15,118 research outputs found
A cost function for similarity-based hierarchical clustering
The development of algorithms for hierarchical clustering has been hampered
by a shortage of precise objective functions. To help address this situation,
we introduce a simple cost function on hierarchies over a set of points, given
pairwise similarities between those points. We show that this criterion behaves
sensibly in canonical instances and that it admits a top-down construction
procedure with a provably good approximation ratio
Integrating Out Astrophysical Uncertainties
Underground searches for dark matter involve a complicated interplay of
particle physics, nuclear physics, atomic physics and astrophysics. We attempt
to remove the uncertainties associated with astrophysics by developing the
means to map the observed signal in one experiment directly into a predicted
rate at another. We argue that it is possible to make experimental comparisons
that are completely free of astrophysical uncertainties by focusing on {\em
integral} quantities, such as and
. Direct comparisons are possible when the
space probed by different experiments overlap. As examples, we
consider the possible dark matter signals at CoGeNT, DAMA and CRESST-Oxygen. We
find that expected rate from CoGeNT in the XENON10 experiment is higher than
observed, unless scintillation light output is low. Moreover, we determine that
S2-only analyses are constraining, unless the charge yield . For DAMA to be consistent with XENON10, we find for
that the modulation rate must be extremely high (\gsim 70% for
m_\chi = 7\, \gev), while for higher quenching factors, it makes an explicit
prediction (0.8 - 0.9 cpd/kg) for the modulation to be observed at CoGeNT.
Finally, we find CDMS-Si, even with a 10 keV threshold, as well as XENON10,
even with low scintillation, would have seen significant rates if the excess
events at CRESST arise from elastic WIMP scattering, making it very unlikely to
be the explanation of this anomaly.Comment: 25 pages, 7 figures; v2 replaced with published versio
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
The effect of the transport property models on the shuttle boundary layer
Theoretical solutions for the nonsimilar, laminar boundary-layer were computed for four points along the shuttle entry trajectory. Since the boundary layer is that region of the flow field where the effects of viscosity and of thermal conductivity are most important, numerical solutions for the boundary layer were generated using different models for the transport properties. These solutions indicate that the displacement thickness and the heat-transfer rates are very sensitive to changes in the models for thermal conductivity and for specific heat. Thus, the solutions are sensitive to the assumed transport-property model
Statistical properties of fractures in damaged materials
We introduce a model for the dynamics of mud cracking in the limit of of
extremely thin layers. In this model the growth of fracture proceeds by
selecting the part of the material with the smallest (quenched) breaking
threshold. In addition, weakening affects the area of the sample neighbour to
the crack. Due to the simplicity of the model, it is possible to derive some
analytical results. In particular, we find that the total time to break down
the sample grows with the dimension L of the lattice as L^2 even though the
percolating cluster has a non trivial fractal dimension. Furthermore, we obtain
a formula for the mean weakening with time of the whole sample.Comment: 5 pages, 4 figures, to be published in Europhysics Letter
Quantitative multielement analysis using high energy particle bombardment
Charged particles ranging in energy from 0.8 to 4.0 MeV are used to induce resonant nuclear reactions, Coulomb excitation (gamma X-rays), and X-ray emission in both thick and thin targets. Quantitative analysis is possible for elements from Li to Pb in complex environmental samples, although the matrix can severely reduce the sensitivity. It is necessary to use a comparator technique for the gamma-rays, while for X-rays an internal standard can be used. A USGS standard rock is analyzed for a total of 28 elements. Water samples can be analyzed either by nebulizing the sample doped with Cs or Y onto a thin formvar film or by extracting the sample (with or without an internal standard) onto ion exchange resin which is pressed into a pellet
Emergency escape system uses self-braking mechanism on fixed cable
Slide-wire system with a twist level slide device incorporates automatic descent and braking for the safe and rapid evacuation of personnel from tall structures. This device is used on any tall structure that might require emergency evacuation. It is also used to transfer materials and equipment
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