8,948 research outputs found
Re-Engineering of the GSI Control System
After more than 12 years of operation without substantial revision a
modernization of the control system at GSI is overdue. A strategy to adapt the
system to future needs is outlined. The system has to support a specific
environment of which the main features are described. More flexibility than in
the current system can be achieved while still using many parts of the actual
system.Comment: ICALEPCS 2001, Talk WEAT002, 3 pages, Late
Channeling of Charged Particles Through Periodically Bent Crystals: on the Possibility of a Gamma Laser
We discuss radiation generated by positrons channeling in a crystalline
undulator. The undulator is produced by periodically bending a single crystal
with an amplitude much larger than the interplanar spacing. Different
approaches for bending the crystal are described and the restrictions on the
parameters of the bending are established. We present the results of numeric
calculations of the spectral distributions of the spontaneous emitted radiation
and estimate the conditions for stimulated emission. Our investigations show
that the proposed mechanism provides an efficient source for high energy
photons, which is worth to be studied experimentally.Comment: contributed to the conference ``Fundamental and Applied Aspects of
Modern Physics'' in Luederitz, Namibia, 200
On the Detectability of Oxygen X-ray Fluorescence and its Use as a Solar Photospheric Abundance Diagnostic
Monte Carlo calculations of the O Kalpha line fluoresced by coronal X-rays
and emitted just above the temperature minimum region of the solar atmosphere
have been employed to investigate the use of this feature as an abundance
diagnostic. While quite weak, we estimate line equivalent widths in the range
0.02-0.2 AA, depending on the X-ray plasma temperature. The line remains
essentially uncontaminated by blends for coronal temperatures T =< 3e6 K and
should be quite observable, with a flux >~ 2 ph/s/arcmin^2. Model calculations
for solar chemical mixtures with an O abundance adjusted up and down by a
factor of 2 indicate 35-60% changes in O Kalpha line equivalent width,
providing a potentially useful O abundance diagnostic. Sensitivity of
equivalent width to differences between recently recommended chemical
compositions with ``high'' and ``low'' complements of the CNO trio important
for interpreting helioseismological observations is less accute, amounting to
20-26% at coronal temperatures T ~< 2e6 K. While still feasible for
discriminating between these two mixtures, uncertainties in measured line
equivalent widths and in the models used for interpretation would need to be
significantly less than 20%. Provided a sensitive X-ray spectrometer with
resolving power >= 1000 and suitably well-behaved instrumental profile can be
built, X-ray fluorescence presents a viable means for resolving the solar
``oxygen crisis''.Comment: To appear in the Astrophysical Journa
The relationship of telomere length to baseline corticosterone levels in nestlings of an altricial passerine bird in natural populations
Indexación: Web of Science; Scopus.Background: Environmental stressors increase the secretion of glucocorticoids that in turn can shorten telomeres via oxidative damage. Modification of telomere length, as a result of adversity faced early in life, can modify an individual's phenotype. Studies in captivity have suggested a relationship between glucocorticoids and telomere length in developing individuals, however less is known about that relationship in natural populations.
Methods: In order to evaluate the effect of early environmental stressors on telomere length in natural populations, we compared baseline corticosterone (CORT) levels and telomere length in nestlings of the same age. We collected blood samples for hormone assay and telomere determination from two geographically distinct populations of the Thorn-tailed Rayadito (Aphrastura spinicauda) that differed in brood size; nestlings body mass and primary productivity. Within each population we used path analysis to evaluate the relationship between brood size, body mass, baseline CORT and telomere length.
Results: Within each distinct population, path coefficients showed a positive relationship between brood size and baseline CORT and a strong and negative correlation between baseline CORT and telomere length. In general, nestlings that presented higher baseline CORT levels tended to present shorter telomeres. When comparing populations it was the low latitude population that presented higher levels of baseline CORT and shorter telomere length.
Conclusions: Taken together our results reveal the importance of the condition experienced early in life in affecting telomere length, and the relevance of integrative studies carried out in natural conditions.https://frontiersinzoology.biomedcentral.com/articles/10.1186/s12983-016-0133-
Synthetic, Spectroscopic and DFT Studies of Iron Complexes with Iminobenzo(semi)quinone Ligands: Implications for o-Aminophenol Dioxygenases
The oxidative CC bond cleavage of o-aminophenols by nonheme Fe dioxygenases is a critical step in both human metabolism (the kynurenine pathway) and the microbial degradation of nitroaromatic pollutants. The catalytic cycle of o-aminophenol dioxygenases (APDOs) has been proposed to involve formation of an FeII/O2/iminobenzosemiquinone complex, although the presence of a substrate radical has been called into question by studies of related ring-cleaving dioxygenases. Recently, we reported the first synthesis of an iron(II) complex coordinated to an iminobenzosemiquinone (ISQ) ligand, namely, [Fe(Tp)(tBuISQ)] (2 a; where Tp=hydrotris(3,5-diphenylpyrazol-1-yl)borate and tBuISQ is the radical anion derived from 2-amino-4,6-di-tert-butylphenol). In the current manuscript, density functional theory (DFT) calculations and a wide variety of spectroscopic methods (electronic absorption, Mössbauer, magnetic circular dichroism, and resonance Raman) were employed to obtain detailed electronic-structure descriptions of 2 a and its one-electron oxidized derivative [3 a]+. In addition, we describe the synthesis and characterization of a parallel series of complexes featuring the neutral supporting ligand tris(4,5-diphenyl-1-methylimidazol-2-yl)phosphine (TIP). The isomer shifts of about 0.97 mm s−1 obtained through Mössbauer experiments confirm that 2 a (and its TIP-based analogue [2 b]+) contain FeII centers, and the presence of an ISQ radical was verified by analysis of the absorption spectra in light of time-dependent DFT calculations. The collective spectroscopic data indicate that one-electron oxidation of the FeII–ISQ complexes yields complexes ([3 a]+ and [3 b]2+) with electronic configurations between the FeIII–ISQ and FeII–IBQ limits (IBQ=iminobenzoquinone), highlighting the ability of o-amidophenolates to access multiple oxidation states. The implications of these results for the mechanism of APDOs and other ring-cleaving dioxygenases are discussed
Motion-Induced Magnetic Resonance of Rb Atoms in a Periodic Magnetostatic Field
We demonstrate that transitions between Zeeman-split sublevels of Rb atoms
are resonantly induced by the motion of the atoms (velocity: about 100 m/s) in
a periodic magnetostatic field (period: 1 mm) when the Zeeman splitting
corresponds to the frequency of the magnetic field experienced by the moving
atoms. A circularly polarized laser beam polarizes Rb atoms with a velocity
selected using the Doppler effect and detects their magnetic resonance in a
thin cell, to which the periodic field is applied with the arrays of parallel
current-carrying wires.Comment: 4 pages, 4 figures; minor corrections, Ref. [9] removed, published in
PR
Spin Nutation Induced by Atomic Motion in a Magnetic Lattice
An atom moving in a spatially periodic field experiences a temporary periodic
perturbation and undergoes a resonance transition between atomic internal
states when the transition frequency is equal to the atomic velocity divided by
the field period. We demonstrated that spin nutation was induced by this
resonant transition in a polarized rubidium (Rb) atomic beam passing through a
magnetic lattice. The lattice was produced by current flowing through an array
of parallel wires crossing the beam. This array structure, reminiscent of a
multiwire chamber for particle detection, allowed the Rb beam to pass through
the lattice at a variety of incident angles. The dephasing of spin nutation was
reduced by varying the incident angle.Comment: 11 pages, 4 figures, submitted to Phys. Rev.
The evidence of quasi-free positronium state in GiPS-AMOC spectra of glycerol
We present the results of processing of Age-Momentum Correlation (AMOC)
spectra that were measured for glycerol by the Gamma-induced positron
spectroscopy (GiPS) facility. Our research has shown that the shape of
experimental s(t) curve cannot be explained without introduction of the
intermediate state of positronium (Ps), called quasi-free Ps. This state yields
the wide Doppler line near zero lifetimes. We discuss the possible properties
of this intermediate Ps state from the viewpoint of developed model. The amount
of annihilation events produced by quasi-free Ps is estimated to be less than
5% of total annihilations. In the proposed model, quasi-free Ps serves as a
precursor for trapped Ps of para- and ortho-states
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