Showing papers by "Atomic Energy of Canada Limited published in 1988"

Spin dynamics and the Haldane gap in the spin-1 quasi-one-dimensional antiferromagnet CsNiCl 3

Abstract: A full description of a series of spin-wave measurements in ${\mathrm{CsNiCl}}_{3}$ in the three-dimensional (3D) and one-dimensional (1D) phases and their analysis to provide the first experimental evidence for the Haldane gap is presented. Neutron scattering experiments were performed on a single crystal of ${\mathrm{CsNiCl}}_{3}$ with both (h,0,l) and (h,h,l) as the scattering plane. The order parameter in both 3D phases was measured and the spin-wave dispersion determined in the lower phase. The spin-wave spectrum calculated from a dynamic susceptibility method was compared with the experimental response in the lower 3D phase to obtain the following values for the exchange and anisotropy constants: J=0.345\ifmmode\pm\else\textpm\fi{}0.008 THz, J'=0.0060\ifmmode\pm\else\textpm\fi{}0.005 THz, and D=-0.0130\ifmmode\pm\else\textpm\fi{}0.0015 THz. These parameters confirm that ${\mathrm{CsNiCl}}_{3}$ in its disordered phase is a good approximation to a one-dimensional Heisenberg antiferromagnet. In the 1D phase the gap frequency for an isolated chain of ${\mathrm{Ni}}^{2+}$ ions is found to be 0.32 THz close to the gap estimated in finite-chain calculations. The results constitute experimental support for the Haldane conjecture that the excitations of integer-spin chains, unlike half-integer--spin chains, exhibit an apparent anisotropy that arises not from the underlying isotropic Hamiltonian, but from many-body effects.

Azimuthal distributions in heavy ion collisions and the nuclear equation of state

Abstract: For near central collisions of Nb on Nb at a laboratory energy of 650 MeV per projectile nucleon we calculate inclusive cross sections as a function of the azimuthal angle where this angle is measured from the reaction plane. The azimuthal dependence is strongly influenced by the nuclear equation of state and is a useful quantity to measure.

The influence of hydride size and matrix strength on fracture initiation at hydrides in zirconium alloys

Abstract: The effects of matrix strength (yield stress) on hydride fracture and alloy ductility have been studied as a function of stress state, hydride content, hydride size, and precipitation stress. Uniaxial and triaxial states of stress were investigated by using smooth and notched tensile specimens, respectively, containing 0.18 or 0.90 at. pct H, with the longest hydride platelet dimension varying from 5 to 400 μm. The majority of the hydrides in the specimens had their plate normals oriented parallel to the tensile axis direction. Crack initiation at hydrides was monitored using acoustic emission, finiteelement calculations were employed to determine the stresses and strains in the notched specimens, and metallographic and fractographic analyses were carried out to determine the state of fractured hydrides/voids near and on the fracture surface. These techniques showed that, up to a hydride platelet length of ∼50 to 100 μm and regardless of the stress state, a critical plastic strain, independent of matrix strength, controls the initiation of fracture in hydrides. The amount of plastic strain needed to fracture hydrides decreases as (a) the average hydride length increases and (b) the axiality of stress increases. The equivalent plastic strain to fracture small hydrides is ∼ 1 pct under a triaxial as opposed to ∼5 pct under a uniaxial state of stress. When the average hydride platelet lengths are longer than ∼50 to 100 μm, negligible plastic deformation is required to fracture hydrides. A critical applied stress then is the governing factor in all three materials, ranging from 750 to 850 MPa, depending on the stress state.

Temperature dependence of S(Q, omega ) in liquid 4He under pressure.

Abstract: The temperature dependence of the dynamic form factor, S(Q,\ensuremath{\omega}), of liquid $^{4}\mathrm{He}$ at p=20 bars has been determined by inelastic neutron scattering measurements. Two wave vectors, Q=1.13 A${\r{}}^{\mathrm{\ensuremath{-}}1}$ and Q=2.03 A${\r{}}^{\mathrm{\ensuremath{-}}1}$, corresponding to the maxon and roton regions of the phonon-roton dispersion curve, were studied over a wide range of energy transfer, \ensuremath{\Elzxh}\ensuremath{\omega}. Based on previous data at SVP, Woods and Svensson proposed that S(Q,\ensuremath{\omega}) could be represented as a sum of two components, one proportional to the superfluid density, ${\ensuremath{\rho}}_{S}$(T), and one proportional to the normal density ${\ensuremath{\rho}}_{N}$(T). The component proportional to ${\ensuremath{\rho}}_{S}$(T) contained the sharp one-phonon peak which vanished at T=${T}_{\ensuremath{\lambda}}$. The aim here is, firstly, to present data on the temperature dependence of S(Q,\ensuremath{\omega}) at 20 bars and, secondly, to explore whether or not the Woods-Svensson decomposition of S(Q,\ensuremath{\omega}) holds at higher pressure. At 20 bars and for the Q values investigated here, we find that the sharp peak of S(Q,\ensuremath{\omega}) does indeed decrease rapidly in intensity as T increases and the corresponding excitation either vanishes or changes abruptly in character at ${T}_{\ensuremath{\lambda}}$.The sharp nature of the one-phonon peak ${T}_{\ensuremath{\lambda}}$ does therefore appear to be associated with superfluidity or a Bose condensate at these Q values. However, the weight of the one-phonon peak does not scale as ${\ensuremath{\rho}}_{S}$(T) and subtracting a contribution proportional to ${\ensuremath{\rho}}_{N}$(T) from S(Q,\ensuremath{\omega}) leads to negative values of the superfluid component of S(Q,\ensuremath{\omega}) at low \ensuremath{\omega}. Thus S(Q,\ensuremath{\omega}) at 20 bars does not naturally separate into a part proportional to ${\ensuremath{\rho}}_{S}$ and one proportional to ${\ensuremath{\rho}}_{N}$. We also explore the consequences of a simple subtraction of the multiphonon component, assumed temperature independent, as an alternative method of extracting one-phonon parameters from the total scattering intensity. The values of the one-phonon properties such as the frequency and the lifetime obtained by the simple multiphonon subtraction method also show a marked change at ${T}_{\ensuremath{\lambda}}$.

Superdeformed band up to spin ~(127/2 in 149 Gd

Abstract: A rotational band of nineteen transitions, extending to spin\ensuremath{\sim}(127/2\ensuremath{\Elzxh} and with an average moment of inertia ${\mathrm{scrI}}^{(2)}$ of 77 ${\ensuremath{\Elzxh}}^{2}$ ${\mathrm{MeV}}^{\mathrm{\ensuremath{-}}1}$, has been observed in $^{149}\mathrm{Gd}$. Its intensity accounts for approximately (1/2% of the total fusion cross section. Lifetime measurements yielded an average quadrupole moment of 17\ifmmode\pm\else\textpm\fi{}2 e-b, in excellent agreement with the value calculated microscopically for a superdeformed shape. The trends in ${\mathrm{scrI}}^{(1)}$ and ${\mathrm{scrI}}^{(2)}$ are consistent with very weak pairing effects. The data also suggest that to populate superdeformed bands significantly one must form cold residual nuclei.

Diffusion of 59Fe in single-crystal α-Zr

Abstract: Diffusion coefficients D of 59Fe have been measured in single crystal α-Zr in the range 765-1193 K (in directions both parallel (D∥) and perpendicular (D⊥) to the c axis) and in a polycrystalline Zr–0•28 at.% Fe alloy, in the range 733–1070 K. The single-crystal results show that D∥/D⊥ has values between 2•2 and 5•9, for T < 1143K, and about 1•22, for 1154–1193 K; the persistence of the anisotropy beyond the nominal α-phase range (T < 1136 K) is surprising. The a-phase D∥ results show a complex temperature dependence which appears to parallel, to a notable degree, that shown by recent measurements of α-Zr self-diffusion. The D values for the alloy are about ten times less than the corresponding (average) single-crystal results.

The effect of dilution with steam on the burning velocity and structure of premixed hydrogen flames

Abstract: A comparative study of the effects of diluents on the burning velocity of H2−O2 mixtures has been carried out focusing on the extraordinary effect of steam on burning velocity and flame structure. Burning velocities are measured by the nozzle-burner, cone angle method with particle tracking by laser doppler anemometry for a range of stoichiometries and diluent fractions (He, Ar, N2, steam). New data are given for H2-air mixtures containing up to 50% steam. The following correlation is obtained, which accurately predicts the physical effects of diluents (flame cooling and heat transport) on burning velocity:Su(αo/α)1/2 = Suo(1-X/XL) . Here, Su is the laminar burning velocity, α is the thermal diffusivity, X is the mole fraction of diluent, XL is the diluent fraction sufficient to inert the mixture, and suffix o denotes the undiluted mixture. This correlation underestimates the burning velocity for steam diluent by 20%, indicating that steam influences the burning velocity by mechanisms other than flame cooling and heat transport. Using a one-dimensional flame model, we calculated in detail the changes to the flame structure caused by the addition of steam and determined that steam effects a redistribution of heat release in the flame. This is caused by the high third-body efficiency of steam in the exothermic reaction, H+O2+M→HO2+M. Thus steam increases in the rate of this reaction and, subsequently, the rates of a cycle of exothermic reactions involving HO2 in the preheat region of the flame. The result is a steeper temperature profile (i.e., a thinner flame) and an increased burning velocity. Sensitivity analysis indicates this mechanism produces the same effect on burning velocity as is observed experimentally upon adding steam.

Growth-curve analysis: a critical reevaluation

Abstract: Growth-curve analysis (Ricklefs 1967) has long enjoyed an ardent following. Recentlyß Brisbin et al. (1987) predicted a bright future for this methodology, but failed to address critical shortcomings of growthcurve analysis and of the Richards (1959) model in particular. They also completely ignored alternatives. Growth-curve analysis is a tool onlyß and not an end in itself. The main question is not which model to use, but whether to use growth-curve analysis at all. For many studies growth-curve analysis is inappropriateß and alternativesß such as simple observedgrowth statistics, may be more effective growth indicators. Although the Richards model with its variable sigmoid shape is intuitively attractiveß other simpler models can be superior for studying growth. There are many reasons growth-curve analysis may not be an ideal methodology. Growth-curve analysis is labor intensive because numerous data are required to satisfy models with several fitted parameters. For passerines, with their short nestling timeß this usually means daily measurement on a rigid schedule so that each nestling is always measured at about the same time. Alternatively, variable time must be taken into account during curve fitting. Growth-curve analysis requires numerous precise data. Small changes in only a single measurement can profoundly affect parameter estimates. Sensitivity depends on the type of model employed, however. High data requirements of growth-curve analysis can stress both parents and nestlings. Frequent disturbances can cause nest abandonment and conse-

Statistics of atmospheric turbulence within a natural black spruce forest canopy

Abstract: Turbulence statistics were measured in a natural black-spruce forest canopy in southeastern Manitoba, Canada. Sonic anemometers were used to measure time series of vertical wind velocity (w), and cup anemometers to measure horizontal wind speed (s), above the canopy and at seven different heights within the canopy. Vertical profiles were measured during 25 runs on eight different days when conditions above the canopy were near-neutral. Profiles of s and of the standard deviation (Σ w ) of w show relatively little scatter and suggest that, for this canopy and these stability conditions, profiles can be predicted from simple measurements made above the canopy. Within the canopy, a negative skewness and a high kurtosis of the w-frequency distributions indicate asymmetry and the persistence of large, high-velocity eddies. The Eulerian time scale is only a weak function of height within the canopy. Although w-power spectra above the canopy are similar to those in the free atmosphere, we did not observe an extensive inertial subrange in the spectra within the canopy. Also, a second peak is present that is especially prominent near the ground. The lack of the inertial subrange is likely caused by the presence of sources and sinks for turbulent kinetic energy within our canopy. The secondary spectral peak is probably generated by wake turbulence caused by form drag on the wide, horizontal spruce branches.

Method of oxidizing gaseous substances

Abstract: A method is provided for oxidizing gases such as, for example, CO, aldehydes having carbon atoms in the range 1 to 5 (eg formaldehyde), ketones having carbon atoms in the range 3 to 5, and alcohols having carbon atoms in the range 1 to 5, wherein the gas at a relative humidity in the range 0 to 100% and an input temperature in the range -50° to 150° C is contacted with a catalyst assembly comprising (a) a hydrophobic support having a surface area in the range 50 to 1,500 m2 /g and selected from the group consisting of silicalite, silylated silica, and styrene divinylbenzene polymers, and, on said support (b) a catalytically active metal selected from the group consisting of chromium, molybdenum, tungsten, manganese, rhenium, iron, ruthenium, osmium, cobalt, rhodium, platinum, iridium, nickel, copper and silver, said metal being present in an amount in the range 01 to 22 wt % of the total weight of the catalyst assembly The catalyst assemblies used are generally catalytically active in the presence of liquid water and water vapor for longer periods than known catalyst assemblies and function at ambient temperatures

Experimental Determination of Single and Two-Phase Cross Flow-Induced Forces on Tube Rows

Abstract: The fluctuating forces induced by water and air/water cross flow on tube rows with pitch to diameter ratios of 1.5 and 3 were measured. Direct measurement of force spectra was possible because the tube was mounted on force transducers. The tubes within the test section were 300 mm in length and 30 mm in diameter. Results were obtained over ranges of void fraction and mass flux from 0 to 95 percent and 375 to 4125 kg/(m2 s), respectively. A reasonably uniform spatial distribution of the void fraction was achieved through the use of an air/water mixing unit. The reported results include the power density spectra of the fluctuating forces and their corresponding force coefficient.

Monoclinic phase of SF6 and the orientational ordering transition

Abstract: The structure of the low temperature phase of SF6 is described in terms of a monoclinic unit cell. The mechanism of the phase transition is discussed for this unit cell setting. It can be analysed in terms of three strain components, two molecular displacements and two molecular orientations.

Region of interest tomography employing a differential scanning technique

Abstract: A method of generating a computed tomographic image of a region-of-interest of a sample is described in which only the region-of-interest need be scanned, i.e. it is not necessary to scan the entire sample. Reference data for the region-of-interest of a complete reference object that is a relatively good match to the sample outside the region-of-interest and has a known attentuation distribution inside the region-of-interest is first obtained. This can be done by scanning only the region-of-interest of an actual complete reference object or by constructing a digital image of the reference object and mathematically scanning only the region-of-interest of the image. Then the region-of-interest of the sample is scanned to obtain sample data from which the reference data is subtracted. A differential image is reconstructed from the differential data thus obtained. The method allows the use of equipment which is simpler, more compact and less costly than conventional CT equipment.

Characteristics and chronology of fracture — fluid infiltration in the Archean, Eye Dashwa Lakes pluton, Superior Province: evidence from H, C, O-isotopes and fluid inclusions

Abstract: The Archean Eye Dashwa Lakes pluton (2672±24 Ma) has domains of mineralogically fresh isotropic granite, domains that have undergone bulk hydrothermal alteration, and at least eleven sets of sequential fracture arrays, each with distinctive mineral assemblages. Fresh granite is characterized by whole rock δ18O=8.1 to 8.6 and primary magmatic quartz-feldspar (+1.3‰), quartz-biotite (5.2 to 5.4‰) and quartz-magnetite (+9.8‰) fractionations. Magmatic fluids had a calculated isotopic composition of δ18O=7.9±0.5, and δD=−80±5. These isotropic volumes of the granite have not experienced significant incursion of external thermal waters. Pegmatites, quartz-molybdenite veins, and phlogopite-muscovite coated fractures are sporadically distributed in the granite, and were precipitated from high-temperature ‘magmatic’ fluids where δ18O=8.0 to 10.3 and δD=−80±5.

Feeding of discrete-line superdeformed bands at very high spin

Abstract: The superdeformed band in $^{149}\mathrm{Gd}$ has been populated with various input angular momenta and excitation energies. These, together with all other available data, indicate that the necessary criterion for populating superdeformed bands is to form cold residual nuclei at spins higher than those where the superdeformed states become yrast. In addition, the decrease in the superdeformed band intensity as it is gated by higher-energy $\ensuremath{\gamma}$ rays does not support the predictions based on a recently proposed feeding mechanism of these states.

Porous composite materials and methods for preparing them

Abstract: Sprayable dispersions or shaped articles are made from a material of high surface area by blending the material with polytetrafluoroethylene in the presence of a liquid. The blended material may be shaped into an article or coated onto the surface of an article. The article is then heated above 327° C. The resulting product retains high surface area, and can be used as a catalyst or catalyst support, a biological support, or an adsorbant.

Valve leakage inspection, testing and maintenance process

Abstract: A valve maintenance process is described that allows for the inspection and testing of valve stem packing and back-seat leakage. The process uses the characteristics of incompressible fluid flow to check the integrity of the valve back-seat, to allow the controlled removal of the complete packing set and to confirm the newly installed packing set is not leaking. The process is designed to reduced down-time and to improve the engineering quality of valve repair.

Colloid and particulate matter studies in the Cigar Lake natural-analog program

Abstract: The Cigar Lake uranium deposit in Northern Saskatchewan is being studied as a natural analog to a used-fuel disposal vault. This paper describes groundwater colloid and particulate matter studies at Cigar Lake, carried out to support the analog program and to obtain basic information about colloid properties and their migration behavior in a natural environment. The concentration of natural groundwater particles (0.6 to 261 mg/1) was found to depend on groundwater chemistry and the amount of local fracturing and alteration of the rock formation. The presence of U, Th and Ra-226 in particulate form is a function of total particle concentration and composition. No significant colloid migration has been observed from the ore deposit to the surrounding sandstone.

Characterization of residual stresses in bent incoloy-800 tubing by neutron diffraction

Abstract: The circumferential distributions of axial, hoop, and radial residual strains in a bent INCOLOY-800* tube have been measured by neutron diffraction. High residual axial tensile strains are observed just above the neutral axis with corresponding compressive strains just below the neutral axis. There are marked and systematic differences in the amount of strain in grains with a [001] direction aligned along the axis of the tube and those with a [111] direction aligned along the tube axis. Model calculations show that these differences originate in the different yield points and elastic moduli for different crystallographic directions.

Regrowth, retention and evolution of tritium from stainless steel

Abstract: This presentation describes some experiments to investigate the removal, regrowth and evolution of tritium from stainless steel planchets that had been exposed to elemental tritium. The total triti...

Density gradients in ceramic pellets measured by computed tomography

Abstract: Density gradients are of fundamental importance in ceramic processing and computed tomography (CT) can provide accurate measurements of density profiles in sintered and unsintered ceramic parts. As a demonstration of this potential, the density gradients in a pellet pressed from an alumina powder were measured by CT scanning. In order to detect small density gradients within the pellet, CT images having high density resolution were measured and corrected for beam-hardening effects. The resulting data are discussed relative to the transmission of forces through the powder mass during the pelletizing process.