3,847 research outputs found
Theory of cooling by flow through narrow pores
We consider the possibility of adding a stage to a dilution refrigerator to
provide additional cooling by ``filtering out'' hot atoms. Three methods are
considered: 1) Effusion, where holes having diameters larger than a mean-free
path allow atoms to pass through easily; 2) Particle waveguide-like motion
using very narrow channels that greatly restrict the quantum states of the
atoms in a channel. 3) Wall-limited diffusion through channels, in which the
wall scattering is disordered so that local density equilibrium is established
in a channel. We assume that channel dimension are smaller than the mean-free
path for atom-atom interactions. The particle waveguide and the wall-limited
diffusion methods using channels on order of the de Broglie wavelength give
cooling. Recent advances in nano-filters give this method some hope of being
practical.Comment: 10 pages, 3 figures. Corrected typos and made some minor wording
change
Characterization of the spore surface and exosporium proteins of Clostridium sporogenes; implications for Clostridium botulinum group I strains.
Clostridium sporogenes is a non-pathogenic close relative and surrogate for Group I (proteolytic) neurotoxin-producing Clostridium botulinum strains. The exosporium, the sac-like outermost layer of spores of these species, is likely to contribute to adhesion, dissemination, and virulence. A paracrystalline array, hairy nap, and several appendages were detected in the exosporium of C. sporogenes strain NCIMB 701792 by EM and AFM. The protein composition of purified exosporium was explored by LC-MS/MS of tryptic peptides from major individual SDS-PAGE-separated protein bands, and from bulk exosporium. Two high molecular weight protein bands both contained the same protein with a collagen-like repeat domain, the probable constituent of the hairy nap, as well as cysteine-rich proteins CsxA and CsxB. A third cysteine-rich protein (CsxC) was also identified. These three proteins are also encoded in C. botulinum Prevot 594, and homologues (75-100% amino acid identity) are encoded in many other Group I strains. This work provides the first insight into the likely composition and organization of the exosporium of Group I C. botulinum spores
Quantum-limited mass flow of liquid He
We consider theoretically the possibility of observing unusual quantum fluid
behavior in liquid He and solutions of He in He systems
confined to nano-channels. In the case of pure ballistic flow at very low
temperature conductance will be quantized in units of . We show that
these steps should be sensitive to increases in temperature. We also use of a
random scattering matrix simulation to study flow with diffusive wall
scattering. Universal conductance fluctuations analogous to those seen in
electron systems should then be observable. Finally we consider the possibility
of the cross-over to a one-dimensional system at sufficiently low temperature
where the system could form a Luttinger liquid
Effects of magnesium treatment in a model of internal capsule lesion in spontaneously hypertensive rats
<p><b>Background and Purpose:</b> The study aim was to assess the effects of magnesium sulfate (MgSO4) administration on white matter damage in vivo in spontaneously hypertensive rats.</p>
<p><b>Methods:</b> The left internal capsule was lesioned by a local injection of endothelin-1 (ET-1; 200 pmol) in adult spontaneously hypertensive rats. MgSO4 was administered (300 mg/kg SC) 30 minutes before injection of ET-1, plus 200 mg/kg every hour thereafter for 4 hours. Infarct size was measured by T2-weighted magnetic resonance imaging (day 2) and histology (day 11), and functional recovery was assessed on days 3 and 10 by the cylinder and walking-ladder tests.</p>
<p><b>Results:</b> ET-1 application induced a small, localized lesion within the internal capsule. Despite reducing blood pressure, MgSO4 did not significantly influence infarct volume (by magnetic resonance imaging: median, 2.1 mm3; interquartile range, 1.3 to 3.8, vs 1.6 mm3 and 1.2 to 2.1, for the vehicle-treated group; by histology: 0.3 mm3 and 0.2 to 0.9 vs 0.3 mm3 and 0.2 to 0.5, respectively). Significant forelimb and hindlimb motor deficits were evident in the vehicle-treated group as late as day 10. These impairments were significantly ameliorated by MgSO4 in both cylinder (left forelimb use, P<0.01 and both-forelimb use, P<0.03 vs vehicle) and walking-ladder (right hindlimb score, P<0.02 vs vehicle) tests.</p>
<p><b>Conclusions:</b> ET-1–induced internal capsule ischemia in spontaneously hypertensive rats represents a good model of lacunar infarct with small lesion size, minimal adverse effects, and a measurable motor deficit. Despite inducing mild hypotension, MgSO4 did not significantly influence infarct size but reduced motor deficits, supporting its potential utility for the treatment of lacunar infarct.</p>
Exclusion Statistics in a two-dimensional trapped Bose gas
We briefly explain the notion of exclusion statistics and in particular
discuss the concept of an ideal exclusion statistics gas. We then review a
recent work where it is demonstrated that a {\em two-dimensional} Bose gas with
repulsive delta function interactions obeys ideal exclusion statistics, with a
fractional parameter related to the interaction strength.Comment: 10 pages, RevTeX. Proceedings of the Salerno workshop "Theory of
Quantum Gases and Quantum Coherence", to appear in a special issue of J.Phys.
B, Dec. 200
Advanced power sources for space missions
Approaches to satisfying the power requirements of space-based Strategic Defense Initiative (SDI) missions are studied. The power requirements for non-SDI military space missions and for civil space missions of the National Aeronautics and Space Administration (NASA) are also considered. The more demanding SDI power requirements appear to encompass many, if not all, of the power requirements for those missions. Study results indicate that practical fulfillment of SDI requirements will necessitate substantial advances in the state of the art of power technology. SDI goals include the capability to operate space-based beam weapons, sometimes referred to as directed-energy weapons. Such weapons pose unprecedented power requirements, both during preparation for battle and during battle conditions. The power regimes for these two sets of applications are referred to as alert mode and burst mode, respectively. Alert-mode power requirements are presently stated to range from about 100 kW to a few megawatts for cumulative durations of about a year or more. Burst-mode power requirements are roughly estimated to range from tens to hundreds of megawatts for durations of a few hundred to a few thousand seconds. There are two likely energy sources, chemical and nuclear, for powering SDI directed-energy weapons during the alert and burst modes. The choice between chemical and nuclear space power systems depends in large part on the total duration during which power must be provided. Complete study findings, conclusions, and eight recommendations are reported
Low-Temperature Spin Diffusion in a Spin-Polarized Fermi Gas
We present a finite temperature calculation of the transverse spin-diffusion
coefficient, , in a dilute degenerate Fermi gas in the presence of a
small external magnetic field, . While the longitudinal diffusion
coefficient displays the conventional low-temperature Fermi-liquid behavior,
, the corresponding results for show three
separate regimes: (a) for ; (b) , for and large spin-rotation
parameter , and (c) for and . Our results are qualitatively consistent with the available
experimental data in weakly spin-polarized and mixtures.Comment: 13 pages, REVTEX, 3 figures available upon request, RU-94-4
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