448 research outputs found
The role of hydrogen in room-temperature ferromagnetism at graphite surfaces
We present a x-ray dichroism study of graphite surfaces that addresses the
origin and magnitude of ferromagnetism in metal-free carbon. We find that, in
addition to carbon states, also hydrogen-mediated electronic states
exhibit a net spin polarization with significant magnetic remanence at room
temperature. The observed magnetism is restricted to the top 10 nm of
the irradiated sample where the actual magnetization reaches emu/g
at room temperature. We prove that the ferromagnetism found in metal-free
untreated graphite is intrinsic and has a similar origin as the one found in
proton bombarded graphite.Comment: 10 pages, 5 figures, 1 table, submitted to New Journal of Physic
Experimental Evidence for Two-Dimensional Magnetic Order in Proton Bombarded Graphite
We have prepared magnetic graphite samples bombarded by protons at low
temperatures and low fluences to attenuate the large thermal annealing produced
during irradiation. An overall optimization of sample handling allowed us to
find Curie temperatures K at the used fluences. The
magnetization versus temperature shows unequivocally a linear dependence, which
can be interpreted as due to excitations of spin waves in a two dimensional
Heisenberg model with a weak uniaxial anisotropy.Comment: 4 pages, 3 figure
Effects of forest management practices on treefrog oviposition site choice
Abstract only availableGlobally, amphibian populations are declining faster than those of birds or mammals. Habitat destruction is considered the primary cause of these declines; however, what remains partly unexplored is the idea that some species may be more greatly affected than others by deforestation. Treefrogs (Family: Hylidae), because of their mobility, may be expected to circumvent disturbed habitats; however, because of their dependency on arboreal habitat, they may be adversely affected by different forms of forest management. As part of the LEAP (Land-Use Effects on Amphibian Populations) study, four forest management practices—clearcut with coarse woody debris (CWD) removed, clearcut with CWD retained, thinning of 25% basal area, and uncut forest—were implemented at four wetlands at the Savannah River Site. In May 2005, we placed wading pools 25 m into each treatment and allowed them to fill with rainwater. To monitor time to first oviposition event and to determine the number of events per treatment, pools were checked daily, eggs were counted, and tadpoles were raised to confirm that all eggs were indeed those of hylids. We measured water depth, canopy cover, and surrounding vegetation. These data will be analyzed to determine if suitable calling/breeding habitat (microhabitat) is a more reliable predictor of oviposition than treatment (macrohabitat). At three of the four wetlands, first oviposition events occurred in the thinning treatments, and second events occurred in the clearcuts with CWD retained. We found that more oviposition events also occurred in the thinning treatments (43%) and the clearcuts with CWD retained (33%) than in the clearcuts with CWD removed (13%) or the uncut forest controls (10%). One explanation for these findings is that hylids have evolved to locate openings in the forest canopy which could indicate a wetland or a fallen tree whose uprooting has caused an ephemeral pool to form.Life Sciences Undergraduate Research Opportunity Progra
Proton-induced magnetic order in carbon: SQUID measurements
In this work we have studied systematically the changes in the magnetic
behavior of highly oriented pyrolytic graphite (HOPG) samples after proton
irradiation in the MeV energy range. Superconducting quantum interferometer
device (SQUID) results obtained from samples with thousands of localized spots
of micrometer size as well on samples irradiated with a broad beam confirm
previously reported results. Both, the para- and ferromagnetic contributions
depend strongly on the irradiation details. The results indicate that the
magnetic moment at saturation of spots of micrometer size is of the order of
emu.Comment: Invited contribution at ICACS2006 to be published in Nucl. Instr. and
Meth. B. 8 pages and 6 figure
Multiwavelength Comparison of Modeled and Measured Remote Tropospheric Aerosol Backscatter Over Pacific Ocean
Aerosol concentrations and size distributions in the middle and upper troposphere over the remote Pacific Ocean were measured with a forward scattering spectrometer probe (FSSP) on the NASA DC-8 aircraft during NASA's Global Backscatter Experiment (GLOBE) in May-June 1990. The FSSP size channels were recalibrated based on refractive index estimates from flight-level aerosol volatility measurements with a collocated laser optical particle counter (LOPC). The recalibrated FSSP size distributions were averaged over 100-s intervals, fitted with lo-normal distributions and used to calculate aerosol backscatter coefficients at selected wavelengths. The FSSP-derived backscatter estimates were averaged over 300-s intervals to reduce large random fluctuations. The smoothed FSSP aerosol backscatter coefficients were then compared with LOPC-derived backscatter values and with backscatter measured at or near flight level from four lidar systems operating at 0.53, 1.06, 9.11, 9.25, and 10.59 micrometers. Agreement between FSSP-derived and lidar-measured backscatter was generally best at flight level in homogeneous aerosol fields and at high backscatter values. FSSP data often underestimated low backscatter values especially at the longer wavelengths due to poor counting statistics for larger particles (greater than 0.8 micrometers diameter) that usually dominate aerosol backscatter at these wavelengths. FSSP data also underestimated backscatter at shorter wavelengths when particles smaller than the FSSP lower cutoff diameter (0.35 micrometers) made significant contributions to the total backscatter
A mechanistic and kinetic analysis of the interactions of the diastereoisomers of adenosine 3′,5′-(cyclic)phosphorothioate with purified cyclic AMP-dependent protein kinase
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FUEL CASK IMPACT LIMITER VULNERABILITIES
Cylindrical fuel casks often have impact limiters surrounding just the ends of the cask shaft in a typical 'dumbbell' arrangement. The primary purpose of these impact limiters is to absorb energy to reduce loads on the cask structure during impacts associated with a severe accident. Impact limiters are also credited in many packages with protecting closure seals and maintaining lower peak temperatures during fire events. For this credit to be taken in safety analyses, the impact limiter attachment system must be shown to retain the impact limiter following Normal Conditions of Transport (NCT) and Hypothetical Accident Conditions (HAC) impacts. Large casks are often certified by analysis only because of the costs associated with testing. Therefore, some cask impact limiter attachment systems have not been tested in real impacts. A recent structural analysis of the T-3 Spent Fuel Containment Cask found problems with the design of the impact limiter attachment system. Assumptions in the original Safety Analysis for Packaging (SARP) concerning the loading in the attachment bolts were found to be inaccurate in certain drop orientations. This paper documents the lessons learned and their applicability to impact limiter attachment system designs
Direct Global Measurements of Tropspheric Winds Employing a Simplified Coherent Laser Radar using Fully Scalable Technology and Technique
Innovative designs of a space-based laser remote sensing 'wind machine' are presented. These designs seek compatibility with the traditionally conflicting constraints of high scientific value and low total mission cost. Mission cost is reduced by moving to smaller, lighter, more off-the-shelf instrument designs which can be accommodated on smaller launch vehicles
Fokker-Planck Equation for Boltzmann-type and Active Particles: transfer probability approach
Fokker-Planck equation with the velocity-dependent coefficients is considered
for various isotropic systems on the basis of probability transition (PT)
approach. This method provides the self-consistent and universal description of
friction and diffusion for Brownian particles. Renormalization of the friction
coefficient is shown to occur for two dimensional (2-D) and three dimensional
(3-D) cases, due to the tensorial character of diffusion. The specific forms of
PT are calculated for the Boltzmann-type of collisions and for the
absorption-type of collisions (the later are typical for dusty plasmas and some
other systems). Validity of the Einstein's relation for the Boltzmann-type
collisions is analyzed for the velocity-dependent friction and diffusion
coefficients. For the Boltzmann-type collisions in the region of very high
grain velocity as well as it is always for non-Boltzmann collisions, such as,
e.g., absorption collisions, the Einstein relation is violated, although some
other relations (determined by the structure of PT) can exist. The generalized
friction force is investigated in dusty plasma in the framework of the PT
approach. The relation between this force, negative collecting friction force
and scattering and collecting drag forces is established.+AFwAXA- The concept
of probability transition is used to describe motion of active particles in an
ambient medium. On basis of the physical arguments the PT for a simple model of
the active particle is constructed and the coefficients of the relevant
Fokker-Planck equation are found. The stationary solution of this equation is
typical for the simplest self-organized molecular machines.+AFwAXA- PACS
number(s): 52.27.Lw, 52.20.Hv, 52.25.Fi, 82.70.-yComment: 18 page
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