Magnetic resonance in beta-active nuclei 8Li at the doubled larmor frequency in LiF polycrystals containing dislocations

The nuclear magnetic resonance in beta-active nuclei ⁸Li at the doubled Larmor frequency in LiF polycrystals is studied before and after treating these polycrystals with an external pressure. A quantitative approach is proposed that makes it possible to calculate the parameters of the resonance line-shape function versus the dislocation structure of crystallites. Data suggesting that the samples under investigation that were subjected to a treatment with an external pressure develop dislocations whose Burgers vectors are parallel are obtained.     

Asymptotic behavior of neutron small-angle multiple scattering spectra

The asymptotic behavior of the spectra for large values of the scattering vector for the case of elastic multiple small-angle neutron scattering (SANS) is investigated theoretically and experimentally. An expansion of the spectrum in terms of the reciprocal of the magnitude of the momentum transfer is obtained taking account of the influence of the instrumental line. It is shown that, to within some factor, the leading term of the expansion is identical to the differential single-scattering cross section averaged over a statistical ensemble of particles; several subsequent terms in the expansion are calculated and the range of applicability of the resulting expressions is determined. The asymptotic behavior of the multiple SANS spectrum is measured, using a two-crystal neutron spectrometer, for samples of an HTSC ceramic, the alloy Fe-Ni, and Al powder. The agreement between the experimental results and the theoretical predictions is analyzed. Zh. éksp. Teor. Fiz. 114, 2194–2203 (December 1998)     

Random walks in disordered systems and their investigations by means of magnetic resonance and relaxation of polarized beta-active nuclei

The experimental investigations of the polarization transport in the disordered spin system⁸Li −⁶Li in LiF crystal are considered. This process is initiated by one originally polarized beta-active⁸Li nucleus produced in a nuclear reaction. The autocorrelation function Foo(t) for the ran dom-walks-in-disordered-system problem has been measured over the range Foo(t)≥0.1 Foo(0) by analysing the β-emission anisotropy. The influence of the resonance process at double larmor frequency and of the thermal translational motion of⁸Li and⁶Li spins is studied. Results are in a good agre ement with the theory.     

Ergodic theorem for an impurity spin subsystem in a paramagnet

A model for verifying and developing the fundamental ideas underlying the ergodic hypothesis is proposed. The model describes the dynamics of the spin subsystem formed by impurity charges with spin I and a small g factor in a crystal immersed in a strong constant external magnetic field under conditions where the spin system of the nuclei in the crystal is isolated from the other degrees of freedom. The additive integral of motion is the projection of the total spin of the subsystem onto the external field. Attention is focused mainly on the case of I=1/2. It is shown that the ergodic hypothesis holds if the correlation radius is finite in the initial state and that the ergodic hypothesis is violated if the initial state is sharply localized or has global correlation. The nonergodicity of the ⁸Li− ⁶Li spin subsystem, which is a convenient object for experimental investigations of spin dynamics, is revealed. An estimate is obtained for the time for transition from a sharply localized disturbance of the canonical distribution to a quasiequilibrium state. Zh. éksp. Teor. Fiz. 116, 1398–1418 (October 1999)     

Investigation of the external-magnetic-field dependence of polarization transfer in a 8Li-6Li disordered nuclear system

Random walks in disorderedmedia are studied by considering the example of the delocalization of nuclear polarization in an ensemble of ⁸Li beta-active nuclei produced in (n, γ) reactions induced by polarized thermal neutrons in LiF single crystals. The process is initiated by magnetic dipole-dipole interactions. It consists in polarization transfer from a primarily polarized ⁸Li nucleus to the closest (stable) ⁶Li nuclei and in subsequent polarizationmigration over 6Li nuclei, the return of the polarization in question to the ⁸Li nucleus being possible. In order to obtain information about the polarization of ⁸Li nuclei, the asymmetry of beta radiation from them was measured with respect to the external magnetic field H ₀. The process was studied until it reached the diffusion asymptotic regime. The concentration of ⁶Li was varied between 3% and 10%, while the field strength H ₀ was changed from 200 to 1200 G. It is shown that microscopic theory is in satisfactory agreement with the experimental results obtained in the way outlined above. A comparison with the investigation of the mathematically allied process of the Förster electricdipole transfer of localized excitons is performed.     

Magnetic Resonance and Relaxation of Polarized Beta-Active Nuclei: Modern State and Visible Trends

A review of experimental and theoretical studies of magnetic resonance and relaxation of polarized beta-active nuclei (beta-NMR) is presented since the invention of the method just after the discovery of parity nonconservation. Basics of the method and layouts of reactor- and accelerator-based installations are considered. Main attention is devoted to studies of radiation after-effects, multi-spin resonances and their application for investigation of the defects and disordered systems, NMR and cross-relaxation line shapes, and random walks in disordered media (magnetic analogy of the Forster resonance energy transfer).     

Mechanism of oxygen response in carbon-based sensors

The mechanism of oxygen response in several newly synthesized oxygen-sensitive chars was studied with the use of EPR spectroscopy. The results suggest that the compounds contain two basic types of paramagnetic centers (PC). The change in oxygen concentration leads to a mutual and reversible transformation of PCs in chars, which is reflected in EPR parameters. The adsorbed molecular oxygen progressively disturbs the wave functions of the PCs and so breaks the Heisenberg exchange between them. At high oxygen concentration, the 2D dipole-dipole interaction between PCs at the surface comes into play and determines the EPR lineshape. A suggested model quantitatively describes the evolution of the basic EPR parameters of each PC as a function of oxygen concentration.