45,648 research outputs found
Advanced dosimetry systems for the space transport and space station
Advanced dosimetry system concepts are described that will provide automated and instantaneous measurement of dose and particle spectra. Systems are proposed for measuring dose rate from cosmic radiation background to greater than 3600 rads/hr. Charged particle spectrometers, both internal and external to the spacecraft, are described for determining mixed field energy spectra and particle fluxes for both real time onboard and ground-based computer evaluation of the radiation hazard. Automated passive dosimetry systems consisting of thermoluminescent dosimeters and activation techniques are proposed for recording the dose levels for twelve or more crew members. This system will allow automatic onboard readout and data storage of the accumulated dose and can be transmitted to ground after readout or data records recovered with each crew rotation
Experimental active and passive dosimetry systems for the NASA Skylab program
Active and passive dosimetry instrumentation to measure absorbed dose, charged particle spectra, and linear energy transfer spectra inside the command module and orbital workshop on the Skylab program were developed and tested. The active dosimetry system consists of one integral unit employing both a tissue equivalent ionization chamber and silicon solid state detectors. The instrument measures dose rates from 0.2 millirad/hour to 25 rads/hour, linear energy transfer spectra from 2.8 to 42.4 Kev/micron, and the proton and alpha particle energy spectra from 0.5 to 75 Mev. The active dosimeter is equipped with a portable radiation sensor for use in astronaut on-body and spacecraft shielding surveys during passage of the Skylab through significant space radiations. Data are transmitted in real time or are recorded by onboard spacecraft tape recorder for rapid evaluation of the radiation levels. The passive dosimetry systems consist of twelve (12) hard-mounted assemblies, each containing a variety of passive radiation sensors which are recoverable at the end of the mission for analysis
A foam model highlights the differences of the macro- and microrheology of respiratory horse mucus
Native horse mucus is characterized with micro- and macrorheology and
compared to hydroxyethylcellulose (HEC) gel as a model. Both systems show
comparable viscoelastic properties on the microscale and for the HEC the
macrorheology is in good agreement with the microrheology. For the mucus, the
viscoelastic moduli on the macroscale are several orders of magnitude larger
than on the microscale. Large amplitude oscillatory shear experiments show that
the mucus responds nonlinearly at much smaller deformations than HEC. This
behavior fosters the assumption that the mucus has a foam like structure on the
microscale compared to the typical mesh like structure of the HEC, a model that
is supported by cryogenic-scanning-electron-microscopy (CSEM) images. These
images allow also to determine the relative amount of volume that is occupied
by the pores and the scaffold. Consequently, we can estimate the elastic
modulus of the scaffold. We conclude that this particular foam like
microstructure should be considered as a key factor for the transport of
particulate matter which plays a central role in mucus function with respect to
particle penetration. The mesh properties composed of very different components
are responsible for macroscopic and microscopic behavior being part of
particles fate after landing.Comment: Accepted for publication in the Journal of the Mechanical Behavior of
Biomedical Material
Spin self-rephasing and very long coherence times in a trapped atomic ensemble
We perform Ramsey spectroscopy on the ground state of ultra-cold 87Rb atoms
magnetically trapped on a chip in the Knudsen regime. Field inhomogeneities
over the sample should limit the 1/e contrast decay time to about 3 s, while
decay times of 58 s are actually observed. We explain this surprising result by
a spin self-rephasing mechanism induced by the identical spin rotation effect
originating from particle indistinguishability. We propose a theory of this
synchronization mechanism and obtain good agreement with the experimental
observations. The effect is general and susceptible to appear in other physical
systems.Comment: Revised version; improved description of the theoretical treatmen
The non-Gaussian tail of cosmic-shear statistics
Due to gravitational instability, an initially Gaussian density field
develops non-Gaussian features as the Universe evolves. The most prominent
non-Gaussian features are massive haloes, visible as clusters of galaxies. The
distortion of high-redshift galaxy images due to the tidal gravitational field
of the large-scale matter distribution, called cosmic shear, can be used to
investigate the statistical properties of the LSS. In particular, non-Gaussian
properties of the LSS will lead to a non-Gaussian distribution of cosmic-shear
statistics. The aperture mass () statistics, recently introduced as
a measure for cosmic shear, is particularly well suited for measuring these
non-Gaussian properties. In this paper we calculate the highly non-Gaussian
tail of the aperture mass probability distribution, assuming Press-Schechter
theory for the halo abundance and the `universal' density profile of haloes as
obtained from numerical simulations. We find that for values of
much larger than its dispersion, this probability distribution is closely
approximated by an exponential, rather than a Gaussian. We determine the
amplitude and shape of this exponential for various cosmological models and
aperture sizes, and show that wide-field imaging surveys can be used to
distinguish between some of the currently most popular cosmogonies. Our study
here is complementary to earlier cosmic-shear investigations which focussed
more on two-point statistical properties.Comment: 9 pages, 5 figures, submitted to MNRA
Geometry of Universal Magnification Invariants
Recent work in gravitational lensing and catastrophe theory has shown that
the sum of the signed magnifications of images near folds, cusps and also
higher catastrophes is zero. Here, it is discussed how Lefschetz fixed point
theory can be used to interpret this result geometrically. It is shown for the
generic case as well as for elliptic and hyperbolic umbilics in gravitational
lensing.Comment: RevTEX4, 13 pages, submitted to J. Math. Phy
Feasibility study of a 110 watt per kilogram lightweight solar array system
An investigation of the feasibility of a solar array panel subsystem which will produce 10,000 watts of electrical output at 1 A.U. with an overall beginning-of-life power-to-weight ratio of at least 110 watt/kg is reported. A description of the current baseline configuration which meets these requirements is presented. A parametric analysis of the single boom, two blanket planar solar array system was performed to arrive at the optimum system aspect ratio. A novel concept for the stiffening of a lightweight solar array by canting the solar cell blankets at a small angle to take advantage of the inherent in-plane stiffness to increase the symmetric out-of-plane frequency is introduced along with a preliminary analysis of the stiffening effect. A comparison of welded and soldered solar cell interconnections leads to the conclusion that welding is required on this ultralightweight solar array. The use of a boron/aluminum composite material in a BI-STEM type deployable boom is investigated as a possible advancement in the state-of-the-art
Dust from AGBs: relevant factors and modelling uncertainties
The dust formation process in the winds of Asymptotic Giant Branch stars is
discussed, based on full evolutionary models of stars with mass in the range
MMM, and metallicities .
Dust grains are assumed to form in an isotropically expanding wind, by growth
of pre--existing seed nuclei. Convection, for what concerns the treatment of
convective borders and the efficiency of the schematization adopted, turns out
to be the physical ingredient used to calculate the evolutionary sequences with
the highest impact on the results obtained. Low--mass stars with MM produce carbon type dust with also traces of silicon carbide. The
mass of solid carbon formed, fairly independently of metallicity, ranges from a
few M, for stars of initial mass M, to
M for MM; the size of dust
particles is in the range mm. On the contrary,
the production of silicon carbide (SiC) depends on metallicity. For the size of SiC grains varies in the range m, while the mass of SiC formed is
. Models of
higher mass experience Hot Bottom Burning, which prevents the formation of
carbon stars, and favours the formation of silicates and corundum. In this case
the results scale with metallicity, owing to the larger silicon and aluminium
contained in higher--Z models. At Z= we find that the most
massive stars produce dust masses M, whereas models of
smaller mass produce a dust mass ten times smaller. The main component of dust
are silicates, although corundum is also formed, in not negligible quantities
().Comment: Paper accepted for publication in Monthly Notices of the Royal
Astronomical Society Main Journal (2014 January 4
Experimental and analytical comparison of flowfields in a 110 N (25 lbf) H2/O2 rocket
A gaseous hydrogen/gaseous oxygen 110 N (25 lbf) rocket was examined through the RPLUS code using the full Navier-Stokes equations with finite rate chemistry. Performance tests were conducted on the rocket in an altitude test facility. Preliminary parametric analyses were performed for a range of mixture ratios and fuel film cooling pcts. It is shown that the computed values of specific impulse and characteristic exhaust velocity follow the trend of the experimental data. Specific impulse computed by the code is lower than the comparable test values by about two to three percent. The computed characteristic exhaust velocity values are lower than the comparable test values by three to four pct. Thrust coefficients computed by the code are found to be within two pct. of the measured values. It is concluded that the discrepancy between computed and experimental performance values could not be attributed to experimental uncertainty
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