Chalk River Laboratories

About: Chalk River Laboratories is a based out in . It is known for research contribution in the topics: Neutron diffraction & Neutron scattering. The organization has 2297 authors who have published 2700 publications receiving 73287 citations.

Electron doping evolution of the anisotropic spin excitations in BaFe2−xNixAs2

Abstract: We use inelastic neutron scattering to systematically investigate the Ni-doping evolution of the low-energy spin excitations in BaFe2-xNixAs2 spanning from underdoped antiferromagnet to overdoped superconductor (0.03 <= x <= 0.18). In the undoped state, BaFe2As2 changes from paramagnetic tetragonal phase to orthorhombic antiferromagnetic (AF) phase below about 138 K, where the low-energy (<=similar to 80 meV) spin waves form transversely elongated ellipses in the [H, K] plane of the reciprocal space. Upon Ni doping to suppress the static AF order and induce superconductivity, the c-axis magnetic exchange coupling is rapidly suppressed and the momentum distribution of spin excitations in the [H, K] plane is enlarged in both the transverse and longitudinal directions with respect to the in-plane AF ordering wave vector of the parent compound. As a function of increasing Ni-doping x, the spin excitation widths increase linearly but with a larger rate along the transverse direction. These results are in general agreement with calculations of dynamic susceptibility based on the random phase approximation (RPA) in an itinerant electron picture. For samples near optimal superconductivity at x approximate to 0.1, a neutron spin resonance appears in the superconducting state. Upon further increasing the electron doping to decrease the superconducting transition temperature T-c, the intensity of the low-energy magnetic scattering decreases and vanishes concurrently with vanishing superconductivity in the overdoped side of the superconducting dome. Comparing with the low-energy spin excitations centered at commensurate AF positions for underdoped and optimally doped materials (x <= 0.1), spin excitations in the overdoped side (x = 0.15) form transversely incommensurate spin excitations, consistent with the RPA calculation. Therefore, the itinerant electron approach provides a reasonable description to the low-energy AF spin excitations in BaFe2-xNixAs2.

A superheated droplet detector for dark matter search

Abstract: We discuss the operation principle of a detector based on superheated droplets of Freon-12 and its feasibility for the search of weakly interacting cold dark matter particles. In particular we are interested in a neutralino search experiment in the mass range from 10 to 10 4 GeV/c 2 and with a sensitivity of better than 10 −2 events/kg/d. We show that our new proposed detector can be operated at ambient pressure and room temperature in a mode where it is exclusively sensitive to nuclear recoils like those following neutralino interaction, which allows a powerful background discrimination. An additional advantage of this technique is due to the fact that the detection material, Freon-12, is cheap and readily available in large quantities. Moreover we were able to show that piezoelectric transducers allow efficient event localization in large volumes.

Static mechanical properties of cast and sinter-annealed cobalt-chromium surgical implants

Abstract: Porous-surface-layered surgical implants may be produced by sintering at elevated temperatures. An investigation was undertaken to determine the effect of these sintering heat treatments on the tensile properties of the cobalt-chromium casting alloy specified by ASTM F75-76. Specimens which were given a sintering treatment and then rapidly cooled from elevated temperature were found to lack ductility. This was due to the incipient melting of an interdendritic material which was subsequently retained in the grain boundaries as a brittle solid after quenching. Two methods were found which would reduce the amount of this brittle solid: (i) modify the heat treatment to include a slow cooling step to temperatures below that at which incipient melting first occurred; and (ii) reduce the carbon content of the alloy. Reduced-carbon alloys gave the greatest post-sintering ductility, but showed a lower 0.2% yield stress. The techniques of thermal activation analysis were used to investigate the effect of second phases upon the initial low-strain work-hardening rates and the 0.2% yield stress. It was found that the work-hardening rate from the elastic limit to a total strain of about 0.01 to 0.02 depends, in part, directly on the volume fraction of second phase.

Pseudoelastic behavior of magnesium alloy during twinning-dominated cyclic deformation

Abstract: In - situ neutron diffraction has been used to examine the pseudoelastic behavior of an extruded Mg–8.5 wt.% Al alloy during twinning-dominated cyclic deformation in both tension and compression. Twinning activity is effectively tracked through the intensity variations of the diffraction peaks for some grain orientations. The results suggest that a fundamental difference in the pseudoelastic behavior between tension and compression cyclic loadings might be due to reversible detwinning–retwinning observed only during cyclic compression.