of laser-plasma accelerated electrons or ions, and the subsequent initiation of nuclear reactions (γ,n), (p,n) or (d,n) with the emission of neutrons. In the present work, a neutron source produced via photodisintegration reactions (γ,n) using an electron beam from a one TW laser-plasma accelerator has been created and characterized. Maximum observed neutron flux was ~10⁵ neutrons/s · srad with a ~10⁶ neutrons per J of laser radiation efficiency. With constant efficiency and 10 times increase in the laser pulse energy the neutron flux will be sufficient for certain applications. Numerical Monte-Carlo simulations of neutron generation by an electron beam with parameters corresponding to those measured experimentally were also carried out. It was demonstrated that the number of generated neutrons can be used to estimate the charge and average energy of accelerated electrons. The obtained values are in good agreement with the values measured by the standard beam diagnostic tools.

Neutron-scattering methods for researching condensed matter allow obtaining information about the inner structure and dynamics of the sample under study. The creation of new high-fluxes neutron sources for studying of promising materials and various biological systems impose special requirements on the operating parameters of detector systems. New generation of thermal neutron detectors should have record-breaking characteristics in spatial and time resolution with high registration efficiency. The paper discusses the possibility of using and expected performance characteristics of ¹⁰B-RPC for thermal neutron detection as part of searching new solutions for the implementation of neutron scattering station detector systems.

The article describes a one-coordinate detector for diffraction experiments on a synchrotron radiation beam. The detector is being developed at the Budker Institute of Nuclear Physics SB RAS. Until recently the Institute was developing gas one-coordinate detectors, in particular a one-coordinate detector with calculated channels OD-3M, based on the technology of multi-wire proportional cameras. To provide a spatial resolution of better than 100 microns at photon energy in a wide energy range (3–30 keV), it is necessary to use solid-state microstrip or matrix sensors in combination with specialized integrated registration circuits. The developed SOCOD detector, using a microstrip sensor based on gallium arsenide as a recording element, operates in the mode of direct counting of photons with an energy of more than 3–4 keV and a speed of up to 1 MHz/channel. The article gives a general description of the current version of the detector, the block diagram of the recording channel, the software that allows users to control the operation of the detector and display the results obtained, and the developed algorithm for leveling the trigger thresholds in the channels. The results of electronic tests, the work of the alignment algorithm and their discussion are presented.

Сomposite solid lubricating coatings TiN–Pb with a thickness of ~2 μm were produced by co-sputtering of Ti and Pb cathodes of two separate magnetrons on titanium alloy VT6. The Pb content in the coating averages ~12 at. %. The inner layer is coating characterized by a uniform distribution of Pb, and the upper layer is characterized by the presence of islands with a high content of Pb. The coating structure is globular, predominantly containing nanometer-sized crystallites. The absence of a columnar structure of the coating is associated with a high content of Pb, which is insoluble in the TiN matrix and interrupts the growth of crystallites. X-ray diffraction analysis showed the presence of Pb, PbO, and TiN phases in the coatings. The diffraction lines are broadened, which indicates that the crystallite size is ~10–20 nm in the coating. Tribological tests of the TiN–Pb coating were carried out under conditions of low-amplitude friction – fretting wear in a wide range of loading parameters. In the full slip mode, a friction coefficient of ~0.25 is observed. During the transition from the full slip mode to the reciprocating slip mode, the energy dissipated during friction drops by more than three times, which is also reflected in a sharp decrease in the friction coefficient from 0.25 to 0.05.

In this work, we demonstrate photoluminescence from the CaF₂/Si multilayer structures formed on the surface of Si(111), Si(100), and SiO₂/Si(100) substrates at ambient temperature followed by annealing. The influence of the substrate structure on the photoluminescence spectra is discussed. Studies of the photoluminescence spectra of the multilayer CaF₂/Si structures have shown that the shape and position of the maxima of the photoluminescence spectra on different substrates are different, despite the fact that the structures are identical. The heterostructures differed only in the substrates, while the thicknesses and number of layers were the same. The photoluminescence spectra of the samples on the single-crystal Si(100) and Si(111) substrates are similar in the shape and have the similar wavelengths corresponding to the maximum of the photoluminescence spectra. The position of the wavelengths corresponding to the maximum of the photoluminescence spectra on the Si(100) and Si(111) single-crystal substrates correspond to the calculations obtained on the basis of the quantum confinement effect. At the same time, the shapes of the photoluminescence spectra on an amorphous silicon oxide layer differ sharply from the spectra on single-crystal substrates. The photoluminescence spectra of the samples on the amorphous SiO₂/Si(100) substrates have two maxima, and the more intense spectral line is shifted to the shorter wavelengths. It is assumed that the nucleation mechanisms of the silicon nanocrystals and their subsequent crystallization during annealing on the amorphous SiO₂/Si(100) substrates are radically different from the formation conditions for the silicon nanocrystals on the single-crystal substrates The different crystallographic structures of the surfaces of the three types of substrates create different conditions for the recrystallization during annealing and, therefore, lead to different properties of both the interfaces of these heterostructures and to different nanocrystalline structures of the silicon layers. Based on the obtained experimental data, a conclusion was made about the influence of the crystallographic structure of the substrates on the photoluminescence spectra.

The effect of excess free volume on the structure and crystallization of amorphous metal alloys is considered. Its change is an important characteristic of such alloys. Changes in the free volume during structural relaxation, aging, heat treatment, deformation, and irradiation are given. It is shown that the excess free volume fraction in the material depends on the alloy composition and the conditions for its production and changes under various external influences, which can contribute to both a decrease and an increase in the fraction. An increased fraction of excess free volume affects the physical properties, the evolution of the structure, and also contributes to the acceleration of the crystallization of the amorphous phase. The ability to control the free volume fraction in a sample opens up new ways to control the structure and, as a result, the properties of materials.