1,683 research outputs found
Roughness of tensile crack fronts in heterogenous materials
The dynamics of planar crack fronts in heterogeneous media is studied using a
recently proposed stochastic equation of motion that takes into account
nonlinear effects. The analysis is carried for a moving front in the
quasi-static regime using the Self Consistent Expansion. A continuous dynamical
phase transition between a flat phase and a dynamically rough phase, with a
roughness exponent , is found. The rough phase becomes possible due
to the destabilization of the linear modes by the nonlinear terms. Taking into
account the irreversibility of the crack propagation, we infer that the
roughness exponent found in experiments might become history-dependent, and so
our result gives a lower bound for .Comment: 7 page
Initial beam-profiling tests with the NML prototype station at the Fermilab A0 Photoinjector
The beam-profile diagnostics station prototype for the superconducting rf
electron linac being constructed at Fermilab at the New Muon Lab has been
tested. The station uses intercepting radiation converter screens for the
low-power beam mode: either a 100-\mu m thick YAG:Ce single crystal
scintillator or a 1-\mu m thin Al optical transition radiation (OTR) foil. The
screens are oriented with the surface perpendicular to the beam direction. A
downstream mirror with its surface at 45 degrees to the beam direction is used
to direct the radiation into the optical transport. The optical system has
better than 20 (10) \mu m rms spatial resolution when covering a vertical field
of view of 18 (5) mm. The initial tests were performed at the A0 Photoinjector
at a beam energy of ~15 MeV and with micropulse charges from 25 to 500 pC for
beam sizes of 45 to 250 microns. Example results will be presented.Comment: 3 pp. Particle Accelerator, 24th Conference (PAC'11) 2011. 28 Mar - 1
Apr 2011. New York, US
Cardiolipin drives cytochrome c proapoptotic and antiapoptotic actions
""\\"Cytochrome c (cytc) is pivotal in mitochondrial respiration and apoptosis. The heme-Fe-atom of native hexacoordinated horse heart cytc (hhcytc) displays a very low reactivity toward ligands and does not exhibit catalytic properties. However, on interaction with cardiolipin (CL), hhcytc changes its tertiary structure disrupting the heme-Fe-Met80 distal bond. The CL-hhcytc complex displays a very low midpoint potential, out of the range required for its physiological role, binds CO and NO with high affinity, facilitates peroxynitrite isomerization to NO(3)(-), and displays peroxidase activity. As a whole, the CL-hhcytc complex could play either proapoptotic effects, catalyzing lipid peroxidation and the subsequent hhcytc release into the cytoplasm, or antiapoptotic actions, such as scavenging peroxynitrite (i.e., protecting the mitochondrion from reactive nitrogen and oxygen species), and binding of CO and NO (i.e., inhibiting lipid peroxidation and hhcytc traslocation). Here, the CL-driven allosteric modulation of hhcytc properties is reviewed, highlighting proapoptotic and antiapoptotic actions. (C) 2011 IUBMB IUBMB Life, 63(3): 160-165, 2011\\""
Chronography of the Milky Way's Halo System with Field Blue Horizontal-Branch Stars
In a pioneering effort, Preston et al. reported that the colors of blue
horizontal-branch (BHB) stars in the halo of the Galaxy shift with distance,
from regions near the Galactic center to about 12 kpc away, and interpreted
this as a correlated variation in the ages of halo stars, from older to
younger, spanning a range of a few Gyrs. We have applied this approach to a
sample of some 4700 spectroscopically confirmed BHB stars selected from the
Sloan Digital Sky Survey to produce the first "chronographic map" of the halo
of the Galaxy. We demonstrate that the mean de-reddened gr color,
, increases outward in the Galaxy from 0.22 to 0.08 (over a
color window spanning [0.3:0.0]) from regions close to the Galactic center
to ~40 kpc, independent of the metallicity of the stars. Models of the expected
shift in the color of the field BHB stars based on modern stellar evolutionary
codes confirm that this color gradient can be associated with an age difference
of roughly 2-2.5 Gyrs, with the oldest stars concentrated in the central ~15
kpc of the Galaxy. Within this central region, the age difference spans a mean
color range of about 0.05 mag (~0.8 Gyrs). Furthermore, we show that
chronographic maps can be used to identify individual substructures, such as
the Sagittarius Stream, and overdensities in the direction of Virgo and
Monoceros, based on the observed contrast in their mean BHB colors with respect
to the foreground/background field population.Comment: 6 pages, 4 figures, ApJ letter
Roughness and multiscaling of planar crack fronts
We consider numerically the roughness of a planar crack front within the
long-range elastic string model, with a tunable disorder correlation length
. The problem is shown to have two important length scales, and the
Larkin length . Multiscaling of the crack front is observed for scales
below , provided that the disorder is strong enough. The asymptotic
scaling with a roughness exponent is recovered for scales
larger than both and . If , these regimes are separated
by a third regime characterized by the Larkin exponent .
We discuss the experimental implications of our results.Comment: 8 pages, two figure
Fracture Roughness Scaling: a case study on planar cracks
Using a multi-resolution technique, we analyze large in-plane fracture fronts
moving slowly between two sintered Plexiglas plates. We find that the roughness
of the front exhibits two distinct regimes separated by a crossover length
scale . Below , we observe a multi-affine regime and the
measured roughness exponent is in
agreement with the coalescence model. Above , the fronts are
mono-affine, characterized by a roughness exponent , consistent with the fluctuating line model. We relate the
crossover length scale to fluctuations in fracture toughness and the stress
intensity factor
Conversion of cytochrome c into a peroxidase: inhibitory mechanisms and implication for neurodegenerative diseases.
A further function of cytochrome c (cyt c), beyond respiration, is realized outside mitochondria in the apoptotic program. In the early events of apoptosis, the interaction of cyt c with a mitochondrion-specific phospholipid, cardiolipin (CL), brings about a conformational transition of the protein and acquirement of peroxidase activity. The hallmark of cyt c with peroxidase activity is its partial unfolding accompanied by loosening of the Fe sixth axial bond and an enhanced access of the heme catalytic site to small molecules like H2O2. To investigate the peroxidase activity of non-native cyt c, different forms of the protein were analyzed with the aim to correlate their structural features with the acquired enzymatic activity and apoptogenic properties (wt cyt c/CL complex and two single cyt c variants, H26Y and Y67H, free and bound to CL). The results suggest that cyt c may respond to different environments by changing its fold thus favouring the exertion of different biological functions in different pathophysiological cell conditions. Transitions among different conformations are regulated by endogenous molecules such as ATP and may be affected by synthetic molecules such as minocycline, thus suggesting a mechanism explaining its use as therapeutic agent impacting on disease-associated oxidative and apoptotic mechanisms
Experimental study of coherent synchrotron radiation in the emittance exchange line at the A0-photoinjector
Next generation accelerators will require a high current, low emittance beam
with a low energy spread. Such accelerators will employ advanced beam
conditioning systems such as emittance exchangers to manipulate high brightness
beams. One of the goals of the Fermilab A0 photoinjector is to investigate the
transverse to longitudinal emittance exchange principle. Coherent synchrotron
radiation could limit high current operation of the emittance exchanger. In
this paper, we report on the preliminary experimental and simulation study of
the coherent synchroton radiation (CSR) in the emittance exchange line at the
A0 photoinjector.Comment: 4 pp. 14th Advanced Accelerator Concepts Workshop, 13-19 Jun 2010:
Annapolis, Marylan
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