2,139 research outputs found
Assurity MRITM & Endurity MRITM 3 Tesla: Séries de Marcapassos Condicionais para Exames de Ressonùncia Magnética Full Body 3 Tesla
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Série de Eletrodos QuartetTM MRI: Quatro Opçoes de Eletrodos Quadripolares Condicionais para RM
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A coherent middle Pliocene magnetostratigraphy, Wanganui Basin, New Zealand
We document magnetostratigraphies for three river sections (Turakina, Rangitikei, Wanganui) in Wanganui Basin and interpret them as corresponding to the Upper Gilbert, the Gauss and lower Matuyama Chrons of the Geomagnetic Polarity Timescale, in agreement with foraminiferal biostratigraphic datums. The Gauss-Gilbert transition (3.58 Ma) is located in both the Turakina and Wanganui River sections, while the Gauss-Matuyama transition (2.58 Ma) is located in all three sections, as are the lower and upper boundaries of the Mammoth (3.33â3.22 Ma) and Kaena (3.11â3.04 Ma) Subchrons. Our interpretations are based in part on the re-analysis of existing datasets and in part on the acquisition and analysis of new data, particularly for the Wanganui River section. The palaeomagnetic dates of these six horizons provide the only numerical age control for a thick (up to 2000 m) mudstone succession (Tangahoe Mudstone) that accumulated chiefly in upper bathyal and outer neritic palaeoenvironments. In the Wanganui River section the mean sediment accumulation rate is estimated to have been about 1.8 m/k.y., in the Turakina section it was about 1.5 m/k.y., and in the Rangitikei section, the mean rate from the beginning of the Mammoth Subchron to the Hautawa Shellbed was about 1.1 m/k.y. The high rates may be associated with the progradation of slope clinoforms northward through the basin. This new palaeomagnetic timescale allows revised correlations to be made between cyclothems in the Rangitikei River section and the global Oxygen Isotope Stages (OIS) as represented in Ocean Drilling Program (ODP) Site 846. The 16 depositional sequences between the end of the Mammoth Subchron and the Gauss-Matuyama Boundary are correlated with OIS MG2 to 100. The cyclothems average 39 k.y. in duration in our age model, which is close to the 41 k.y. duration of the orbital obliquity cycles. We support the arguments advanced recently in defence of the need for local New Zealand stages as a means of classifying New Zealand sedimentary successions, and strongly oppose the proposal to move stage boundaries to selected geomagnetic polarity transitions. The primary magnetisation of New Zealand mudstone is frequently overprinted with secondary components of diagenetic origin, and hence it is often difficult to obtain reliable magnetostratigraphic records. We suggest specific approaches, analytical methods, and criteria to help ensure robustness and coherency in the palaeomagnetic identification of chron boundaries in typical New Zealand Cenozoic mudstone successions
Relativistic bound states in Yukawa model
The bound state solutions of two fermions interacting by a scalar exchange
are obtained in the framework of the explicitly covariant light-front dynamics.
The stability with respect to cutoff of the J= and J=
states is studied. The solutions for J= are found to be stable for
coupling constants below the critical value
and unstable above it. The asymptotic behavior of the
wave functions is found to follow a law. The coefficient
and the critical coupling constant are calculated from an
eigenvalue equation. The binding energies for the J= solutions
diverge logarithmically with the cutoff for any value of the coupling constant.
For a wide range of cutoff, the states with different angular momentum
projections are weakly split.Comment: 22 pages, 13 figures, .tar.gz fil
Seismological structure of the 1.8 Ga Trans-Hudson Orogen of North America
Precambrian tectonic processes are debated: what was the nature and scale of orogenic events on the younger, hotter, and more ductile Earth? Northern Hudson Bay records the Paleoproterozoic collision between the Western Churchill and Superior platesâthe âŒ1.8 Ga Trans-Hudson Orogeny (THO)âand is an ideal locality to study Precambrian tectonic structure. Integrated field, geochronological, and thermobarometric studies suggest that the THO was comparable to the present-day Himalayan-Karakoram-Tibet Orogen (HKTO). However, detailed understanding of the deep crustal architecture of the THO, and how it compares to that of the evolving HKTO, is lacking. The joint inversion of receiver functions and surface wave data provides new Moho depth estimates and shear velocity models for the crust and uppermost mantle of the THO. Most of the Archean crust is relatively thin (âŒ39 km) and structurally simple, with a sharp Moho; upper-crustal wave speed variations are attributed to postformation events. However, the Quebec-Baffin segment of the THO has a deeper Moho (âŒ45 km) and a more complex crustal structure. Observations show some similarity to recent models, computed using the same methods, of the HKTO crust. Based on Moho character, present-day crustal thickness, and metamorphic grade, we support the view that southern Baffin Island experienced thickening during the THO of a similar magnitude and width to present-day Tibet. Fast seismic velocities at >10 km below southern Baffin Island may be the result of partial eclogitization of the lower crust during the THO, as is currently thought to be happening in Tibet
X-rays from Colliding Stellar Winds: the case of close WR+O binary systems
We have analysed the X-ray emission from a sample of close WR+O binaries
using data from the public Chandra and XMM-Newton archives. Global spectral
fits show that two-temperature plasma is needed to match the X-ray emission
from these objects as the hot component (kT > 2 keV) is an important ingredient
of the spectral models. In close WR+O binaries, X-rays likely originate in
colliding stellar wind (CSW) shocks driven by the massive winds of the binary
components. CSW shocks in these objects are expected to be radiative due to the
high density of the plasma in the interaction region. Opposite to this, our
analysis shows that the CSW shocks in the sample of close WR+O binaries are
adiabatic. This is possible only if the mass-loss rates of the stellar
components in the binary are at least one order of magnitude smaller than the
values currently accepted. The most likely explanation for the X-ray properties
of close WR+O binaries could be that their winds are two-component flows. The
more massive component (dense clumps) play role for the optical/UV emission
from these objects, while the smooth rarefied component is a key factor for
their X-ray emission.Comment: MNRAS, accepted for publication (Feb 6, 2012); 13 pages, 6 figures, 3
table
Heavy Flavours in Collider Experiments
Current issues in the studies of Heavy Flavours in colliders are described
with particular emphasis on experiments in which the UK is involved. Results on
charm production at HERA are examined and compared to those at the Tevatron. B
production rates at the Tevatron as well as the status of B lifetimes and
mixing in the LEP collaborations and at the Tevatron are highlighted. The
measurement of sin2beta from CDF is described as well as the most recent
results on top physics at the Tevatron
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Two-Fermion Bound States within the Bethe-Salpeter Approach
To solve the spinor-spinor Bethe-Salpeter equation in Euclidean space we
propose a novel method related to the use of hyperspherical harmonics. We
suggest an appropriate extension to form a new basis of spin-angular harmonics
that is suitable for a representation of the vertex functions. We present a
numerical algorithm to solve the Bethe-Salpeter equation and investigate in
detail the properties of the solution for the scalar, pseudoscalar and vector
meson exchange kernels including the stability of bound states. We also compare
our results to the non relativistic ones and to the results given by light
front dynamics.Comment: 32 pages, XIII Tables, 8 figure
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