Showing papers by "Argonne National Laboratory published in 1976"

Laws for Work-Hardening and Low-Temperature Creep

Abstract: The true stress-strain curves of polycrystalline aluminum, copper, and stainless steel are shown to be adequately represented by an exponential approach to a saturation stress over a significant range. This empirical law, which was first proposed by Voce, is expanded to describe the temperature and strain-rate dependence, and is put on a physical foundation in the framework of dislocation storage and dynamic recovery rates. The formalism can be applied to the steady-state limit of creep in the same range of temperatures and strain rates; the stress exponent of the creep rate must, as a consequence, be strongly temperature dependent, the activation energy weakly stress dependent. Near half the melting temperature, where available work-hardening data and available creep data overlap, they match. Extrapolation of the proposed law to higher temperatures suggests that no new mechanisms may be necessary to describe high-temperature creep. A new differential equation for transient creep also follows from the empirical work-hardening law.

Scission-point model of nuclear fission based on deformed-shell effects

Abstract: A static model of nuclear fission is proposed based on the assumption of statistical equilibrium among collective degrees of freedom at the scission point. The relative probabilities of formation of complementary fission fragment pairs are determined from the relative potential energies of a system of two nearly touching. coaxial spheroids with quadrupole deformations. The total potential energy of the system at the scission point is calculated as the sum of liquid-drop and shell- and pairing-correction terms for each spheroid, and Coulomb and nuclear potential terms describing the interaction between them. The fissioning system at the scission point is characterized by three parameters---the distance between the tips of the spheroids ($d$), the intrinsic excitation energy of the fragments (${\ensuremath{\tau}}_{\mathrm{int}}$), and a collective temperature (${T}_{\mathrm{coll}}$). No attempt is made to adjust these parameters to give optimum fits to experimental data, but rather, a single choice of values for $d$, ${\ensuremath{\tau}}_{\mathrm{int}}$ and ${T}_{\mathrm{coll}}$ is used in the calculations for all fissioning systems. The general trends of the distributions of mass, nuclear charge, and kinetic energy in the fission of a wide range of nuclides from Po to Fm are well reproduced in the calculations. The major influence of the deformed-shell corrections for neutrons is indicated and provides a convenient framework for the interpretation of observed trends in the data and for the prediction of new results. The scission-point configurations derived from the model provide an interpretation of the "saw-tooth" neutron emission curve as well as previously unexplained observations on the variation of $\stackrel{-}{\mathrm{TKE}}$ for isotopes of U, Pu, Cm, and Cf; structure in the width of total kinetic energy release as a function of fragment mass ratio; and a difference in threshold energies for symmetric and asymmetric mass splits in the fission of Ra and Ac isotopes. In spite of a number of recognized simplifications in the model, quantitative fits to the data are generally within expected errors of the shell corrections determined by the Strutinski prescription.NUCLEAR REACTIONS, FISSION Po, Ra, U, Cm, Cf, Fm; calculated $A$, $Z$, and KE distribution of fragments. Liquid-drop model. Deformed-shell corrections. Strutinski prescription. Asymmetric and symmetric fission.

Lipid Storage and Utilization in Reptiles

Abstract: Lipids represent a biochemically efficient mechanism for storing energy to be used at a later date for maintenance and/or reproduction. This storage and utilization at different times results in seasonal patterns of lipid cycling. Reptiles exhibit a number of lipid cycling patterns which can be explained by seasonal patterns of food availability. Seasonality in food availability determines the quantity of lipids stored, when lipids are stored, and for what purposes these lipids are utilized. Lipid cycling patterns are in turn correlated with life histories.

Theory of the angular distribution of photoelectrons ejected from optically active molecules and molecular negative ions

Abstract: It is shown that the angular distribution of photoelectrons ejected on absorption of left or right circularly polarized light by a mixture of randomly oriented dextrorotatory and levorotatory optical isomers behaves as $A\ifmmode\pm\else\textpm\fi{}Bcos\ensuremath{\theta}+C{cos}^{2}\ensuremath{\theta}$, where $\ensuremath{\theta}$ is the angle between the photoelectron momentum and photon direction of incidence and $\ifmmode\pm\else\textpm\fi{} Bcos\ensuremath{\theta}$ is introduced by the presence of unequal numbers of each isomer. All coefficients of ${cos}^{n}\ensuremath{\theta}$ are of order $\ensuremath{\alpha}$, and analysis shows that $B$ can be as large as $A$ and $C$.

Marine diatoms grown in chemostats under silicate or ammonium limitation. II. Transient response of Skeletonema costatum to a single addition of the limiting nutrient

Abstract: Skeletonema costatum was grown at different steady-state growth rates in ammonium or silicate-limited chemostats. The culture was perturbed from its steady-state condition by a single addition of the limiting nutrients ammonium or silicate. The transient response was followed by measuring nutrient disappearance of the liliting perturbation experiment indicate that three distinct modes of uptake of the limiting nutrient can be distinguished; surge uptake (V s ), internally controlled uptake (V i ), and externally controlled uptake (V e ). An interpretation of these three modes of uptake is given and their relation to control of uptake of the limiting nutrient is discussed. The uptake rates of the non-limiting nutrients were shown to be depressed during the surge of the uptake of the limiting nutrient. Kinetic uptake parameters, K s and V max, were obtained from data acquired during the externally controlled uptake segment, V e . The same V max value of 0. 12 h-1, was obtained under either silicate or ammonium limitation. Estimates of K s were 0.4 μg-at NH4-N l-1 and 0.7 μg-at Si l-1. Short-term 15N uptake-rate measurements conducted on nitrogen-limited cultures appear to be a combination of V s or V i , or at lower substrate concentrations V s and V e . It is difficult to separate these different uptake modes in batch or tracer experiments, and ensuing problems in interpretation are discussed.

The Combined Effect of Temperature and Humidity Fluctuations on Refractive Index

Abstract: Depending on whether the radiation under consideration is acoustic, visible or microwave, either temperature or humidity fluctuations are ordinarily assumed to be an insignificant source of refractive index fluctuations. For applications in the atmospheric surface layer or in a free convection layer, the value of the local Bowen ratio β, which is the ratio of sensible to latent heat flux densities, can be used to determine when variations in both temperature T and water vapor pressure e are important considerations. When |β| 0.32 for microwave radiation, both T and e fluctuations have at least a 10% effect on the amplitudes of refractive index fluctuations, provided T and e are highly correlated. If T and e are uncorrelated, this 10% level is |β| 1.45 for acoustic, visible and microwave radiation, respectively. With knowledge of β and the extent of the T and e correlation, r...

The effective interaction between nucleons deduced from nuclear spectra

Abstract: Two-body matrix elements of the residual nucleon-nucleon interaction are extracted from experimental data throughout the periodic table and are used to determine the ranges and well depths of various components of a local interaction. The $T=1$ even and odd components of the central interaction both definitely require two wells with different ranges; a shorter-range attractive well with a longer-range repulsive one. The need for a tensor interaction and a two-body spin-orbit interaction is also explored and their inclusion improves the fit slightly.

The distribution of cortical and trabecular bone mass along the lengths of the radius and ulna and the implications for in vivo bone mass measurements.

Abstract: The amounts of cortical and trabecular bone mineral mass were measured by means of microdissection and an ashing technique at approximately 2.5 mm intervals along the most distal 12 cm of radii and ulnae from four women aged 21, 43, 63, and 85. The data show that the distributions of mineral mass and percentage of trabecular bone are similar in both bones. At sites in the radius and ulna commonly used in the photon absorptiometric method of bone mineral mass measurement the percentage of trabecular bone varies between 10% and 50%. The percentage of trabecular bone in the most distal 10% of the length of the radius and ulna remains approximately constant with age but the percentage in the segment which lies between 30% and 40% of the length, measured from the styloid process, increases with age.

Temper embrittlement of Ni-Cr Steels by phosphorus

Abstract: Temper embrittlement in 3.5 pct Ni, 1.7 pct Cr steels doped with P and isothermally aged at several temperatures was studied by measurements of ductile-to-brittle transition temperature and hardness, which were correlated with observations of the intergranular fracture surfaces by Auger electron spectroscopy and scanning electron fractography. It is shown that if all other factors remain constant, the effect of a small change in the matrix hardness can be very large; “overaging” (a maximum in embrittlement with respect to aging time) was found to result from softening rather than from a reversal of segregation of P. Nickel was found to be segregated at the grain boundaries, and both Ni and Cr appear to enhance the amount of segregated P. The major role of Cr was found to be its effect of increasing matrix hardness (by enhancing hardenability and resistance to softening during tempering), resulting in an increased susceptibility to temper embrittlement. The effect of variations in the roughness of grain boundary topography appears to be small. It is shown that the segregation of P to grain boundaries can be accounted for by diffusion from the matrix and is consistent with the hypothesis of equilibrium (Gibbsian) segregation. The results are in qualitative agreement with the thermo-dynamic theory of Guttmann.

Neutron diffraction study of UO/sub 2/: Antiferromagnetic state

Abstract: The elastic neutron magnetic scattering cross section from the ${\mathrm{U}}^{4+}$ ions in antiferromagnetic U${\mathrm{O}}_{2}$ has been measured at 4.2 K. The measurements included all reciprocal-lattice points for which $\frac{(sin\ensuremath{\theta})}{\ensuremath{\lambda}}=\frac{\ensuremath{\kappa}}{4\ensuremath{\pi}}\ensuremath{\le}0.83$ ${\mathrm{\AA{}}}^{\ensuremath{-}1}$. The values of $\ensuremath{\mu}f(\stackrel{\ensuremath{\rightarrow}}{\ensuremath{\kappa}})$, obtained from the measurements, fall on a smooth curve at small $\ensuremath{\kappa}$, but show considerable anisotropy for $\frac{(sin\ensuremath{\theta})}{\ensuremath{\lambda}}g0.5$ ${\mathrm{\AA{}}}^{\ensuremath{-}1}$. The ordered magnetic moment is $(1.74\ifmmode\pm\else\textpm\fi{}0.02){\ensuremath{\mu}}_{B}$ per uranium atom at 4.2 K. Theoretical calculations of the magnetic cross section from a number of possible states (including the effects of intermediate coupling and $J$ mixing) are unable to reproduce the experimental data at large $\ensuremath{\kappa}$. Subtracting the calculated magnetic cross section from the observed scattering cross section, we have determined that additional scattering is present only for reflections with $h$, $k$ even and $l$ odd. The additional intensity is a result of a small (0.014 \AA{}) shift of the oxygen atoms from their equilibrium position. These shifts are examined within the framework of Allen's microscopic theory for U${\mathrm{O}}_{2}$. An analysis in terms of the homogeneous deformations proposed by Allen cannot explain the neutron results. However, when Allen's concepts are extended to include inhomogeneous deformations, corresponding to a zone-boundary $q=(\frac{2\ensuremath{\pi}}{a}) (1,0,0)$ phonon, excellent agreement is obtained between theory and experiment. The oxygen displacement is 0.014 \AA{} from the fluorite lattice positions and, in addition, the inhomogeneous deformation ${T}_{2g}\ensuremath{-}{T}_{1g}$ does not require a corresponding change in the unit-cell dimensions. The dominance of this deformation mode in the spin-lattice interactions suggests the presence of a noncollinear magnetic structure in U${\mathrm{O}}_{2}$.

Wind profile relationships from the ‘wangara’ experiment

Abstract: From over 1000 half-hour observations of near-surface wind profiles at Hay, NSW, more than 500 high quality sets of data are selected. In unstable conditions, these closely confirm previous analyses and suggest near equality between z/L and Ri. The data are well described by either the KEYPS relationship (Panofsky) or that of Businger: ϕM = (1–16z/L)−1/4. In stable conditions, a log-linear formulation (ϕM = 1+az/L) is found to give an adequate description of the wind profile up to z/L ≃ 0.5, with some evidence for a slight variation in α between the values 4.0 at neutral and about 6 when z/L ≃ 0.2. The average value of α between these limits is found to be 5.0±0.2. In conditions of very high stablity, a linear profile (du/dz = cu*/(kL)) is suggested above the height z ≃ 10L. In the transition region between the log-linear and linear profile regimes, the log-linear formulation appears to tend towards a purely logarithmic law as stability increases. There is no evidence for any sudden change in behaviour, nor is there any suggestion that a purely logarithmic relationship is ever attained as an average situation. The value of c in the purely linear relationship is found to be between 0.4 and 0.9. The data also indicate that in extremely stable conditions (L ≃ 50cm) the dimensionless gradients of heat and of momentum may differ by about a factor of two, with ϕH being the larger. The roughness length of the site used is found to be 1.2±0.1 mm, considerably less than the values appropriate to earlier experiments performed in the same general area. There is some evidence for an increase in z0 with decreasing wind speed (reaching about 3 mm when the wind at 1 m is about 1 ms−1), in accord with Deacon's hypothesis concerning the form drag of roughness elements. From the point of view of applying a low-level drag coefficient in order to estimate friction velocities, the errors arising from the change in roughness length are sometimes comparable to those resulting from stability effects. Not surprisingly, the data show that in slightly stable conditions dewfall was low, whereas in extremely stable situations dewfall accounted for most of the heat loss from the air. The data used here form part of a much larger body of information obtained during 1967 and known as the ‘Wangara’ experiment.

Melting Heat Transfer in Steady Laminar Flow Over a Flat Plate

Abstract: Exact similarity solutions for melting heat transfer in steady laminar flow over a flat plate were collected from the diffusional mass transfer literature to provide solutions for Prandtl numbers from 0.001 to 100. An exact closed-form solution is introduced for the case of melting in porous media. The results are presented and discussed in such a way as to emphasize the similarity between melting and diffusional mass transfer.

An Experimental Investigation of the Thermally Induced Flow Oscillations in Two-Phase Systems

Abstract: An experimental study on the onset of thermally induced two-phase flow oscillations has been carried out in a uniformly heated boiling channel using Freon-113 as the operating fluid. The effects of inlet subcooling, system pressure, inlet and exit restrictions, and inlet velocity have been studied. The experimental data have been compared with the equilibrium as well as the nonequilibrium theory including the effect of subcooled boiling. It has been found that the effect of thermal nonequilibrium should be included in a theoretical model for accurate prediction of the onset and the frequency of thermally induced flow oscillations. A simplified stability criterion has also been presented and compared with the experimental data.

Ab initio SCF and CI studies on the ground state of the water molecule. II. Potential energy and property surfaces

Abstract: Self‐consistent field and configuration interaction calculations for the energy and one‐electron properties of the ground state of the water molecule were carried out with a (5s4p2d/3s1p) 39‐function STO basis set. The CI treatment included all single and double excitation configurations (SD) relative to the SCF configuration, and a simple formula due to Davidson was used to estimate the energy contribution of quadruple excitations and thus produce a set of corrected (SDQ) energies. The calculations were carried out for 36 molecular geometries (22 symmetric and 14 asymmetric) in the neighborhood of the equilibrium geometry. Least‐squares analysis was used to determine theoretical equilibrium geometries and to derive quartic and cubic expansions for the energy and one‐electron properties in terms of internal displacement coordinates. Considerable improvement was found in the SD and particularly in the SDQ results relative to SCF, with the SDQ equilibrium geometry being in excellent agreement with experimen...

Ideal concentrators for finite sources and restricted exit angles

Abstract: Design procedures for ideal radiation concentrators are described which are applicable to finite sources and/or restricted exit angles. Finite sources are relevant for second stage concentrators which collect and further concentrate radiation from a primary focusing element (mirror or lens) in a manner similar to the field optic element in a telescope. Restricting the exit angle is useful for improving the optical efficiency of solar collectors by eliminating grazing angles of incidence of the absorber. It also serves to extend the useful range of angular acceptance values available from solid dielectric concentrators that function by total internal reflection. Concentrators of this type can be used to construct highly efficient radiation traps (spectrally selective filters).

Role of Erythronium americanum Ker. in Energy Flow and Nutrient Dynamics of a Northern Hardwood Forest Ecosystem.

Abstract: The aboveground activity of the spring herb, Erythronium americanum, is restricted to the period between snowmelt and forest canopy development. Its phenology and production capacity closely adapt the species to this temporal niche in northern deciduous forests. While E. americanum has a minor effect on energy flow, it may reduce losses of potassium and nitrogen from the ecosystem during the period of maximum removal by incorporating these elements in accumulating biomass. Later, during the summer, these nutrients are made available when the above-ground, nonperennating tissues decay.

Polarization induced in a solid by the passage of fast charged particles

Abstract: A theoretical description is given of the polarization charge-density distribution induced in a solid by the passage of a fast charged particle. The stopping power of the solid (including the effects of close collisions) is well accounted for by the braking effect of the polarization charges. Calculations based on this model for the polarization give good agreement with the results of recent experiments on the dissociation of fast molecular ions in foils. (AIP)

New proposal for structure of special-pair chlorophyll.

Abstract: A new model is proposed for the structure of the special pair of chlorophyll a molecules believed to correspond to the P700 species in green plants and algae. The proposed model, although admittedly speculative, is based upon exciton-theoretical considerations and on in vitro infrared and visible absorption spectra of a 700 nm absorbing ethanol adduct of chlorophyll a. In the new model, two chlorophyll a molecules are held together by (a) two strong ring V keto C[unk]O[unk]H[unk]O (or keto C[unk]O[unk]H[unk]N, or keto C[unk]O[unk]H[unk]S) hydrogen bonds and by (b) π-π van der Waals stacking interactions between the two chlorophyll a macrocycles. Macrocycle stacking provides the intermolecular π-π overlap necessary to promote the (experimentally observed) delocalization of the unpaired electron of the in vivo radical over two chlorophyll π systems. The new model provides an explicit role for the participation of protein in the formation of the chlorophyll special pair. The in vitro system yields an electron spin resonance signal indistinguishable from that of oxidized P700 in Chlorella vulgaris.

Ferromagnetism in iron-chromium alloys. II. Neutron scattering studies

Abstract: Diffuse neutron scattering measurements have been made on ferromagnetic iron-chromium alloys containing 15-, 30-, 50-, and 73-at.% Cr with the long-wavelength neutron spectrometer at Harwell. The results have been analyzed, within the framework of the Marshall model, to determine both the average moments at the iron and chromium sites and the disturbances in the individual moments arising from fluctuations in the near-neighbor environent. The average moment per iron atom rises to a small maximum around 15-at.% Cr and then decreases as the chromium concentration increases. The average chromium moment is negative and becomes smaller in magnitude as the chromium concentration increases. The sum of the disturbances at the iron and chromium sites changes sign as a function of concentration. The range of the disturbance appears to be smaller in chromium-rich than in iron-rich alloys.

Solar concentrators with maximal concentration for cylindrical absorbers.

Abstract: The differential equation is derived that describes the reflector of an ideal two-dimensional radiation concentrator with an absorber of arbitrary convex shape. For the special case of an absorber with circular cross section, the equation can be solved in closed form if suitable coordinates are used. The effect of absorption at the reflector is considered, and formulas are presented for determining the attenuation of radiation on its passage from aperture to absorber.

Sensitivity of strains of escherichia coli differing in repair capability to far uv, near uv and visible radiations*

Abstract: — In stationary phase, strains of Escherichia coli deficient in excision (B/r Her) or recombination repair (K.12 AB2463) were more sensitive than a repair proficient strain (B/r) to monochromatic near-ultraviolet (365 nm) and visible (460 nm) radiations. The relative increase in sensitivity of mutants deficient in excision or recombination repair, in comparision to the wildtype, was less at 365 nm than at 254 nm. However, a strain deficient in both excision and recombination repair (K12 AB2480) showed a large, almost equal, increase in sensitivity over mutants deficient in either excision or recombination repair at 365 nm and 254 nm. All strains tested were highly resistant to 650 nm radiation. Action spectra for lethality of strains B/r and B/r Her in stationary phase reveal small peaks or shoulders in the 330–340, 400–410 and 490–510 nm wavelength ranges. The presence of 5μg/ml acriflavine (an inhibitor of repair) in the plating medium greatly increased the sensitivity of strain B/r to radiation at 254, 365 and 460 nm, while strains E. coli B/r Her and K12 AB2463 were sensitized by small amounts. At each of the wavelengths tested, acriflavine in the plating medium had at most a small effect on E. coli K.12 AB2480. Acriflavine failed to sensitize any strain tested at 650 nm. Evidence supports the interpretation that lesions induced in DNA by 365 nm and 460 nm radiations play the major role in the inactivation of E. coli by these wavelengths. Single-strand breaks (or alkali-labile bonds), but not pyrimidine dimers are candidates for the lethal DNA lesions in uvrA and repair proficient strains. At high fluences lethality may be enhanced by damage to the excision and recombination repair systems.

Relativistic energy levels and bonding in actinide hexaflourides

Abstract: A fully relativistic self‐consistent Dirac–Slater model has been used to calculate one‐electron energy levels and charge distributions for UF6, NpF6, and PuF6. Analysis of ground state charge densities reveals 5fn populations similar to free atom states; we are able to distinguish between metal–ligand bonding levels and the optically active levels described by crystal field theory. Ultraviolet and optical excitations, as well as ionziation energies, are calculated using an approximate transition state procedure and found to be in reasonable agreement with experiment. The Coulomb correlation integral appearing in the determination of the nature of 5f orbitals in solids is discussed on the basis of the systematics observed in our transition state calculations.