Moscow University Physics Bulletin最新文献

An approach employing the transmittance matrix has been implemented to calculate the intensity of Bragg reflections in resonant X-ray diffraction. This enables the inclusion of anisotropic effects that lead to a change in the polarization of radiation during propagation. Using the example of processing the spectra of forbidden reflections in an iron orthoborate crystal, performed with a program based on this approach, an improvement in the agreement between calculated results and experimental data is demonstrated in comparison with standard methods.

The problem of the sound field of a point sound source placed in a nonideal marine waveguide is considered. The boundaries of the waveguide are assumed to be plane-parallel, and three layers are distinguished: a water layer with an inhomogeneous refractive component of the sound speed, a layer of liquid sediments, and an elastic bottom layer on a rigid base. At the top, the water layer adjoins a homogeneous air half-space. For the given waveguide model, analytical expressions for the impedance functions at the boundaries of the water layer are obtained for the first time, which makes it possible to construct the Green’s function of the vertical boundary-value problem. Moreover, exact values for the residues of the Green’s function are found, which allows for writing down an exact analytical representation for the acoustic pressure potential in the form of a sum of normal modes. It is shown that, in this model, the influence of the continuous spectrum (integrals along the branch cuts) is small even at short distances from the source. As the thickness of the elastic layer decreases, the discrete spectrum of the problem tends to the values for the classical waveguide; with sufficiently large thickness and attenuation, it resembles the discrete spectrum of the Pekeris elastic waveguide. Examples of numerical implementation for waveguide parameters typical for the Black Sea region are given.

In this work, the main characteristics of the crystal–homogeneous phase transition in argon are calculated on the basis of the correlation cell-cluster expansion. Along the crystallization and melting line, the heat capacity at constant pressure is calculated for the crystalline phase and for the homogeneous phase. A monotonic decrease of the heat capacity at constant pressure with increasing temperature is observed. A comparison between theoretical data and experimental results is carried out, showing good agreement.

This paper presents an analysis of spring atmospheric blockings (SAB) in the Northern Hemisphere (NH) for the period 1992–2023. The results of the analysis indicate a significant increase in blocking activity in the NH atmosphere in recent decades. This trend is observed against the background of a general increase in near-surface temperature in the NH during the spring seasons. The following SAB parameters were used in the study: quantity, average duration, total duration, intensity, and size. Statistically significant increases were obtained for the total duration, quantity, and size of SAB, associated with an increase in the average spring near-surface temperature in the NH.

Within the framework of parameterized post-Maxwellian vacuum electrodynamics, the propagation of an X-ray and gamma radiation pulse through the electromagnetic field of a relativistically rotating pulsar is investigated. Expressions are obtained for the trajectory of this pulse and for the law of its motion from the point (mathbf{r}_{s}={x_{s},y_{s},z_{s}},) where at the moment of time (t=t_{s}) an X-ray or gamma-ray burst occurs, to the point (mathbf{r}_{d}={x_{s},y_{s},z_{d}},) where the detector of this radiation is located. In the case when the post-Maxwellian parameters of the theory differ, (eta_{1}neqeta_{2}), the time of nonlinear electrodynamic delay of electromagnetic signals carried by different normal modes is calculated. The change in the polarization state of the X-ray or gamma radiation pulse after passing through the electromagnetic field of a relativistically rotating pulsar is analyzed. The possibilities of observing the nonlinear effect of the electromagnetic field of a relativistically rotating pulsar on X-ray and gamma-ray pulses are discussed.

Simulation of measurements of the scattering characteristics of a reference object in the form of a flat disk in a low-frequency tapered anechoic electromagnetic chamber with a lens has been carried out. Two lens collimators are considered: a conventional hyperbolic lens and the same lens coated on both sides with a matching layer optimized for the frequency of 600 MHz. The contribution of the nonuniformity of the field distribution in the quiet zone of the chamber to the measurement accuracy of the radar cross section of the object has been investigated. It is shown that the collimator with the lens coated with the matching layer provides higher accuracy in measuring the radar cross section of the disk in the side lobes of the scattering pattern due to providing lower nonuniformity of the field amplitude distribution in the quiet zone of the chamber. The influence of re-reflections between the lens and the disk on the accuracy of its radar cross section measurements has also been investigated. It is shown that the lens coated with the matching layer reduces the effect of re-reflections not only at the frequency of matching but also in a sufficiently wide frequency range.

A criterion has been developed for detecting anthropogenic debris floating on the water surface during water column profiling using a pulsed ultraviolet laser with a sounding radiation wavelength of 355 nm. The criterion has been applied to lidar sounding data of the near-surface sea layer from several expeditions in the Black Sea. The conditions for the applicability of the proposed marine debris detection criterion are formulated. A comparison has been carried out between the results obtained using this criterion and visual observation data. The potential for using the criterion to assess the scale of pollution in the World Ocean water areas is demonstrated.

The paper presents a systematic analysis of physical factors influencing the development of secondary malignant neoplasms in patients who have undergone radiation therapy. Based on clinical data, animal experiments, and epidemiological studies (Hiroshima, Chernobyl), key aspects are considered: the biological effects of ionizing radiation, age- and gender-related differences (increased risks in children and women), localisation of the primary tumor (breast, prostate, head, and neck), as well as radiation therapy methods (photon, proton). Special attention is paid to the role of secondary particles (neutrons, protons) and medical imaging (CT, PET), which increase the cumulative radiation dose. Mathematical models for risk assessment are discussed, taking into account dose, fractionation, and tissue radiosensitivity. The results highlight the need to optimize radiation therapy methods, limit the frequency of diagnostic procedures involving ionizing radiation, and develop personalized approaches to reduce long-term risks.

The statistical distributions of output signals and their derivatives have been investigated for a superconducting bolometer exposed to weak photon pulses at a frequency of 1 THz. The terahertz pulses were generated via strongly nondegenerate parametric down-conversion in a LiNbO({}_{3}) crystal cooled to 4.8 K, under conditions of pumping the crystal with laser pulses of 28 ps duration. Various scenarios of the superconducting bolometer’s response to excitation by a small number of terahertz photons are discussed.

A pump-probe experimental system was developed that delivers single-cycle terahertz (THz) pulses with energies of up to 10 (mu)J optically synchronized with laser pulses in the near-infrared range (1030 nm) with a duration of 25 fs. The THz pulses were generated in two-color laser-induced plasma in atmospheric air with mid-infrared laser radiation (3.9 (mu)m wavelength) and its second harmonic with a controlled phase difference between them. We measured the waveform and polarization of the THz pulse, and the size of the THz beam focal waist. The ability to switch the orientation of the THz electric field vector to the opposite direction was demonstrated. The developed experimental pump-probe setup can be used to study the properties of materials exposed to THz pulses with peak field strength of several MV/cm.