215 research outputs found
Preliminary analysis of the potential of LANDSAT imagery to study desertification
The use of LANDSAT imagery to define and delimit areas under process of desertification was investigated. Imagery for two different years (1973 and 1978) and two different seasons (dry and rainy seasons in 1976), were used to identify terrain morphology and vegetation cover. The analysis of LANDSAT interpretation, combined with geological and soil information obtained from published literature, allowed the identification of eleven ecological units which were classified corresponding to the degree of the Xique Xique region of Rio Sao Francisco
High-flux beam source for cold, slow atoms or molecules
We demonstrate and characterize a high-flux beam source for cold, slow atoms
or molecules. The desired species is vaporized using laser ablation, then
cooled by thermalization in a cryogenic cell of buffer gas. The beam is formed
by particles exiting a hole in the buffer gas cell. We characterize the
properties of the beam (flux, forward velocity, temperature) for both an atom
(Na) and a molecule (PbO) under varying buffer gas density, and discuss
conditions for optimizing these beam parameters. Our source compares favorably
to existing techniques of beam formation, for a variety of applications.Comment: 5 Pages, 4 figure
Determination of the s-wave Scattering Length of Chromium
We have measured the deca-triplet s-wave scattering length of the bosonic
chromium isotopes Cr and Cr. From the time constants for
cross-dimensional thermalization in atomic samples we have determined the
magnitudes and ,
where . By measuring the rethermalization rate of Cr over a
wide temperature range and comparing the temperature dependence with the
effective-range theory and single-channel calculations, we have obtained strong
evidence that the sign of is positive. Rescaling our Cr
model potential to Cr strongly suggests that is positive,
too.Comment: v3: corrected typo in y-axis scaling of Figs. 3 and
A Floating Node Method for the Modelling of Discontinuities Within a Finite Element
This paper focuses on the accurate numerical representation of complex networks of evolving discontinuities in solids, with particular emphasis on cracks. The limitation of the standard finite element method (FEM) in approximating discontinuous solutions has motivated the development of re-meshing, smeared crack models, the eXtended Finite Element Method (XFEM) and the Phantom Node Method (PNM). We propose a new method which has some similarities to the PNM, but crucially: (i) does not introduce an error on the crack geometry when mapping to natural coordinates; (ii) does not require numerical integration over only part of a domain; (iii) can incorporate weak discontinuities and cohesive cracks more readily; (iv) is ideally suited for the representation of multiple and complex networks of (weak, strong and cohesive) discontinuities; (v) leads to the same solution as a finite element mesh where the discontinuity is represented explicitly; and (vi) is conceptually simpler than the PNM
Buffer gas cooling and trapping of atoms with small magnetic moments
Buffer gas cooling was extended to trap atoms with small magnetic moment
(mu). For mu greater than or equal to 3mu_B, 1e12 atoms were buffer gas cooled,
trapped, and thermally isolated in ultra high vacuum with roughly unit
efficiency. For mu < 3mu_B, the fraction of atoms remaining after full thermal
isolation was limited by two processes: wind from the rapid removal of the
buffer gas and desorbing helium films. In our current apparatus we trap atoms
with mu greater than or equal to 1.1mu_B, and thermally isolate atoms with mu
greater than or equal to 2mu_B. Extrapolation of our results combined with
simulations of the loss processes indicate that it is possible to trap and
evaporatively cool mu = 1mu_B atoms using buffer gas cooling.Comment: 17 pages, 4 figure
Association of Exposures to Seated Postures With Immediate Increases in Back Pain: A Systematic Review of Studies With Objectively Measured Sitting Time
have determined sitting time by self-report and/or LBP by recall following exposure. Given known problems with recall and the validity of estimated sitting time, we conducted a systematic review of studies using objectively measured sitting time to determine if sitting time is associated with immediate LBP in adults. Methods: Four databases (PubMed, EMBASE, SPORTDiscus and CINAHL) were searched from inception to September 1, 2018. Randomized controlled trials, cohort and cross-sectional studies, where objectively measured sitting time was temporally matched with a measure of LBP in adults, were included. Studies without a control session conducted on a separate day were excluded. Screening, full text review, data extraction and risk of bias assessment (QUIPS) of included papers were performed independently by two reviewers, with a third available to resolve disagreements. Results: In total, 609 articles were identified, 361 titles/abstracts were screened,75 full-text articles were assessed for eligibility and 10 met the inclusion criteria. All but one reported sitting time to be associated with immediate increase in LBP. Six of these reported clinically relevant pain levels (n=330). Half of the included studies were rated as having low risk of bias and the remaining as moderate risk of bias. Conclusion: Prolonged sitting increases immediate reporting of LBP in adults; however, no conclusion between sitting and clinical episodes of LBP can be made. Future prospective studies should match objectively measured sitting with temporally related pain measurements to determine whether prolonged sitting can trigger a clinical episode of LBP
Automatic interpretation of LANDSAT data to characterize the dispersion of suspended sediments in Artificial Reservoirs
There are no author-identified significant results in this report
Floating Node Method and Virtual Crack Closure Technique for Modeling Matrix Cracking-Delamination Interaction
A novel approach is proposed for high-fidelity modeling of progressive damage and failure in composite materials that combines the Floating Node Method (FNM) and the Virtual Crack Closure Technique (VCCT) to represent multiple interacting failure mechanisms in a mesh-independent fashion. In this study, the approach is applied to the modeling of delamination migration in cross-ply tape laminates. Delamination, matrix cracking, and migration are all modeled using fracture mechanics based failure and migration criteria. The methodology proposed shows very good qualitative and quantitative agreement with experiments
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Zeeman spectroscopy of CaH molecules in a magnetic trap
In a recent experiment [Weinstein et al., Nature 395, 148 (1998)] we magnetically trapped 108108ground-statecalcium monohydride molecules, CaH(X2Ī£,vā³=0,Jā³=0).CaH(Xā2Ī£,vā³=0,āJā³=0). The molecules were prepared by laser ablation of a solid sample of CaH2CaH2 and loaded via thermalization with a cold (<1(<1 K) 3He3He buffer gas. The magnetic trap was formed by superconducting coils arranged in the anti-Helmholtz configuration. The detection was done by laser fluorescence spectroscopy excited at 635635 nm (in the B2Ī£,vā²=0āX2Ī£,vā³=0Bā2Ī£,vā²=0āXā2Ī£,vā³=0 band) and detected at 692692 nm (within the B,vā²=0āX,vā³=1B,vā²=0āX,vā³=1 band). Both a photomultiplier tube and a CCD camera were used. Due to the thermalization of molecular rotation, only a transition from the lowest rotational state could be detected at zero field, Nā²=1,Jā²=3/2āNā³=0,Jā³=1/2.Nā²=1,āJā²=3/2āNā³=0,āJā³=1/2. In the magnetic field this rotational transition splits into two features, one shifted towards lower and one towards higher frequencies. The measured shifts are linear in field strength and indicate a small difference (0.02 Ī¼B)Ī¼B) in the magnetic moments between the ground and excited states. Here we present a theoretical analysis of the observed magnetic shifts. These are identified as arising from a rotational perturbation of the B2Ī£,vā²=0Bā2Ī£,vā²=0 state by a close-lying A2Ī ,vā²=1Aā2Ī ,vā²=1 state that lends the B state some of its A character. We find that the Hamiltonian can be well approximated by a 3Ć33Ć3 matrix built out of elements that connect states from within the Ī£-doublet and the 2Ī 3/22Ī 3/2 manifolds. The interaction parameter describing the Ī£āĪ coupling in the Zeeman Hamiltonian is determined from the observed shifts and the field-free molecular parameters of CaH given by Berg and Klyning [Phys. Scr. 10, 331 (1974)] and by Martin [J. Mol. Spectrosc 108, 66 (1984)].Physic
Sympathetic Cooling with Two Atomic Species in an Optical Trap
We simultaneously trap ultracold lithium and cesium atoms in an optical
dipole trap formed by the focus of a CO laser and study the exchange of
thermal energy between the gases. The cesium gas, which is optically cooled to
K, efficiently decreases the temperature of the lithium gas through
sympathetic cooling. The measured cross section for thermalizing
Cs-Li collisions is cm, for both species in
their lowest hyperfine ground state. Besides thermalization, we observe
evaporation of lithium purely through elastic cesium-lithium collisions
(sympathetic evaporation).Comment: 4 pages 3 fig
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