1,366 research outputs found
Analytical formulas for calculating the thermal diffusivity of cylindrical shell and spherical shell samples
Calculating the thermal diffusivity of solid materials is commonly carried
out using the laser flash experiment. This classical experiment considers a
small (usually thin disc-shaped) sample of the material with parallel front and
rear surfaces, applying a heat pulse to the front surface and recording the
resulting rise in temperature over time on the rear surface. Recently, Carr and
Wood [Int J Heat Mass Transf, 144 (2019) 118609] showed that the thermal
diffusivity can be expressed analytically in terms of the heat flux function
applied at the front surface and the temperature rise history at the rear
surface. In this paper, we generalise this result to radial unidirectional heat
flow, developing new analytical formulas for calculating the thermal
diffusivity for cylindrical shell and spherical shell shaped samples. Two
configurations are considered: (i) heat pulse applied on the inner surface and
temperature rise recorded on the outer surface and (ii) heat pulse applied on
the outer surface and temperature rise recorded on the inner surface. Code
implementing and verifying the thermal diffusivity formulas for both
configurations is made available.Comment: 12 pages, 5 figure
Upper critical field in {BaKBiO}: magnetotransport versus magnetotunneling
Elastic tunneling is used as a powerful direct tool to determine the upper
critical field in the high- oxide BaKBiO. The
temperature dependence of inferred from the tunneling follows the
Werthamer-Helfand-Hohenberg prediction for type-II superconductors. A
comparison will be made with resistively determined critical field data.Comment: 4 pages incl. 5 figure
Simplified models of diffusion in radially-symmetric geometries
We consider diffusion-controlled release of particles from -dimensional
radially-symmetric geometries. A quantity commonly used to characterise such
diffusive processes is the proportion of particles remaining within the
geometry over time, denoted as . The stochastic approach for computing
is time-consuming and lacks analytical insight into key parameters while
the continuum approach yields complicated expressions for that obscure
the influence of key parameters and complicate the process of fitting
experimental release data. In this work, to address these issues, we develop
several simple surrogate models to approximate by matching moments with
the continuum analogue of the stochastic diffusion model. Surrogate models are
developed for homogeneous slab, circular, annular, spherical and spherical
shell geometries with a constant particle movement probability and
heterogeneous slab, circular, annular and spherical geometries, comprised of
two concentric layers with different particle movement probabilities. Each
model is easy to evaluate, agrees well with both stochastic and continuum
calculations of and provides analytical insight into the key parameters
of the diffusive transport system: dimension, diffusivity, geometry and
boundary conditions.Comment: 22 pages, 3 figures, submitte
Constraints on Dark Matter from Colliders
We show that colliders can impose strong constraints on models of dark
matter, in particular when the dark matter is light. We analyze models where
the dark matter is a fermion or scalar interacting with quarks and/or gluons
through an effective theory containing higher dimensional operators which
represent heavier states that have been integrated out of the effective field
theory. We determine bounds from existing Tevatron searches for monojets as
well as expected LHC reaches for a discovery. We find that colliders can
provide information which is complementary or in some cases even superior to
experiments searching for direct detection of dark matter through its
scattering with nuclei. In particular, both the Tevatron and the LHC can
outperform spin dependent searches by an order of magnitude or better over much
of parameter space, and if the dark matter couples mainly to gluons, the LHC
can place bounds superior to any spin independent search.Comment: 23 pages, 16 figure
BICEP3: a 95GHz refracting telescope for degree-scale CMB polarization
Bicep3 is a 550 mm-aperture refracting telescope for polarimetry of radiation in the cosmic microwave background at 95 GHz. It adopts the methodology of Bicep1, Bicep2 and the Keck Array experiments | it possesses sufficient resolution to search for signatures of the inflation-induced cosmic gravitational-wave background while utilizing a compact design for ease of construction and to facilitate the characterization and mitigation of systematics. However, Bicep3 represents a significant breakthrough in per-receiver sensitivity, with a focal plane area 5x larger than a Bicep2/Keck Array receiver and faster optics (f=1:6 vs. f=2:4). Large-aperture infrared-reflective metal-mesh filters and infrared-absorptive cold alumina filters and lenses were developed and implemented for its optics. The camera consists of 1280 dual-polarization pixels; each is a pair of orthogonal antenna arrays coupled to transition-edge sensor bolometers and read out by multiplexed SQUIDs. Upon deployment at the South Pole during the 2014-15 season, Bicep3 will have survey speed comparable to Keck Array 150 GHz (2013), and will signifcantly enhance spectral separation of primordial B-mode power from that of possible galactic dust contamination in the Bicep2 observation patch
Caracterização espectrorradiométrica de minerais e rochas sedimentares.
bitstream/item/33610/1/documento-172.pd
Degree-scale Cosmic Microwave Background Polarization Measurements from Three Years of BICEP1 Data
BICEP1 is a millimeter-wavelength telescope designed specifically to measure the inflationary B-mode polarization of the cosmic microwave background at degree angular scales. We present results from an analysis of the data acquired during three seasons of observations at the South Pole (2006-2008). This work extends the two-year result published in Chiang et al., with additional data from the third season and relaxed detector-selection criteria. This analysis also introduces a more comprehensive estimation of band power window functions, improved likelihood estimation methods, and a new technique for deprojecting monopole temperature-to-polarization leakage that reduces this class of systematic uncertainty to a negligible level. We present maps of temperature, E- and B-mode polarization, and their associated angular power spectra. The improvement in the map noise level and polarization spectra error bars are consistent with the 52% increase in integration time relative to Chiang et al. We confirm both self-consistency of the polarization data and consistency with the two-year results. We measure the angular power spectra at 21 ≤ ℓ ≤ 335 and find that the EE spectrum is consistent with Lambda cold dark matter cosmology, with the first acoustic peak of the EE spectrum now detected at 15σ. The BB spectrum remains consistent with zero. From B-modes only, we constrain the tensor-to-scalar ratio to r = 0.03^(+0.27)_(-0.23), or r < 0.70 at 95% confidence level
Dark Matter Capture in the First Stars: a Power Source and Limit on Stellar Mass
The annihilation of weakly interacting massive particles can provide an
important heat source for the first (Pop. III) stars, potentially leading to a
new phase of stellar evolution known as a "Dark Star". When dark matter (DM)
capture via scattering off of baryons is included, the luminosity from DM
annihilation may dominate over the luminosity due to fusion, depending on the
DM density and scattering cross-section. The influx of DM due to capture may
thus prolong the lifetime of the Dark Stars. Comparison of DM luminosity with
the Eddington luminosity for the star may constrain the stellar mass of zero
metallicity stars; in this case DM will uniquely determine the mass of the
first stars. Alternatively, if sufficiently massive Pop. III stars are found,
they might be used to bound dark matter properties.Comment: 19 pages, 4 figures, 3 Tables updated captions and graphs, corrected
grammer, and added citations revised for submission to JCA
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