Showing papers by "Chalk River Laboratories published in 2011"

Short range ballistic motion in fluid lipid bilayers studied by quasi-elastic neutron scattering

Abstract: Diffusion is the primary mechanism for movement of lipids and proteins in the lateral direction of a biological membrane. In this paper we have used quasi-elastic neutron scattering to examine the diffusion process of lipid molecules in fluid DMPC membranes. We found that the motion over length scales greater than the lipid diameter could be characterized as a continuous diffusion process, with a diffusion coefficient of D = 64 × 10−12 m2/s. The continuous diffusion model has been successfully used in the past to describe the motion of lipid over long length scales. However, the focus of this measurement was to determine how the character of the molecular motion changes on length scales shorter than the nearest neighbour distance. At very short length scales (<2.37 A), we see first experimental evidence for a short-range flow-like ballistic motion.

Triplet and in-gap magnetic states in the ground state of the quantum frustrated fcc antiferromagnet Ba 2 YMoO 6

Abstract: The geometrically frustrated double perovskite Ba${}_{2}$YMoO${}_{6}$ is characterized by quantum $s=1/2$ spins at the Mo${}^{5+}$ sites of an undistorted fcc lattice. Previous low-temperature characterization revealed an absence of static long-range magnetic order and suggested a nonmagnetic spin-singlet ground state. We report unique time-of-flight and triple-axis neutron spectroscopy of Ba${}_{2}$YMoO${}_{6}$ that shows a 28 meV spin excitation with a bandwidth of $\ensuremath{\sim}$4 meV, which vanishes above $\ensuremath{\sim}$125 K. We identify this as the singlet-triplet excitation that arises out of a singlet ground state, and further identify a weaker continuum of magnetic states within the gap, reminiscent of spin-polaron states arising due to weak disorder.

New insights into particle detection with superheated liquids

Abstract: We report new results obtained from calibrations of superheated liquid droplet detectors used in dark matter searches with different radiation sources (n, α, γ). In particular, detectors were spiked with α-emitters located inside and outside the droplets. It is shown that the responses have different temperature thresholds, depending on whether α-particles or recoil nuclei create the signals. The measured temperature threshold for recoiling 210Pb nuclei from 214Po α-decays was found to be in agreement with test beam measurements using mono-energetic neutrons. A comparison of the threshold data with theoretical predictions shows deviations, especially at high temperatures. It is shown that signals produced simultaneously by recoil nuclei and α-particles have more acoustic energy than signals produced by one or the other separately. A model is presented that describes how the observed intensities of particle-induced acoustic signals can be related to the dynamics of bubble growth in superheated liquids. A growth scenario that is limited by the inertia of the surrounding liquid shows a trend that is supported by the data. An improved understanding of the bubble dynamics is an important first step in obtaining better discrimination between particle types interacting in detectors of this kind.

Helical magnetic order in MnSi thin films

Abstract: We present a study of the magnetic structure of crystalline MnSi(111) thin films grown by molecular beam epitaxy. A combination of polarized neutron reflectometry (PNR) and superconducting quantum interference device magnetometry show that the films have helical magnetic order with a pitch vector $\mathbf{Q}$ along the film normal. The helix wavelength of $2\ensuremath{\pi}/Q=13.9\ifmmode\pm\else\textpm\fi{}0.1$ nm is found to be independent of thickness below 40 nm. PNR shows that the magnetic structure has both left-handed and right-handed chiralities due to the presence of inversion domains observed by transmission electron microscopy.

Formation of Kinetically Trapped Nanoscopic Unilamellar Vesicles from Metastable Nanodiscs

Abstract: Zwitterionic long-chain lipids (e.g., dimyristoyl phosphatidylcholine, DMPC) spontaneously form onion-like, thermodynamically stable structures in aqueous solutions (commonly known as multilamellar vesicles, or MLVs). It has also been reported that the addition of zwitterionic short-chain (i.e., dihexanoyl phosphatidylcholine, DHPC) and charged long-chain (i.e., dimyristoyl phosphatidylglycerol, DMPG) lipids to zwitterionic long-chain lipid solutions results in the formation of unilamellar vesicles (ULVs). Here, we report a kinetic study on lipid mixtures composed of DMPC, DHPC, and DMPG. Two membrane charge densities (i.e., [DMPG]/[DMPC] = 0.01 and 0.001) and two solution salinities (i.e., [NaCl] = 0 and 0.2 M) are investigated. Upon dilution of the high-concentration samples at 50 °C, thermodynamically stable MLVs are formed, in the case of both weakly charged and high salinity solution mixtures, implying that the electrostatic interactions between bilayers are insufficient to cause MLVs to unbind. Impo...

Temperature driven annealing of perforations in bicellar model membranes.

Abstract: Bicellar model membranes composed of 1,2-dimyristoylphosphatidylcholine (DMPC) and 1,2-dihexanoylphosphatidylcholine (DHPC), with a DMPC/DHPC molar ratio of 5, and doped with the negatively charged lipid 1,2-dimyristoylphosphatidylglycerol (DMPG), at DMPG/DMPC molar ratios of 0.02 or 0.1, were examined using small angle neutron scattering (SANS), 31P NMR, and 1H pulsed field gradient (PFG) diffusion NMR with the goal of understanding temperature effects on the DHPC-dependent perforations in these self-assembled membrane mimetics. Over the temperature range studied via SANS (300−330 K), these bicellar lipid mixtures exhibited a well-ordered lamellar phase. The interlamellar spacing d increased with increasing temperature, in direct contrast to the decrease in d observed upon increasing temperature with otherwise identical lipid mixtures lacking DHPC. 31P NMR measurements on magnetically aligned bicellar mixtures of identical composition indicated a progressive migration of DHPC from regions of high curvatu...

Chain formation in a magnetic fluid under the influence of strong external magnetic fields studied by small angle neutron scattering

Abstract: We studied the aggregation of magnetic particles into chain-like structures in a cobalt-based magnetic fluid, exposed to external magnetic fields. The length of chain segments in very strong magnetic fields of up to 2 Tesla was measured in situ using small angle neutron scattering. Arrangement of the magnetic particles was studied with the scattering vector aligned parallel to the magnetic field lines, and at angles of 30° and 60° with respect to . Although chains several hundred particles in length were predicted, we observe a maximum correlation length of ∼65 nm equivalent to 3–4 particles up to highest fields. We speculate that the interaction between chains, i.e., the interplay between the entropy and energy of the system combined with the particular properties of the magnetic dipole–dipole interaction ultimately decide the length of the particle chains.

Highly stable cooperative distortion in a weak Jahn-Teller d2 cation: perovskite-type ScVO3 obtained by high-pressure and high-temperature transformation from bixbyite.

Abstract: A novel ScVO3 perovskite phase has been synthesized at 8 GPa and 1073 K from the cation-disordered bixbyite-type ScVO3. The new perovskite has orthorhombic symmetry at room temperature, space group Pnma, and lattice parameters a = 5.4006(2) A, b = 7.5011(2) A, and c = 5.0706(1) A with Sc3+ and V3+ ions fully ordered on the A and B sites of the perovskite cell. The vanadium oxygen octahedra [V–O6] display cooperative Jahn–Teller (JT) type distortions, with predominance of the tetragonal Q3 over the orthorhombic Q2 JT modes. The orthorhombic perovskite shows Arrhenius-type electrical conductivity and undergoes a transition to triclinic symmetry space group P-1 close to 90 K. Below 60 K, the magnetic moments of the 4 nonequivalent vanadium ions undergo magnetic long-range ordering, resulting in a magnetic superstructure of the perovskite cell with propagation vector (0.5, 0, 0.5). The magnetic moments are confined to the xz plane and establish a close to zigzag antiferromagnetic mode.

Solidification Analysis of an Al-19 Pct Si Alloy Using In-Situ Neutron Diffraction

Abstract: In-situ neutron diffraction and thermal analysis techniques were used simultaneously to evaluate the kinetics of the nonequilibrium solidification process of an Al-19 pct Si binary alloy. Feasibility studies concerning the application of neutron diffraction for advanced solidification analysis were undertaken to explore its potential for high resolution phase analysis coupled with fraction solid/liquid analysis of phase constituents. Neutron diffraction patterns were collected in a stepwise mode during solidification between 983 K and 793 K (710 °C and 520 °C). The variation of intensity of the diffraction peaks was analyzed and compared to the results of conventional cooling curve analysis. Neutron diffraction was capable of detecting nucleation of the Si phase (primary and eutectic), as well as the Al phase during Al-Si eutectic nucleation. Moreover, neutron diffraction indicated the possibility of detecting the presence of Si peaks at near liquidus temperature and premature nucleation of α-Al prior to Al-Si eutectic temperature. The solid and liquid volume fractions were determined based on the change of intensity of neutron diffraction peaks over the solidification interval. Overall, the volume fraction determined was in good agreement with the results of the cooling curve thermal analysis, as well as calculations using the FactSage software. The potential of neutron diffraction for high resolution melt analysis required for advanced studies of grain refining, eutectic modification, etc. was illustrated. This study will help us better understand the solidification mechanism of Al-Si alloys used for various casting component applications.

Small unilamellar vesicles: a platform technology for molecular imaging of brain tumors

Abstract: Molecular imaging enables the non-invasive investigation of cellular and molecular processes. Although there are challenges to overcome, the development of targeted contrast agents to increase the sensitivity of molecular imaging techniques is essential for their clinical translation. In this study, spontaneously forming, small unilamellar vesicles (sULVs) (30 nm diameter) were used as a platform to build a bimodal (i.e., optical and magnetic resonance imaging (MRI)) targeted contrast agent for the molecular imaging of brain tumors. sULVs were loaded with a gadolinium (Gd) chelated lipid (Gd-DPTA-BOA), functionalized with targeting antibodies (anti-EGFR monoclonal and anti-IGFBP7 single domain), and incorporated a near infrared dye (Cy5.5). The resultant sULVs were characterized in vitro using small angle neutron scattering (SANS), phantom MRI and dynamic light scattering (DLS). Antibody targeted and nontargeted Gd loaded sULVs labeled with Cy5.5 were assessed in vivo in a brain tumor model in mice using time domain optical imaging and MRI. The results demonstrated that a spontaneously forming, nanosized ULVs loaded with a high payload of Gd can selectively target and image, using MR and optical imaging, brain tumor vessels when functionalized with anti-IGFBP7 single domain antibodies. The unique features of these targeted sULVs make them promising molecular MRI contrast agents.

Local and Average Structures and Magnetic Properties of Sr2FeMnO5+y, y = 0.0, 0.5. Comparisons with Ca2FeMnO5 and the Effect of the A-Site Cation

Abstract: Sr2FeMnO5+y was synthesized under two different conditions, in air and in argon, both of which resulted in a cubic, Pm3m, structure with no long-range ordering of oxygen vacancies. The unit cell constants were found to be a0 = 3.89328(1) A for argon (y = 0.0) and a0 = 3.83075(3) A for air (y = 0.5). In contrast, Ca2FeMnO5 retains long-range brownmillerite oxygen vacancy ordering for either air or argon synthesis. Remarkably, Sr2FeMnO5.0 oxidizes spontaneously in air at room temperature. A neutron pair distribution function (NPDF) study of Sr2FeMnO5.0(Ar) showed evidence for local, brownmillerite-like ordering of oxygen vacancies for short distances up to 5 A. Mossbauer spectroscopy results indicate more than one Fe site for Sr2FeMnO5+y(Ar and air), consistent with the noncubic local structure found by NPDF analysis. The isomer shifts and quadrupole splittings in both air- and argon-synthesized materials are consistent with the 3+ oxidation state for Fe in sites with coordination number four or five. This...

Ethanol enhances collective dynamics of lipid membranes.

Abstract: From inelastic neutron-scattering experiments and all atom molecular dynamics simulations we present evidence for a low-energy dynamical mode in the fluid phase of a 1,2-dimyristoyl-sn-glycero-3-phoshatidylcholine (DMPC) bilayer immersed in a 5% water/ethanol solution. In addition to the well-known phonon that shows a liquidlike dispersion with energies up to 4.5 meV, we observe an additional mode at smaller energies of 0.8 meV, which shows little or no dispersion. Both modes show transverse properties and might be related to molecular motion perpendicular to the bilayer.

The Microstructure of Unirradiated and Neutron Irradiated Inconel X750

Abstract: Extended abstract of a paper presented at Microscopy and Microanalysis 2011 in Nashville, Tennessee, USA, August 7–August 11, 2011.

Fast neutron spectroscopy using neutron-induced charged particle reactions

Abstract: The invention provides a method of performing fast neutron detection or spectroscopy comprising selecting at least one isotope which exhibits fast neutron-induced charged particle reactions, selecting a host medium capable of performing radiation energy spectroscopy, combining the isotope and host medium into an interactive spectroscopic combination, exposing the combination structure to radiation comprising fast neutrons to provide a spectroscopic output, which includes at least one peak in the pulse-height spectrum whose height and amplitude correlate to the energy and intensity respectively of the incident neutrons; and processing the output to detect or to provide measurements of the energy and intensity of incident fast neutron radiation. The invention also provides a fast neutron spectrometer for use with the method.

Design of a multi-element TEPC for neutron monitoring.

Abstract: Tissue-equivalent proportional counters (TEPCs) have long been considered suitable candidate instruments for more accurate neutron monitors in nuclear power plants. It has also been recognised that the production of truly light-weight devices based on TEPCs requires further effort directed towards increasing their sensitivity and decreasing their physical size. This paper deals with the construction of a multi-element TEPC (METEPC) designed to have the sensitivity of a 12.7-cm (5-in.) diameter spherical TEPC, but with approximately one-tenth of its physical size. Construction of the METEPC is achieved by machining 61 elongated cylindrical cavities in a single block of A150 TE plastic. Comparative measurements carried out in neutron fields with mean energies ranging from 34 to 354 keV demonstrate that the METEPC constructed does match the sensitivity of a 5-in. spherical TEPC and that microdosimetric lineal energy spectra measured with both detectors have the same features and show the same changes with neutron radiation quality.

Tritium dynamics in large fish – a model test

Abstract: Tritium can represent a key radionuclide in the aquatic environment, in some cases, contributing significantly to the doses received by aquatic non-human biota and by humans due to aquatic releases. Recently, the necessity to have a robust assessment of tritium routine and accidental risk emissions for large nuclear installations increased the interest in the topic. In the present study, the recent experiments concerning tritium transfer in adult rainbow trout are described. The updated model concerning the dynamics of tritium transfer in aquatic food chain (AQUATRIT model) developed by the authors is applied and tested for these experimental data. The model predicts the experimental data with a factor of 2 to 3 and the potential improvements of the model are discussed. The present model results emphasize that in the field conditions, the major factors influencing the OBT biological loss rate are the temperature and the prey availability while, the OBT uptake is mainly influenced by the fish growth rates. The main goals of this study are to enhance the robustness of aquatic models for tritium risk assessment and to fulfil a gap for aquatic pathways in environment.

Monotonic and Cyclic Plasticity Response of Magnesium Alloy. Part I. Experimental Response of a High‐Pressure Die Cast AM60B

Abstract: The mechanical behaviours of a high-pressure die cast AM60B magnesium alloy were studied. Monotonic and cyclic tests (with a strain ratio R = −1) were carried out at ambient temperature. The results of the mechanical testing were discussed with respect to the crystallographic texture and the microstructure of the alloy in terms of clusters of pores, grain size, second phase strengthening as well as the orientation-dependent activation of different deformation mechanisms such as slip and twinning.

In vivo measurement of lead in the bones of smelter workers using the four-element 'clover-leaf' geometry detector system.

Abstract: A total of 497 smelter employees from New Brunswick participated in a bone lead survey conducted by McMaster University in 2008 to examine the efficiency of lead exposure control programmes and a four-element 'clover-leaf' geometry detector system. Nearly 42% of the subjects had participated in both the previous surveys performed in 1994 and 1999. After developing the clover-leaf geometry system in 2006, the reliability of the system based on examining the consistency of four detectors and improving the minimum detection limit (MDL) was tested for the first time in 2008 by measuring lead levels of a large population that was occupationally exposed to lead. The Z test was used to study the distribution of the lead concentration calculated based on Kα and Kβ lead x-rays, where the results were broadly consistent with a normal distribution criterion, with relatively small means and standard deviations of between 1 and 2. The MDL of the clover-leaf geometry system was improved on average for tibia and calcaneus by a factor of 3.1 compared to the 1999 and 1994 surveys in which a conventional system (one detector) was used. Furthermore, by comparing the results of the three mentioned surveys, the 2008 results were found to represent the highest precision.

Aberration effects in quick X-Ray reflectivity of curved samples

Abstract: Quick x-ray reflectivity (QXRR) is a new surface examination technique for studying fast processes at the surface and interface of the materials on a nano-scale. The currently available models for classical scanning-type x-ray reflectivity cannot be applied directly to the QXRR simulations. The present article proposes and discusses models for simulation of QXRR measurements of curved samples, which are applicable for analysis of liquid materials.