[en] Short communication

[en] Wave propagation in periodic waveguides is analyzed by decomposing the eigen Bloch waves into traveling-wave components. It is shown that the principal components consist of a primary forward wave, a primary backward wave, and their Bragg-scattered secondary waves. One important parameter is the coupling constant s due to Bragg scattering, which relates the secondary wave to the respective primary wave. Laser threshold condition is then obtained by applying the continuity of tangential E and H at the two boundaries. The results thus obtained are general and applicable to thin-film lasers with various waveguide structures. The laser threshold condition of thin-film Bragg lasers is expressed in terms of two effective reflection coefficients for easy comparison with conventional lasers. For appreciable reflection, a significant change either in the propagation constant or in the coupling constant is required. Two basic types of thin-film Bragg lasers are distributed-feedback (DFB) lasers in which Bragg scattering is confined to the active medium and distributed-Bragg-reflector (DBR) lasers in which Bragg scattering is limited to regions beyond the active medium. The threshold gain, frequency control, and mode selectivity for both types are analyzed and the analyses are applied to GaAs and Nd lasers. It is shown that DBR lasers should have a lower threshold gain and a better mode selectivity than DFB lasers. For distributed-feedback effect to play a significant role in thin-film Bragg lasers, the product kappaL/sub int/ must be greater than unity where kappa is the distributed-feedback coefficient and L/sub int/ is the interaction length. Advantages for having periodic structures outside the active medium so as to relax constraints on kappa and L/sub int/ are also discussed. (U.S.)

No abstract available

No abstract available

[en] The features of quasicharacteristic radiation based on direct radiative transitions between the above-barrier energy levels corresponding to the dynamic diffraction of electrons and the levels of channeled electron motion in crystals are considered. The structure and features of such radiative transitions are studied using diffraction in the Laue and Bragg geometry. The main advantages of these transitions in relation to channeling are associated with a high probability of excitation of the diffraction state, low scattering and deceleration of moving particles, long duration of the orientational (i.e., not chaotic, related to certain planes) motion regime, and large corresponding matrix element of the dipole momentum of the radiative transition. It is shown that the radiation based on these transitions can display a large integral and spectral intensity and can be implemented at a lower particle energy than the traditionally considered quasicharacteristic radiation between channeling levels.

[en] A large solid-angle Bragg Curve Spectrometer (BCS) is being developed for use on the new ATLAS scattering chamber which is presently being constructed. The detector which is of cylindrical design has an active area of approximately 300 cm² and active length of 27 cm. It is designed to be used with a position-sensitive Breskin detector trigger for experiments in good timing and a large solid angle is necessary. Several other groups have built small solid angle BCSs which provided excellent energy and good nuclear charge (Z) resolutions. In the coming year, the authors will test this detector to determine the feasibility of employing such a device with a large solid angle. If these tests are successful, the BCS will be used in conjunction with Breskin detectors to construct a TOF arm to measure evaporation residues in studies of complete and incomplete fusion reactions

[en] Gold nanoparticles are highly active catalysts for CO oxidation, although bulk gold is considered quite inert even to reactive molecules, the nanometer-size particles are efficient catalysts. Several effects contribute to the catalytic properties of supported nanoparticles catalysts such as particle size, morphology, exposed crystal facets, interaction with the support, low-coordinated atoms, strain effects and crystalline defects. During the reaction, nanoparticle surface goes under dynamic restructuration and expose different surface sites and defects with distinct reactivity. Understanding catalysts dynamic behaviours is crucial and requires the study of active sites IN-SITU and operando conditions. A peculiarity of CO oxidation reaction is the temperature hysteresis observed when during a complete cycle the catalytic properties do not match during heating and cooling. Explanations about the origin of this hysteresis loop are still conflicting. In order to investigate the facet-selectivity and dynamics involved in the active sites formation and the hysteresis behaviour we imaged the defects dynamics of gold nanocubes during CO oxidation reaction by Bragg CDI. Bragg CDI uses a coherent X-ray beam and phase-retrieval algorithms to image the 3D morphology of crystals and probe dynamic changes in strain and defects. I will show how the anisotropic strain, occurring during the oxidation reaction, provides new understanding of the formation of the active sites in gold nanocrystals. Significant changes were observed in the strain distribution during the reaction, regions of high tensile and compressed strain are concentrated on preferential facets in shape-controlled nanoparticles. The lattice strain can alter the reactivity of metal surfaces, strained metal surfaces have different chemical properties from those of unstrained surfaces due the shift in the d-band center. The tensile strain generation and release during the reaction directly influences the temperature hysteresis in CO oxidation. IN-SITU/operando strain imaging provide important insights to elucidate catalytic processes and designing catalysts with optimized morphology and defect control. (author)

[en] Some dynamical neutron diffraction phenomena studied with using positively the properties of wave fan and total reflection are described. The intrinsic rocking curve measured in the Bragg-case diffraction with using a monolithic double-crystal system is schematically shown and discussed. Proposals are made for the construction of a crystal system using multiple total reflection, especially with π/2 Bragg angle. A method of measuring a minute deviation angle of the beam is given. The construction of a two-component neutron interferometer is envisaged

[en] Atom-atom interactions between the molecules of plastic crystals or between reorientable molecules and the rigid frame of orientationally disordered crystals may be summed up and represented by multipole rotational potential V(ω). We review briefly the derivation of the rotational structure factor F_ROT(Q) for the general case of arbitrary molecular and site symmetry from Boltzmann as well as from quantum statistics. V(ω) has been derived from Bragg neutron diffraction data for some molecules, e.g., C₂Cl₆, NO₃, NH₄, in various environments. Quantum disorder is demonstrated in (NH₄)₂SnCl₆. V(ω) can, at this time, not yet be predicted from theoretical considerations: this is demonstrated by comparing V_TH(ω) and V_EXP(ω) for (NH₄)2PtCl₆. (author) 29 refs., 7 figs

[en] A thin-crystal Si (400) Bragg transmission x-ray phase plate has been constructed for the production of 5 to 12 keV circularly polarized x-rays. Using multiple beam diffraction from a GaAs crystal, a direct measurement of the degree of circular polarization as a function of off-Bragg position was made. These measurements indicated nearly complete circular polarization (|P_c| ≥ 0.95) and full helicity reversal on opposite sides of the rocking curve