Journal of Surface Investigation: X-ray, Synchrotron and Neutron Techniques最新文献

Multiwire position-sensitive neutron detector with two layers of boron-10 has been developed to detect both thermal and fast neutrons. Sensitive dimensions of two coordinate neutron detectors are 50 × 50 mm. New detector characteristics are compared with those of a 100 × 100 mm detector built earlier which we used in neutron flux spatial distribution measurements. Plane-parallel design of the new detector has symmetrical structure with respect to wire anode and also includes two intermediate grids and two cathodes made of parallel wires with 2 mm pitch and two silicon substrates coated with boron-10 layers of 0.003 mm thickness. Detector geometry and working gas mixture and pressure are chosen so as to ensure full absorption of secondary alpha particle from reaction with thermal neutron within detector gas medium half thickness. Neutron coordinates are determined from measured ionization loss pulse heights produced by secondary nuclei. The detector expected efficiency to thermal neutrons is about 5%. The detector can be used in small-angle and diffraction scattering setups in condensed matter physics.

Neutron scintillation detectors based on ZnS(Ag)/LiF, solid-state photomultipliers, and an organic glass light guide developed at the Institute for Nuclear Research, Russian Academy of Sciences (INR RAS) are successfully used in neutron diffractometers facilities at the INR RAS as a replacement for standard counters based on ³He. These detectors use optical light guides with diffuse reflection, which makes it possible to multiply the recorded signal (up to 95 photoelectrons) in comparison with detectors with wavelength shifting fibers. The article describes two types of bias circuit for silicon photomultipliers. A method of dynamic bias has been proposed, which makes it possible to reduce the recovery time of a silicon photomultiplier and to increase the loading capacity of neutron detectors by a factor of 8. Simulation and comparison of two types of preamplifiers showed an increase in the loading capacity. The new electronics makes it possible to increase the loading capacity of the detectors up to 400 kHz. A circuit for digital control of discrimination thresholds has been developed and described. A new data acquisition system for time-of-flight neutron diffractometers for 80 detectors with the possibility of scaling has been developed.

An analytical theory of the reflection of light ions from solids is presented. The theory is based on the method of solving the elastic scattering problem (the Oswald–Kasper–Gauckler method), successfully tested in the theory of electron scattering. The solution of a boundary value problem for light ion reflection from solids based on the invariant imbedding method is constructed. Particle interaction with amorphous and polycrystalline samples is considered. Analytical formulas for calculating the integral reflection coefficients of particles and energy are obtained. It is shown that an analytical solution can be obtained only within the framework of a small-angle approximation. The obtained analytical solutions are based on the path length distribution function taking into account the maximum residual range. It is demonstrated that within the framework of the analytical theory the reflection coefficients are determined by two dimensionless parameters: the ratio of the residual range to the transport path length and the screening parameter. The results of theoretical consideration are compared with the data of computer simulation. Numerical calculations are performed for the case of reflection of protons with initial energy E₀ = 1–10 keV from Be, C, Cu, and W targets for different scattering geometries. The results of the calculated integral reflection coefficients of particles and energy show satisfactory agreement between analytics and computer simulation.

The densities of electronic states in quasi-two-dimensional vanadium nitrides have been studied using density functional theory and the method of the crystal orbital Hamilton population. The contribution of various orbital pairs and their influence on the stability of the magnetic subsystem of these compounds have been analyzed using the crystal orbital Hamilton population (COHP) algorithm. The calculation results and their analysis suggest that the formation of long-range magnetic order plays a role in the structural stabilization of magnetic quasi-two-dimensional transition metal nitrides. Comparing COHP curves for different vanadium nitrides shows that the nitrogen stoichiometry in V_xN_y compounds affects the electronic properties and the nature of the chemical bond during the transition to the ferromagnetic state. Calculation data and total energies prove the structure-stabilizing effect of long-range magnetic ordering in quasi-two-dimensional vanadium–nitrogen compounds.

Using the methods of secondary ion mass spectrometry, elastic peak electron spectroscopy, and Auger electron spectroscopy, the elemental and chemical composition of the surface and concentration profiles of the distribution of atoms over the depth of silicon implanted with ({text{O}}_{2}^{ + }) ions with energy E₀ = 1 keV at a dose of D = 6 × 10¹⁶ cm^–2 were studied. It was found that oxides and suboxides of Si (SiO₂, Si₂O, and SiO_0.5) were formed in the ion-doped layer, which also contained unbound O and Si atoms. Post-implantation annealing at 850–900 K led to the formation of a stoichiometric SiO₂ layer ~25–30 Å thick.

An impact mill has been developed to produce powders from shavings of refractory metals using the impact grinding method for reuse in electrometallurgy in devices with screw feed, for example, in 3D printers. The proposed device provides high uniformity of grinding with a minimum content of dust fraction and impurity content at low technical and economic costs. The result is achieved using a Laval nozzle, which operates in the supersonic jet formation mode. In the area of the first Mach disk, there are rod fenders arranged in a cascade, and the impact plate is located in the turbulence zone and is equipped with winglets with holes for separating crushed metal.

To impregnate graphite of the GMZ brand with liquid aluminum alloy D16 a high-pressure method and apparatus previously used for impregnating carbon frames with liquid coal pitch in the isostatic technology for the production of carbon–carbon composite materials were used. The graphite blank and the amount of aluminum alloy calculated from the initial porosity of graphite were placed in a thin-walled container and degassed in a vacuum furnace, and the container with the contents was sealed. Thermobaric treatment was carried out at a temperature of 750°C and pressure of 100 and 200 MPa. After finishing the treatment, the density and porosity of the obtained metallographic composites, as well as their compressive strength, were determined. As a result of these thermobaric treatments, the density and compressive strength of the obtained composites increased significantly while the porosity decreased markedly.

Using an up-to-date full molecular dynamics model of sputtering of single crystals and taking into account the incidence of ions on the surface, the peculiarities and mechanisms of atom sputtering during bombardment of the (001) Ni face with 200 eV Ar ions are studied for two target temperatures. It is shown for the first time that with an increase in the energy of sputtered atoms not only the maximum of their polar angular distribution in the azimuthal direction toward the Wehner spots but also the maximum of the polar angular distribution integrated over the azimuthal angles shifts first toward the normal to the surface and then in the opposite direction. The polar distribution integrated over the azimuthal angles is formed by atoms ejecting from the surface at much larger polar angles than that in the final (observed) distribution. Both effects are explained in terms of the surface mechanism of single crystal sputtering.

A theoretical study of new polymorphic varieties of boron nitride, which have diamond-like structures with boron and nitrogen atoms in equivalent structural positions, was carried out. The model construction of new phases of boron nitride was performed in the process of crosslinking of precursor nanostructures. Single-walled boron nitride nanotubes with chirality indices (3;0), (4;0), and (6;0) were chosen as precursors for the model construction of diamond-like phases. Using the density functional theory method in the generalized gradient approximation, the possibility of stable existence of three new structural varieties of boron nitride with a diamond-like structure, namely, BN-TA4, BN-TA5, and BN-TA6, was established: the structure of the BN-TA7 diamond-like phase turned out to be unstable and, in the process of geometric optimization, was transformed into the initial structure, a boron nitride nanotube (6;0). As a structural characteristic, the bulk density of new polymorphs was determined to be in the range from 2.613 to 3.0836 g/cm³. The sublimation energy of new polymorphic varieties ranged from 17.16 to 17.63 eV/(BN). The value of the band gap near the Fermi energy varied from 5.37 to 5.74 eV.

The capabilities of lead slowing-down neutron spectrometry are demonstrated. The paper reviews the results of a series of studies on the measurement of the fission cross sections of curium isotopes ²⁴³Cm, ²⁴⁴Cm, ²⁴⁵Cm, ²⁴⁶Cm, ²⁴⁷Cm, and ²⁴⁸Cm by neutrons with energies below 100 keV. These studies were carried out by a joint group of researchers from the Institute for Nuclear Research, Russian Academy of Sciences (INR RAS), and the State Scientific Center of the Russian Federation, Leypunsky Institute of Physics and Power Engineering (SSC RF LIPPE) using an SVZ-100 lead slowing-down neutron spectrometer. This third-generation spectrometer possesses high luminosity, enabling the investigation of neutron-nuclear processes in microgram-scale samples of radioactive nuclides, which is unattainable in experiments using time-of-flight spectrometry. The results of research at the INR RAS and the SSC RF LIPPE are reflected in international nuclear data repositories, supplementing known experimental data and indicating, in several cases, the need for adjustments to recommended values.