4,775 research outputs found
Gravitational Collapse of Dust with a Cosmological Constant
The recent analysis of Markovic and Shapiro on the effect of a cosmological
constant on the evolution of a spherically symmetric homogeneous dust ball is
extended to include the inhomogeneous and degenerate cases. The histories are
shown by way of effective potential and Penrose-Carter diagrams.Comment: 2 pages, 2 figures (png), revtex. To appear in Phys. Rev.
Recommended from our members
Nascent polypeptide chains exit the ribosome in the same relative position in both eucaryotes and procaryotes.
We located the polypeptide nascent chain as it leaves cytoplasmic ribosomes from the plant Lemna gibba by immune electron microscopy using antibodies against the small subunit of the enzyme ribulose-1,5-bisphosphate carboxylase. Similar studies with Escherichia coli ribosomes, using antibodies directed against the enzyme beta-galactosidase, show that the polypeptide nascent chain emerges in the same relative position in plants and bacteria. The eucaryotic ribosomal exit site is on the large subunit, approximately 75 A from the interface between subunits and nearly 160 A from the central protuberance, the presumed site for peptidyl transfer. This is the first functional site on both the eucaryotic and procaryotic ribosomes to be determined
Oscillating Fracture in Rubber
We have found an oscillating instability of fast-running cracks in thin
rubber sheets. A well-defined transition from straight to oscillating cracks
occurs as the amount of biaxial strain increases. Measurements of the amplitude
and wavelength of the oscillation near the onset of this instability indicate
that the instability is a Hopf bifurcation
Biodistribution and PET Imaging of pharmacokinetics of manganese in mice using Manganese-52
<div><p>Manganese is essential to life, and humans typically absorb sufficient quantities of this element from a normal healthy diet; however, chronic, elevated ingestion or inhalation of manganese can be neurotoxic, potentially leading to <i>manganism</i>. Although imaging of large amounts of accumulated Mn(II) is possible by MRI, quantitative measurement of the biodistribution of manganese, particularly at the trace level, can be challenging. In this study, we produced the positron-emitting radionuclide <sup>52</sup>Mn (<i>t</i><sub><i>1/2</i></sub> = 5.6 d) by proton bombardment (<i>E</i><sub><i>p</i></sub><15 MeV) of chromium metal, followed by solid-phase isolation by cation-exchange chromatography. An aqueous solution of [<sup>52</sup>Mn]MnCl<sub>2</sub> was nebulized into a closed chamber with openings through which mice inhaled the aerosol, and a separate cohort of mice received intravenous (IV) injections of [<sup>52</sup>Mn]MnCl<sub>2</sub>. <i>Ex vivo</i> biodistribution was performed at 1 h and 1 d post-injection/inhalation (p.i.). In both trials, we observed uptake in lungs and thyroid at 1 d p.i. Manganese is known to cross the blood-brain barrier, as confirmed in our studies following IV injection (0.86%ID/g, 1 d p.i.) and following inhalation of aerosol, (0.31%ID/g, 1 d p.i.). Uptake in salivary gland and pancreas were observed at 1 d p.i. (0.5 and 0.8%ID/g), but to a much greater degree from IV injection (6.8 and 10%ID/g). In a separate study, mice received IV injection of an imaging dose of [<sup>52</sup>Mn]MnCl<sub>2</sub>, followed by <i>in vivo</i> imaging by positron emission tomography (PET) and <i>ex vivo</i> biodistribution. The results from this study supported many of the results from the biodistribution-only studies. In this work, we have confirmed results in the literature and contributed new results for the biodistribution of inhaled radiomanganese for several organs. Our results could serve as supporting information for environmental and occupational regulations, for designing PET studies utilizing <sup>52</sup>Mn, and/or for predicting the biodistribution of manganese-based MR contrast agents.</p></div
Junctions and thin shells in general relativity using computer algebra I: The Darmois-Israel Formalism
We present the GRjunction package which allows boundary surfaces and
thin-shells in general relativity to be studied with a computer algebra system.
Implementing the Darmois-Israel thin shell formalism requires a careful
selection of definitions and algorithms to ensure that results are generated in
a straight-forward way. We have used the package to correctly reproduce a wide
variety of examples from the literature. We present several of these
verifications as a means of demonstrating the packages capabilities. We then
use GRjunction to perform a new calculation - joining two Kerr solutions with
differing masses and angular momenta along a thin shell in the slow rotation
limit.Comment: Minor LaTeX error corrected. GRjunction for GRTensorII is available
from http://astro.queensu.ca/~grtensor/GRjunction.htm
Observation of long range magnetic ordering in pyrohafnate Nd2Hf2O7: A neutron diffraction study
We have investigated the physical properties of a pyrochlore hafnate Nd2Hf2O7
using ac magnetic susceptibility \chi_ac(T), dc magnetic susceptibility
\chi(T), isothermal magnetization M(H) and heat capacity C_p(T) measurements,
and determined the magnetic ground state by neutron powder diffraction study.
An upturn is observed below 6 K in C_p(T)/T, however both C_p(T) and \chi(T) do
not show any clear anomaly down to 2 K. The \chi_ac(T) shows a well pronounced
anomaly indicating an antiferromagnetic transition at T_N = 0.55 K. The long
range antiferromagnetic ordering is confirmed by neutron diffraction. The
refinement of neutron diffraction pattern reveals an all-in/all-out
antiferromagnetic structure, where for successive tetrahedra, the four Nd3+
magnetic moments point alternatively all-into or all-out-of the tetrahedron,
with an ordering wavevector k = (0, 0, 0) and an ordered state magnetic moment
of m = 0.62(1) \mu_B/Nd at 0.1 K. The ordered moment is strongly reduced
reflecting strong quantum fluctuations in ordered state.Comment: 10 pages, 9 figures and 2 tables; to appear in Phys. Rev.
Microscopic theory of quantum-transport phenomena in mesoscopic systems: A Monte Carlo approach
A theoretical investigation of quantum-transport phenomena in mesoscopic
systems is presented. In particular, a generalization to ``open systems'' of
the well-known semiconductor Bloch equations is proposed. The presence of
spatial boundary conditions manifest itself through self-energy corrections and
additional source terms in the kinetic equations, whose form is suitable for a
solution via a generalized Monte Carlo simulation. The proposed approach is
applied to the study of quantum-transport phenomena in double-barrier
structures as well as in superlattices, showing a strong interplay between
phase coherence and relaxation.Comment: to appear in Phys. Rev. Let
- …