Alternative explanation of stiffening in cross-linked semiflexible networks

Strain stiffening of filamentous protein networks is explored by means of a finite strain analysis of a two-dimensional network model of cross-linked semiflexible filaments. The results show that stiffening is caused by nonaffine network rearrangements that govern a transition from a bending-dominated response at small strains to a stretching-dominated response at large strains. Filament undulations, which are key in the existing explanation of stiffening, merely postpone the transition.     

Scale size of magnetic turbulence in tokamaks probed with 30-MeV electrons

Measurements of synchrotron radiation emitted by 30-MeV runaway electrons in the TEXTOR-94 tokamak show that the runaway population decays after switching on neutral beam injection (NBI). The decay starts only with a significant delay, which decreases with increasing NBI heating power. This delay provides direct evidence of the energy dependence of runaway confinement, which is expected if magnetic modes govern the loss of runaways. Application of the theory by Mynick and Strachan [Phys. Fluids 24, 695 (1981)] yields estimates for the "mode width" (delta) of magnetic perturbations: delta<0.5 cm in Ohmic discharges, increasing to delta = 4.4 cm for 0. 6 MW NBI.     

Boron implantations in silicon: A comparison of charge carrier and boron concentration profiles

The concentration profiles of boron implanted in silicon were measured using secondary ion mass spectrometry. The accompanying charge carrier profiles were determined by Hall-effect sheet-resistivity measurements combined with layer removal by anodic oxidation and etching. From a mutual comparison of these profiles an electrically inactive boron fraction was found to exist in the region of maximum boron concentration. This fraction can be correlated with boron precipitates. In high dose implantations the precipitates still exist after annealing at 1000°C. In the tail of the profile a small electrically inactive boron fraction was observed. This fraction was correlated with fast diffusing non-substitutional boron. Near the surface a charge carrier peak was found that can be correlated with the damage caused by implantation. The interpretation of the observed electrical effects was facilitated by investigations on boron concentration profiles of layers implanted with different doses and annealed in accordance with different time-temperature schedules.     

A multi-method approach for determination of radionuclide distribution in trinitite

The spatial distribution of radiation within trinitite thin sections have been mapped using alpha track radiography and beta autoradiography in combination with optical microscopy and scanning electron microscopy. Alpha and beta maps have identified areas of higher activity, and these are concentrated predominantly within the surficial glassy component of trinitite. Laser ablation-inductively coupled plasma mass spectrometry (LA-ICP-MS) analyses conducted at high spatial resolution yield weighted average ²³⁵U/²³⁸U and ²⁴⁰Pu/²³⁹Pu ratios of 0.00718 ± 0.00018 (2σ) and 0.0208 ± 0.0012 (2σ), respectively, and also reveal the presence of some fission (¹³⁷Cs) and activation products (^152,154Eu). The LA-ICP-MS results indicate positive correlations between Pu ion signal intensities and abundances of Fe, Ca, U and ¹³⁷Cs. These trends suggest that Pu in trinitite is associated with remnants of certain chemical components from the device and surrounding Trinity test-related structures at ground zero. In contrast, negative correlations between Pu ion signals and SiO₂ and K₂O contents were observed within the glassy matrix of trinitite. This LA-ICP-MS result was corroborated by combined back-scattered electron imaging and alpha radiography, and indicates that Pu was not incorporated into unmelted crystalline grains of precursor minerals (i.e., quartz-SiO₂ and K-feldspar-KAlSi₃O₈) present within the desert sand at the Trinity site. The results from this study indicate that the device-related radionuclides were preferentially incorporated into the glassy matrix in trinitite.     

Rapid catalytic oxidation of CO to CO(2) - On the development of a new approach to on-line oxygen isotope analysis of organic matter

A new approach to on-line oxygen isotope analysis has been developed which utilises existing elemental analyser and mass spectrometry technology to produce a sample of carbon dioxide gas for oxygen isotople analysis. The method relies on on-line high temperature pyrolysis of the sample over a carbon source followed by a rapid, non-contributive partial catalytic oxidation over nickel powder at between 550 and 600 degrees C. Initial results demonstrate both good precision (better than 0.2 per thousand) and accuracy for both cellulose and silver nitrate samples. Copyright 1999 John Wiley & Sons, Ltd.