Showing papers by "Chalk River Laboratories published in 2002"

Direct evidence for neutrino flavor transformation from neutral current interactions in the Sudbury Neutrino Observatory

Abstract: Observations of neutral-current nu interactions on deuterium in the Sudbury Neutrino Observatory are reported. Using the neutral current (NC), elastic scattering, and charged current reactions and assuming the standard 8B shape, the nu(e) component of the 8B solar flux is phis(e) = 1.76(+0.05)(-0.05)(stat)(+0.09)(-0.09)(syst) x 10(6) cm(-2) s(-1) for a kinetic energy threshold of 5 MeV. The non-nu(e) component is phi(mu)(tau) = 3.41(+0.45)(-0.45)(stat)(+0.48)(-0.45)(syst) x 10(6) cm(-2) s(-1), 5.3sigma greater than zero, providing strong evidence for solar nu(e) flavor transformation. The total flux measured with the NC reaction is phi(NC) = 5.09(+0.44)(-0.43)(stat)(+0.46)(-0.43)(syst) x 10(6) cm(-2) s(-1), consistent with solar models.

Measurement of Day and Night Neutrino Energy Spectra at SNO and Constraints on Neutrino Mixing Parameters

Abstract: The Sudbury Neutrino Observatory (SNO) has measured day and night solar neutrino energy spectra and rates. For charged current events, assuming an undistorted 8B spectrum, the night minus day rate is 14.0%+/-6.3%(+1.5%)(-1.4%) of the average rate. If the total flux of active neutrinos is additionally constrained to have no asymmetry, the nu(e) asymmetry is found to be 7.0%+/-4.9%(+1.3%)(-1.2%). A global solar neutrino analysis in terms of matter-enhanced oscillations of two active flavors strongly favors the large mixing angle solution.

Pressure-induced crystallization of a spin liquid

Abstract: Liquids are expected to crystallize at low temperature. The only exception is helium, which can remain liquid at 0 K, owing to quantum fluctuations1, 2. Similarly, the atomic magnetic moments (spins) in a magnet are expected to order at a temperature scale set by the Curie–Weiss temperature thetaCW (ref. 3). Geometrically frustrated magnets represent an exception. In these systems, the pairwise spin interactions cannot be simultaneously minimized because of the lattice symmetry4. This can stabilize a liquid-like state of short-range-ordered fluctuating moments well below thetaCW (refs 5–7). Here we use neutron scattering to observe the spin liquid state in a geometrically frustrated system, Tb2Ti2O7, under conditions of high pressure (approx9 GPa) and low temperature (approx1 K). This compound is a three-dimensional magnet with thetaCW = -19 K, where the negative value indicates antiferromagnetic interactions. At ambient pressure Tb2Ti2O7 remains in a spin liquid state down to at least 70 mK (ref. 8). But we find that, under high pressure, the spins start to order or 'crystallize' below 2.1 K, with antiferromagnetic order coexisting with liquid-like fluctuations. These results indicate that a spin liquid/solid mixture can be induced by pressure in geometrically frustrated systems.

Ferroelectric ordering in the relaxor P b ( M g 1 / 3 Nb 2 / 3 ) O 3 as evidenced by low-temperature phonon anomalies

Abstract: Neutron scattering measurements of the lowest-energy TO phonons in the relaxor $\mathrm{Pb}({\mathrm{Mg}}_{1/3}{\mathrm{Nb}}_{2/3}){\mathrm{O}}_{3}$ (PMN) are reported for $10l~Tl~750 \mathrm{K}.$ The soft mode, which is overdamped by the polar nanoregions below the Burns temperature ${T}_{d}=620 \mathrm{K},$ surprisingly recovers below 220 K. The square of the soft-mode energy $(\ensuremath{\Elzxh}{\ensuremath{\omega}}_{0}{)}^{2}$ increases linearly with decreasing temperature and is consistent with the behavior of a ferroelectric soft mode. At 10 K, $\ensuremath{\Elzxh}{\ensuremath{\omega}}_{0}$ reaches 11 meV, the same value observed in ferroelectric $\mathrm{Pb}({\mathrm{Zn}}_{1/3}{\mathrm{Nb}}_{2/3}){\mathrm{O}}_{3}$ at low T. An unusual broadening of the TA phonon starts at ${T}_{d}$ and disappears at 220 K, coincident with the recovery of the TO mode. These dynamics suggest that a well-developed ferroelectric state is established below 220 K.

Spin-glass behavior in the S = 1 / 2 fcc ordered perovskite Sr 2 CaReO 6

Abstract: The ordered perovskite ${\mathrm{Sr}}_{2}{\mathrm{CaReO}}_{6}$ of monoclinic symmetry [space group ${P2}_{1}/n,a=5.7556(3)\AA{},b=5.8534(3)\AA{},c=8.1317(4)\AA{},\ensuremath{\beta}=90.276(5)\ifmmode^\circ\else\textdegree\fi{}$ at $T=4\mathrm{K}$] has been synthesized using standard solid-state chemistry techniques. The difference in the size and charge of the cations induces an ordering of the B site ${\mathrm{Ca}}^{2+}$ and ${\mathrm{Re}}^{6+}$ ions which leads to a distorted fcc lattice of spin-$\frac{1}{2}$ ${\mathrm{Re}}^{6+}$ ${(5d}^{1})$ moments. dc magnetic susceptibility measurements indicate a maximum at ${T}_{G}\ensuremath{\sim}14\mathrm{K}$ and an irreversibility in the field-cooled and zero-field-cooled data at \ensuremath{\sim}22 K that is believed to be caused by the geometric frustration inherent in the fcc structure. Neutron-scattering measurements confirm the absence of magnetic long-range order, and muon spin relaxation experiments indicate the presence of an abrupt spin freezing at ${T}_{G}.$ Specific heat measurements reveal a broad anomaly typical of spin glasses and no sharp feature. 65% of the spin entropy is released at low temperatures. The low-temperature data do not show the expected linear temperature dependence, but rather a ${T}^{3}$ relationship, as is observed, typically, for antiferromagnetic spin waves. The material is characterized as an unconventional, essentially disorder-free, spin glass.

Enhancement of steric repulsion with temperature in oriented lipid multilayers.

Abstract: We have studied the temperature dependence of the stacking periodicity, $d$, of oriented phospholipid multilayers using grazing angle neutron scattering techniques. $d$ is found to increase substantially at higher temperatures, just before the bilayers peel off from the substrate. Although we do not observe thermal unbinding, our results are consistent with the notion that the unbinding transition is driven by steric repulsion arising from thermal fluctuations of the membranes, in contrast to those of a recent study by Vogel et al. [Phys. Rev. Lett. 84, 390 (2000)].

Design and Operational Experience with a Pilot-Scale CECE Detritiation Process

Abstract: A 5 Mg/annum Combined Electrolysis Catalytic Exchange (CECE) Facility was designed, constructed and operated to demonstrate the CECE process for heavy water detritiation. In this demonstration faci...

An accelerator based system for in vivo neutron activation analysis measurements of manganese in human hand bones.

Abstract: Manganese (Mn) is an essential nutrient for growth and development. Unfortunately, overexposure can lead to neurological damage, which is manifested as a movement disorder marked by tremors. Preclinical symptoms have been found in populations occupationally exposed to the element, and it is suggested that in late stages of the disorder, removing the Mn exposure will not prevent symptoms from progressing. Hence, it is desirable to have a means of monitoring Mn body burden. In vivoneutronactivation analysis (IVNAA) is a technique which allows the concentration of some elements to be determined within sites of the body without invasive procedures. Data in the literature suggests that the Mn concentration in bone is greater than other tissues, and that it may be a long term storage site following exposure. Therefore, using the McMaster KN-accelerator to produce neutrons through the 7 Li (p,n) 7 Be reaction, the feasibility of IVNAA for measuring Mn levels in the human hand bone was investigated. Mn is activated through the 55 Mn (n,γ) 56 Mn reaction, and the 847 keV gamma rays emitted when 56 Mn decays are measured outside the body using NaI(Tl) detectors. An optimal incident proton energy of 2.00 MeV was determined from indium foil and microdosimetry measurements. Hand phantom data suggest a minimum detectable limit of approximately 1.8 ppm could be achieved with a reasonably low dose of 50 mSv to the hand (normal manganese levels in the human hand are approximately 1 ppm). It is recommended the technique be developed further to make human in vivo measurements.

Neutron scattering search for static magnetism in oxygen-ordered YBa2Cu3O6.5

Abstract: We present elastic and inelastic neutron-scattering results on highly oxygen-ordered ${\mathrm{YBa}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{6.5}$ ortho-II. We find no evidence of the presence of long-ranged ordered magnetic moments to a sensitivity of $\ensuremath{\sim}0.003{\ensuremath{\mu}}_{B},$ an order of magnitude smaller than has been suggested in theories of orbital or d-density-wave (DDW) currents. The absence of sharp elastic peaks rules out the existence of well-correlated static DDW currents in our crystal. We cannot exclude the possibility that a broad peak may exist with extremely short-range DDW correlations. For less ordered or more doped crystals it is possible that disorder may lead to static magnetism. We have also searched for the large normal-state spin gap that is predicted to exist in an ordered DDW phase. Instead of a gap we find that the Q-correlated spin susceptibility persists to the lowest energies studied, $\ensuremath{\sim}6 \mathrm{meV}.$ Our results are only compatible with the coexistence of superconductivity and orbital currents if the latter are dynamic and do not participate in a sharp phase transition to a highly ordered DDW state.

Correlations between charge ordering and local magnetic fields in overdoped YBa 2 Cu 3 O 6 + x

Abstract: Zero-field muon spin-relaxation $(\mathrm{ZF}\ensuremath{-}\ensuremath{\mu}\mathrm{SR})$ measurements were undertaken on under- and overdoped samples of superconducting ${\mathrm{YBa}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{6+x}$ to determine the origin of the weak static magnetism recently reported in this system. The temperature dependence of the muon spin-relaxation rate in overdoped crystals displays an unusual behavior in the superconducting state. A comparison to the results of NQR and lattice structure experiments on highly doped samples provides compelling evidence for strong coupling of charge, spin, and structural inhomogeneities.

Low-temperature electrical transport in bilayer manganite La 1.2 Sr 1.8 Mn 2 O 7

Abstract: The temperature T and magnetic-field H dependences of anisotropic in-plane ${\ensuremath{\rho}}_{\mathrm{ab}}$ and out-of-plane ${\ensuremath{\rho}}_{c}$ resistivities are investigated in single crystals of the bilayer manganite ${\mathrm{La}}_{1.2}{\mathrm{Sr}}_{1.8}{\mathrm{Mn}}_{2}{\mathrm{O}}_{7}.$ Below the Curie transition temperature ${T}_{c}$=125 K, ${\ensuremath{\rho}}_{\mathrm{ab}}$ and ${\ensuremath{\rho}}_{c}$ display almost the same temperature dependence with an up-turn around 50 K. In the metallic regime (50 K $l~Tl~$ 110 K), both ${\ensuremath{\rho}}_{\mathrm{ab}}(T)$ and ${\ensuremath{\rho}}_{c}(T)$ follow a ${T}^{9/2}$ dependence, consistent with the two-magnon scattering. We found that the value of the proportionality coefficient ${B}_{\mathrm{ab}}^{\mathrm{fit}}$ and the ratio of the exchange interaction ${J}_{\mathrm{ab}}{/J}_{c},$ obtained by fitting the data, are in excellent agreement with the calculated ${B}_{\mathrm{ab}}$ based on the two-magnon model and ${J}_{\mathrm{ab}}{/J}_{c}$ deduced from neutron scattering, respectively. This provides further support for this scattering mechanism. At even lower T, in the nonmetallic regime $(Tl50 \mathrm{K}),$ both the in-plane ${\ensuremath{\sigma}}_{\mathrm{ab}}$ and out-of-plane ${\ensuremath{\sigma}}_{c}$ conductivities obey a ${T}^{1/2}$ dependence, consistent with weak-localization effects. Hence this demonstrates the three-dimensional metallic nature of the bilayer manganite ${\mathrm{La}}_{1.2}{\mathrm{Sr}}_{1.8}{\mathrm{Mn}}_{2}{\mathrm{O}}_{7}$ at $Tl{T}_{c}.$

A study of twinning in zirconium using neutron diffraction and polycrystalline modeling

Abstract: An experimental study using neutron diffraction quantified the evolution of twinning in pure clockrolled zirconium that was subsequently deformed under uniaxial compression. The clock rolling introduced an initial texture of approximately 5 times random, and the compression specimens were cut with their loading axes nearly parallel to the predominant c-axes direction. Seven specimens deformed to strains between −2 and −17 pct and an undeformed specimen (0 pct strain) were examined. The deformation was performed at an applied strain rate of 0.001/s at 77 K. Twin volume fractions were estimated from diffraction data. Changes in texture and twin volume fractions were compared to predictions from a visco-plastic self-consistent (VPSC) polycrystal model, which described both slip and twinning. This work demonstrates the feasibility of using neutron diffraction to track the evolution of twinning. These results help benchmark the polycrystalline model, validate the description of twinning, and potentially lead to a better understanding of its role in hardening.

Demonstration of Very High Detritiation Factors with a Pilot-Scale CECE Facility

Abstract: Analysis of process data from initial detritiation tests in a pilot-scale Combined Electrolysis and Catalytic Exchange (CECE) Facility1 indicated that very high detritiation factors (DFs), at least...

The influence of martensite on line broadening in neutron diffraction spectra of a DP steel

Abstract: Neutron diffraction experiments were performed on dual-phase steels to determine the feasibility of measuring the martensite volume fraction through ferrite peak broadening. Mechanical properties were also investigated to determine if a correlation exists with ferrite peak broadening. Results indicate that the correlation between martensite volume fraction and ferrite peak broadening is carbon dependent. Furthermore, two mechanisms regarding the nature of the ferrite peak broadening in dual-phase steels are proposed. Tensile testing demonstrated that a correlation between peak broadening and mechanical properties exists for dual-phase steels.

The magnetic structures of Nd5Si4 and Nd5Ge4

Abstract: We have determined the magnetic structures of Nd5Si4 and Nd5Ge4 by means of high-resolution neutron powder diffraction. The crystal space groups of these compounds are tetragonal P41212 and orthorhombic Pnma, respectively. Their magnetic ordering temperatures are 66(2) and 51(2) K, respectively, and both magnetic structures can be described as canted structures, with net antiferromagnetism in one plane with ferromagnetism perpendicular to that plane. The magnetic space groups are P4121'2' and Pnm'a' and their ferromagnetic ordering directions are [001] and [100], respectively.

Temperature dependence of the magnetic penetration depth in the vortex state of the pyrochlore superconductor, Cd2Re2O7.

Abstract: We report transverse-field and zero-field muon spin rotation and relaxation studies of the superconducting rhenium oxide pyrochlore, Cd2Re2O7. Transverse-field measurements (H=0.007 T) show line broadening below T-c, which is characteristic of a vortex state, demonstrating conclusively the type-II nature of this superconductor. The penetration depth is seen to level off below about 400 mK (T/T(c)similar to0.4), with a rather large value of lambda(T=0)similar to7500 Angstrom. The temperature independent behavior below similar to400 mK is consistent with a nodeless superconducting energy gap. Zero-field measurements indicate no static magnetic fields developing below the transition temperature.

Residual stresses in a welded superalloy disc: Characterization using synchrotron diffraction and numerical process modeling

Abstract: The residual stress state in an electron-beam welded assembly of IN718 nickel-based superalloy has been studied both experimentally and using computer modeling. Diffraction measurements were made at the European Synchrotron Radiation Facility (ESRF), using the ID11 beamline. The assembly consisted of a 1.75-mm-thick web of rolled IN718 sheet, welded between two 14-mm-thick circular forgings. Four radial scans each of 400 measuring points were made across the web at 90-deg intervals. Diffraction patterns were recorded using a charge-coupled device (CCD) detector, which enabled the collection of information from several diffraction peaks and, thus, the determination of in-plane lattice strains; the macroscopic residual strain and stress fields were then estimated using appropriate values of the diffraction elastic constants. The experimental results were compared with the predictions from a sequentially coupled thermal-mechanical model based upon the finite-element (FE) method. The agreement between the experimental data and the results from the model is reasonable. It is shown that the residual strain/stress field is strongly influenced by the constraint imposed by the geometry of the assembly, i.e., by the inner and outer forgings, and that, as a result, the web is in a state of biaxial tension. The FE model predicts that a steady state is reached during welding and, thus, that there are systematic errors associated with the strain scanning measurements. These are considered to arise from the uncertainty associated with the positioning of the assembly and a sensitivity analysis for this effect is presented.

Variable Property Mixed Convection Heat Transfer to Air Flowing Through a Vertical Passage of Annular Cross Section: Part 1

Abstract: In this paper, an experimental study is reported of variable property mixed convection heat transfer to air flowing through a vertical passage of annular cross-section of diameter ratio 2.1, having a uniformly heated core and a thermally insulated outer casing. This is the first of three related papers each of which is concerned with buoyancy-influenced flow and heat transfer in vertical passages with one surface heated and the opposite one unheated. The second paper also reports mixed convection heat transfer experiments with a vertical passage of annular cross-section. The diameter ratio of 1.9 is similar. However, in that case the working fluid was water rather than air and effects of fluid property variations were negligibly small. The third paper reports mixed convection heat transfer experiments with a vertical plane passage of relatively short length. The working fluid was air. In all three cases the need for the experiments arose in connection with nuclear power plant design and operation. However, the studies are all of a fundamental nature and address an interesting and important aspect of buoyancy-influenced convective heat transfer in vertical passages. The results presented in this first paper show, that with a passage of annular cross-section having a heated inner surface and an adiabatic outer one, the heat transfer behaviour is generally similar to that found with uniformly heated circular tubes but that there are certain important differences. Thermal development is slower and the effects of buoyancy and variable properties are both weaker. As with circular tubes, the mixed convection data for downward flow show systematic enhancement of heat transfer with an increase of buoyancy influence and a fully developed heat transfer situation is readily achieved. The results for such conditions correlate satisfactorily in terms of Nusselt number ratio (mixed to forced) and a buoyancy parameter which combines Grashof number, Reynolds number and Prandtl number in a particular manner designed to characterize the strength of buoyancy influences. The results are fitted satisfactorily by a simple correlation equation based on a semi-empirical model. In the case of mixed convection with upward flow, heat transfer is always less effective than with downward flow. Impairment of heat transfer occurs with the onset of buoyancy influences, but more gradually and at a somewhat higher values of buoyancy parameter than in circular tubes. Non-monotonic axial distributions of Nusselt number are found and the data do not correlate in terms of purely local parameters. A fully developed situation is not readily achieved. With further increase of buoyancy influence, heat transfer recovers and eventually becomes enhanced.

Origins of intermediate velocity particle production in heavy ion reactions

Abstract: In this paper, we report for the first time clear distinctions between these prompt processes and an alternative phenomenon of delayed aligned asymmetric breakup that populates the intermediate-velocity zone by a deformation rupture of mainly the heavier of the colliding partners in mass asymmetric collisions. These distinctions were observed experimentally with intermediate-velocity particle correlation analysis of Ni+C and Ni+Au reac

The Effect of Processing History on a Cold Rolled and Annealed Mo-Nb Microalloyed TRIP Steel

Abstract: In this paper, a Mo-Nb microalloyed TRIP Steel was subjected to several heat treatments designed to generate different microstructures. These microstructures were then cold rolled and TRIP-annealed, and the resulting tensile properties and retained austenite characteristics were determined. The results reveal that prior heat treatment has a significant effect on the cold rolled and annealed behavior. Generally, an increasing volume fraction of proeutectoid ferrite prior to cold rolling and TRIP annealing leads to improved tensile ductility. This is due to increased work-hardening rates, which, in turn, correlate to increased carbon enrichment in the retained austenite. These results can clearly be used to optimize the hot rolling process to produce hot strip for the subsequent production of cold rolled Mo-Nb TRIP Steel.

Environmental Tritium Concentrations due to Continuous Atmospheric Sources

Abstract: In the past few years, five separate studies have been undertaken at AECL's Chalk River Laboratories to determine the levels of tritium in the environment due to routine releases from an experimental heavy water reactor. Data from these studies are presented in terms of the ratios of the long-term average tritium concentrations in precipitation, soil and plants to the long-term average HTO concentration in air moisture. The data are used to assess the performance of models that predict long-term average tritium concentrations in the environment.

Characterization of miniature tissue-equivalent proportional counters for neutron radiotherapy applications.

Abstract: A miniature tissue-equivalent proportional counter (TEPC) system has been developed to facilitate microdosimetric measurements in high-flux mixed fields. Counters with collecting volumes of 12.3 and 2.65 mm3 have been constructed using various tissue-equivalent wall materials, including those loaded with 10B for evaluation of the effects of the boron neutron capture reaction. These counters provide a measure of both the absorbed dose and associated radiation quality, allowing an assessment of the utility and relative effectiveness of various neutron radiotherapy techniques such as boron neutron capture therapy (BNCT), boron neutron capture enhanced fast neutron therapy (BNCEFNT) and intensity modulated neutron radiotherapy (IMNRT). An evaluation of the physical parameters affecting the measured microdosimetric spectrum, the gas multiplication characteristics and the measurement of absorbed dose is presented. In addition, important aspects of the calibration and low energy extrapolation techniques for the microdosimetric spectrum are provided.