Showing papers by "Chalk River Laboratories published in 1992"
TL;DR: The application of thermal neutron scattering to the study of the structure and dynamics of condensed matter requires a knowledge of the scattering lengths and the corresponding scattering and absorption cross sections of the elements as discussed by the authors.
Abstract: The application of thermal neutron scattering to the study of the structure and dynamics of condensed matter requires a knowledge of the scattering lengths and the corresponding scattering and absorption cross sections of the elements. Ln some cases, values for the individual isotopes are needed as well. This information is required to obtain an absolute normalization ofthe scatteredneutron distributions, tocalculate unit-cell structure factors in neutron crystallography, and to correct for effects such as absorption, self-shielding, extinction, multiple scattering, incoherent scattering, and detector efficiency.
3,077 citations
TL;DR: It is suggested that the maintenance of diversity in the absence of frequent modern-day recruitment, and resistance to further geographic differentiation in this spatially heterogeneous environment reflect occasional seedling establishment through "windows of opportunity" and more importantly, the species' clonal morphology.
Abstract: Trembling aspen (Populus tremuloides Michx.), a clonal angiosperm, is the most geographically widespread tree in North America. It is generally thought that most extant populations in the western interior of Canada and the United States became established shortly after glacial retreat, but sexual recruitment then effictively ceased owing to inimical climatic conditions. Six populations of trembling aspen were studied in the prairie and montane environments of Waterton Lakes National Park, Alberta. Vegetative tissues were analyzed for electrophoretically detectable variation in 13 enzymes encoded by 14 polymorphic loci and three monomorphic loci. All populations maintained high levels of interand intrapopulation diversity (P = 0.891; H = 0.319; A = 2.4). The mean fixation index, F, was -0.102, indicating some deviation from Hardy-Weinberg expectations. Genetic differentiation (FST = 3.0) was apparent in this ecologically diverse, but geographically small-scale, spatial setting. It is suggested that the maintenance of diversity in the absence of frequent modern-day recruitment, and resistance to further geographic differentiation in this spatially heterogeneous environment reflect occasional seedling establishment through "windows of opportunity" and more importantly, the species' clonal morphology. The phalanx growth form and concomitant physiological integration between ramets combine to spread the risk of death and buffer the effects of selection over time and space.
178 citations
TL;DR: The magnetic metal ions in the cubic pyrochlore form an infinite three-dimensional network of corner-sharing tetrahedra with a very high potential for frustration in the presence of antiferromagnetism as mentioned in this paper.
Abstract: The magnetic metal ions in the cubic pyrochlore ${\mathrm{Tb}}_{2}$${\mathrm{Mo}}_{2}$${\mathrm{O}}_{7}$ form an infinite three-dimensional network of corner-sharing tetrahedra with a very high potential for frustration in the presence of antiferromagnetism. We have performed neutron scattering measurements which show short-range spatial correlations that develop continuously with decreasing temperature, while the characteristic time scale for the fluctuating moments decreases dramatically below ${\mathit{T}}_{\mathit{f}}$\ensuremath{\sim}25 K. Therefore, this pure material, which possess frustration that is purely geometrical in origin, displays a spin-glass state at low temperatures.
133 citations
15 Aug 1992-Nuclear Instruments & Methods in Physics Research Section A-accelerators Spectrometers Detectors and Associated Equipment
TL;DR: In this article, the response of CsI(Tl) scintillators to heavy ions is investigated as a function of E, A, and Z, in addition to the expected dependence of light output on Z and E, a significant dependence on mass number is observed.
Abstract: The response of CsI(Tl) scintillators to heavy ions is investigated as a function of E , A , and Z . In addition to the expected dependence of light output on Z and E , we observe a significant dependence on mass number. A simple parameterization of the quenching in terms of a few physical variables permits characterization of the light output for a variety of nuclear species with a single quenching constant.
82 citations
TL;DR: In this article, the IAEA's International Chernobyl Project to assess the radiological consequences of the nuclear reactor accident, the Agency's Laboratories at Seibersdorf participated in the collection and analyses of environmental samples from the Soviet Union.
80 citations
69 citations
TL;DR: In this paper, the authors measured the thermal diffusivity and specific heat of simulated high-burnup UO 2 fuel with an equivalent burnup of 3 and 8 at% and found that the reduction in thermal conductivity was approximately linear with burnup.
56 citations
TL;DR: This review describes a major portion of the published work on neutron scattering experiments aimed at measuring large scale motions in proteins and the importance of these motions for enzyme function and oxygen transport is indicated.
Abstract: This review describes a major portion of the published work on neutron scattering experiments aimed at measuring large scale motions in proteins. The importance of these motions for enzyme function and oxygen transport is indicated. The theory applicable to each type of neutron scattering measurement is given and results are discussed with a view to biological relevance. New experiments are suggested and a comparison of neutron scattering data is made with results from other techniques such as raman scattering, infrared absorption, photolysis and molecular dynamics simulations.
55 citations
TL;DR: In this article, the relationship between visually estimated tree defoliation and spectral features was linear for trees with >20% defoliations, and curvilinear if the entire defoliation range from healthy to dead trees was considered.
55 citations
TL;DR: In this article, a shell model calculation of the nuclear-structure-dependent axial vector contribution of the Born graphs to the radiative correction in superallowed Fermi β-decay is presented.
51 citations
TL;DR: In this article, a cavity perturbation technique was used to measure the complex scalar permittivity of materials at frequencies between 50 MHz and 3 CHz, and up to a temperature of 1400°C.
Abstract: A system has been developed, based on the cavity perturbation technique, for measuring the complex scalar permittivity of materials at frequencies between 50 MHz and 3 CHz, and up to a temperature of 1400°C. Measurements have been made on several classes of materials, including metallic oxides, conventional and advanced ceramics, fabrics, earthenwares, geological and biological materials, foods, glasses, organics and ferrites. Some materials remain unchanged by thermal cycling, but many undergo irreversible changes; the latter are of interest in microwave processing. The system measures the complex dielectric constant of materials during conventional heating, curing or sintering. Correlating these microwave measurements with other diagnostic techniques will help in the understanding of these processes in several classes of materials.
TL;DR: In this article, the enhancement factor ϵmec = 1 + δmec, defined as the ratio of the axial-charge matrix element in first-forbidden beta decay to its impulse-approximation value, is calculated in a meson exchange model for both light and heavy nuclei.
TL;DR: The London penetration depth is calculated and the results interpolate smoothly between weak- and strong-coupling regimes, and within the framework of Ginzburg-Landau (GL) theory the GL coherence length and the upper critical field are calculated.
Abstract: In the model of hole superconductivity, the pairing interaction enhances the mobility of the carriers. This gives rise to an additional contribution to the London kernel besides the one that is usually considered arising from the single-particle mobility. Thus, the penetration depth in a system described by this model will be shorter than what would be expected from estimates of the carrier effective mass in the normal state. We calculate the London penetration depth and examine its behavior for parameters believed to be appropriate to the high-${\mathit{T}}_{\mathit{c}}$ oxide superconductors. Our results interpolate smoothly between weak- and strong-coupling regimes. The contribution to the kernel from the pairing interaction becomes dominant in the strong-coupling limit, which within this model is achieved for low density of carriers. We also calculate within the framework of Ginzburg-Landau (GL) theory the GL coherence length and the upper critical field. The zero-temperature GL coherence length is not directly related to the average size of the pair wave function, as is usually the case, when the effect of the pairing interaction on the mobility is taken into account, and can be substantially larger. The relevance of these results to the interpretation of experimental data in high-${\mathit{T}}_{\mathit{c}}$ oxides is discussed.
TL;DR: Distributions for beta emitters are in excellent agreement with both experimental results and ETRAN and EGS4 calculations, except at very short distances.
Abstract: The ACCEPT Monte Carlo code has been used to calculate radial dose distributions around isotropic point sources of monoenergetic electrons between 0.01 and 10 MeV in an infinite water medium. The results were averaged over beta spectra to derive distributions for 147 beta emitters of which 32 are presented. More extensive tables of distributions will be presented in a report. Distributions for monoenergetic electrons agree with recent ETRAN-code calculations of Berger and Seltzer within 2%, except at very short distances where there are differences up to several percent. Results calculated by the EGS4 code differ by up to a few percent. Distributions for beta emitters are in excellent agreement with both experimental results and ETRAN and EGS4 calculations, except at very short distances.
TL;DR: In this article, Fischer-Tropsch synthesis was performed at semi-industrial conditions (300°C, 1515 kPa, H2/CO = 1) on various oxide (SiO2, A1203, TiO2 and ZrO2)-supported iron catalysts.
TL;DR: In this article, the structure of the low temperature phase of acetonitrile was determined from neutron powder diffraction data and the lattice parameters were measured as a function of temperature.
Abstract: The structure of the low temperature phase of acetonitrile was determined from neutron powder diffraction data. The lattice parameters were measured as a function of temperature and the structure of the high temperature phase, as previously determined, was confirmed. At 12 K, the structure is orthorhombic, Cmc21, a = 5·9895(3) A, b = 5·2079(2) A, c = 7·7317(4) A. A transformation to a monoclinic phase, P21/c, a = 4·1182(7) A, b = 8·2865(10) A, c = 7·9946(11) A, β = 100·4(6)°, takes places between 220 and 229 K. Equilibrium structures for the two phases were calculated using simple model potentials and the calculated structures are very similar to those observed. The nature of the phase transition was investigated by means of these model potentials.
15 Jan 1992-Nuclear Instruments & Methods in Physics Research Section A-accelerators Spectrometers Detectors and Associated Equipment
TL;DR: The theory of the measurement of multipole fields with rotating coil devices with particular emphasis on the analysis of the sources of error in these measurements was developed in cylindrical coordinates for the full three-dimensional magnetic field as mentioned in this paper.
Abstract: The theory of the measurement of multipole fields with rotating coil devices is developed particular emphasis on the analysis of the sources of error in these measurements. The theory is developed in cylindrical coordinates for the full three-dimensional magnetic field. The presentation is quite detailed so that the reader can easily follow the development and, more importantly, adapt the results presented to his or her own requirements.
TL;DR: Within the framework of the International Chernobyl Project, radionuclides were studied in food and total diet samples collected in selected communities in the areas of the USSR which were contaminated to various degrees by fallout from the accident at the Chernobyl nuclear power station as discussed by the authors.
TL;DR: In this article, a detailed level scheme containing approximately 380 transitions was presented, and the quasiparticle structure of the various rotational bands was discussed, where the interaction at the first backbend in the ground band was found to be strongly signature dependent.
TL;DR: The self-energy of {bold q}={bold 0} phonons is calculated in both the superconducting and normal states, with use of a real-axis formulation, valid for arbitrary temperature, impurity scattering rate, and electron-phonon coupling strength.
Abstract: The self-energy of q=0 phonons is calculated in both the superconducting and normal states, with use of a real-axis formulation, valid for arbitrary temperature, impurity scattering rate, and electron-phonon coupling strength. The calculations are performed within the Eliashberg framework. We qualitatively confirm much of the previous work done using Pade\ifmmode\acute\else\textasciiacute\fi{} approximants. Besides providing quantitative corrections, we also predict an observable effect with low-frequency phonons.
TL;DR: In this paper, tritium and tritiated water (HTO) desorption and sorption on materials to be used in the construction of fusion facilities are studied.
Abstract: In this paper, sorption and desorption of tritium (HT) and tritiated water (HTO) on materials to be used in the construction of fusion facilities are studied. In ca. 24-hour exposures in argon or room air, metal samples sorbed 8-200 {mu}Ci/m{sup 2} (1 Ci = 3.7 {times} 10{sup 10} Bq) of tritium form atmospheres of 5-9 Ci/m{sup 3} HT, and non-metallic samples sorbed 60-800 {mu}Ci/m{sup 2} from atmospheres of 14 Ci/m{sup 3} HT. Sorption of HTO varied much more widely than HT sorption for different samples, ranging from 4 {mu}Ci/m{sup 2} for glass to 1,300,000 {mu}Ci/m{sup 3} HTO in room air. Time dependence of desorption in dry air showed a rapid initial process and a slower secondary process.
TL;DR: The formation of polycrystalline α-alumina during the heat treatment of aluminium oxychloride fibres occurs via transitions through various intermediate crystalline alumina phases.
Abstract: The formation of polycrystalline α-alumina during the heat treatment of aluminium oxychloride fibres occurs via transitions through various intermediate crystalline alumina phases. The different phases show a strict pseudomorphism, i.e. the external shape of the crystal is retained and there is an orientational relationship between the crystal axes, before and after the phase transition. Consequently, the phase sequences and microstructure are interdependent and controlled by the heat-treatment conditions, which determine the final properties of the fibres. The effects of water content in the fibre formulation, and sintering temperature and heating rate on the phase development from amorphous oxychloride to crystalline α-alumina fibres, were investigated. The transition path for the phase evolution during heat treatment to 1200°C (when the formation of α-alumina is completed) was established and correlated with fibre microstructure. In fibres with higher water content, the transition temperature for γ- and δ-alumina phase formation was higher. Only for a very high heating rate was θ-alumina observed as an intermediate phase during phase evolution. The axial porosity appeared to be eliminated more rapidly in the fibres at temperatures below 1200°C. Consequently, residual porosity, comprising closed pores, in the final stage of sintering, was mainly observed radially oriented toward, and on, the fibre surface. A high diametral shrinkage was measured and plotted as a function of the sintering temperatures used for fibre heat-treatment.
TL;DR: The dependence of the superconducting critical temperature and the isotope coefficient on bandwidth, band-filling, and the direct Coulomb repulsion within Eliashberg theory was investigated in this paper.
Abstract: We investigate the dependence of the superconducting critical temperature and the isotope coefficient on bandwidth, band-filling, and the direct Coulomb repulsion, within Eliashberg theory. The Migdal approximation is assumed throughout, and the Coulomb repulsion is modelled by the Hubbard U and treated in the simplest approximation. We assume a constant density of states with a finite bandwidth. We find that while, in principle, small isotope coefficients are possible, it is unlikely that the isotope coefficient can ever be negative within this model. Furthermore, it is difficult to achieve small isotope coefficients for realistic parameters. Finally, we discuss a possible means by which large isotope coefficients can occur at low filling.
TL;DR: Elastic neutron scattering has been used to study the magnetic structure of erbium in an applied magnetic field of up to 2.8 T along the c axis, and a lock-in structure with q=(1/4)c * appears in the cone phase for fields above 1.
Abstract: Elastic neutron scattering has been used to study the magnetic structure of erbium in an applied magnetic field of up to 2.8 T along the c axis. There are three major types of ordered structure: the cone phase, an intermediate-temperature phase with moments modulated both along the c axis and perpendicular to the c axis, and a phase at high temperatures where only the c component of the moment is ordered. Within these phases the modulation of the moments may lock in to the lattice. The field tends to stabilize the cone phase and to destabilize the intermediate phase. A lock-in structure with q=(1/4)c * appears in the cone phase for fields above 1
TL;DR: In this paper, the energy of the first plasmon loss peak for the ~-Zr hydride phase in pure Zr was determined and the peak energy is sufficiently different from that of the 03B4-hydride to permit EELS fingerprinting with ease using a parallel EELS system, but with greater difficulty using serial EELS, because of drift associated with the slower collection rates.
Abstract: The energy of the first plasmon loss peak has been determined for the ~-Zr hydride phase in pure Zr. The peak energy is sufficiently different from that of the 03B4-hydride to permit EELS "fingerprinting" with ease using a parallel EELS system, but with greater difficulty using serial EELS, because of drift associated with the slower collection rates. Experiments on Zr-2.5Nb alloys showed that the hydride phases can also be distinguished using EELS, facilitating the analysis of hydride precipitates and blisters in nuclear reactor pressure tubes.
01 Feb 1992-Nuclear Instruments & Methods in Physics Research Section A-accelerators Spectrometers Detectors and Associated Equipment
TL;DR: Three methods of standardizing 125I have been compared and it is concluded that systematic differences, if any, between the results of the methods are ≤ 0.2%.
Abstract: Three methods of standardizing 125I have been compared: (1) sum-peak coincidence counting with a Ge detector, (2) X-ray-X-ray coincidence counting, and (3) 4πe-X coincidence counting with efficiency extrapolation. The object of the study was to look for any evidence at the 0.1% level of systematic differences in the derived activity concentration. Enough data were accumulated to reduce the statistical uncertainties for methods (2) and (3) to 0.05%. It is concluded that systematic differences, if any, between the results of the methods are ≤ 0.2%.
TL;DR: In this paper, a horizontal pulsed solvent extraction column (HPC) has been proposed to replace vertical pulsed columns (VPC) in the separation of heavy elements in the nuclear industry.
Abstract: A horizontal pulsed solvent extraction column (HPC) has great potential in replacing vertical pulsed columns (VPC) in the separation of heavy elements in the nuclear industry. Published work examining the performance of the HPC suggests that the mass transfer efficiency for this column is comparable to the VPC, but information on the hydrodynamic characteristics of the HPC is sparse. This paper describes the flow characteristics of the HPC and presents experimental results and correlations for flooding conditions and holdup of the dispersed phase. Correlations obtained for the HPC for these hydrodynamic parameters are compared to published results for the VPC.
La colonne pulsee horizontale (HPC) servant a l'extraction de solvants montre un fort potentiel pour remplacer les colonnes pulsees verticales (VPC) dans la separation des elements lourds dans l'industrie nucleaire. Les travaux publies examinant la performance de la colonne horizontale suggerent que l'efficacite du transfert du matiere pour cette colonne est comparable a celle de la colonne verticale, mais les donnees sur les caracteristiques de la HPC sont rares. On decrit dans cet article les caracteristiques d'ecoulement de la HPC et on presente des resultats experimentaux et des correlations pour des conditions d'engorgement et la retention de la phase dispersee. Les corre'lations obtenues pour la colonne horizontale pour ces parametres hydrodynamiques sont comparees aux resultats publies pour la colonne verticale.
TL;DR: In this paper, the importance of charged particle detection in channel selection for in-beam spectroscopic studies is pointed out, and the main emphasis of the initial experiments with this apparatus has been on a study of the charged particles associated with superdeformed bands, and on the search for nuclear-structure effects in the evaporation spectra.
TL;DR: In this paper, the application of four methods to predict subcooled boiling void fraction measured in an experiment aimed at separate-effect measurements of sub-cooled void condensation and generation is discussed.
Abstract: While subcooled boiling at high pressures has been studied extensively, the phenomenon is, as yet, no sufficiently characterized at low pressures. In this paper the application of four methods to predict subcooled boiling void fraction measured in an experiment aimed at separate-effect measurements of subcooled void condensation and generation is discussed. The methods include a simple correlation and a hierarchy of three models, each of which addresses void generation and condensation at a different level of complexity. Comparisons are given between the experimental data and results from each of the prediction methods.