26,109 research outputs found
Helium, neon, and tritium in the Black Sea
Measurements of the 3He/4He ratio, and concentrations of helium, neon, and tritium have been made in samples collected at station 1355 in the Black Sea during the 1975 cruise of R/V Chain. Helium concentrations increase rapidly from 400 m to about 1000 m and then less rapidly below 1000 m. The maximum He concentration excess is about 16% above solubility equilibrium. Neon concentrations are constant, within a few percent of solubility equilibrium, below 400 m. The He-Ne data thus clearly indicate a large component of radiogenic 4He in the deep water from decay of U and Th in the bottom sediments. Tritium concentrations decrease from 67.2 T.U. at the surface to near-zero values at 726 m, 968 m, and 1358 m. Two deeper samples at 1745 m and 1939 m contain some tritium—0.6 ± 0.3 T.U. and 0.7 ± 0.2 T.U. respectively, which shows that a small amount of high-tritium surface water has descended to the bottom during the past twenty years. A one-dimensional three-box model using our tritium concentrations and the geothermal heat flux values measured by Erickson and Simmons (1974) gives vertical exchange times of 440 ± 180 years between the deep water (1000 m-2000 m) and the middle water (400 m-1000 m), and 125 ± 75 years between the middle water and a layer above from 200 m to 400 m in the salinity interval 21.50‰-22.00‰. The fluxes of radiogenic 4He and primordial 3He into the deep water are found to be 1.3 ± 0.5 × 105 atoms cm—2 sec—1 and 1.1 ± 0.6 atoms cm—2 sec—1 respectively which may be compared with world-ocean estimates of Craig et al. (1975)—3 ± 1.5 x 105 atoms cm—2 sec—1 and 4 ± 1 atoms cm—2 sec—1
Simplified Onsager theory for isotropic-nematic phase equilibria of length polydisperse hard rods
Polydispersity is believed to have important effects on the formation of
liquid crystal phases in suspensions of rod-like particles. To understand such
effects, we analyse the phase behaviour of thin hard rods with length
polydispersity. Our treatment is based on a simplified Onsager theory, obtained
by truncating the series expansion of the angular dependence of the excluded
volume. We describe the model and give the full phase equilibrium equations;
these are then solved numerically using the moment free energy method which
reduces the problem from one with an infinite number of conserved densities to
one with a finite number of effective densities that are moments of the full
density distribution. The method yields exactly the onset of nematic ordering.
Beyond this, results are approximate but we show that they can be made
essentially arbitrarily precise by adding adaptively chosen extra moments,
while still avoiding the numerical complications of a direct solution of the
full phase equilibrium conditions.
We investigate in detail the phase behaviour of systems with three different
length distributions: a (unimodal) Schulz distribution, a bidisperse
distribution and a bimodal mixture of two Schulz distributions which
interpolates between these two cases. A three-phase isotropic-nematic-nematic
coexistence region is shown to exist for the bimodal and bidisperse length
distributions if the ratio of long and short rod lengths is sufficiently large,
but not for the unimodal one. We systematically explore the topology of the
phase diagram as a function of the width of the length distribution and of the
rod length ratio in the bidisperse and bimodal cases.Comment: 18 pages, 16 figure
Thermal-neutron cross section for 10(n,t)2_ via 3-4He mass spectrometry
The thermal cross section for the reaction 10He is determined to be 4.47±0.15 mb by neutron irradiation of H3BO3 followed by measurement of 3He (from decay of 3H) and 4He in a static mass spectrometer. Some samples contained boron of normal isotopic composition and some were enriched in 10B, and irradiations were carried out in a highly thermalized neutron flux and in the core of a light-water-moderated reactorPeer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/86120/1/PhysRevC.39.1633.pd
Active galactic nucleus feedback in clusters of galaxies
Observations made during the last ten years with the Chandra X-ray
Observatory have shed much light on the cooling gas in the centers of clusters
of galaxies and the role of active galactic nucleus (AGN) heating. Cooling of
the hot intracluster medium in cluster centers can feed the supermassive black
holes found in the nuclei of the dominant cluster galaxies leading to AGN
outbursts which can reheat the gas, suppressing cooling and large amounts of
star formation. AGN heating can come in the form of shocks, buoyantly rising
bubbles that have been inflated by radio lobes, and the dissipation of sound
waves.Comment: Refereed review article published in Chandra's First Decade of
Discovery Special Feature edition of the Proceedings of the National Academy
of Science
Dynamics of a self-gravitating thin cosmic string
We assume that a self-gravitating thin string can be locally described by
what we shall call a smoothed cone. If we impose a specific constraint on the
model of the string, then its central line obeys the Nambu-Goto equations. If
no constraint is added, then the worldsheet of the central line is a totally
geodesic surface.Comment: 20 pages, latex, 1 figure, final versio
Experimental demonstration of Aharonov-Casher interference in a Josephson junction circuit
A neutral quantum particle with magnetic moment encircling a static electric
charge acquires a quantum mechanical phase (Aharonov-Casher effect). In
superconducting electronics the neutral particle becomes a fluxon that moves
around superconducting islands connected by Josephson junctions. The full
understanding of this effect in systems of many junctions is crucial for the
design of novel quantum circuits. Here we present measurements and quantitative
analysis of fluxon interference patterns in a six Josephson junction chain. In
this multi-junction circuit the fluxon can encircle any combination of charges
on five superconducting islands, resulting in a complex pattern. We compare the
experimental results with predictions of a simplified model that treats fluxons
as independent excitations and with the results of the full diagonalization of
the quantum problem. Our results demonstrate the accuracy of the fluxon
interference description and the quantum coherence of these arrays
The effect of phenytoin, phenobarbitone, dexamethasone and flurbiprofen on misonidazole neurotoxicity in mice.
Using a quantitative cytochemical technique for measuring beta-glucuronidase activity in the peripheral nerves of mice, we have investigated the effectiveness of four potential adjuncts for reducing the dose limiting neurotoxicity of misonidazole (MISO) in the clinic. Under the conditions used, the most effective adjunct was the steroid anti-inflammatory agent dexamethasone. When given over the week previous to MISO treatment, this agent almost completely eliminated the MISO neurotoxicity as determined at week 4 after commencement of MISO dosing. The second most effective adjunct was phenytoin, the third flurbiprofen and the last adjunct, phenobarbitone, was ineffective. Dexamethasone, phenytoin and phenobarbitone all reduced the clearance half-life of MISO and hence the drug exposure dose calculated as the area under the curve of MISO tissue concentration against time. However, no correlation was evident with these parameters and MISO neurotoxicity in the mouse. Dexamethasone, whilst affording protection against MISO toxicity, did not alter the radiosensitivity of the anaplastic MT tumour
Transform-limited X-ray pulse generation from a high-brightness self-amplified spontaneous-emission free-electron laser
A method to achieve high-brightness self-amplified spontaneous emission (HB-SASE) in the free-electron laser (FEL) is described. The method uses repeated nonequal electron beam delays to delocalize the collective FEL interaction and break the radiation coherence length dependence on the FEL cooperation length. The method requires no external seeding or photon optics and so is applicable at any wavelength or repetition rate. It is demonstrated, using linear theory and numerical simulations, that the radiation coherence length can be increased by approximately 2 orders of magnitude over SASE with a corresponding increase in spectral brightness. Examples are shown of HB-SASE generating transform-limited FEL pulses in the soft x-ray and near transform-limited pulses in the hard x-ray. Such pulses may greatly benefit existing applications and may also open up new areas of scientific research
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