Showing papers by "Chalk River Laboratories published in 2010"

The location and behavior of α‐tocopherol in membranes

Abstract: Vitamin E (alpha-tocopherol) has long been recognized as the major antioxidant in biological membranes, and yet many structurally related questions persist of how the vitamin functions. For example, the very low levels of alpha-tocopherol reported for whole cell extracts question how this molecule can successfully protect the comparatively enormous quantities of PUFA-containing phospholipids found in membranes that are highly susceptible to oxidative attack. The contemporary realization that membranes laterally segregate into regions of distinct lipid composition (domains), we propose, provides the answer. We hypothesize alpha-tocopherol partitions into domains that are enriched in polyunsaturated phospholipids, amplifying the concentration of the vitamin in the place where it is most needed. These highly disordered domains depleted in cholesterol are analogous, but organizationally antithetical, to the well-studied lipid rafts. We review here the ideas that led to our hypothesis. Experimental evidence in support of the formation of PUFA-rich domains in model membranes is presented, focusing upon docosahexaenoic acid that is the most unsaturated fatty acid commonly found. Physical methodologies are then described to elucidate the nature of the interaction of alpha-tocopherol with PUFA and to establish that the vitamin and PUFA-containing phospholipids co-localize in non-raft domains.

Cholesterol in Bilayers with PUFA Chains: Doping with DMPC or POPC Results in Sterol Reorientation and Membrane-Domain Formation

Abstract: Using neutron diffraction Harroun et al. [(2006) Biochemistry 45, 1227-1233; (2008) Biochemistry 47, 7090-7096] carried out studies that unequivocally demonstrated cholesterol preferentially sequestering in the middle of bilayers (i.e., flat orientation) made of lipids with polyunsaturated fatty acids (PUFA), in contrast to its "usual" position where its hydroxyl group locates near the lipid/water interface (i.e., upright orientation). Here we clearly show, using neutron diffraction, cholesterol's orientational preference in different lipid bilayers. For example, although it requires 50 mol % POPC (16:0-18:1 PC) in DAPC (di20:4 PC) bilayers to cause cholesterol to revert to its upright orientation, only 5 mol % DMPC (di14:0 PC) is needed to achieve the same effect. This result demonstrates not only cholesterol's affinity for saturated hydrocarbon chains, but also its aversion for PUFAs. Molecular dynamics (MD) simulations performed on similar systems show that in high PUFA content bilayers cholesterol is simultaneously capable of assuming different orientations within a bilayer. Although this result is known from previous MD studies by Marrink et al. [(2008) J. Am. Chem. Soc. 130, 10-11], it has yet to be confirmed experimentally. Importantly, MD simulations predict the formation of DMPC-rich domains, data corroborated by experiment (i.e., 10 mol % DMPC-doped DAPC bilayers), where cholesterol preferentially locates in its upright orientation, while in DMPC-depleted domains cholesterol is found mostly in the bilayer center (i.e., flat orientation). These results lend credence to DMPC's aversion for PUFAs, supporting the notion that domain formation is primarily driven by lipids.

Magnetic properties of the geometrically frustrated S=(1)/(2) antiferromagnets, La 2 LiMoO 6 and Ba 2 YMoO 6 , with the B-site ordered double perovskite structure: Evidence for a collective spin-singlet ground state

Abstract: Two B-site ordered double perovskites, ${\text{La}}_{2}{\text{LiMoO}}_{6}$ and ${\text{Ba}}_{2}{\text{YMoO}}_{6}$, based on the $S=\frac{1}{2}$ ion, ${\text{Mo}}^{5+}$, have been investigated in the context of geometric magnetic frustration. Powder neutron diffraction, heat capacity, susceptibility, muon-spin relaxation $(\ensuremath{\mu}\text{SR})$, and $^{89}\text{Y}$ NMR-including magic-angle spinning (MAS) NMR data have been collected. ${\text{La}}_{2}{\text{LiMoO}}_{6}$ crystallizes in $P{2}_{1}/n$ with $a=5.59392(19)\text{ }\text{\AA{}}$, $b=5.69241(16)\text{ }\text{\AA{}}$, $c=7.88029(22)\text{ }\text{\AA{}}$, and $\ensuremath{\beta}=90.2601(30)\ifmmode^\circ\else\textdegree\fi{}$ at 299.7 K, while ${\text{Ba}}_{2}{\text{YMoO}}_{6}$ is cubic, $Fm3m$, with $a=8.39199(65)\text{ }\text{\AA{}}$ at 297.8 K. ${\text{Ba}}_{2}{\text{YMoO}}_{6}$ shows no distortion from cubic symmetry even at 2.8 K in apparent violation of the Jahn-Teller theorem for a ${t}_{2\text{g}}^{1}$ ion. $^{89}\text{Y}$ NMR MAS data indicate about a 3% level of Y/Mo site mixing. ${\text{La}}_{2}{\text{LiMoO}}_{6}$ deviates strongly from simple Curie-Weiss (C-W) paramagnetic behavior below 150 K and zero-field-cooled/field-cooled (ZFC/FC) irreversibility occurs below 20 K with a weak, broad susceptibility maximum near 5 K in the ZFC data. A Curie-Weiss fit shows a reduced ${\ensuremath{\mu}}_{\text{eff}}=1.42{\ensuremath{\mu}}_{\text{B}}$, (spin $\text{only}=1.73{\ensuremath{\mu}}_{\text{B}}$) and a Weiss temperature, ${\ensuremath{\theta}}_{\text{C}}$, which depends strongly on the temperature range of the fit. Powder neutron diffraction and heat capacity show no evidence for long-range magnetic order to 2 K. On the other hand oscillations develop below 20 K in $\ensuremath{\mu}\text{SR}$ indicating at least short-range magnetic correlations. Susceptibility data for ${\text{Ba}}_{2}{\text{YMoO}}_{6}$ also deviate strongly from the C-W law below 150 K with a nearly spin only ${\ensuremath{\mu}}_{\text{eff}}=1.72{\ensuremath{\mu}}_{\text{B}}$ and ${\ensuremath{\theta}}_{\text{C}}=\ensuremath{-}219(1)\text{ }\text{K}$. There is no discernible ZFC/FC irreversibility to 2 K. Heat capacity, neutron powder diffraction, and $\ensuremath{\mu}\text{SR}$ data show no evidence for long-range order to 2 K but a very broad, weak maximum appears in the heat capacity. The $^{89}\text{Y}$ NMR paramagnetic Knight shift shows a remarkable local spin susceptibility behavior below about 70 K with two components from roughly equal sample volumes, one indicating a singlet state and the other a strongly fluctuating paramagnetic state. Further evidence for a singlet state comes from the behavior of the relaxation rate, $1/{T}_{1}$. These results are discussed and compared with those from other isostructural $S=\frac{1}{2}$ materials and those based on $S=3/2$ and $S=1$.

Residual stress characterization in low transformation temperature 13%Cr-4%Ni stainless steel weld by neutron diffraction and the contour method

Abstract: This study presents the results of residual stress characterization by neutron diffraction and the contour method on 13%Cr–4%Ni welds made using 410NiMo weld filler metal. The transverse, longitudinal and normal components of stress were determined by neutron diffraction. The longitudinal stress distribution was also measured by the contour method. The last bead of the weld was found to be in a state of triaxial compression while a part of the heat-affected zone as well as a region beneath the weld were in a state of longitudinal tension. These results are explained with reference to the low martensitic transformation start temperature (Ms) of the alloy. The same measurements were made on an identical weld that had undergone a standardized post-weld heat treatment. The maximum tensile stress was reduced from 534 to 136 MPa, and the maximum compressive stress was reduced from 371 to 152 MPa.

Effects of basal texture on mechanical behaviour of magnesium alloy AZ31B sheet

Abstract: The effect of the basal texture on the mechanical behaviour of AZ31B magnesium alloy sheet is studied numerically using a recently developed Elastic Visco-Plastic Self-consistent (EVPSC) model, in which both slip and twinning contribute to plastic deformation. We simulate uniaxial tension within the sheet plane using starting textures having the basal poles tilted at varying degrees from the normal direction. It is found that increasing the tilt angle increases the uniform strain, but decreases the 0.2% proof strength and the R -value. The effect of texture on sheet metal forming is further assessed by calculating the limit strain under in-plane plane strain tension. It is demonstrated that the limit strain increases dramatically with decreasing intensity of the basal texture. The numerical results are found to be in good qualitative agreement with experimental observations.

Experimental study of residual stresses in laser clad AISI P20 tool steel on pre-hardened wrought P20 substrate

Abstract: Laser cladding is to deposit desired material onto the surface of a base material (or substrate) with a relatively low heat input to form a metallurgically sound and dense clad. This process has been successfully applied for repairing damaged high-value tooling to reduce their through-life cost. However, laser cladding, which needs to melt a small amount of a substrate along with cladding material, inevitably introduces residual stresses in both clad and substrate. The tensile residual stresses in the clad could adversely affect mechanical performance of the substrate being deposited. This paper presents an experimental study on process-induced residual stresses in laser clad AISI P20 tool steel onto pre-hardened wrought P20 base material and the correlation with microstructures using hole-drilling and neutron diffraction methods. Combined with X-ray diffraction and scanning electron microscopic analyses, the roles of solid-state phase transformations in the clad and heat-affected zone (HAZ) of the substrate during cladding and post-cladding heat treatments on the development and controllability of residual stresses in the P20 clad have been investigated, and the results could be beneficial to more effective repair of damaged plastic injection molds made by P20 tool steel.

Orientational ordering, tilting and lone-pair activity in the perovskite methylammonium tin bromide, CH3NH3SnBr3.

Abstract: Synchrotron powder diffraction data from methylammonium tin bromide, CH3NH3SnBr3, taken as a function of temperature, reveal the existence of a phase between 230 and 188 K crystallizing in Pmc21, a = 5.8941 (2), b = 8.3862 (2), c = 8.2406 (2) A. Strong ferroelectric distortions of the octahedra, associated with stereochemical activity of the Sn 5s2 lone pair, are evident. A group analysis and decomposition of the distortion modes of the inorganic framework with respect to the cubic parent is given. The primary order parameters driving this upper transition appear to be an in-phase tilt (rotation) of the octahedra coupled to a ferroelectric mode. The precise nature of the lower-temperature phase remains uncertain, although it appears likely to be triclinic. Density-functional theory calculations on such a triclinic cell suggest that directional bonding of the amine group to the halide cage is coupled to the stereochemical activity of the Sn lone pair via the Br atoms, i.e. that the bonding from the organic component may have a strong effect on the inorganic sublattice (principally via switching the direction of the lone pair with little to no energy cost).

Intralayer Cation Ordering in a Brownmillerite Superstructure: Synthesis, Crystal, and Magnetic Structures of Ca2FeCoO5

Abstract: The synthesis, crystal, and magnetic structures and the bulk magnetic properties of Ca2FeCoO5, a brownmillerite type oxide, are presented. The crystal structure, solved and refined from single crystal X-ray and powder neutron diffraction data, is described in Pbcm with cell parameters, a = 5.3652(3) A, b = 11.0995(5) A, c = 14.7982(7) A. Thus, one axis, b in this setting, is doubled in comparison with the standard brownmillerite structure description giving rise to two sets of octahedral and tetrahedral sites. Aided by the strong scattering contrast between Fe and Co for neutrons, a nearly perfect intralayer cation site ordering, not observed for any brownmillerite before, is detected in the tetrahedral layers. There is a lesser degree of cation site ordering in the octahedral sites. Overall, the Fe/Co site ordering is of the NaCl type both within and between the tetrahedral and octahedral layers. There are also both intra- and interlayer ordering of tetrahedral chain orientations. The left- and right-han...

Structure and magnetic properties of the S = 1 geometrically frustrated double perovskites La 2 LiReO 6 and Ba 2 YReO 6

Abstract: Two B-site ordered double perovskites, ${\text{La}}_{2}{\text{LiReO}}_{6}$ and ${\text{Ba}}_{2}{\text{YReO}}_{6}$, with $S=1$ were investigated as geometrically frustrated antiferromagnets, using x-ray and neutron diffraction, superconducting quantum interference device magnetometry, heat capacity, muon spin relaxation $(\ensuremath{\mu}\text{SR})$, and $^{89}\text{Y}$ magic-angle spinning (MAS) NMR. ${\text{La}}_{2}{\text{LiReO}}_{6}$ has a monoclinic structure $(\text{P}{2}_{1}/\text{n})$ with cell parameters at room temperature; $a=5.58262(22)\text{ }\text{\AA{}}$, $b=5.67582(20)\text{ }\text{\AA{}}$, $c=7.88586(27)\text{ }\text{\AA{}}$, and $\ensuremath{\beta}=90.240(4)\ifmmode^\circ\else\textdegree\fi{}$. A zero-field cooled/field cooled (ZFC/FC) divergence at 50 K was observed in the susceptibility. The ZFC susceptibility is zero below $\ensuremath{\sim}5\text{ }\text{K}$ for polycrystalline samples, suggesting a cooperative singlet ground state but weak moments are induced by cooling in very small fields $\ensuremath{\sim}1\text{ }\text{mT}$. No evidence of long-range ordering is evident in heat capacity, neutron-diffraction, or $\ensuremath{\mu}\text{SR}$ data. The ZF spin dynamics from $\ensuremath{\mu}\text{SR}$ are anomalous and can be fitted to a stretched exponential rather than the Kubo-Toyabe form expected for random frozen spins but the muon spins are decoupled in longitudinal fields (LF), consistent with spin freezing of the fraction of spins relaxing within the muon time scale. The internal fields sensed by the muons are anomalously small, consistent with an electronic spin-singlet state. ${\text{Ba}}_{2}{\text{YReO}}_{6}$ is found to be cubic (Fm3m) with cell parameter $a=8.36278(2)\text{ }\text{\AA{}}$ at 300 K with no change in symmetry at 3.8 K, at variance with the Jahn-Teller theorem for a ${t}_{2\text{g}}^{2}$ configuration for ${\text{Re}}^{5+}$. $^{89}\text{Y}$ MAS NMR shows a single peak indicating that Y/Re site disorder is at most 0.5%. The susceptibility shows two broad peaks around 50 and 25 K but no evidence for long-range order from heat capacity, neutron diffraction, or $\ensuremath{\mu}\text{SR}$. The ZF $\ensuremath{\mu}\text{SR}$ result shows a two-component ground state with both slow and fast relaxations and decoupling results in a 1 kG LF, indicating spin freezing. These results are in sharp contrast to the long-range AF order found in the $S=3/2$ isostructural materials, ${\text{La}}_{2}{\text{LiRuO}}_{6}$ and ${\text{Ba}}_{2}{\text{YRuO}}_{6}$, indicating that the reduction to $S=1$ plays a major role in ground state determination.

Effect of the pseudogap on suppressing high energy inelastic neutron scattering in superconducting YBa 2 Cu 3 O 6.5

Abstract: We have measured the spin fluctuations in the ${\text{YBa}}_{2}{\text{Cu}}_{3}{\text{O}}_{6.5}$ $({\text{YBCO}}_{6.5},{T}_{c}=59\text{ }\text{K})$ superconductor at high-energy transfers above $\ensuremath{\sim}100\text{ }\text{meV}$. Within experimental error, the momentum dependence is isotropic at high energies, similar to that measured in the insulator for two-dimensional spin waves, and the dispersion extrapolates back to the incommensurate wave vector at the elastic position. This result contrasts with previous expectations based on measurements around 50 meV which were suggestive of a softening of the spin-wave velocity with increased hole doping. Unlike the insulator, we observe a significant reduction in the intensity of the spin excitations for energy transfers above $\ensuremath{\sim}100\text{ }\text{meV}$ similar to that observed above $\ensuremath{\sim}200\text{ }\text{meV}$ in the ${\text{YBCO}}_{6.35}$ $({T}_{c}=18\text{ }\text{K})$ superconductor as the spin waves approach the zone boundary. We attribute this high-energy scale with a second gap and find agreement with measurements of the pseudogap in the cuprates associated with electronic anomalies along the antinodal positions. In addition, we observe a sharp peak at around 400 meV whose energy softens with increased hole doping. We discuss possible origins of this excitation including a hydrogen-related molecular excitation and a transition of electronic states between $d$ levels.

Diffusion in single supported lipid bilayers studied by quasi-elastic neutron scattering

Abstract: It seems to be increasingly accepted that the diversity and composition of lipids play an important role in the function of biological membranes. A prime example of this is the case of lipid rafts; regions enriched with certain types of lipids which are speculated to be relevant to the proper functioning of membrane embedded proteins. Although the dynamics of membrane systems have been studied for decades, the microscopic dynamics of lipid molecules, even in simple model systems, is still an active topic of debate. Neutron scattering has proven to be an important tool for accessing the relevant nanometre length scale and nano to picosecond time scales, thus providing complimentary information to macroscopic techniques. Despite their potential relevance for the development of functionalized surfaces and biosensors, the study of single supported membranes using neutron scattering poses the challenge of obtaining relevant dynamic information from a sample with minimal material. Using state of the art neutron instrumentation we were, for the first time, able to model lipid diffusion in single supported lipid bilayers. We find that the diffusion coefficient for the single bilayer system is comparable to the multi-lamellar lipid system. More importantly, the molecular mechanism for lipid motion in the single bilayer was found to be a continuous diffusion, rather than the flow-like ballistic motion reported in the stacked membrane system. We observed an enhanced diffusion at the nearest neighbour distance of the lipid molecules. The enhancement and change of character of the diffusion can most likely be attributed to the effect the supporting substrate has on the lipid organization.

Bicellar mixtures containing pluronic F68: morphology and lateral diffusion from combined SANS and PFG NMR studies.

Abstract: Small angle neutron scattering (SANS) and pulsed field gradient (PFG) nuclear magnetic resonance (NMR) diffusion measurements were applied to examine morphology and diffusion in dimyristoyl- plus dihexanoyl-phosphatidylcholine bicellar mixtures, either neutral or negatively charged, incorporating a Pluronic triblock copolymer (F68). Negatively charged bicellar mixtures, doped with dimyristoylphosphatidylglycerol (DMPG), exhibited SANS profiles consistent with a perforated lamellar morphology for the magnetically alignable phase. Correspondingly, F68 diffusion in this magnetically aligned phase was normal Gaussian, in that the mean square displacements increased linearly with the experimental diffusion time, with a lateral diffusion coefficient of 1.9 x 10(-11) m(2) s(-1) consistent with a lipid bilayer inserted configuration. Neutral bicellar mixtures, that is, lacking DMPG, in contrast, displayed SANS profiles characteristic of ribbons arranged in such a fashion as to produce extended lamellae. Within the lamellae, the ribbons exhibited an in-plane periodicity (interribbon) of between 120 and 140 A. Correspondingly, F68 diffusion was non-Gaussian, exhibiting a square root diffusion time dependence of the mean square displacement indicative of one-dimensional curvilinear diffusion. The presence or absence of DMPG, rather than of F68, dictated the ribbon versus lamellar morphology, with F68 reflecting this difference via its lateral diffusion behavior. Although ribbons have been reported previously, this is the first study to show that they aggregate, most likely into extended lamellar sheets, and eventually fold into multilamellar vesicles.

Neutron scattering study of the classical antiferromagnet MnF2: a perfect hands-on neutron scattering teaching courseSpecial issue on Neutron Scattering in Canada.

Abstract: We present the classical antiferromagnet MnF2 as a perfect demonstration system for teaching a remarkably wide variety of neutron scattering concepts. The nature of antiferromagnetism and the magne...

The Effect of Microstructure on Delayed Hydride Cracking Behavior of Zircaloy-4 Fuel Cladding—An International Atomic Energy Agency Coordinated Research Program

Abstract: The rate of delayed hydride cracking (DHC) has been measured in Zircaloy-4 fuel cladding in several metallurgical conditions using the pin-loading tension technique. In light water reactor (LWR) cladding in the cold-worked and cold-worked and stress-relieved conditions, the cracking rate followed Arrhenius behavior up to about 280 °C, but at higher temperatures the rate declined with no cracking above 300°C. Non-LWR cladding appeared to behave in the same manner. In LWR cladding in the recrystallized condition, the cracking rate was highly variable because it depended on KI within the test range up to 25 MPa√m, whereas in the other LWR claddings, cracking rate was independent of KI, indicating that KIH was below 11 MPa√m. The main role of microstructure was to control the material strength; the cracking rate increased as the strength increased. Although all the claddings had a radial texture, it did not protect the cladding from DHC. The DHC fracture surface consisted of flat broken hydrides, often in arcs, but no striations were observed, except in one specimen subjected to thermal cycles.

Universal magnetic and structural behaviors in the iron arsenides

Abstract: Commonalities among the order parameters of the ubiquitous antiferromagnetism present in the parent compounds of the iron arsenide high-temperature superconductors are explored. Additionally, comparison is made between the well established two-dimensional Heisenberg-Ising magnet, ${\text{K}}_{2}{\text{NiF}}_{4}$, and iron arsenide systems residing at a critical point whose structural and magnetic phase transitions coincide. In particular, analysis is presented regarding two distinct classes of phase-transition behavior reflected in the development of antiferromagnetic and structural order in the three main classes of iron arsenide superconductors. Two distinct universality classes are mirrored in their magnetic phase transitions which empirically are determined by the proximity of the coupled structural and magnetic phase transitions in these materials.

Magnetic phase transition in V2O3 nanocrystals

Abstract: V{sub 2}O{sub 3} nanocrystals can be synthesized through hydrothermal reduction in VO(OH){sub 2} using hydrazine as a reducing agent. Addition of different ligands to the reaction produces nanoparticles, nanorods, and nanoplatelets of different sizes. Small nanoparticles synthesized in this manner show suppression of the magnetic phase transition to lower temperatures. Using muon spin relaxation spectroscopy and synchrotron x-ray diffraction, we have determined that the volume fraction of the high-temperature phase, characterized by a rhombohedral structure and paramagnetism, gradually declines with decreasing temperature, in contrast to the sharp transition observed in bulk V{sub 2}O{sub 3}.

Small-Angle Scattering from Homogenous and Heterogeneous Lipid Bilayers

Abstract: Advances in colloid and interface science have stimulated a renewed interest in the study of lipid–water systems. At the same time, much progress has been made in regards to the analysis of small-angle X-ray and neutron scattering data. The popularity of small-angle scattering for the study of biologically relevant materials stems from the fact that it provides detailed information on the size, shape, and conformation of molecular assemblies in solution. Additionally, neutron scattering has the capability to accentuate, or nullify, the scattering from individual parts of a macromolecular complex with its unique ability to distinguish between hydrogen and its isotope deuterium. As a result, structural biophysics has taken advantage of recent developments in small-angle scattering to accurately determine the structure of lipid bilayers in both the transverse and lateral directions. An example is the joint refinement of X-ray and neutron scattering data, which has been used to improve the values of lipid areas that are commonly used in molecular dynamics simulations. Advances in neutron scattering theory have also made possible the determination of the membrane's in-plane organization by enabling the detection and characterization of nanoscopic domains.

Neutron diffraction measurements of residual stresses around a crack tip developed under variable‐amplitude fatigue loadings

Abstract: The spatially resolved neutron-diffraction residual stress mappings were performed on five compact-tension (CT) specimens subjected to various variable-amplitude fatigue loadings (e.g. overload, underload and their mixed loads) during fatigue crack propagation. Three principal residual-stress components (i.e. longitudinal, transverse and normal stresses) were measured as a function of the distance from the crack tip along the crack-propagation direction. The shape of respective crack tips on the five CT specimens was examined using scanning electron microscope. The results show the distinct residual-stress fields near the crack tip and significant changes in the crack-tip geometry for five different loading cases. It is thought that the combined effects of the changes in the residual-stress state and crack-tip geometry seem to be a key factor to account for the observed transient crack-growth phenomena.

USING NEUTRON DIFFRACTION AND MÖSSBAUER SPECTROSCOPY TO STUDY MAGNETIC ORDERING IN THE R3T4Sn4 FAMILY OF COMPOUNDS

Abstract: We review the complementary roles that 119Sn Mossbauer spectroscopy and neutron diffraction are playing in developing a complete description of magnetic ordering in the R3 Cu4Sn4 and R3Ag4Sn4 intermetallic compound series. We show that the two techniques yield consistent pictures of the order, and in many cases both are essential to obtaining a complete description. The recent neutron diffraction work on Sm3Cu4Sn4, Sm3Ag4Sn4 and Gd3Ag4Sn4 is highlighted.

Cosmic ray muon tomography system using drift chambers for the detection of Special Nuclear Materials

Abstract: The smuggling of illicit Special Nuclear Materials (SNM) and Radiological Materials (RM) is a major security concern Current radiation detection systems for cargo are not sensitive to well-shielded nuclear materials The Muon Scattering Tomography (MST) method that we are developing might be a solution to this problem It is based on the measurement of multiple scattering of cosmic ray-induced muons, traversing high-Z materials such as uranium and plutonium This is possible due to the muons' highly penetrating nature The technique involves measuring the angular deflections of these muons with charged particle tracking detectors placed around the object to be probed One candidate detector is the single wire drift chamber It can measure the 2-D impact position of a muon The CRIPT (Cosmic Ray Inspection and Passive Tomography) collaboration has performed computer simulations of detectors designed to detect SNM via MST We have also worked on the development of image reconstruction algorithms, and simulated the performances of different muon spectrometer designs and the response of the drift chambers In addition to these efforts, the collaboration has built three prototypes which are being tested

Non-collinear antiferromagnetism in FeCrAs Special issue on Neutron Scattering in Canada.

Abstract: FeCrAs undergoes a magnetic ordering transition at TN ≃ 125 K. The magnetic structure of FeCrAs, which crystallizes in space group P6¯2m and is isostructural to Fe2P, was determined by constant wav...

Diffusion of Hydrogen in Single Crystals of Monoclinic-ZrO2 and Yttrium Stabilized Cubic Zirconia

Abstract: Transport of hydrogen through oxide layers formed on zirconium alloys has been the focus of studies associated with hydrogen ingress into nuclear reactor core components. The studies have shown that microstructure and microchemistry of the underlying alloy can affect the characteristics of the oxide and in turn the transport of hydrogen through the oxide and into the underlying metal. In certain cases the oxide layer can be a homogeneous medium for hydrogen diffusion, while in most cases it is found to be heterogeneous and comprised of homogeneous, nonporous and fully oxidized, cells surrounded by a network of fast diffusion paths for hydrogen. Since in such heterogeneous systems the derived diffusion parameter and the cell size are interdependent, an absolute diffusion parameter could only be derived if the cell size were known. However, no such information is available from the microstructural studies on the oxides grown on these alloys. Another alternative is to carry out the measurements in a material, which is homogeneous at least within the dimensions of a few μm - the range of the measurements. With this in mind, hydrogen diffusion measurements were carried out in a set of single crystal zirconia specimens with the prospect that the single crystals would provide a non-porous and homogeneous medium to study the diffusivity of hydrogen. These measurements show that the single crystal specimens, contrary to the initial thinking, are not entirely homogeneous and the results do not yield an absolute diffusion coefficient for hydrogen in zirconium oxides. The details and analyses of the results from the single crystal zirconia specimens are discussed in this paper.