Configurations of the zero-birefringence and zero-dichroism lines of electromagnetic waves in weakly anisotropic crystals

Configurations of two types of lines—those of zero birefringence and zero dichroism—drawn by the end of the directional vector of the wave normal over the unit sphere, which correspond to the coincident phase velocities or the absorption coefficients of isonormal electromagentic waves in weakly anisotropic crystals, have been studied analytically. It is shown that under the conditions of weak dielectric anisotropy, even slight changes in anisotropy result in the dramatic deformation of the lines of both types. The corresponding evolution of the configurations with the change of the anisotropy parameters is considered by example of absorptive orthorhombic crystals.     

Properties of nonlinear elastic waves propagating along acoustic axes

Amplitudes and polarization of nonlinear elastic harmonics exited along an arbitrary acoustic axis of the conic type have been studied as functions of the rotation angle of the polarization vector of the degenerate reference wave. When the reference polarization is rotated by an angle of π, the resulting rotation angle for the polarization vector of the second harmonic turns out to be equal to either zero or 2π or ?2π. The actual type of the behavior of the second harmonic propagating along the given acoustic axis is determined by an algebraical criterion for elastic moduli of the second and the third order for the medium. For polarization of the reference wave, we determined the orientations corresponding to the maximum and minimum for the amplitude of the second harmonic. Similar results were obtained for different cases of collinear interactions between degenerate waves, in particular, for the excitation of longitudinal harmonics. The possible control of nonlinear interactions with the use of degenerate-wave polarization in optics and acoustooptics is also discussed.     

Experimental Limit on the Probability of Neutral Pion Emission in Neutron-Induced Fission of 235U Nuclei

Results of an experiment on the search for the emission of ⁰ mesons in neutron-induced fission of ²³⁵U nuclei based on the detection in coincidence of two -quanta from the ⁰-meson decay performed with the nuclear reactor developed at the Scientific-Research Nuclear Physics Institute at Tomsk Polytechnic University are described. The experimental setup comprised two erenkov spectrometers of total absorption for the detection and measurement of -quantum energy placed in two coaxial horizontal channels of the nuclear reactor. Water filters placed in the channels were used to protect the detectors from -, -, and neutron fluxes. To reject the cosmic radiation, scintillation counters of large area, operating in anticoincidence mode, were placed above each spectrometer. The experimentally measured limitation on the probability of neutral pion emission in neutron-induced fission of nuclei was established 4.1·10^–11 with a 90% confidence level. The energy spectrum of -quanta from the active reactor zone was measured in the range 32–65 MeV. The parameters of the spectrum deviated from the results of theoretical calculations and from the experiment performed with the nuclear pulse reactor developed at the Joint Institute for Nuclear Research (Dubna).     

Localized vibration modes in free anisotropic wedges

Propagation of flexural localized vibration modes along edges of anisotropic wedges is considered in the framework of the geometrical-acoustics approach. Its application allows for straightforward evaluation of the wedge-mode velocities in the general case of arbitrary elastic anisotropy. The velocities depend on the wedge apex angle and on the mode number in the same way as in the isotropic case, but there appears to be additional dependence on elastic coefficients. The velocities in tetragonal wedges (with the midplane orthogonal to the four-fold axis) and in "weakly" monoclinic wedges are explicitly calculated and analyzed. Bounds of the wedge-wave velocity variation in tetragonal materials are established.     

Nuclear magnetic resonance study of Na (DxH1−x)3(SeO3)2 crystals

A comparison of the quadrupole perturbed deuteron magnetic resonance spectra of the α,β, and γ phase of partially deuterated sodium trihydrogen selenite crystals shows that not only in the monoclinic γ but also in the triclinic β phase the deutrons are ordered whereas they are disordered in the α phase. Spin-lattice relaxation time measurements show the existence of slow hindered rotations of the HSeO₃ and H₂SeO₃ groups at high temperatures as well as the existence of an overdamped ferroelectric soft mode which condenses at the paraelectric to ferroelectric transition.