345 research outputs found
Developing a second generation Laue lens prototype: high reflectivity crystals and accurate assembly
Laue lenses are an emerging technology that will enhance gamma-ray telescope
sensitivity by one to two orders of magnitude in selected energy bands of the
\sim 100 keV to \sim 1.5 MeV range. This optic would be particularly well
adapted to the observation of faint gamma ray lines, as required for the study
of Supernovae and Galactic positron annihilation. It could also prove very
useful for the study of hard X-ray tails from a variety of compact objects,
especially making a difference by providing sufficient sensitivity for
polarization to be measured by the focal plane detector. Our group has been
addressing the two key issues relevant to improve performance with respect to
the first generation of Laue lens prototypes: obtaining large numbers of
efficient crystals and developing a method to fix them with accurate
orientation and dense packing factor onto a substrate. We present preliminary
results of an on-going study aiming to enable a large number of crystals
suitable for diffraction at energies above 500 keV. In addition, we show the
first results of the Laue lens prototype assembled using our beamline at SSL/UC
Berkeley, which demonstrates our ability to orient and glue crystals with
accuracy of a few arcsec, as required for an efficient Laue lens telescope.Comment: Published in the proceedings of the SPIE conference held in San Diego
in August 201
Impact of water saturation on seismoelectric transfer functions: a laboratory study of coseismic phenomenon
Seismic waves propagating in a porous medium, under favourable conditions, generate measurable electromagnetic fields due to electrokinetic effects. It has been proposed, following experimental and numerical studies, that these so-called ‘seismoelectromagnetic' couplings depend on pore fluid properties. The theoretical frame describing these phenomena are based on the original Biot's theory, assuming that pores are fluid-filled. We study here the impact of a partially saturated medium on amplitudes of those seismoelectric couplings by comparing experimental data to an effective fluid model. We have built a 1-m-length-scale experiment designed for imbibition and drainage of an homogeneous silica sand; the experimental set-up includes a seismic source, accelerometers, electric dipoles and capacitance probes in order to monitor seismic and seismoelectric fields during water saturation. Apparent velocities and frequency spectra (in the kiloHertz range) are derived from seismic and electrical measurements during experiments in varying saturation conditions. Amplitudes of seismic and seismoelectric waves and their ratios (i.e. transfer functions) are discussed using a spectral analysis performed by continuous wavelet transform. The experiments reveal that amplitude ratios of seismic to coseismic electric signals remain rather constant as a function of the water saturation in the Sw=[0.2-0.9] range, consistently with theoretically predicted transfer function
Guide ROSELT/OSS pour l'évaluation et le suivi des pratiques d'exploitation des ressources naturelles
Hemodynamic and antifibrotic effects of a selective liver nitric oxide donor V-PYRRO/NO in bile duct ligated rats.
AIM: To assess whether a liver specific nitric oxide (NO) donor (V-PYRRO/NO) would prevent the development of portal hypertension and liver fibrosis in rats with bile duct ligation (BDL).
METHODS: Treatment (placebo or V-PYRRO/NO 0.53 micromol/kg per hour) was administered i.v. to rats 2 d before BDL (D-2) and maintained until the day of hemodynamic measurement (D26). Intra-hepatic NO level was estimated by measuring liver cGMP level. Effects of V-PYRRO/NO on liver fibrosis and lipid peroxidation were also assessed.
RESULTS: Compared to placebo treatment, V-PYRRO/NO improved splanchnic hemodynamics in BDL rats: portal pressure was significantly reduced by 27% (P<0.0001) and collateral circulation development was almost completely blocked (splenorenal shunt blood flow by 74%, P=0.007). Moreover, V-PYRRO/NO significantly prevented liver fibrosis development in BDL rats (by 30% in hepatic hydroxyproline content and 31% in the area of fibrosis, P<0.0001 respectively), this effect being probably due to a decrease in lipid peroxidation by 44% in the hepatic malondialdehyde level (P=0.007). Interestingly, we observed a significant and expected increase in liver cGMP, without any systemic hemodynamic effects (mean arterial pressure, vascular systemic resistance and cardiac output) in both sham-operated and BDL rats treated with V-PYRRO/NO. This result is in accordance with studies on V-PYRRO/NO metabolism showing a specific release of NO in the liver.
CONCLUSION: Continuous administrations of V-PYRRO/NO in BDL rats improved liver fibrosis and splanchnic hemodynamics without any noxious systemic hemo-dynamic effects
Planar subgraphs without low-degree nodes
We study the following problem: given a geometric graph G and an integer k, determine if G has a planar spanning subgraph (with the original embedding and straight-line edges) such that all nodes have degree at least k. If G is a unit disk graph, the problem is trivial to solve for k = 1. We show that even the slightest deviation from the trivial case (e.g., quasi unit disk graphs or k = 1) leads to NP-hard problems.Peer reviewe
Foncier et désertification, quelle gestion patrimoniale ? : approche foncière environnementale pour un développement durable au Sahel
Gathering in Dynamic Rings
The gathering problem requires a set of mobile agents, arbitrarily positioned
at different nodes of a network to group within finite time at the same
location, not fixed in advanced.
The extensive existing literature on this problem shares the same fundamental
assumption: the topological structure does not change during the rendezvous or
the gathering; this is true also for those investigations that consider faulty
nodes. In other words, they only consider static graphs. In this paper we start
the investigation of gathering in dynamic graphs, that is networks where the
topology changes continuously and at unpredictable locations.
We study the feasibility of gathering mobile agents, identical and without
explicit communication capabilities, in a dynamic ring of anonymous nodes; the
class of dynamics we consider is the classic 1-interval-connectivity.
We focus on the impact that factors such as chirality (i.e., a common sense
of orientation) and cross detection (i.e., the ability to detect, when
traversing an edge, whether some agent is traversing it in the other
direction), have on the solvability of the problem. We provide a complete
characterization of the classes of initial configurations from which the
gathering problem is solvable in presence and in absence of cross detection and
of chirality. The feasibility results of the characterization are all
constructive: we provide distributed algorithms that allow the agents to
gather. In particular, the protocols for gathering with cross detection are
time optimal. We also show that cross detection is a powerful computational
element.
We prove that, without chirality, knowledge of the ring size is strictly more
powerful than knowledge of the number of agents; on the other hand, with
chirality, knowledge of n can be substituted by knowledge of k, yielding the
same classes of feasible initial configurations
NuSTAR observation of a Type I X-ray burst from GRS 1741.9-2853
We report on two NuSTAR observations of GRS 1741.9-2853, a faint neutron star
low mass X-ray binary burster located 10' away from the Galactic center. NuSTAR
detected the source serendipitously as it was emerging from quiescence: its
luminosity was erg~s on 2013 July 31, and erg~s in a second observation on 2013 August 3. A bright, 800-s
long, H-triggered mixed H/He thermonuclear Type I burst with mild photospheric
radius expansion (PRE) was present during the second observation. Assuming that
the luminosity during the PRE was at the Eddington level, a H mass fraction
in the atmosphere, and a neutron star mass , we
determine a new lower limit on the distance for this source of
kpc. Combining with previous upper limits, this places GRS 1741.9-2853 at a
distance of 7 kpc. Energy independent (achromatic) variability is observed
during the cooling of the neutron star, which could result from the disturbance
of the inner accretion disk by the burst. The large dynamic range of this burst
reveals a long power-law decay tail. We also detect, at a 95.6\% confidence
level (1.7 ), a narrow absorption line at keV during the
PRE phase of the burst, reminiscent of the detection by Waki et al. (1984). We
propose that the line, if real, is formed in the wind above the photosphere of
the neutron star by a resonant K transition from H-like Cr
gravitationally redshifted by a factor , corresponding to a radius
range of 29.0 -- 41.4 km for a mass range of 1.4 -- 2.0 .Comment: Accepted for publication in The Astrophysical Journa
The SIGMA rat brain templates and atlases for multimodal MRI data analysis and visualization
Preclinical imaging studies offer a unique access to the rat brain, allowing investigations that go beyond what is possible in human studies. Unfortunately, these techniques still suffer from a lack of dedicated and standardized neuroimaging tools, namely brain templates and descriptive atlases. Here, we present two rat brain MRI templates and their associated gray matter, white matter and cerebrospinal fluid probability maps, generated from ex vivo [Formula: see text]-weighted images (90 µm isotropic resolution) and in vivo T2-weighted images (150 µm isotropic resolution). In association with these templates, we also provide both anatomical and functional 3D brain atlases, respectively derived from the merging of the Waxholm and Tohoku atlases, and analysis of resting-state functional MRI data. Finally, we propose a complete set of preclinical MRI reference resources, compatible with common neuroimaging software, for the investigation of rat brain structures and functions.This work is part of the SIGMA project with the reference FCT-ANR/NEU-OSD/0258/2012, co-financed by the French public funding agency ANR (Agence Nationale pour laRecherche, APP Blanc International II 2012), the Portuguese FCT (Fundação para aCiência e Tecnologia) and the Portuguese North Regional Operational Program (ON.2—O Novo Norte) under the National Strategic Reference Framework (QREN), through theEuropean Regional Development Fund (FEDER) as well as the Projecto Estratégico co-funded by FCT (PEst-C/SAU/LA0026-/2013) and the European Regional DevelopmentFund COMPETE (FCOMP-01-0124-FEDER-037298). D.A.B. and A.N. were funded bygrants from FCT-ANR/NEU-OSD/0258/2012. R.M. was supported by the FCT fellow-ship grant with the reference PDE/BDE/113604/2015 from the PhDiHES program. A.C.was supported by a grant from the foundation NRJ. P.M. was funded by FundaçãoCalouste Gulbenkian (Portugal;‘Better mental health during ageing based on temporalprediction of individual brain ageing trajectories TEMPO’) with Grant Number P-139977. France Life Imaging is acknowledged for its support in funding the NeuroSpinplatform of preclinical MRI scanners. The authors also acknowledge and thank EdwardGanz, MD, for proof reading our work
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