Showing papers by "Chalk River Laboratories published in 2005"

Risk of cancer after low doses of ionising radiation: retrospective cohort study in 15 countries

Abstract: Objectives To provide direct estimates of risk of cancer after protracted low doses of ionising radiation and to strengthen the scientific basis of radiation protection standards for environmental, occupational, and medical diagnostic exposures. Design Multinational retrospective cohort study of cancer mortality. Setting Cohorts of workers in the nuclear industry in 15 countries. Participants 407 391 workers individually monitored for external radiation with a total follow-up of 5.2 million person years. Main outcome measurements Estimates of excess relative risks per sievert (Sv) of radiation dose for mortality from cancers other than leukaemia and from leukaemia excluding chronic lymphocytic leukaemia, the main causes of death considered by radiation protection authorities. Results The excess relative risk for cancers other than leukaemia was 0.97 per Sv, 95% confidence interval 0.14 to 1.97. Analyses of causes of death related or unrelated to smoking indicate that, although confounding by smoking may be present, it is unlikely to explain all of this increased risk. The excess relative risk for leukaemia excluding chronic lymphocytic leukaemia was 1.93 per Sv ( < 0 to 8.47). On the basis of these estimates, 1-2% of deaths from cancer among workers in this cohort may be attributable to radiation. Conclusions These estimates, from the largest study of nuclear workers ever conducted, are higher than, but statistically compatible with, the risk estimates used for current radiation protection standards. The results suggest that there is a small excess risk of cancer, even at the low doses and dose rates typically received by nuclear workers in this study.

Residual stresses in LENS® components using neutron diffraction and contour method

Abstract: During manufacturing of components by laser engineered net shaping (LENS®), a solid freeform fabrication process, the introduction of residual stresses causes deformation or in the worst case, cracking. The origin is attributed to thermal transients encountered during solidification. In the absence of reliable predictive models for the residual stresses, measurements are necessary. Residual stresses were measured in LENS® samples of 316 stainless steel and Inconel 718 having simple geometrical shapes by both neutron diffraction and the contour methods. The results by the two methods are compared and discussed in the context of the growth direction during the LENS® process. Surprisingly, the residual stresses are practically uni-axial, with high stresses in the growth direction.

From incommensurate to dispersive spin-fluctuations: The high-energy inelastic spectrum in superconducting YBa2Cu3O6.5

Abstract: We have investigated the spin-fluctuations at energy transfers up to $\ensuremath{\sim}110\phantom{\rule{0.3em}{0ex}}\mathrm{meV}$, well above the resonance energy $(33\phantom{\rule{0.3em}{0ex}}\mathrm{meV})$ in the $\mathrm{Y}{\mathrm{Ba}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{6.5}$ ortho-II superconductor using neutron time-of-flight and triple-axis techniques. The spectrum at high energies differs from the low-energy incommensurate modulations previously reported where the incommensurate wave vector is largely independent of energy. Well above the resonance the peak of the spin response lies at wave vectors that increase with energy. Within error the excitations at all energies above the resonance are best described by a ring around the $(\ensuremath{\pi},\ensuremath{\pi})$ position. The isotropic wave-vector pattern differs from a recently reported square pattern in different but related systems. The spin spectral weight at high energies is similar to that in the insulator but the characteristic velocity is $\ensuremath{\sim}40%$ lower. We introduce a method of extracting the acoustic and optic weights at all energies from time-of-flight data. We find that the optic spectral weight extends to surprisingly low energies of $\ensuremath{\sim}25\phantom{\rule{0.3em}{0ex}}\mathrm{meV}$, and infer that the bilayer spin correlations weaken with increase in hole doping. When the low-energy optic excitations are taken into account we measure the total integrated weight around $(\ensuremath{\pi},\ensuremath{\pi})$, for energies below $120\phantom{\rule{0.3em}{0ex}}\mathrm{meV}$, to agree with that expected from the insulator. As a qualitative guide, we compare spin-wave calculations based on models with ordered and disordered stripes and describe the inadequacy of these and other stripe models for the high-energy fluctuations.

Bicellar lipid mixtures as used in biochemical and biophysical studies

Abstract: Over the past decade “bicellar” lipid mixtures composed of the long-chain dimyristoyl phosphatidylcholine (DMPC) and the short-chain dihexanoyl PC (DHPC) molecules have emerged as a powerful medium for studying membrane associated, biologically relevant macromolecules and assemblies. Depending on temperature, lipid concentration and composition these lipid mixtures can assume a variety of morphologies, some of them alignable in the presence of a magnetic field. This article will examine the biophysical studies that have elucidated the various morphologies assumed by these lipid mixtures, and their use in the biochemical studies of biomolecules.

Detection of submicron-sized raft-like domains in membranes by small-angle neutron scattering.

Abstract: Using coarse grained models of heterogeneous vesicles we demonstrate the potential for small-angle neutron scattering (SANS) to detect and distinguish between two different categories of lateral segregation: 1) unilamellar vesicles (ULV) containing a single domain and 2) the formation of several small domains or “clusters” (~10 nm in radius) on a ULV. Exploiting the unique sensitivity of neutron scattering to differences between hydrogen and deuterium, we show that the liquid ordered (lo) DPPC-rich phase can be selectively labeled using chain deuterated dipalymitoyl phosphatidylcholine (dDPPC), which greatly facilitates the use of SANS to detect membrane domains. SANS experiments are then performed in order to detect and characterize, on nanometer length scales, lateral heterogeneities, or so-called “rafts”, in ~30 nm radius low polydispersity ULV made up of ternary mixtures of phospholipids and cholesterol. For 1:1:1 DOPC:DPPC:cholesterol (DDC) ULV we find evidence for the formation of lateral heterogeneities on cooling below 30 °C. These heterogeneities do not appear when DOPC is replaced by SOPC. Fits to the experimental data using coarse grained models show that, at room temperature, DDC ULV each exhibit approximately 30 domains with average radii of ~10 nm.

Melilite-type borates Bi2ZnB2O7 and CaBiGaB2O7

Abstract: The bismuth borates Bi2ZnB2O7 and CaBiGaB2O7 have been synthesized by solid-state reactions at temperatures in the 650−825 °C range at 1 atm pressure. These compounds are the only synthetic diborate members of the melilite family, A2XZ2O7, in which layers of A cations alternate with XZ2O7 tetrahedral layers. Except for CdBiGaB2O7, the synthesis of other substituted bismuth borate melilites has been unsuccessful. The crystal structures of Bi2ZnB2O7 and CaBiGaB2O7 have been determined by powder X-ray diffraction and refined by the Rietveld method using powder neutron diffraction data. CaBiGaB2O7 adopts the regular tetragonal melilite structure (P421m space group, Z = 2) containing B2O7 tetrahedral dimers. The refinement of split eight-coordinated sites for the Ca2+ and Bi3+ interlayer cations suggests the presence of additional disorder. Bi2ZnB2O7 adopts a unique orthorhombic melilite superstructure (Pba2 space group, Z = 4) containing both tetrahedral B2O7 and triangular B2O5 dimers. The Bi3+ cations occu...

Comparative Study of Microwave Sintering of Zinc Oxide at 2.45, 30, and 83 GHz

Abstract: Temperature gradients that develop in ceramic materials during microwave heating are known to be strongly dependent on the applied microwave frequency. To gain a better understanding of this dependence, identical samples of ZnO powder compacts were microwave heated at three distinct widely separated frequencies of 2.45, 30, and 83 GHz and the core and surface temperatures were simultaneously monitored. At 2.45 GHz, the approximately uniform “volumetric” heating tends to raise the temperature of the sample as a whole, but the interior becomes hotter than the exterior because of heat loss from the surface. At 30 and 83 GHz, this interior to exterior temperature difference was found to be reversed, especially for high heating rates. This reversal resulted from increased energy deposition close to the sample's surface associated with reduced skin depth. A model for solving Maxwell's equations was incorporated into a newly developed two-dimensional (2-D) heat transport simulation code. The numerical simulations are in agreement with the experimental results. Simultaneous application of two or more widely separated frequencies is expected to allow electronic tailoring of the temperature profile during sintering.

Response of superheated droplet detectors of the PICASSO dark matter search experiment

Abstract: We present results of systematic studies of the radiation response of superheated liquid droplet detectors, which are used in the PICASSO dark matter search experiment. This detection technique is based on the phase transitions of superheated liquid Freon droplets dispersed and trapped in a polymerized gel. Phase transitions can be induced by nuclear recoils following particle interactions and, in particular, interactions with Weakly Interacting Massive Particles (WIMPs). These detectors are threshold devices since a minimal energy deposition is necessary to induce a phase transition and their sensitivity to various types of radiation depends strongly on the operating temperature and pressure. The sensitivity to neutrons, α-particles and γ-rays was determined as a function of these operating parameters and the results are compared with simulations. In particular, we present a complete characterization of the response of detector modules already in use for a dark matter search at the SNO site to detect WIMPs and discuss possible background sources.

Corrosion Behavior of Uranium Dioxide in Alpha Radiolytically Decomposed Water

Abstract: The response of the corrosion potential of a UO 2 electrode to oxidants (H 2 O 2 ,O 2 ) produced by the alpha radiolysis of water has been measured in a thin-layer electrochemical cell. This cell allows the electrode to be brought within ∼25 μm of a gold-plated alpha source, thereby ensuring the uniform distribution of radiolytic oxidants in the aqueous solution filling the gap. The kinetics of water radiolysis, coupled with the surface-catalyzed decomposition of H 2 O 2 and diffusive transport of radiolytic species out of the electrode-source gap, was modeled using a finite difference description of the cell and the commercial numerical integration software, Facsimile. Using this approach, the mechanism of UO 2 corrosion in the presence of alpha radiolysis was shown to be dominated by reaction of the fuel with radiolytically produced H 2 O 2 . To explain the results, two surface-catalyzed H 2 O 2 decomposition processes were invoked: one with H 2 to produce H 2 O, and a second process leading to the production of O 2 and H 2 O. For sufficiently high alpha source strengths, the fuel behavior becomes redox-buffered, i.e., independent of alpha source strength, due to the presence of a corrosion product deposit.

Strong Influence of the Diffuse Component on the Lattice Dynamics in Pb(Mg1/3Nb2/3)O3

Abstract: The temperature and zone dependence of the lattice dynamics in Pb(Mg 1/3 Nb 2/3 )O 3 is characterized using neutron inelastic scattering. Through a detailed study of the phonon lineshape and a comparison of the structure factors to SrTiO 3 , we conclude that the coupling between the acoustic and optic modes is weak. However, based on a direct comparison between the (110) Brillouin zone where the diffuse scattering is strong to the (220) zone where the diffuse scattering is weak, a strong coupling between the diffuse component and low-energy acoustic phonon mode is observed. We conclude that the coupling to the diffuse component is the reason for several recent conflicting interpretations of the lattice dynamics based on data from zones with a strong diffuse component.

Novel coexistence of superconductivity with two distinct magnetic orders.

Abstract: The heavy fermion ${\mathrm{CeRh}}_{1\ensuremath{-}x}{\mathrm{Ir}}_{x}{\mathrm{In}}_{5}$ system exhibits properties that range from an incommensurate antiferromagnet for small $x$ to an exotic superconductor on the Ir-rich end of the phase diagram. At intermediate $x$ where antiferromagnetism coexists with superconductivity, two types of magnetic order are observed: the incommensurate one of ${\mathrm{CeRhIn}}_{5}$ and a new, commensurate antiferromagnetism that orders separately. The coexistence of $f$-electron superconductivity with two distinct $f$-electron magnetic orders is unique among unconventional superconductors, adding a new variety to the usual coexistence found in magnetic superconductors.

Neutron powder diffraction study of the orientational order-disorder phase transition in calcite, CaCO3

Abstract: Neutron powder diffraction studies of calcite on heating towards the orientational order–disorder phase transition show that the phase transition is not a simple analogue of an Ising-like transition, but more similar to a rotational analogue of Lindemann melting. The transition is precipitated by the librational amplitude of the carbonate molecular ions exceeding a critical value rather than a result of a statistical entropy of ‘wrong’ orientations. Using tested interatomic potentials the single-particle orientational potential and nearest-neighbour orientational interactions have been calculated.

Tilt and acoustic instabilities in ABX4, A2BX4 and ABX3 perovskite structure types: their role in the incommensurate phases of the organic-inorganic perovskites.

Abstract: An examination of the tilt modes and other low-frequency modes is made for an isolated, untilted perovskite layer, which maps very simply to the ABX4 perovskites. A sheet of pure tilts exists at the Brillouin Zone boundary at {ξ, ½, ζ}. An instability is also found at all wavevectors which can be described as continuously varying from pure tilts to pure layer displacements as a function of the wavevector. Analysis is extended by considering the stacking of layers in the I-centered A2BX4-layer perovskites. The effect of freezing in the commensurate tilt required to generate the Cmca tilt system, pertinent to the modulated phases of propylammonium salts, is examined. The zero-frequency modes are restricted to two planes in the Brillouin Zone. All of the observed wavevectors associated with modulated phases, and the commensurately tilted propylammonium tetrachlorocadmate, are consistent with this calculation. The effect of full three-dimensional connectivity is briefly reviewed for the true ABX3 perovskites. While pure tilt incommensurates appear to be hypothetically possible, they do not appear to have been observed to date.

Correlation of diffraction peak broadening to crystal strengthening in finite element simulations

Abstract: Trends in the evolution of the distributions of crystal slip system strength and lattice strain were collected from finite element simulations of tensile loading of polycrystals. The loading programmes replicated physical experiments on two metallic alloys (AA5182 aluminum and AL6XN stainless steel) in which Bragg peaks were measured by neutron diffraction. Correlations between the broadening of the Bragg peaks and the evolution of slip system strength are demonstrated.

Structural Phase Behavior of High-Concentration, Alignable Biomimetic Bicelle Mixtures

Abstract: The structures of bicelle mixtures composed of dimyristoyl and dihexanoyl phosphatidylcholines (DMPC and DHPC) with DMPC/DHPC molar ratios of 3.2 and 5 are characterized using polarized optical microscopy (POM) and small angle neutron scattering (SANS). Three phases, isotropic (I), chiral nematic (N*) and smectic (S) are observed as temperature (T) varies from 10 to 70 °C. The structure of the magnetically alignable N phase, which was previously considered to be made up of discoidal micelles, is found to be composed of ribbons. Doping with the charged lipid, dimyristoyl phosphatidylglycerol (DMPG), which has the same 14:0 hydrocarbon chains as DMPC, results in a structural change of the aggregates where only the isotropic and smectic phases are observed. The smectic phase for the mixtures doped with DMPG is shear-alignable and follows one-dimensional swelling. However, at high-T zwitterionic DMPC/DHPC mixtures form multi-lamellar vesicles (MLV) with a relatively constant lamellar spacing of 66 A, independent of water content.

Combined neutron powder and X-ray single-crystal diffraction refinement of the atomic structure and hydrogen bonding of goslarite (ZnSO4·7H2O)

Abstract: The atomic structure of synthetic, deuterated goslarite (ZnSO4·7D2O), a = 11.8176(6) A, b = 12.0755(7) A, c = 6.8270(4) A, space group P 212121, Z = 4, has been refined in a combined neutron powder diffraction and X-ray single-crystal data refinement to wRp 1.92%, Rp 1.45% and R ( F 2) 12.66% for the neutron powder data contribution and R ( F 2) 8.72% for the X-ray single-crystal data contribution. Both data sets were necessary to achieve the best overall fit agreement in the Rietveld refinement and reasonable geometry within structural units. The results of this study confirm that the H-bonding scheme for goslarite is the same as that of the other epsomite group minerals. Small but significant variations of the Zn–O bond lengths can be attributed to details of the H bonds to the O atoms of the Zn octahedra. This investigation of the atomic structure and hydrogen bonding of goslarite is groundwork for future studies into phase relationships and the mechanisms of hydration and dehydration in the ZnSO4–H2O system.

Magnetic properties of the overdoped superconductor La 2 − x Sr x Cu O 4 with and without Zn impurities

Abstract: The magnetic properties of the Zn-substituted overdoped high-${T}_{c}$ superconductor ${\mathrm{La}}_{2\ensuremath{-}x}{\mathrm{Sr}}_{x}{\mathrm{Cu}}_{1\ensuremath{-}y}{\mathrm{Zn}}_{y}{\mathrm{O}}_{4}$ have been studied by magnetization measurements and neutron scattering. Magnetization measurements reveal that for Zn-free samples with $x\ensuremath{\geqslant}0.22$ a Curie term is induced in the temperature dependence of the magnetic susceptibility implying the existence of local paramagnetic moments. The induced Curie constant corresponds to a moment of $0.5\phantom{\rule{0.3em}{0ex}}{\ensuremath{\mu}}_{B}$ per additional ${\mathrm{Sr}}^{2+}$ ion that exceeds $x=0.22$. Zn substitution in the overdoped ${\mathrm{La}}_{2\ensuremath{-}x}{\mathrm{Sr}}_{x}\mathrm{Cu}{\mathrm{O}}_{4}$ also induces a Curie term that corresponds to $1.2\phantom{\rule{0.3em}{0ex}}{\ensuremath{\mu}}_{B}$ per ${\mathrm{Zn}}^{2+}$ ion, simultaneously suppressing ${T}_{c}$. The relationship between ${T}_{c}$ and the magnitude of the Curie term for Zn-free ${\mathrm{La}}_{2\ensuremath{-}x}{\mathrm{Sr}}_{x}\mathrm{Cu}{\mathrm{O}}_{4}$ with $x\ensuremath{\geqslant}0.22$ and for Zn-substituted ${\mathrm{La}}_{2\ensuremath{-}x}{\mathrm{Sr}}_{x}\mathrm{Cu}{\mathrm{O}}_{4}$ with $x=0.22$ are closely similar. This signifies a general competitive relationship between the superconductivity and the induced paramagnetic moment. Neutron scattering measurements show that Zn substitution in overdoped ${\mathrm{La}}_{2\ensuremath{-}x}{\mathrm{Sr}}_{x}\mathrm{Cu}{\mathrm{O}}_{4}$ anomalously enhances the inelastic magnetic scattering spectra around the $(\ensuremath{\pi},\ensuremath{\pi})$ position, peaking at $\ensuremath{\omega}\ensuremath{\sim}7\phantom{\rule{0.3em}{0ex}}\mathrm{meV}$. These facts are discussed on the basis of a ``swiss-cheese'' model of Zn-substituted systems as well as a microscopic phase separation scenario in the overdoped region indicated by muon-spin-relaxation measurements.