Physics of Atomic Nuclei最新文献

The melting front thickness of ice has been estimated from the measured kinetics of the melting of ice balls in air at room temperature (≈22°C) and in cooled water (( lesssim 1^circ {text{C}})) taking into account the temperature of the ice ball surface and the bulk temperature inside the balls. In both cases, the input heat is absorbed by ice in the form of the latent heat of fusion within the layer called the melting front. In order to describe the kinetics of the ice ball melting, we have developed a model with allowance for the heat transfer through the entire surface of the ice ball, which decreases during melting. The measured temperatures of the ice surface and the temperature inside the balls are approximately 0.4 and 0°C, respectively. The ice melting kinetics in cold water has been determined by means of the continuous measurement of the weight of the submerged ice ball (containing a lead ball frozen inside) as a function of time. In both cases (melting in air and water), the thickness of the ice melting front estimated from the fit the proposed model of experimental data is approximately 3 mm.

Strontium hexaferrite with the formula SrFe₁₂O₁₉ is prepared using citrate technology. A feature of this synthesis is a relatively low production temperature of 700°C. X-ray diffraction study reveals the single-phase state of the resulting material, due to the full correspondence of the positions of the diffraction maxima on the experimental XRD pattern and the positions on the reference XRD pattern of strontium hexaferrite from the ICDD database. A SEM image of the sample at a magnification of 50 000 testifies to the nanodispersed state of strontium hexaferrite particles. DSC reveals a Curie point of 450.9°C. Analysis of the hysteresis loops at 300 and 50 K indicates a magnetically hard material in a single-domain state and the growth of magnetic parameters at low temperatures, which slows upon cooling to 100 K. Studies are performed on a Rigaku Ultima IV diffractometer with CuKα radiation and a scanning speed of 2°/min, a JEOL JSM-7001F electron microscope with an EDS Oxford INCA X-max 80 energy-dispersive spectrometer, a Netzsch STA449C F1 Jupiter thermal analyzer upon heating to 600°C at a rate of 10°С/min in air, and a Quantum Design PPMS VersaLab vibrating magnetometer at temperatures of 300 and 50 K with an applied magnetic field of up to 3 T.

The results on the extraction of a proton beam of a varying intensity from the U-70 accelerator using bent silicon single crystals are presented. Stable beam extraction using crystals has been demonstrated for two experiments where smooth adjustment of the beam intensity over a large dynamic range has been required.

Universal features of dynamic failure and turbulence phenomena are considered on the basis of results of research and analyses of published data. The absorbed energy density causing the failure is comparable to the energy parameter of the crystal lattice E, which is fractions of electronvolt per atom in the range of nonequilibrium states t ~ 3 × 10^–6–10^–10 s. Macrofailure, changing the body connectivity, arises when approaching the critical density of the failure center cascade, i.e., percolation cluster, through the cascade of bifurcations. Fully developed turbulence is a chaotic dynamics, which is related to the stochastic instability of the transition through a sequence of period-doubling bifurcations, through intermittency, etc. At present, a strict mathematically grounded theory of dynamic failure phenomena of condensed matters and turbulence theories are absent, which implies the application of scaling relations and phenomenological approaches for their description. On the basis of design-theoretical studies and analysis of published data, it has been shown that phenomena of dynamic metal failure and fully developed turbulence are analogous, meaning that these processes have close values of critical exponents and belong to one universality class.

This article deals with fuzzy and fuzzy PI controllers for the automatic power control system of a linearized mathematical model of the VVER-1200 nuclear reactor. The automatic power control system includes a mathematical model of a step-by-step electromagnetic drive (SED), a model of in-core and ex-core neutron flux sensors, and a refined mathematical model of the group 12 of control and protection system of control rods (CPS CR) obtained by approximating experimental data using the Levenberg–Marquardt algorithm for the nonlinear least squares problem. Based on the state space model, ten transfer function matrices of the nuclear reactor corresponding to a power range of from 10 to 100% of the nominal power were determined, and ten classical PI controllers were designed to ensure stability margins of at least 60° in phase and at least 10 decibels in amplitude for each power level of the plant. Simulation results show a significant advantage of the developed fuzzy controllers both in the steady-state power maintenance mode considering noise in the neutron flux sensor channel and in the load-following mode at different power levels.

An analysis is performed of the effectiveness of using spherical gold nanoparticles (AuNPs) with a size of 50 nm in combination with proton irradiation at a dose of 30 Gy (energy of 150 MeV) in order to enhance the cytotoxic effect of binary therapy. The analysis is performed with in vivo experiments using the model invertebrate animal Daphnia magna. The biodistribution of the AuNPs in the animal body is studied, and the biochemical effect of the separate and combined action of AuNPs with irradiation is analyzed. The accumulation of AuNPs in the intestine and the eggs and embryos of the next generation is detected. Analysis of the level of free radicals and malondialdehyde reveals an increase in the cytotoxic effect of irradiation with nanoparticles.

The influence of an ultrafine-grained tungsten structure with an average grain size of 300 nm on the formation of blisters on the surface under high-fluence irradiation by 30-keV He⁺ ions has been studied. Fine-grained tungsten has been used for comparative studies. The microstructure and surface morphology of the samples have been investigated.

The design and characteristics of the detection unit of a gaseous medium spectrometer for measuring gamma rays from inert radioactive gases formed in nuclear power plant reactors and entering the environment as a part of gas-aerosol emissions are described. Gamma rays from inert radioactive gases are detected by one scintillation detector based on a 51 × 51-mm CeBr₃ crystal in two chambers with a volume of 10⁴ and 2 cm³. The energy range of the spectrometer is 0.05–3.0 MeV. Switching of gas inputs and outputs allows both simultaneous and alternate measurements in both chambers, which ensures the calculated dynamic range of measured inert radioactive gas activities from 10³ to 10¹³ Bq/m³ with a relative measurement error of 50% in the nuclide emission lines for time intervals from 1 to 500 s, depending on the activity.

In the PAMELA experiment, precipitation of electrons from the Earth’s radiation belt was detected at moments of recording gamma ray bursts of extraterrestrial origin, leading to a hypothesis about the relationship between these phenomena. This work provides estimates of the number of electrons that have interacted with gamma quanta through the Compton effect and changed their energy and trajectories in the so-called toy model approximation. A formula is obtained for determining the cross section of the interaction between a gamma quantum and a stationary electron, depending on the angle of electron emission and the energy spectrum of the emitted electrons. A narrow peak is observed near the maximum energy in the energy distribution of secondary electrons, which is close to the energy of the initial gamma quantum. An estimate is obtained for the upper limit of the contribution from the considered process to the excess of the electron count rate over the background value recorded in the PAMELA experiment. It is found that the proposed mechanism does not explain the observed effect because the calculated electron count rate is several orders of magnitude lower and can be explained by the small Compton scattering cross section.

The prospect of creation of the accumulator of energy on the basis of nuclear isomer ^186mRe (a half-life of 2 × 10⁵ yr) does actual a question of the choice of conditions of natural rhenium irradiation by reactor neutrons for production of big amounts of the isomer. Calculations of dependence of the isomeric ratio, known in literature, at formation of ¹⁸⁶Re nuclei in ground and isomeric states show insignificant difference in efficiency of production of isomer by thermal neutrons or in active zone of reactor. This conclusion has been confirmed experimentally with an accuracy up to 20% earlier. In the present work, a method of more exact experimental check of results of the isomeric ratio calculation is suggested.