10,503 research outputs found
Recertification of the air and methane storage vessels at the Langley 8-foot high-temperature structures tunnel
This center operates a number of sophisticated wind tunnels in order to fulfill the needs of its researchers. Compressed air, which is kept in steel storage vessels, is used to power many of these tunnels. Some of these vessels have been in use for many years, and Langley is currently recertifying these vessels to insure their continued structural integrity. One of the first facilities to be recertified under this program was the Langley 8-foot high-temperature structures tunnel. This recertification involved (1) modification, hydrotesting, and inspection of the vessels; (2) repair of all relevant defects; (3) comparison of the original design of the vessel with the current design criteria of Section 8, Division 2, of the 1974 ASME Boiler and Pressure Vessel Code; (4) fracture-mechanics, thermal, and wind-induced vibration analyses of the vessels; and (5) development of operating envelopes and a future inspection plan for the vessels. Following these modifications, analyses, and tests, the vessels were recertified for operation at full design pressure (41.4 MPa (6000 psi)) within the operating envelope developed
OH hyperfine ground state: from precision measurement to molecular qubits
We perform precision microwave spectroscopy--aided by Stark deceleration--to
reveal the low magnetic field behavior of OH in its ^2\Pi_{3/2} ro-vibronic
ground state, identifying two field-insensitive hyperfine transitions suitable
as qubits and determining a differential Lande g-factor of
1.267(5)\times10^{-3} between opposite parity components of the
\Lambda-doublet. The data are successfully modeled with an effective hyperfine
Zeeman Hamiltonian, which we use to make a tenfold improvement of the
magnetically sensitive, astrophysically important \Delta F=\pm1 satellite-line
frequencies, yielding 1720529887(10) Hz and 1612230825(15) Hz.Comment: 4+ pages, 3 figure
Trees for Shelter: The Implications in Agroforestry System
The objective of this study was to determine the horizontal and vertical variations in soil penetration resistance (PR) observed at tree-scale in silvopastoral plots that were grazed by sheep with and without trees. Sycamore trees (Acer pseudoplatanus L) were planted in the spring of 1988 at 10 m x 10 m spacing (100 stems/ha) at Glensaugh NE of Scotland on plots replicated over three blocks in Randomized Complete Block design on a predominantly rye grass (Lolium perenne L) pasture. Included in the design were pasture plots without trees (Control). The experiment is grazed by sheep yearly from April to October. Soil PR was measured in Mega Pascals (MPa) around two randomly selected trees in each plot in 8 directions of the compass - N, NE, E, SE, S, SW, W and NW at 1 m intervals starting at 0.5 m from the tree base to mid point of the separation distance (4.5 m) between the trees. The soil PR data were measured at 3.5 cm soil depth intervals at points around the tree up to depth limit of 21.0 cm. This gave six depth intervals of d1 (3.5), d2 (7.0), d3 (10.5), d4 (14.0), d5 (17.5) and d6 (21.0). In the Control plots, soil PR was measured as in the Sycamore plots around two hypothetical tree positions chosen randomly in each plot. The soil PR was found to decrease significantly within the horizontal distance of 4.5 m from the tree and depth for up to d3 (10.5 cm) only in the grazed Sycamore plots. Soil penetrometer resistance was found to increase significantly within the vertical distance of 0-14 cm of the soil around the tree in grazed Sycamore and Control plots. Beyond this soil depth, soil PR was no longer significant in these treatments
Spacecraft instrument calibration and stability
The following topics are covered: instrument degradation; the Solar Backscatter Ultraviolet (SBUV) Experiment; the Total Ozone Mapping Spectrometer (TOMS); the Stratospheric Aerosol and Gas Experiment 1 (SAGE-1) and SAGE-2 instruments; the Solar Mesosphere Explorer (SME) UV ozone and near infrared airglow instruments; and the Limb Infrared Monitor of the Stratosphere (LIMS)
Initial estimate of NOAA-9 SBUV/2 total ozone drift: Based on comparison with re-calibrated TOMS measurements and pair justification of SBUV/2
Newly recalibrated version 6 Total Ozone Mapping Spectrometer (TOMS) data are used as a reference measurement in a comparison of monthly means of total ozone in 10 degree latitude zones from SBUV/2 and the nadir measurements from TOMS. These comparisons indicate a roughly linear long-term drift in SBUV/2 total ozone relative to TOMS of about 2.5 Dobson units per year at the equator over the first three years of SBUV/2. The pari justification technique is also applied to the SBUV/2 measurements in a manner similar to that used for SBUV and TOMS. The higher solar zenith angles associated with the afternoon orbit of NOAA-9 and the large changes in solar zenith angle associated with its changing equator crossing time degrade the accuracy of the pair justification method relative to its application to SBUV and TOMS, but the results are consistent with the SBUV/2-TOMS comparisons, and show a roughly linear drift in SBUV/2 of 2.5 to 4.5 Dobson units per year in equatorial ozone
Magneto-electrostatic trapping of ground state OH molecules
We report the magnetic confinement of neutral, ground state hydroxyl radicals
(OH) at a density of cm and temperature of 30
mK. An adjustable electric field of sufficient magnitude to polarize the OH is
superimposed on the trap in either a quadrupole or homogenous field geometry.
The OH is confined by an overall potential established via molecular state
mixing induced by the combined electric and magnetic fields acting on the
molecule's electric dipole and magnetic dipole moments, respectively. An
effective molecular Hamiltonian including Stark and Zeeman terms has been
constructed to describe single molecule dynamics inside the trap. Monte Carlo
simulation using this Hamiltonian accurately models the observed trap dynamics
in various trap configurations. Confinement of cold polar molecules in a
magnetic trap, leaving large, adjustable electric fields for control, is an
important step towards the study of low energy dipole-dipole collisions.Comment: 4 pages, 4 figure
The Interstellar Rubidium Isotope Ratio toward Rho Ophiuchi A
The isotope ratio, 85Rb/87Rb, places constraints on models of the
nucleosynthesis of heavy elements, but there is no precise determination of the
ratio for material beyond the Solar System. We report the first measurement of
the interstellar Rb isotope ratio. Our measurement of the Rb I line at 7800 A
for the diffuse gas toward rho Oph A yields a value of 1.21 +/- 0.30 (1-sigma)
that differs significantly from the meteoritic value of 2.59. The Rb/K
elemental abundance ratio for the cloud also is lower than that seen in
meteorites. Comparison of the 85Rb/K and 87Rb/K ratios with meteoritic values
indicates that the interstellar 85Rb abundance in this direction is lower than
the Solar System abundance. We attribute the lower abundance to a reduced
contribution from the r-process. Interstellar abundances for Kr, Cd, and Sn are
consistent with much less r-process synthesis for the solar neighborhood
compared to the amount inferred for the Solar System.Comment: 12 pages with 2 figures and 1 table; will appear in ApJ Letter
Unknown Quantum States: The Quantum de Finetti Representation
We present an elementary proof of the quantum de Finetti representation
theorem, a quantum analogue of de Finetti's classical theorem on exchangeable
probability assignments. This contrasts with the original proof of Hudson and
Moody [Z. Wahrschein. verw. Geb. 33, 343 (1976)], which relies on advanced
mathematics and does not share the same potential for generalization. The
classical de Finetti theorem provides an operational definition of the concept
of an unknown probability in Bayesian probability theory, where probabilities
are taken to be degrees of belief instead of objective states of nature. The
quantum de Finetti theorem, in a closely analogous fashion, deals with
exchangeable density-operator assignments and provides an operational
definition of the concept of an ``unknown quantum state'' in quantum-state
tomography. This result is especially important for information-based
interpretations of quantum mechanics, where quantum states, like probabilities,
are taken to be states of knowledge rather than states of nature. We further
demonstrate that the theorem fails for real Hilbert spaces and discuss the
significance of this point.Comment: 30 pages, 2 figure
Strain Modulated Superlattices in Graphene
Strain engineering of graphene takes advantage of one of the most dramatic
responses of Dirac electrons enabling their manipulation via strain-induced
pseudo-magnetic fields. Numerous theoretically proposed devices, such as
resonant cavities and valley filters, as well as novel phenomena, such as snake
states, could potentially be enabled via this effect. These proposals, however,
require strong, spatially oscillating magnetic fields while to date only the
generation and effects of pseudo-gauge fields which vary at a length scale much
larger than the magnetic length have been reported. Here we create a periodic
pseudo-gauge field profile using periodic strain that varies at the length
scale comparable to the magnetic length and study its effects on Dirac
electrons. A periodic strain profile is achieved by pulling on graphene with
extreme (>10%) strain and forming nanoscale ripples, akin to a plastic wrap
pulled taut at its edges. Combining scanning tunneling microscopy and atomistic
calculations, we find that spatially oscillating strain results in a new
quantization different from the familiar Landau quantization observed in
previous studies. We also find that graphene ripples are characterized by large
variations in carbon-carbon bond length, directly impacting the electronic
coupling between atoms, which within a single ripple can be as different as in
two different materials. The result is a single graphene sheet that effectively
acts as an electronic superlattice. Our results thus also establish a novel
approach to synthesize an effective 2D lateral heterostructure - by periodic
modulation of lattice strain.Comment: 18 pages, 5 figures and supplementary informatio
An Ultra-High-Resolution Survey of the Interstellar ^7Li-to-^6Li Isotope Ratio in the Solar Neighborhood
In an effort to probe the extent of variations in the interstellar ^7Li/^6Li
ratio seen previously, ultra-high-resolution (R ~ 360,000), high
signal-to-noise spectra of stars in the Perseus OB2 and Scorpius OB2
Associations were obtained. These measurements confirm our earlier findings of
an interstellar ^7Li/^6Li ratio of about 2 toward o Per, the value predicted
from models of Galactic cosmic ray spallation reactions. Observations of other
nearby stars yield limits consistent with the isotopic ratio ~ 12 seen in
carbonaceous chondrite meteorites. If this ratio originally represented the gas
toward o Per, then to decrease the original isotope ratio to its current value
an order of magnitude increase in the Li abundance is expected, but is not
seen. The elemental K/Li ratio is not unusual, although Li and K are formed via
different nucleosynthetic pathways. Several proposals to account for the low
^7Li/^6Li ratio were considered, but none seems satisfactory.
Analysis of the Li and K abundances from our survey highlighted two sight
lines where depletion effects are prevalent. There is evidence for enhanced
depletion toward X Per, since both abundances are lower by a factor of 4 when
compared to other sight lines. Moreover, a smaller Li/H abundance is observed
toward 20 Aql, but the K/H abundance is normal, suggesting enhanced Li
depletion (relative to K) in this direction. Our results suggest that the
^7Li/^6Li ratio has not changed significantly during the last 4.5 billion years
and that a ratio ~ 12 represents most gas in the solar neighborhood. In
addition, there appears to be a constant stellar contribution of ^7Li,
indicating that one or two processes dominate its production in the Galaxy.Comment: 54 pages, accepted for publication in the Astrophysical Journa
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