Experimental observation of parametric X-ray radiation directed along the propagation velocity of relativistic electrons in a tungsten crystal

Parametric X-ray radiation (PXR) due to dynamic diffraction of relativistic electrons is experimentally observed at small angles to the propagation velocity of electrons in a tungsten crystal. The specific features of the experimental method are described, and forward PXR reflections from two crystallographic planes of tungsten are reliably measured.     

Advanced exact structure searching in large databases of chemical compounds

Efficient recognition of tautomeric compound forms in large corporate or commercially available compound databases is a difficult and labor intensive task. Our data indicate that up to 0.5% of commercially available compound collections for bioscreening contain tautomers. Though in the large registry databases, such as Beilstein and CAS, the tautomers are found in an automated fashion using high-performance computational technologies, their real-time recognition in the nonregistry corporate databases, as a rule, remains problematic. We have developed an effective algorithm for tautomer searching based on the proprietary chemoinformatics platform. This algorithm reduces the compound to a canonical structure. This feature enables rapid, automated computer searching of most of the known tautomeric transformations that occur in databases of organic compounds. Another useful extension of this methodology is related to the ability to effectively search for different forms of compounds that contain ionic and semipolar bonds. The computations are performed in the Windows environment on a standard personal computer, a very useful feature. The practical application of the proposed methodology is illustrated by several examples of successful recovery of tautomers and different forms of ionic compounds from real commercially available nonregistry databases.     

New mesoionic systems of the azolopyridine series. 1. Synthesis and structures of thiazolo[3,2-a]pyridinium 2-thiolates

A procedure was developed for the synthesis of representatives of the previously unknown bicyclic mesoionic thiazolo[3,2-a]pyridinium 2-thiolate system by the reaction of 2-X-N-phenacylpyridinium salts (X = Cl, SMe) with CS₂ in the presence of Et₃N. The three-dimensional structure of 3-(p-nitrobenzoyl)thiazolo[3,2-a]pyridinium 2-thiolate was established by X-ray diffraction analysis.     

On the Effect of Anomalous Photoabsorption in Parametric X Radiation

The parametric X radiation of relativistic electrons passing through a monocrystal is considered. It is demonstrated that, contrary to widespread opinion, conditions for the effect of anomalous photoabsorption to show up can be realized in the Laue scattering geometry. It is expected that the predicted effect will increase the spectral-angular density of the radiation several times.     

Anisotropy of torsional rigidity of sheet polymer composite materials

Wide application of polymer composite materials (PCM) in modern technology calls for detailed evaluation of their stress-strain properties in a broad temperature range. To obtain such information, we use the dynamic mechanical analysis and with the help of a reverse torsion pendulum measure the dynamic torsional rigidity of PCM bars of rectangular cross section in the temperature range up to 600 K. It is found that the temperature dependences of the dynamic rigidity of the calculated values of dynamic shear moduli are governed by the percentage and properties of the binder and fibers, the layout of fibers, the phase interaction along interfaces, etc. The principles of dynamic mechanical spectrometry are used to substantiate and analyze the parameters of anisotropy by which the behavior of a composite can be described in the temperature range including the transition of the binder from the glassy into a highly elastic state. For this purpose, the values of dynamic rigidity are measured under low-amplitude vibrations of the PCM specimens with a fiber orientation angle from 0 to 90°. It is shown that for unidirectional composites the dependence between the dynamic rigidity and the fiber orientation angle is of extreme character. The value and position of the peak depend on the type of the binder and fibers and change with temperature. It is found that the anisotropy degree of PCM is dictated by the molecular mobility and significantly changes in the temperature range of transition of the binder and reinforcement from the glassy into a highly elastic state (in the case of SVM fibers). The possibility of evaluating the anisotropy of composites with other reinforcement schemes, in particular, of orthogonally reinforced PCMs, is shown.Translated from Mekhanika Kompozitnykh Materialov, Vol. 35, No. 3, pp. 291–308, May–June, 1999.     

Coherence of Active Noise Interference in Radar Systems with Antenna Arrays

Radiophysics and Quantum Electronics - We analyze the coherence degree of active noise interference in radar systems. The entropy of the distribution of eigenvalues of the interference correlation...     

Photoluminescence of bulk Eu-doped GaN crystals

A study of the variation of photoluminescence spectra of bulk Eu-doped GaN samples revealed that the dopant can reside in the crystal in various charge states depending on the total defect concentration in the starting semiconductor host matrix. In crystals with the lowest concentration of shallow-level defects, the ion can exist only in one charge state, Eu³⁺. At higher concentrations of such defects, Eu can be observed in two charge states, Eu²⁺ and Eu³⁺. A rare-earth impurity was found to act as a getter of defects in the starting GaN matrix.