Preliminary studies on a variable energy positron annihilation lifetime spectroscopy system

There are many advantages in being able to perform positron annihilation lifetime spectroscopy (PALS) using a variable energy positron beam, the most obvious being the easy identification of different defect types at different depths. The difficulty in conducting variable energy (VE) PALS studies lies in the fact that a “start” signal is required to signal the entry of the positron into the target. Two methods have been used to overcome this problem, namely the bunching technique, which employs radio frequency (RF) cavities and choppers, and secondly the use of secondary electrons emitted from the target. The latter technique is in terms of experimental complexity much simpler, but has in the past suffered from poor time resolution (typically ∼500 ps). In this work, we present a series of computer simulations of a design based on the secondary electron emission from thin C-foils in transmission mode which shows that significant improvements in time resolution can be made with resolutions ∼200 ps being in principle possible.     

Dispersion modeling by kinematic simulation: Cloud dispersion model

A new technique has been developed to compute mean and fluctuating concentrations in complex turbulent flows (tidal current near a coast and deep ocean). An initial distribution of material is discretized into any small clouds which are advected by a combination of the mean flow and large scale turbulence. The turbulence can be simulated either by kinematic simulation (KS) or direct numerical simulation. The clouds also diffuse relative to their centroids; the statistics for this are obtained from a separate calculation of the growth of individual clouds in small scale turbulence, generated by KS. The ensemble of discrete clouds is periodically re-discretized, to limit the size of the small clouds and prevent overlapping. The model is illustrated with simulations of dispersion in uniform flow, and the results are compared with analytic, steady state solutions. The aim of this study is to understand how pollutants disperses in a turbulent flow through a numerical simulation of fluid particle motion in a random flow field generated by Fourier modes. Although this homogeneous turbulent is rather a “simple” flow, it represents a building block toward understanding pollutant dispersion in more complex flow. The results presented here are preliminary in nature, but we expect that similar qualitative results should be observed in a genuine turbulent flow.     

Orientational ordering of liquid crystals containing a difluoro-substituted phenyl ring

The orientational ordering of several liquid crystals containing a difluorosubstituted phenyl ring has been studied through the use of C-13 NMR. The fluorinated phenyl ring of these liquid crystals have C_s symmetry, so three order parameters are required to completely describe the ordering of this ring. All three of these order parameters have been calculated from carbon-fluorine dipolar coupling constants obtained from the carbon-fluorine splittings in the C-13 NMR spectra. Because of the complexity of the fluorine-coupled spectra, variable angle spinning (VAS) was used to resolve the carbon-fluorine splittings. In order to study the orientational ordering over wide ranges of temperature, we have developed an empirical correlation between the order parameter and the value of a carbon-fluorine dipolar coupling constant. This enabled us to study the change in the order parameter with temperature. The results of applying this method to several structurally similar liquid crystals containing the same type of difluorinated phenyl ring are presented. A comparison is made to a similar mono-fluorinated liquid crystal.     

Multivariable identification and controller design of an integrated flight control system

This paper investigates the multivariable identification and controller design for the longitudinal channel of a Boeing 747 transport. The transfer function matrix of the system is identified using the prediction error (PE) identification method with multivariable ARX model. An ellipsoidal parametric uncertainty set is constructed from the covariance matrix of the identified parameters. It contains the parameters of actual system at a certain probability level. The identified models and the associated uncertainty sets are validated by measuring the worst-case ν-gap and then compared with the maximum value of the generalized stability margin. In automatic flight control system or autopilots, multiple specifications criteria are needed to be satisfied concurrently, such as good holding (small static altitude holding error), fast response, smooth transition (less oscillation, overshoot). The design of a Multiple Simultaneous Specifications (MSS) controller effectively and practically is a very significant and challenging job. Liu and Mills [H.H.T. Liu, J.K. Mills, Multiple specification design in flight control system, in: Proceedings of the American Control Conference, Chicago, Illinois, 2000, pp. 1365–1369] proposed a MSS controller design method using a convex combination approach. In this paper, we apply the method [H.H.T. Liu, J.K. Mills, Multiple specification design in flight control system, in: Proceedings of the American Control Conference, Chicago, Illinois, 2000, pp. 1365–1369; H.H.T. Liu, Design combination in integrated flight control, in: Proceedings of the American Control Conference, Arlington, Virginia, 2001, pp. 494–499; H.H.T. Liu, Multi-objective design for an integrated flight control system: a combination with model reduction approach, in: Proceedings of IEEE International Symposium on Computer Aided Control System Design, Glasgow, 2002, pp. 21–26] to design a MSS controller based on the identified models of the Boeing 747 transport aircraft longitudinal channel. The controllers are also validated by simulation using the true plant transfer functions.     

Preparation and Characterization of a Cobalt‐Containing Mono‐Phosphine Ligand and its Coordinated Molybdenum Complex

A cobalt‐containing monodentate phosphine [(μ₂‐PPh₂CH₂PPh₂‐κ²P)Co₂(CO)₄][μ₂‐η²‐PhC≡CP(i‐Pr)₂] 2f , was prepared from the reaction of (μ₂‐PPh₂CH₂PPh₂‐κ²P)Co₂(CO)₆ 1 with PhC≡CP(i‐Pr)₂. It was accompanied by an oxidized compound, [(μ₂‐PPh₂CH₂PPh₂‐κ²P)Co₂(CO)₄][μ₂‐η²‐PhC≡CP(=O)(i‐Pr₂)] 2fo during the chromatographic process. Further reaction of 2f with Mo(CO)₆ resulted in the formation of a 2f ‐ligated molybdenum complex 4 , [(μ₂‐PPh₂CH₂PPh₂‐κ²P)Co₂(CO)₄][μ₂‐η²‐PhC≡CP(i‐Pr₂)‐κP]‐Mo(CO)₅.     

Composition of particles emitted at largep T and medium angles inpp,dd, andαα collisions at the CERN intersecting storage rings

The relative yields of kaons and protons compared with the yield of pions at highp _T and polar angles θ away from 90° (in the range from 10° to 45°) are presented forpp, dd, and αα interactions at a centre-of-mass energy \(\sqrt {s_{NN} } = 31\) GeV per nucleon-nucleon collision. The measured particle ratios depend on the atomic mass numberA of the beam particles and on θ. TheA dependence of the ratios becomes stronger for largerp _T and is more pronounced at smaller polar angles.     

Darstellung und Kristallstruktur von Na2Mn3O7

Synthesis and Crystal-Structure of Na₂Mn₃O₇ Single crystals of Na₂Mn₃O₇ have been grown hydrothermally applying high oxygen pressure (p = 2 kbar). The new compound cystallizes triclinic; space group P1 ; a = 6.636(6) Å, b = 6.854(6) Å, c = 7.548(6) Å, α = 105.76(6)°, β = 106.86(6)°, γ = 111.60(6)°; Z = 2. The crystal structure has been solved and refined to R = 7.9% and R_w = 6.2% (diffractometer data, 1044 independent reflexions). The crystal structure consists of Mn₃O₇^2? anions with manganese coordinated octahedrally by oxygen. These layered anions are hold together by Na⁺ ions (coordination numbers 5 and 6).     

Determination of phenols in soil by supercritical fluid extraction-capillary electrochromatography

A new analytical procedure is developed to couple supercritical fluid extraction with capillary electrochromatography (SFE-CEC) to extract and determine phenols in soil. Ten phenols consisting of phenol, methylphenols (p-cresol and o-cresol), dimethylphenols (3,5-xylenol, 3,4-xylenol and 2,6-xylenol), trimethylphenol, ethylphenols (p-ethylphenol and o-ethylphenol), and o-isopropylphenol are investigated. The use of supercritical CO2 with 10% methanol as the organic modifier was found to give satisfactory extraction of alkylphenols from soil at 1200 p.s.i. and 50 degrees C for 45 min under a total extractant flow-rate of 0.2 ml/min (1 p.s.i.=6894.76 Pa). Baseline resolution was achieved for the 10 selected phenols under optimised CEC conditions at 20 kV in a mobile phase of acetonitrile-4 mM Tris, pH 7.0 (35:65) in a 45 cm (25 cm packed with 3 microm ODS) x 75 microm I.D. fused-silica capillary column. Using SFE with a 10-fold preconcentration factor, all alkyl-substituted phenols in soil can be determined with detection limits ranging from 0.0032 to 0.014 mg/kg and working range from 0.019 to 2.72 mg/kg. The SFE-CEC procedure developed has been applied successfully to determine phenols extracted from real soil sample contaminated with medical disinfectant. It will provide a rapid method for the direct determination of phenol and alkyl-substituted phenol in soils, with capability for confirmation of unknown peaks.