Showing papers by "Paul Scherrer Institute published in 2000"

Review of Particle Physics

Abstract: This biennial Review summarizes much of particle physics. Using data from previous editions., plus 2778 new measurements from 645 papers, we list, evaluate, and average measured properties of gauge bosons, leptons, quarks, mesons, and baryons. We also summarize searches for hypothetical particles such as Higgs bosons, heavy neutrinos, and supersymmetric particles. All the particle properties and search limits are listed in Summary Tables. We also give numerous tables, figures, formulae, and reviews of topics such as the Standard Model, particle detectors., probability, and statistics. Among the 108 reviews are many that are new or heavily revised including those on CKM quark-mixing matrix, V-ud & V-us, V-cb & V-ub, top quark, muon anomalous magnetic moment, extra dimensions, particle detectors, cosmic background radiation, dark matter, cosmological parameters, and big bang cosmology.

Linking stable oxygen and carbon isotopes with stomatal conductance and photosynthetic capacity: a conceptual model.

Abstract: Based on measurements of δ18O and δ13C in organic matter of C3-plants, we have developed a conceptual model that gives insight into the relationship between stomatal conductance (g l) and photosynthetic capacity (A max) resulting from differing environmental constraints and plant-internal factors. This is a semi-quantitative approach to describing the long-term effects of environmental factors on CO2 and H2O gas exchange, whereby we estimate the intercellular CO2 concentration (c i) from δ13C and the air humidity from δ18O. Assuming that air humidity is an important factor influencing g l, the model allows us to distinguish whether differences in c i are caused by a response of g l or of A max. As an application of the model we evaluated the isotope data from three species in plots differing in intensity of land use (hay meadows and abandoned areas) at three sites along a south north transect in the Eastern Alps. We found three different δ18O-δ13C response patterns in native and planted grassland species (cultivated in the greenhouse). After preliminary confirmation by gas-exchange measurements we conclude that the proposed model is a promising tool for deriving carbon water relations in different functional groups from δ18O and δ13C isotope data.

Power spectrum and detrended fluctuation analysis: Application to daily temperatures

Abstract: The variability measures of fluctuation analysis (FA) and detrended fluctuation analysis (DFA) are expressed in terms of the power spectral density and of the autocovariance of a given process. The diagnostic potential of these methods is tested on several model power spectral densities. In particular we find that both FA and DFA reveal an algebraic singularity of the power spectral density at small frequencies corresponding to an algebraic decay of the autocovariance. A scaling behavior of the power spectral density in an intermediate frequency regime is better reflected by DFA than by FA. We apply FA and DFA to ambient temperature data from the 20th century with the primary goal to resolve the controversy in literature whether the low frequency behavior of the corresponding power spectral densities are better described by a power law or a stretched exponential. As a third possible model we suggest a Weibull distribution. However, it turns out that neither FA nor DFA can reliably distinguish between the proposed models.

Unbinding forces of single antibody-antigen complexes correlate with their thermal dissociation rates

Abstract: Point mutants of three unrelated antifluorescein antibodies were constructed to obtain nine different single-chain Fv fragments, whose on-rates, off-rates, and equilibrium binding affinities were determined in solution. Additionally, activation energies for unbinding were estimated from the temperature dependence of the off-rate in solution. Loading rate-dependent unbinding forces were determined for single molecules by atomic force microscopy, which extrapolated at zero force to a value close to the off-rate measured in solution, without any indication for multiple transition states. The measured unbinding forces of all nine mutants correlated well with the off-rate in solution, but not with the temperature dependence of the reaction, indicating that the same transition state must be crossed in spontaneous and forced unbinding and that the unbinding path under load cannot be too different from the one at zero force. The distance of the transition state from the ground state along the unbinding pathway is directly proportional to the barrier height, regardless of the details of the binding site, which most likely reflects the elasticity of the protein in the unbinding process. Atomic force microscopy thus can be a valuable tool for the characterization of solution properties of protein-ligand systems at the single molecule level, predicting relative off-rates, potentially of great value for combinatorial chemistry and biology.

Grain-boundary structures in polycrystalline metals at the nanoscale

Abstract: We present a detailed analysis of grain-boundary structures in computer-generated Cu and Ni three-dimensional nanocrystalline samples. The study includes both totally random and textured microstructures with grain sizes in the range of 5\char21{}12 nm. A detailed direct visualization technique is used at the atomic scale for studying the grain-boundary structural features. The study focuses on determining the presence of regions in the boundary exhibiting order and structural units normally expected for high-angle boundaries. For low-angle boundaries we investigate the presence of dislocation networks accommodating the misfit between the grains. A significant degree of crystalline order is found for all the boundaries studied. The highest degree of structural order was identified for boundaries with misfits within about 10\ifmmode^\circ\else\textdegree\fi{} deviation from the perfect twin. These grain boundaries contain a repeated building structure consisting of structural units typical of a $\ensuremath{\Sigma}=3$ symmetrical tilt twin boundary and highly disordered steps between those structural units. For all other types of misfit, we also observe some degree of structural coherence, and misfit accommodation occurs in a regular pattern. The cases studied include grain boundaries with a high-index common axis and show structural coherency that is independent of the grain size. Similar results are obtained for textured samples containing only low-angle grain boundaries, where regular dislocation arrays that are typical of larger grain materials are observed. These results provide evidence against the view of grain boundaries in nanocrystals as highly disordered, amorphous, or liquidlike interfaces. The results suggest that the grain-boundary structure in nanocrystalline materials is actually similar to that found in larger grain polycrystals.

Intersubband electroluminescence from silicon-based quantum cascade structures.

Abstract: The quantum cascade laser, which uses electronic transitions within a single band of a semiconductor, constitutes a possible way to integrate active optical components into silicon-based technology. This concept necessitates a transition with a narrow linewidth and an upper state with a sufficiently long lifetime. We report the observation of intersubband electroluminescence from a p-type silicon/silicon-germanium quantum cascade structure, centered at 130 millielectron volts with a width of 22 millielectron volts, with the expected polarization, and discernible up to 180 kelvin. The nonradiative lifetime is found to depend strongly on the design of the quantum well structure, and is shown to reach values comparable to that of an equivalent GaInAs/AlInAs laser structure.

High-resolution powder diffractometer HRPT for thermal neutrons at SINQ

Abstract: Design characteristics and first experience concerning the new high-resolution powder diffractometer for thermal neutrons at the Swiss spallation neutron source SINQ are summarized. It is based on a linear position-sensitive 3 He detector with 1600 wires and angular separation of 0.1°, permitting also real-time experiments.

Extreme-ultraviolet flare activity in late-type stars

Abstract: Extreme Ultraviolet Explorer Deep Survey observations of cool stars (spectral type F to M) have been used to investigate the distribution of coronal —are rates in energy and its relation to activity indicators and rotation parameters. Cumulative and diUerential —are rate distributions were constructed and —tted with diUerent methods. Power laws are found to approximately describe the distributions. A trend toward —atter distributions for later type stars is suggested in our sample. Assuming that the power laws continue below the detection limit, we have estimated that the superposition of —ares with radiated ener- gies of about 1029¨1031 ergs could explain the observed radiative power loss of these coronae, while the detected —ares are contributing only B10%. Although the power-law index is not correlated with rota- tion parameters (rotation period, projected rotational velocity, Rossby number) and only marginally with the X-ray luminosity, the —are occurrence rate is correlated with all of them. The occurrence rate of —ares with energies larger than 1032 ergs is found to be proportional to the average total stellar X-ray luminosity. Thus, energetic —ares occur more often in X-ray bright stars than in X-ray faint stars. The normalized occurrence rate of —ares with energies larger than 1032 ergs increases with increasing L X /L bol and stays constant for saturated stars. A similar saturation is found below a critical Rossby number. The —ndings are discussed in terms of simple statistical —are models in an attempt to explain the previously observed trend for higher average coronal temperatures in more active stars. It is concluded that —ares can contribute a signi—cant amount of energy to coronal heating in active stars. Subject headings: stars: activitystars: coronaestars: —arestars: late-typestars: rotation ¨ X-rays: stars

Resummation of double logarithms in electroweak high-energy processes

Abstract: At future linear ${e}^{+}{e}^{\ensuremath{-}}$ collider experiments in the TeV range, Sudakov double logarithms originating from massive boson exchange can lead to significant corrections to the cross sections of the observable processes. These effects are important for the high precision objectives of the Next Linear Collider. We use the infrared evolution equation, based on a gauge invariant dispersive method, to obtain double logarithmic asymptotics of scattering amplitudes and discuss how it can be applied, in the case of broken gauge symmetry, to the standard model of electroweak processes. We discuss the double logarithmic effects to both non-radiative processes and to processes accompanied by soft gauge boson emission. In all cases the Sudakov double logarithms are found to exponentiate. We also discuss double logarithmic effects of a non-Sudakov type which appear in Regge-like processes.

Dilatometric Investigations of Graphite Electrodes in Nonaqueous Lithium Battery Electrolytes

Abstract: A relatively uncommon technique known as in situ electrochemical dilatometry can be used to record the macroscopic expansion (dilatation) and contraction of graphite samples during charge/discharge in cation‐containing nonaqueous electrolytes. Several electrolytes based on solvent mixtures such as ethylene carbonate/dimethyl carbonate, pure propylene carbonate (PC), and PC with additional solvents (ethylene sulfite or chlorethylene carbonate) have been investigated. The dilatometer yields a clear distinction between solvated lithium intercalation/deintercalation occuring in pure PC (relative expansion of the order of >100%) and the corresponding unsolvated processes occuring in the other electrolytes (theoretical relative expansion of the order of 10%). Exfoliation of graphite due to solvated lithium intercalation may destroy the graphite sample. The penetration of electrolyte into pores or fissures of the exfoliated sample can also be monitored by dilatometry. Hence, dilatometry provides relevant information concerning the feasibility of a given electrolytic solution for rechargeable lithium‐ion cells with graphite as the negative electrode. Limitations and advantages of the electrochemical dilatometer and specific experimental features of the instrument are addressed. © 2000 The Electrochemical Society. All rights reserved.

The new SANS instrument at the Swiss spallation source SINQ

Abstract: With the start-up of the neutron spallation source SINQ at PSI an instrument for small angle neutron scattering became operational and is open for the user community. The instrument is equipped with state-of-the-art components and compares well with the world's largest and most powerful facilities of this kind. Great emphasis was put on providing a flexible, universal multi-user facility which guaranties a comfortable and reliable operation. In the present paper, the layout of the instrument is presented, the main components are described, and the performance is illustrated by selected examples.

Sensory tricks in cervical dystonia: Perceptual dysbalance of parietal cortex modulates frontal motor programming

Abstract: Cervical dystonia is a disabling basal ganglia disorder characterized by an involuntary head deviation to one side. A typical but also mysterious feature is the impressive improvement of muscle spasms and involuntary head posture by application of a sensory facial stimulus (sensory trick). Here, we report the effect of a sensory trick on cortical activation patterns in 7 patients with cervical dystonia by using H2(15)O positron emission tomography. The application of the sensory trick stimulus, resulting in a near-neutral head position, led to an increased activation mainly of the superior and inferior parietal lobule (ipsilateral to the original head turn) and bilateral occipital cortex and to a decreased activity of the supplementary motor area and the primary sensorimotor cortex (contralateral to the head turn). We propose that a perceptual dysbalance induced by a sensory trick maneuver leads to a relative displacement of the egocentric midvertical reference to the opposite side and a decrease in motor cortex activity. This modulation of motor programming gives novel insights into the mechanisms involved in sensorimotor integration in movement disorders.

A novel tracking technique for the continuous precise measurement of tumour positions in conformal radiotherapy.

Abstract: Changing tumour positions induced by organ motion can impede the full exploitation of the strengths of conformal radiotherapy. The unnecessary irradiation of healthy tissue surrounding the target volume can be the consequence. To overcome this, one should measure tumour positions directly and continuously with high resolution in space and time. We have developed a novel tracking technique which will allow this. The method can also be used to survey and monitor the patient positioning. The proper functioning of our method has been technically demonstrated at PSI with the help of phantom irradiation with protons. Implementation into the clinical environment is now beginning.

Hydrolysis of Isocyanic Acid on SCR Catalysts

Abstract: The hydrolysis of isocyanic acid (HNCO) was investigated on various oxidic catalysts, including TiO2, V2O5/TiO2, and V2O5−WO3/TiO2. The kinetics were studied using powdered samples in a microreactor. The conversions of HNCO are high even at the high space velocities (106 h-1) and low temperatures (150 °C) used in the tests. The highest rate for the hydrolysis of HNCO was found on pure TiO2 powder, while the addition of vanadia and tungsta decreased the activity slightly. To compare the rate of HNCO hydrolysis with the rate of the SCR reaction, uncorrected first-order rate constants were calculated for both reactions using the integral equation for a plug flow reactor. For a catalyst sample containing both WO3 and V2O5, the rate constant for hydrolysis of HNCO at low temperatures is about 2 orders of magnitude higher than the respective rate constant for the SCR reaction, whereas at high temperatures, the rate constants are of comparable magnitude. The apparent activation energy of the hydrolysis reaction ...

Diesel Aerosol Sampling in the Atmosphere

Abstract: The University of Minnesota Center for Diesel Research along with a research team including Caterpillar, Cummins, Carnegie Mellon University, West Virginia University (WVU), Paul Scherrer Institute in Switzerland, and Tampere University in Finland have performed measurements of Diesel exhaust particle size distributions under real-world dilution conditions. A mobile aerosol emission laboratory (MEL) equipped to measure particle size distributions, number concentrations, surface area concentrations, particle bound PAHs, as well as CO 2 and NO x concentrations in real time was built and will be described. The MEL was used to follow two different Cummins powered tractors, one with an older engine (L10) and one with a state-of-the-art engine (ISM), on rural highways and measure particles in their exhaust plumes. This paper will describe the goals and objectives of the study and will describe representative particle size distributions observed in roadway experiments with the truck powered by the ISM engine.

Magnetic correlations in nanostructured ferromagnets

Abstract: Small-angle neutron scattering experiments on nanostructured Fe, Co, and Ni reveal grain-size dependent magnetic correlations across grain boundaries. In Fe, a minimum of the correlation length is observed for grain sizes of the order of the bulk domain-wall width where the coercive field has a maximum. The results are explained within a generalization of the random-anisotropy model that takes into account domain-wall formation within grains and reduced interface coupling.