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

In the first part of this work, using the found vector Green’s function, the electromagnetic fields in an arbitrarily chosen volume are presented in vector form using surface and volume integrals, at which the surface integrals describe the diffraction of the field of charges external to the volume, and the volume integrals describe the fields of the charges, moving within the volume. One of the two found forms is used for the Green’s function. In this work, expressions for the electromagnetic fields are obtained using the second form of the found Green’s function. When using the second form of the vector Green’s function, the fields of charges moving inside the volume are expressed in terms of the generalized Hertz vector. The expressions obtained in this work have their advantages for determining the fields in a selected volume.

Fraunhofer diffraction of an electromagnetic wave by a slit on an opaque screen located between a vacuum and an anisotropic medium with an open wave vector surface (WVS) is considered. A formula for diffraction in a material medium is obtained, including the case of diffraction into a uniaxial anisotropic medium in the absence of absorption, and diffraction minima and maxima formation is studied.

At the linear electron accelerator AANL (YerPhI), at mean electron energies Ê_е = 28 and 30 MeV (with a Gaussian width σ_е ≈ 0.7 MeV), an attempt is undertaken to search for near-threshold production of the four-neutron system in the reaction ²⁰⁹Bi(γ,4n)²⁰⁵Bi (with the threshold energy (E_{gamma }^{{{text{th}}}}) = 29.5 MeV) induced by bremsstrahlung photons. The induced activity method was applied in the experiment. The gamma-spectroscopic measurements were carried out in the underground low background laboratory of the AANL. Owing to low-background conditions in the laboratory, one achieves, at Ê_е = 30 MeV, reliable identification of ²⁰⁵Bi radionuclide and the first determination of the near-threshold cross sections of this rare process, namely, the weighted by the spectrum of bremsstrahlung photons cross-section σ_w = (4.42 ± 0.48) × 10^–4 mb and the averaged over the spectrum of bremsstrahlung photons cross-section (leftlangle sigma rightrangle ) = (1.95 ± 0.22) mb. No ²⁰⁵Bi production was observed at Ê_е = 28 MeV. Low-background conditions allow one also to identify the rare decays of the ²⁰⁸Bi nucleus produced in the reaction ²⁰⁹Bi(γ, n)²⁰⁸Bi, and measure its weighted and averaged cross sections at Ê_е = 28 and 30 MeV: σ_w = (19.6 ± 3.5) and (16.4 ± 2.2) mb and (leftlangle sigma rightrangle ) = (139 ± 25) and (142 ± 19) mb, respectively. From the data at Ê_е= 28 MeV, an upper limit of the yield of a hypothetical four-neutron bound state (tetraneutron) production reaction relative to the yield of the reaction ²⁰⁹Bi(γ, n)²⁰⁸Bi is estimated to be 10^–5. The obtained experimental data are compared with predictions of the TALYS1.9 and FLUKA models. It is shown that the measured at Ê_е = 30 MeV cross-section of the reaction ²⁰⁹Bi(γ,4n)²⁰⁵Bi strongly overestimates (by more than one order of magnitude) the theoretical value. This disagreement can be caused by the fact that the models do not foresee the production of correlated neutron clusters, in particular, the four-neutron resonance state with 2.4 MeV excitation energy observed recently in the reaction ¹H(⁸He, ¹H + ⁴He)4n. The near-threshold production of the four-neutron system, observed in the present work, can, probably, serve as an indirect indication of its resonance nature.

A prototype PIN-photodiode-based electron system for flux measurement of the AREAL accelerator electron beam (energy up to 5 MeV) was developed and tested. The system can be eventually used to measure beam losses from the vacuum chamber of the SASE100 undulator, which is intended for the generation of radiation in the terahertz range and will be installed in the AREAL accelerator tract during its modernization. The method of using the PIN-photodiodes as a beam loss monitor is based on the effect of electron–hole pairs formation when ionizing particles pass through the photodiode barrier layer. Calculations of the interaction of electrons with the substance of the barrier layer are performed using the PCLab program. The experiments carried out on the accelerator electron beam showed that the developed system can effectively register the electron fluxes of both the main beam of the AREAL accelerator and its dark current.

New and simple expressions are obtained to quickly assess the absorption coefficients of longitudinal and transverse hypersonic waves in lithium niobate single crystals in the Akhiezer regime. It is shown that the attenuation coefficients of hypersonic waves at room temperature computed from the obtained expressions and the values determined experimentally are in good agreement.