Search Results - (Author, Cooperation:K. W. Brown)

2018-05-18

American Physical Society (APS)

0556-2813

1089-490X

Physics

Nuclear Reactions

2018-03-28

American Physical Society (APS)

0556-2813

1089-490X

Physics

Nuclear Reactions

2018-05-17

American Physical Society (APS)

0556-2813

1089-490X

Physics

Nuclear Structure

2013-05-24

Nature Publishing Group (NPG)

0028-0836

1476-4687

Biology

Chemistry and Pharmacology

Medicine

Natural Sciences in General

Physics

*Bibliometrics ; Financing, Organized/economics/statistics & numerical data ; Journal Impact Factor ; Research Personnel/*standards/*statistics & numerical data

1089-7550

AIP Digital Archive

Physics

In this paper we have calculated the free-electron density in a finite superlattice. Resonant tunneling causes a buildup of particle density in the well regions, giving rise to an accumulation of electrons in those regions. Using our results, we have estimated the change in barrier heights and well depths caused by the electrostatic force. A negligible change is found for a double-well structure having well widths of 40 A(ring) and barrier widths of 20 A(ring). Our approach could be extended to calculate the tunneling current self-consistently. Additionally we have used a time-dependent solution of Schrödinger's equation to estimate the trapping time of the electrons due to the resonant effect. The results show that the probability density oscillates several times between the two wells, leaking out gradually at each step. After about 2.4×10−13 s, most of the waves centered about the resonant energies have been transmitted.

Electronic Resource

1089-7690

AIP Digital Archive

Physics

Chemistry and Pharmacology

Coherent anti-Stokes Raman spectra are reported for solid, nanometer-sized C2H2 and CO2 aggregates formed in free jet expansions. From comparisons with spectra of equilibrium samples, the clusters are seen to be face centered cubic with a crystalline character indicated by the appearance of vibrational factor group splittings and, for CO2, by the observation of two external lattice librations.

Electronic Resource

1399-3054

Blackwell Publishing Journal Backfiles 1879-2005

Biology

Young growth-chamber-grown cotton plants were subjected to a series of eight periods of soil water stress, which served as a preconditioning treatment. After preconditioning, water was withheld and changes in the stomatal resistance and leaf water potential were determined and compared with similar well watered control plants.The stomatal response of stress preconditioned plants adjusted such that the diffusion resistance of the lower surface of the leaf did not reach a value greater than 20 s cm−1 until the leaf water potential dropped 14 bars below that required to reach the same resistance on previously unstressed plants. The resistance—leaf water potential relation for the adaxial surface was unaltered by the preconditioning treatment. Adjustment of the osmotic potential of the guard cells on the abaxial surface provides at least a partial explanation of this change in response. The lack of adjustment of stomatal response on the adaxial surface of the leaves was correlated with a lack of adjustment in osmotic potential of guard cells on that surface.

Electronic Resource

1476-4687

Nature Archives 1869 - 2009

Biology

Chemistry and Pharmacology

Medicine

Natural Sciences in General

Physics

[Auszug] The suggestion that Ad 12-induced rodent tumours originate specifically from primitive neural epithelium10, together with the discovery that this virus induces retinal tumours following intraocular inoculation of rats and baboons11'12, prompted us to attempt to transform human embryo retinoblasts ...

Electronic Resource

1399-3054

Blackwell Publishing Journal Backfiles 1879-2005

Biology

A model is proposed for the relationship between net photosynthetic rate (N) and light Intensity at a given concentration of CO2 in the air ([CO2]a). The model provides a prediction of the sum of the diffusion resistances (Σr), the capacity (K) of the leaf to fix CO2, the concentration of CO2 at the point of photosynthesis ([CO2]g), and the respiration rate (R). The model fits the available data well and provides a frame work by which future research may be guided.The calculated values of [CO2]g decreased from [CO2]g at the compensation point to a nearly constant value at high tight intensities. [CO2]g high light infensitit-s range from 32 to 144 μ/l (volume) depending on the species. When these values of [CO2]g, are used in the diffusion equation, the resulting levels of the mesophyll resistance (rm) are lower than those calculated by using the assumptions that [CO2]g equals zero. The plants which had (he higher photosynthetic rates at a given light intensity and [CO2]a had grealer values of [CO2]g than those with lower photo-synthetic rates.The calculated rates of respiration of wheat leaves were twice as high as those measured in the dark. This suggests that the light respiration rate may be twice as great as the dark respiration rate at the same temperature.The calculated values of K demonstrate variability within and between species. The maximum N was independent of K. A relationship between K and the maximum quantum efficiency, at constant levels of [CO2]g, was demonstrated in several species.The Σr was inversely related to the maximum rate of photosynthesis for the species investigated. The values of rm calculated for cotton were inversely related to [CO2]a suggesting that the transfer of [CO2] in the cell may involve a concentration dependent chemical reaction in addition to or rather than a physical diffusion process.

Electronic Resource

1399-3054

Blackwell Publishing Journal Backfiles 1879-2005

Biology

The respiration rate of individual leaves of cotton (Gossypium hirsutum L. cv. Stoneville 213), beans (Phaseolus vulgaris L. cv. Bush Blue Lake), and sorghum (Sorghum vulgare Pers.) which had been fully expanded prior to a series of severe water stresses was compared with those of unstressed leaves of similar age. Measurements were made over a range of leaf temperatures. The respiration rate per unit area of the leaves of all rewatered plants were significantly lower than those of the plants which had not undergone water stress. During the stress periods, the leaves of all species suffered dry matter loss. The respiration rates per unit dry matter for cotton and beans were the same for the plants which had undergone stress as they were for the plants which had not undergone stress, thus for these two C3 plants the decrease in dark respiration due to water stress may be explained entirely by the loss of dry matter. Respiration rates of the water stressed sorghum leaves expressed on a per unit weight basis were significantly lower than those which had not undergone water stress preconditioning. The lower respiration rates of the stressed leaves when expressed on both a per unit area basis and a per unit weight basis for the C4 species indicate that water stress adaptation of C4 plants may include alterations in the respiratory mechanism or on the amount of residual respirable substrate. The light compensation points of all the species were not altered by the water stress preconditioning. This indicates that the mechanisms controlling the net photosynthetic exchange per unit leaf area underwent adaptations as a result of repeated water stress which decreased its efficiency.

Electronic Resource

1432-0800

Springer Online Journal Archives 1860-2000

Energy, Environment Protection, Nuclear Power Engineering

Medicine

Electronic Resource

1432-0800

Springer Online Journal Archives 1860-2000

Energy, Environment Protection, Nuclear Power Engineering

Medicine

Electronic Resource

1432-0800

Springer Online Journal Archives 1860-2000

Energy, Environment Protection, Nuclear Power Engineering

Medicine

Electronic Resource

1432-0703

Springer Online Journal Archives 1860-2000

Energy, Environment Protection, Nuclear Power Engineering

Medicine

Abstract TheSalmonella/microsome assay was used to monitor the mutagenic potential of the organic extract of plants grown on municipal sewage sludge amended soils. The solvent-extractabie organic chemicals from sewage sludge, unamended Padina fine sand (Grossarenic Paleustalf), sludge amended Padina fine sand, and two forage crops; alfalfa (Medicago sativa) and coastal bermudagrass (Cynodon dactylon) were tested withSalmonella strain TA98 with and without metabolic activation. The mutagenic potential of the sludge-amended soil was greater than that of the unamended soil. Fractions extracted from the sludge amended soil over a 154 day period displayed an increase in mutagenicity. None of the plant extracts, whether collected from plants grown on sludge amended or unamended soils, induced a positive response (doubling of revertants at two consecutive dose levels) in the bioassay. Thus, for the evaluated conditions, plants are not likely to translocate significant quantities of organic mutagens from municipal sludge amended soil.

Electronic Resource

1432-0703

Springer Online Journal Archives 1860-2000

Energy, Environment Protection, Nuclear Power Engineering

Medicine

Abstract Examination of standard metaphase chromosome preparations was employed to evaluate the use of resident small mammals as indicators of environmental mutagenesis. Small mammals of two species, (Peromyscus leucopus andSigmodon hispidus) were trapped over a two-year period at a locality polluted with a complex mixture of petrochemical waste products, heavy metals, and PCBs (polychlorinated biphenyls), and from two uncontaminated localities. Significant differences in levels of chromosomal aberrations between animals collected at the contaminated site and the uncontaminated sites were clearly indicated. Increases in lesions per cell and aberrant cells per individual were shown for both species at the contaminated site compared to the control sites. Levels of chromosomal aberrations were not different between the two control sites, however. This study suggests that cytogenetic analysis of resident small mammals is a feasible test model for assessment of environmental mutagenesis.

Electronic Resource

1432-1009

Springer Online Journal Archives 1860-2000

Energy, Environment Protection, Nuclear Power Engineering

Electronic Resource

1432-184X

Springer Online Journal Archives 1860-2000

Biology

Abstract A continuous flow soil respirometer was used to evaluate the effect of nutrient addition, application rate, and application frequency on biodegradation of 2 complex oily sludges in soil. The most rapid biodegradation of the refinery sludge occurred when nitrogen was added to reduce the carbon to nitrogen (C∶N) ratio to 9∶1. The petrochemical sludge was degraded most rapidly when nitrogen, phosphorus, and potassium were added at a rate of 124∶1, C∶NPK; CO2evolution from both wastes increased with increasing application rates, but the fraction of applied sludge which degraded decreased with increasing application rates. Small frequent applications resulted in a slight increase in respiration rate per unit applied over a single equivalent application, indicating that repeated applications of smaller amounts of sludge result in a more rapid rate of decomposition. The population of total soil bacteria was greatest when 1% of either sludge was added to the soil, whereas 5 and 10% sludge additions resulted in slightly lower microbial populations.

Electronic Resource

1573-2932

Springer Online Journal Archives 1860-2000

Energy, Environment Protection, Nuclear Power Engineering

Abstract Clay liners remain an important component of composite liners used in landfill construction. Because their hydraulic integrity is frequently lost due to desiccation cracking, either during construction or shortly thereafter. The present study was initiated to evaluate the effects of common soil additives including lime, cement, and sand on the shrinkage and hydraulic conductivity of compacted clay soils commonly used in clay liner construction. Three soils having predominant clay minerals of smectite, illite and kaolinite were amended with varying amounts of lime, cement or sand; compacted using the Harvard miniature compactor; and the volumetric shrinkage was measured on the compacted samples. Additional samples of each treated soil were compacted according to ASTM 698 and used for measurement of the hydraulic conductivity. The results show that the majority of shrinkage occurs when the samples were dried to 25 °C with little additional shrinkage at temperatures up to 105 °C. The amendments of either 4% lime or 40 to 50% sand resulted in reduced shrinkage and increased hydraulic conductivity. The addition of 3% cement reduced shrinkage by up to 50% and simultaneously reduced hydraulic conductivity by 2 orders of magnitude. Thus, amendment of clay soils having a high shrink-swell potential with Type I Portland cement has the greatest poetential for field application as an amendment to help maintain the integrity and improve the long term performance of compacted clay liners.

Electronic Resource

1573-2932

Permeability ; landfill liner ; cracking ; drying ; laboratory testing

Springer Online Journal Archives 1860-2000

Energy, Environment Protection, Nuclear Power Engineering

Abstract Despite our best efforts to reduce the waste stream, there will always remain some residues which cannot be further treated and must be disposed in landfills. One critical aspect of landfill construction is the integrity of the landfill liner. Current landfill liner technology includes a composite liner which consists of a FML component and a compacted soil component. The primary characteristic for selecting a soil for use in composite liner construction is that the soil have a saturated hydraulic conductivity of 1 × 10−7 cm s−1 or less. In the present study the effects of desiccation cracks on the hydraulic conductivity of the compacted soil were measured. Two soils of diverse mineralogy and typical of soils used for clay liner construction were selected for use. Each was tested in its native state plus after the addition of 30% sand. Laboratory measurements were made of the volumetric shrinkage of each soil. In addition, the hydraulic conductivity was determined using 10 cm diameter fixed wall permeameters. Additional conductivity measurements were made using 60 cm diameter fixed wall double ring permeameters which had been exposed to 0, 1, and 2 periods of desiccation prior to hydraulic conductivity determinations. The data show that laboratory measurements using 10- cm diameter fixed wall permeameters underestimate the hydraulic conductivity of the same soils when packed in large diameter permeameters. It was also found that exposure to two cycles of desiccation resulted in large increases in hydraulic conductivity. The time required to reach a steady outflow volume decreased as the amount of desiccation increased. The hydraulic conductivities of soils which had been allowed to dry were greater than those which were not allowed to dry prior to measurement. The relationship between volumetric shrinkage and the increase in hydraulic conductivity after desiccation indicates that soils which exhibit less than 11% shrinkage in the laboratory, exhibit increases in K of less than a factor of 2 upon desiccation. Clay soils with greater than 11% shrinkage can potentially be amended with sand to decrease the volumetric shrinkage and their response to desiccation.

Electronic Resource

1618-2650

Springer Online Journal Archives 1860-2000

Chemistry and Pharmacology

Electronic Resource