14,853 research outputs found
An omnidirectional retroreflector based on the transmutation of dielectric singularities
In the field of transformation optics, metamaterials mimic the effect of
coordinate transformations on electromagnetic waves, creating the illusion that
the waves are propagating through a virtual space. Transforming space by
appropriately designed materials makes devices possible that have been deemed
impossible. In particular, transformation optics has led to the demonstration
of invisibility cloaking for microwaves, surface plasmons and infrared light.
Here we report the achievement of another "impossible task". We implement, for
microwaves, a device that would normally require a dielectric singularity, an
infinity in the refractive index. We transmute a singularity in virtual space
into a mere topological defect in a real metamaterial. In particular, we
demonstrate an omnidirectional retroreflector, a device for faithfully
reflecting images and for creating high visibility, from all directions. Our
method is robust, potentially broadband and similar techniques could be applied
for visible light
Solar modulation of cosmic ray intensity and solar flare events inferred from (14)C contents in dated tree rings
The delta 14C values in 42 rings of a white spruce grown in Mackenzie Delta was measured as a continuing effort of tracing the history of solar modulation of cosmic ray intensity. The delta 14C values in six rings were measured, in search of a 14C increase due to two large solar flares that occurred in 1942. The results are presented
Exposure to violence and PTSD symptoms among Somali women
Posttraumatic stress disorder (PTSD) symptoms, exposure to traumatic stressors, and health care utilization were examined in 84 women attending a primary health care clinic in Mogadishu, Somalia. The Somalia-Posttraumatic Diagnostic Scale was used in this active warzone to measure symptoms. Nearly all women reported high levels of confrontations with violence; half described being exposed to a potentially traumatizing event. Nearly one third had significant PTSD symptoms. Compared to those who did not, women who reported exposure to a traumatic stressor reported more confrontations with violence (7.1 vs. 3.3; p < . 001), health complaints (3.8 vs. 2.9; p = .03), and nearly 3 times as much (p = .03) health service utilization. A potentially traumatizing event was found to be a simplified proxy for assessing mental health distress in women attending a primary health care facility in highly insecure, unpredictable, resource-limited settings
Outcomes of a remote, decentralized health center-based HIV/AIDS antiretroviral program in Zambia, 2003 to 2007
A cross-sectional study of patients living with HIV/ AIDS treated during 2003 to 2007 in decentralized, rural health centers in Zambia was performed to measure virological outcomes after 12 months of antiretroviral therapy and identify factors associated with virological failure. Data from 228 patients who started antiretroviral therapy >12 months prior were analyzed. In all, 93% received stavudine + lamivudine + nevirapine regimens, and median antiretroviral therapy duration was 23.5 months (interquartile range 20-28). Of the 205 patients tested for viral load, 177 (86%) had viral load <1000 copies/mL. Probability of developing virological failure (viral load >1000 copies/mL) was 8.9% at 24 months and 19.6% at 32 months. Predictors for virological failure were <100% adherence, body mass index <18.5 kg/m(2), and women <40 years old. Of those with virological failure who underwent 3 to 6 months of intensive adherence counseling, 45% obtained virological success. In a remote, resource-limited setting in decentralized health centers, virological and immunological assessments of patients on antiretroviral therapy >12 months showed that positive health outcomes are achievable
Powerful High Velocity-Dispersion Molecular Hydrogen Associated with an Intergalactic Shock Wave in Stephan's Quintet
We present the discovery of strong mid-infrared emission lines of molecular
hydrogen of apparently high velocity dispersion (~870 km/s) originating from a
group-wide shock wave in Stephan's Quintet. These Spitzer Space Telescope
observations reveal emission lines of molecular hydrogen and little else. this
is the first time an almost pure H_2 line spectrum has been seen in an
extragalactic object. Along with the absence of PAH features and very low
excitation ionized gas tracers, the spectra resemble shocked gas seen in
Galactic supernova remnants, but on a vast scale. The molecular emission
extends over 24 kpc along the X-ray emitting shock-front, but has ten times the
surface luminosity as the soft X-rays, and about one-third the surface
luminosity of the IR continuum. We suggest that the powerful H_2 emission is
generated by the shock wave caused when a high-velocity intruder galaxy
collides with filaments of gas in the galaxy group. Our observations suggest a
close connection between galaxy-scale shock-waves and strong broad H_2 emission
lines, like those seen in the spectra of Ultraluminous Infrared Galaxies where
high-speed collisions between galaxy disks are common.Comment: 4 pages, 4 figures and 1 tabl
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