Relaxation and phonons in viscous and glassy orthoterphenyl by neutron scattering

We present an extended set of incoherent neutron scattering measurements on the van der Waals liquido-terphenyl, obtained by time-of-flight and backscattering spectroscopy. In the supercooled liquid regime, data from three instruments are combined and analysed in terms of the selfcorrelationS(Q, t). In the time range 1...100 ps, the crossover from -to -relaxation is well described by the masterfunction of mode coupling theory, and fitted parameters are consistent with the previously established critical temperatureT _c [Z. Phys. B83, 175 (1991)]. In the glassy regime, vibrations are harmonic and can be described by a density of states. Deviations at lowQ are quantitatively explained by a multiple scattering simulation. Throughout the article, experimental difficulties are discussed in some detail.     

Additive pulse mode-locked Nd: YAG laser

Additive pulse mode locking applied to lamppumped Nd: YAG lasers results in an attractive source of picosecond pulses at 1.06 m or 1.32 m with average powers at the Watt level. We provide detailed information on construction and operation and give data on performance. A modified active stabilization scheme allows not only improved stability of operation but also insight into the dynamics of pulse formation.     

Thermal diffusivity measurements of natural and isotopically enriched diamond by picosecond infrared transient grating experiments

The photopyroelectric (PPE) technique is used to measure the specific heat of YBa₂Cu₃O_7–X thin films. Hysteresis in the PPE amplitude is observed in the temperature range 90 K to 250 K. Peaks in the inverse PPE amplitude at T=110 K in the cooling path and at T230 K in the heating path are observed. The magnitude of hysteresis depends upon the thermal history of the sample.     

CO(2) laser with modulated losses: Theoretical models and experiments in the chaotic regime

We compare two different theoretical models for a CO(2) laser, namely, the two- and four-level model, and show that the second one traces with much better accuracy the experimentally observed chaotic dynamics when the cavity losses are sinusoidally modulated. Even though the two-level model provides a qualitative explanation of the chaotic dynamics, only the four-level one assures a quantitative fitting. We also show that, at the onset of chaos, the chaotic dynamics is low dimensional and can be described in terms of a noninvertible one-dimensional map.     

Phonon anomalies and martensitic transitions in bcc materials

The general systematics of phonon spectral anomalies are considered for materials with bcc and B2 structures. In a model of central pair-wise interactions, it is shown that anomalies on the TA₂ branch at =1/3 (211), the LA phonon branch at =2/3 (111), and on the TA branch at =1/2 (111) arise simultaneously with the softening of the shear elastic moduli C' and C₄₄. The TA phonon anomaly at =1/3 (110) is not due to softening of the elastic moduli, but rather is due to the long range nature of the pair-wise or multiparticle interaction. A model is proposed for the nucleation of the martensitic phase for B2 compounds, the distinctive feature of which is a consideration of the softening of the shear moduli, which is modulated by lattice relaxation around ordering defects, along with the possibility of additional defect ordering near the transition.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, No. 10, pp. 31–39, October, 1993.     

Imaging radiotracer model parameters in PET: a mixture analysis approach

Two methodologies for fitting radiotracer models on a pixel-wise basis to PET data are considered. The first method does parameter optimization for each pixel considered as a separate region of interest. The second method also does pixel-wise analysis but incorporates an additive mixture representation to account for heterogeneity effects induced by instrumental and biological blurring. Several numerical and statistical techniques including cluster analysis, constrained nonlinear optimization, subsampling, and spatial filtering are used to implement the methods. A computer simulation experiment, modeling a standard F-18 deoxyglucose (FDG) imaging protocol using the UW-PET scanner, is conducted to evaluate the statistical performance of the parametric images obtained by the two methods. The results obtained by mixture analysis are found to have substantially improved mean square error performance characteristics. The total computation time for mixture analysis is on the order of 0.7 s/pixel on a 16 MIPS workstation. This results in a total computation time of about 1 h per slice for a typical FDG brain study.     

Temporal alignment of tissue and arterial data and selection of integration start times for the H(2)(15)O autoradiographic CBF model in PET

A technique has been developed and tested that provides an automated method of temporally aligning the PET tissue activity curve with the arterial activity curve for quantification of cerebral blood flow using the H(2)(15)O autoradiographic model. This technique not only determines the relative time delay between the two curves, but also provides the start time of integration. Variability in computing global cerebral blood flow using this technique is shown to be less than that obtained by trained observers manually selecting parameters and at least as good as that obtained by using another automated alignment technique.     

Rate distortion function for nth-order Gaussian Markov process

The foundations of information theory as a whole, and the rate distortion theory in particular, were ably laid by Shannon in 1948. The rate distortion function R(D) is defined as the effective rate (R) at which the source produces information subject to the constraint that the user can tolerate an average distortion of D. The rate distortion functions for Gaussian Markov Processes were studied by Berger [1] and Bunin [2] for small distortion values. This paper presents the generalization of this study fornth-order Gaussian Markov processes for all distortion levels from 0 to ². The function is first derived, computer simulated, and then calculated for first and second order cases, and the corresponding results are commented.     

Total synthesis and properties of prostaglandins. 36. Condensation of (+)2-oxabicyclo[3.3.0]-6-octen-3-one with furylacrolein and furylpropionic aldehyde

A study was made of the condensation of bicyclic -lactone I with furylacrolein and furylpropionic aldehyde. Significant differences were observed in the reactivity of the aldols obtained, and in the stereochemical trend of their dehydration reaction.For Communication 35, see [1].Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 8, pp. 1035–1039, August, 1993.     

Characterisation of chlorinated hydrocarbons in a third generation candidate fish reference material (Limanda limanda) prepared under cryogenic conditions

Summary This paper describes the cryogenic preparation of 50 individual specimens of dab (Limanda limanda) from the Baltic Sea to produce a homogeneous fish material. 950 subsamples (8 g each) are stored in the gaseous phase over liquid nitrogen. Five series of analyses were randomly selected out of store. The results show that the obtained precision of 8 to 9% could be used as an indication unit for the deviation of the mean values of CHC concentrations from true values. If the analytical variability is 5%, the maximum inhomogeneity in the subsamples is 3–4%. It may be concluded that the subsamples of the candidate fish reference material (RM) are homogeneous enough for analytical purposes. Thus ESB (German Environmental Specimen Bank) material prepared in accordance with the standard operating procedure (SOP) [1] are valuable and appropriate reference materials. The prepared fish RM is suitable for the analysis of chlorinated hydrocarbons in fat containing marine samples.