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Contains fulltext : 216745.pdf (publisher's version ) (Open Access)

Conventionally, the stretching of rubber is modeled exclusively by rotations of segments of the embedded polymer chains; i.e. changes in entropy. However models have not been tested on all relevant length scales due to a lack of appropriate probes. Here we present a universal X-ray based method for providing data on the structure of rubbers in the 2-50 angstrom range. First results relate to the elongation of a silicone rubber. We identify several non-entropic contributions to the free energy and describe the associated structural changes. By far the largest contribution comes from structural changes within the individual monomers, but among the contributions is also an elastic strain, acting between chains, which is 3-4 orders of magnitude smaller than the macroscopic strain, and of the opposite sign, i.e. extension of polymer chains in the direction perpendicular to the stretch. This may be due to trapped entanglements relaxing to positions close to the covalent crosslinks.

Contains fulltext : 144941.pdf (publisher's version ) (Closed access)

A gamma-ray telescope mission concept [gamma ray imager (GRI)] based on Laue focusing techniques has been proposed in reply to the European Space Agency call for mission ideas within the framework of the next decade planning (Cosmic Vision 2015-2025). In order to optimize the design of a focal plane for this satellite mission, a CdZnTe detector prototype has been tested at the European Synchrotron Radiation Facility under an similar to 100% polarized gamma-ray beam. The spectroscopic, imaging, and timing performances were studied and in particular its potential as a polarimeter was evaluated. Polarization has been recognized as being a very important observational parameter in high energy astrophysics (> 100 keV) and therefore this capability has been specifically included as part of the GRI mission proposal. The prototype detector tested was a 5 mm thick CdZnTe array with an 11 x 11 active pixel matrix (pixel area of 2.5 x 2.5 mm(2)). The detector was irradiated by a monochromatic linearly polarized beam with a spot diameter of about 0.5 mm over the energy range between 150 and 750 keV. Polarimetric Q factors of 0.35 and double event relative detection efficiency of 20% were obtained. Further measurements were performed with a copper Laue monochromator crystal placed between the beam and the detector prototype. In this configuration we have demonstrated that a polarized beam does not change its polarization level and direction after undergoing a small angle (< 1 degrees) Laue diffraction inside a crystal. (C) 2008 American Institute of Physics. [DOI:10.1063/1.3000092]

We have determined experimentally optical constants for eight thin film materials that can be used in hard X-ray multilayer coatings. Thin film samples of Ni_(.97)V_(.03), Mo, W, Pt, C, B_4C, Si and SiC were deposited by magnetron sputtering onto superpolished optical flats. Optical constants were determined from fits to reflectance-vs-incidence angle measurements made using synchrotron radiation over the energy range E=35-180 keV. We have also measured the X-ray reflectance of a prototype W/SiC multilayer coating over the energy range E=35-100 keV, and we compare the measured reflectance with a calculation using the newly derived optical constants.

We have developed a new depth-graded multilayer system comprising W and SiC layers, suitable for use as hard x-ray reflective coatings operating in the energy range 100-200 keV. Grazing-incidence x-ray reflectance at E = 8 keV was used to characterize the interface widths, as well as the temporal and thermal stability in both periodic and depth-graded W/SiC structures, whereas synchrotron radiation was used to measure the hard x-ray reflectance of a depth-graded multilayer designed specifically for use in, the range Esimilar to150-170 keV. We have modeled the hard x-ray reflectance using newly derived optical constants, which we determined from reflectance versus incidence angle measurements also made using synchrotron radiation, in the range E = 120-180 keV. We describe our experimental investigation in detail, compare the new W/SiC multilayers with both W/Si and W/B4C films that have been studied previously, and discuss the significance of these results with regard to the eventual development of a hard x-ray nuclear line telescope.

We have studied the electron-momentum density distribution in Al using the coincidence as well as the conventional high-resolution Compton scattering technique. In order to interpret the results, corresponding band theory based computations of the electron momentum density (EMD) and the Compton profiles (CPs) have been carried out. Our focus here is on determining the size of the break ZF in the EMD at the Fermi momentum. For this purpose, differences between measurements and theoretical predictions are analyzed in terms of a simple model for describing electron correlation effects which are missing from the independent particle band theory framework; the model involves ZF as the only adjustable parameter. A good fit with the coincidence measurements is obtained for ZF of about 0.7, while the CP data yields ZF = 0.7 to 0.8. This study suggests that, in sharp contrast to the case of Li where recent high-resolution Compton work indicates ZF≈0, the standard picture of the interacting electron gas is substantially correct in Al.