Journal of Contemporary Physics (Armenian Academy of Sciences)最新文献

The work presents a study of the parameters of secondary X-ray radiation generated during the operation of a sterilization setup based on a high-current pulse-periodic electron accelerator, SINUS-320. The energy of accelerated electrons equals to 500 keV. Therefore, the issues of radiation protection during the operation of the installation are highly topical, which results in the importance of studying the energy spectrum and dose rate of the secondary X-ray radiation. The studies were carried out by statistical simulation using the GEANT4 toolkit.

Based on a detailed numerical analysis and comparison with experimental observations, a nonlinear process of the generation of a new type of similaritons in fibers is presented in the region of negative dispersion. These similaritons are needle-like in shape with a sharp top and a large base and are formed in passive single-mode optical fibers with weak nonlinearity and strong group velocity dispersion. Consistent boundary conditions are found for the parameters of the input pulse and the characteristics of the optical fiber under which such self-similarly propagating pulses are generated.

The possibility of improving the quality of X-ray images formed using diffraction on a two-block crystalline system is considered. It is shown that the low spatial coherence of the initial radiation improves imaging quality. Conventional wide-focus X-ray tubes can be used as a source for such radiation. The feasibility of using a scanning scheme to reduce the background level in the formed image was also considered by numerical simulation.

The oscillatory behavior of a heavy substitutional impurity (Cr³⁺) in Al₂O₃ has been studied by the Brillouin scattering method. The oscillations of chromium ions are carried out by their photoexcitation. Discrete shifts of the ruby luminescence lines R₁ and R₂ at room temperature depending on the absorbed laser pump power upon excitation of the ²T₁ state observed for the first time. These shifts are caused by the excitation of quasi-local vibrations of chromium ions. The frequency of quasi-local oscillations is equal to 22 GHz, which corresponds to transverse acoustic oscillations in ruby. An explanation of the observed phenomenon is given based on the Born–Oppenheimer adiabatic approximation and the Pekar function.

Below, we theoretically study the generation of narrowband terahertz (THz) radiation by optical rectification in an artificial periodically poled lithium niobate (LN) crystal with a wide aperture. The latter is formed by placing a multi-slit phase mask in front of a conventional (single-domain) LN crystal. It is shown that the bandwidth of THz generation can be varied over a wide range by changing the linear size of the pump beam in the crystal. In addition, it is possible to tune up the generation frequency in the range of 0.4–0.8 THz by building a mask image in the crystal with different magnifications. According to estimates, the energy of narrowband THz pulses at a frequency of 0.5 THz in a stoichiometric LN crystal at a temperature of 100 K is about 265 μJ for a pump pulse energy of 220 mJ.

The SiO₂/ZnO nanocomposite with a core-shell structure for photocatalysis from water-soluble zinc salts and sodium silicate was synthesized using the hydrothermal-microwave method. The physicochemical properties of the synthesized SiO₂/ZnO were studied and its photocatalytic activity was tested. The band gaps of the heat-treated composite (E_{{text{g}}}^{{{text{dir}}}}) and (E_{{text{g}}}^{{{text{indir}}}}) are 3.35 and 3.32 eV, respectively. The photocatalytic activity of the resulting SiO₂/ZnO was determined by the decomposition reaction of methylene blue under UV irradiation. The conversion of methylene blue was determined by optical method. The resulting SiO₂/ZnO has high photocatalytic activity. The conducted studies showed the effectiveness of microwave synthesis of SiO₂/ZnO with a core-shell structure in comparison with traditional methods.

The vacuum expectation value of the current density for a massive scalar field in a (D + 1)-dimensional flat space-time with compact dimension is investigated. Along a compact dimension, the field operator obeys the helical quasiperiodicity condition with a constant phase. The results previously studied in the literature are obtained in the special case of a zero helicity parameter. In contrast to this case, the vacuum current, in addition to the component along the compact dimension, has a non-zero component in the direction of the non-compact dimension along which the field operator is shifted. A simple relation between these components is obtained. They are odd periodic functions of magnetic flux penetrating the compact dimension, with the period equal to the magnetic flux quantum. For a given non-zero value of the helicity parameter, the component of the vacuum current along the compact dimension tends to zero in the limit when the length of that dimension tends to zero. In the problem considered earlier with the usual quasiperiodic condition, the vacuum current diverges in that limit.

This article provides data on the development and creation of a vacuum window module, which will bring out the proton beam of the C18 cyclotron (IBA, Belgium) with energy of 18 MeV from a vacuum environment into the atmosphere. The module is made of aluminum and consists of a flange with a collimator and a helium (He) chamber of foil plates cooling. The module was installed at the end of the cyclotron’s beam line. Profile measurements were made for various current values of proton beam from 1 to 30 µA. The vacuum window showed its functionality.

The use of a drone as an aerial vehicle to realize the antenna measurement method is considered. The drone is controlled by software, using unified radio-electronic equipment containing a transmitter, a half-wave vibrator, and a GPS navigator. The parameters of the directional properties of the antenna system are the result of software processing of the output signals of the radar receiver synchronized with the coordinates of the drone flight path points.

Using computer simulation, the processes of heat propagation were studied and noise was determined in a three-layer detection pixel of a thermoelectric single-photon detector, consisting of an absorber (Pt), a thermoelectric sensor (La_0.99Ce_0.01B₆) and a heat sink (Pt, Mo). The absorption of photons with energies of 0.8–7.1 eV (1550–175 nm) in absorbers of various thicknesses, providing high absorption efficiency, was studied. The simulation was based on the heat propagation equations from a limited volume. The temporal dependencies of the signal appearing on the sensor were studied. Signal power, noise equivalent power, and signal-to-noise ratio were determined. It was shown that a detection pixel with a platinum absorber can reliably detect single photons with the energy of 3.1–7.1 eV.