Effects of the primary recoil spectrum on long-range migration of defects

Abstract

For quantitative predictions and comparisons of microstructures that evolve during exposure to different radiation environments at elevated temperature one needs to develop methods that go beyond those based on the number of displacements per atom. The number of freely migrating defects that contribute to the microstructural development is far less than the total number of defects produced, as has been recognized for some time from measurements of radiation-induced segregation and of radiation-enhanced diffusion. Defect production in various neutron and ion irradiation environments is discussed in light of this fact. A method to calculate the fraction of freely migrating defects from the cluster size distribution of defects produced in cascades is suggested. The results are in good agreement with available data.     

Surface segregation measurements during electron irradiation

Abstract

The growth of Ni3Si surface films on Ni-12.7at%Si alloys has been measured during lMeV electron irradiation. Stereoscopic techniques were used to determine film thickness from dark field images formed from Ni₃Si superlattice reflections. Parabolic growth kinetics are observed at lower temperatures. However, at higher temperatures, deviations from parabolic kinetics are observed after short irradiation times. Such deviations have not been observed in bulk specimens during bombardment with energetic ions and, therefore, may be due to foil thickness effects.     

Protein elasticity determined by pressure tuning of the tyrosine residue of ubiquitin

We determined the isotropic, isothermal compressibility of ubiquitin by pressure tuning spectral holes burnt into the red edge of the absorption spectrum of the single tyrosine residue. The pressure shift is perfectly linear with burn frequency. From these data, a compressibility of 0.086 GPa(-1) in the local environment of the tyrosine residue could be determined. This value fits nicely into the range known for proteins. Although the elastic behavior at low temperatures does not show any unusual features, the pressure tuning behavior at room temperature is quite surprising: the pressure-induced spectral shift is close to zero, even up to very high pressure levels of 0.88 GPa, well beyond the denaturation point. The reason for this behavior is attributed to equally strong blue as well as red spectral pressure shifts resulting in an average pressure-induced solvent shift that is close to zero.     

On the theory of void formation during irradiation

A simple theory of the swelling of materials subjected to high energy particle irradiation is developed. Chemical reaction rate equations are used as a basis. Point defects, interstitials and vacancies, are assumed to be produced randomly throughout the solid. They move by random walk through the material until they cease to exist either by recombination with the opposite type of defect or by incorporation into the crystal at sinks such as dislocations, grain boundaries and voids. The rate equations for interstitials and for vacancies, which are coupled via the recombination term, are solved for steady state conditions under irradiation. Defect concentrations, supersaturations, recombination and total sink annihilation rates are obtained in terms of the production rate, sink annihilation probabilities, jump frequencies and thermal equilibrium concentrations of defects. The swelling rate is derived using sink annihilation probabilities at three principally different types of sinks, i.e. voids, sinks which have a bias with regard to the annihilation of interstitials and vacancies (such as dislocations), and sinks with no bias. The defect annihilation probabilities at void, precipitate, dislocation and grain boundary sinks are estimated by using a cellular model and solving the diffusion equation for geometries approximating that of the cells, e.g. a concentric sphere around a void. The relative effects of different types of sinks, i.e. the microstructure, on the swelling rate is discussed. The swelling rate is integrated to give swelling-time or swelling-dose relations, making some simplifying assumptions about the changes in the sink structure as the irradiation proceeds. It is shown that the relation obtained is rather sensitive to the type of assumptions made.     

Spectral shape of relaxations in silica glass

Precise low-frequency light scattering experiments on silica glass are presented, covering a broad temperature and frequency range ( 9 GHz相似文献   

Combining imaging ellipsometry and grazing incidence small angle X-ray scattering for in situ characterization of polymer nanostructures

A combination of microbeam grazing incidence small angle X-ray scattering (μGISAXS) and imaging ellipsometry is introduced as a new versatile tool for the characterization of nanostructures. μGISAXS provides a local lateral and depth-sensitive structural characterization, and imaging ellipsometry adds the position-sensitive determination of the three-dimensional morphology in terms of thickness, roughness, refractive index, and extinction coefficient. Together μGISAXS and imaging ellipsometry enable a complete characterization of structure and morphology. On the basis of an example of buildup of nanostructures from monodisperse colloidal polystyrene nanospheres on a rough solid support, the scope of this new combination is demonstrated. Roughness is introduced by a dewetting structure of a diblock copolymer film with one block being compatible with the colloidal nanoparticles and one block being incompatible. To demonstrate the potential for kinetic investigations, μGISAXS and imaging ellipsometry are applied to probe the drying process of an aqueous dispersion of nanospheres on such a type of rough substrate.     

Ion bombardment-induced subsurface composition modifications in alloys at elevated temperatures

Modifications of subsurface alloy composition during high-temperature sputtering were studied using a comprehensive kinetic model that Includes Gibbsian adsorption, preferential sputtering, displacement mixing, radiation-enhanced diffusion, and radiation-induced segregation. Numerical solutions were obtained for a Cu-40 at.% Ni alloy under 5-keV Ar⁺ ion bombardment as functions of time and temperature. The model predictions are in good qualitative agreement with recent experimental measurements.     

Anomaly of the nonergodicity parameter and crossover to white noise in the fast relaxation spectrum of a simple glass former

We present quasielastic light scattering and dielectric spectra of the glass former alpha-picoline. At high temperatures the evolution of the susceptibility minimum is well described by the mode coupling theory (MCT). Below the critical temperature T(c) the simple scaling laws of MCT fail due to the appearance of the excess wing of the alpha process, which shows a universal evolution as a function of log(10)tau(alpha). Taking this into account, however, we observe the predicted cusplike anomaly of the nonergodicity parameter as well as a crossover to "white noise."     

Experiments with proteins at low temperature: what do we learn on properties in their functional state?

The authors compared the spectral response of Zn-substituted horseradish peroxidase in a glycerol/water solvent to hydrostatic pressure at 2 K and ambient temperature. The low temperature experiments clearly demonstrate the presence of at least three different conformations with drastically different elastic properties. However, the main conformation, which determines the fluorescence spectrum at ambient temperature, did not show any significant difference between low and high temperature and pressure. The authors conclude that the local compressibility of the heme pocket of the protein depends only very weakly on temperature.