18,418 research outputs found
A procedural development for the analysis of <sup>56/54</sup>Fe and <sup>57/54</sup>Fe isotope ratios with new generation IsoProbe MC-ICP-MS
We have developed a procedure for iron isotope analysis using a hexapole collision cell MC-ICP-MS which is capable of Fe isotope ratio analysis using two different extraction modes. Matrix effects were minimised and the signal-to-background ratio was maximised using high-concentration samples (~ 5μg Fe) and introducing 1.8 mL/min<sup>-1</sup> Ar and 2 mL/min H<sub>2</sub> into the collision cell to decrease polyatomic interferences. The use of large intensity on the faraday cups considerably decreases the internal error of the ratios and ultimately, improves the external precision of a run. Standard bracketing correction for mass bias was possible when using hard extraction. Mass bias in soft extraction mode seems to show temporal instability that makes the standard bracketing inappropriate. The hexapole rf amplitude was decreased to 50 % to further decrease polyatomic interferences and promote the transmission of iron range masses. We routinely measure Fe isotopes with a precision of ± 0.05 ‰ and ± 0.12 ‰ (2σ) for δ<sup>56</sup>Fe and δ<sup>57</sup>Fe respectively
Are quasars accreting at super-Eddington rates?
In a previous paper, Collin & Hur\'e (2001), using a sample of Active
Galactic Nuclei (AGN) where the mass has been determined by reverberation
studies (Kaspi et al. 2000), have shown that if the optical luminosity is
emitted by a steady accretion disc, about half of the objects are accreting
close to or higher than the Eddington rate. We conclude here that this result
is unavoidable, unless the masses are strongly underestimated by reverberation
studies, which does not seem to be the case. There are three issues to the
problem: 1. Accretion proceeds at Eddington or super-Eddington rates through
thick discs. Several consequences follow: an anti-correlation between the line
widths of the lines and the Eddington ratios, and a decrease of the Eddington
ratio with an increasing black hole mass. Extrapolated to all quasars, these
results imply that the amount of mass locked in massive black holes should be
larger than presently thought. 2. The optical luminosity is not produced
directly by the gravitational release of energy, and super-Eddington rates are
not required. The optical luminosity has to be emitted by a dense and thick
medium located at large distances from the center (10 to
gravitational radii). It can be due to reprocessing of the X-ray photons from
the central source in a geometrically thin warped disc, or in dense "blobs"
forming a geometrically thick system, which can be a part of the accretion flow
or the basis of an outflow. 3. Accretion discs are completely "non standard".
Presently neither the predictions of models nor the observed spectral
distributions are sufficient to help choosing between these solutions.Comment: 16 pages, 11 figures, accepted in A&
Disoriented Chiral Condensate: Theory and Experiment
It is thought that a region of pseudo-vacuum, where the chiral order
parameter is misaligned from its vacuum orientation in isospin space, might
occasionally form in high energy hadronic or nuclear collisions. The possible
detection of such disoriented chiral condensate (DCC) would provide useful
information about the chiral structure of the QCD vacuum and/or the chiral
phase transition of strong interactions at high temperature. We review the
theoretical developments concerning the possible DCC formation in high-energy
collisions as well as the various experimental searches that have been
performed so far. We discuss future prospects for upcoming DCC searches, e.g.
in high-energy heavy-ion collision experiments at RHIC and LHC.Comment: 120 pages, 52 figures. Uses elsart.cls. To appear in Physics Reports.
Minor corrections, references adde
Strong dynamical effects during stick-slip adhesive peeling
We consider the classical problem of the stick-slip dynamics observed when
peeling a roller adhesive tape at a constant velocity. From fast imaging
recordings, we extract the dependencies of the stick and slip phases durations
with the imposed peeling velocity and peeled ribbon length. Predictions of
Maugis and Barquins [in Adhesion 12, edited by K.W. Allen, Elsevier ASP,
London, 1988, pp. 205--222] based on a quasistatic assumption succeed to
describe quantitatively our measurements of the stick phase duration. Such
model however fails to predict the full stick-slip cycle duration, revealing
strong dynamical effects during the slip phase.Comment: Soft Matter 201
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