355 research outputs found
Scattering of positronium by H, He, Ne, and Ar
The low-energy scattering of ortho positronium (Ps) by H, He, Ne, and Ar
atoms has been investigated in the coupled-channel framework by using a
recently proposed time-reversal-symmetric nonlocal electron-exchange model
potential with a single parameter . For H and He we use a three-Ps-state
coupled-channel model and for Ar and Ne we use a static-exchange model. The
sensitivity of the results is studied with respect to the parameter .
Present low-energy cross sections for He, Ne and Ar are in good agreement with
experiment.Comment: 8 pages Latex with 4 postscript figure
Complete Break Up of Ortho Positronium (Ps)- Hydrogenic ion System
The dynamics of the complete breakup process in an Ortho Ps - He+ system
including electron loss to the continuum (ELC) is studied where both the
projectile and the target get ionized. The process is essentially a four body
problem and the present model takes account of the two centre effect on the
electron ejected from the Ps atom which is crucial for a proper description of
the ELC phenomena. The calculations are performed in the framework of Coulomb
Distorted Eikonal Approximation. The exchange effect between the target and the
projectile electron is taken into account in a consistent manner. The proper
asymptotic 3-body boundary condition for this ionization process is also
satisfied in the present model. A distinct broad ELC peak is noted in the fully
differential cross sections (5DCS) for the Ps electron corroborating
qualitatively the experiment for the Ps - He system. Both the dynamics of the
ELC from the Ps and the ejected electron from the target He+ in the FDCS are
studied using coplanar geometry. Interesting features are noted in the FDCS for
both the electrons belonging to the target and the projectile.Comment: 14 pages,7 figure
Cross-talk between GABAergic postsynapse and microglia regulate synapse loss after brain ischemia.
Microglia interact with neurons to facilitate synapse plasticity; however, signal(s) contributing to microglia activation for synapse elimination in pathology are not fully understood. Here, using in vitro organotypic hippocampal slice cultures and transient middle cerebral artery occlusion (MCAO) in genetically engineered mice in vivo, we report that at 24 hours after ischemia, microglia release brain-derived neurotrophic factor (BDNF) to downregulate glutamatergic and GABAergic synapses within the peri-infarct area. Analysis of the cornu ammonis 1 (CA1) in vitro shows that proBDNF and mBDNF downregulate glutamatergic dendritic spines and gephyrin scaffold stability through p75 neurotrophin receptor (p75 <sup>NTR</sup> ) and tropomyosin receptor kinase B (TrkB) receptors, respectively. After MCAO, we report that in the peri-infarct area and in the corresponding contralateral hemisphere, similar neuroplasticity occurs through microglia activation and gephyrin phosphorylation at serine-268 and serine-270 in vivo. Targeted deletion of the Bdnf gene in microglia or GphnS268A/S270A (phospho-null) point mutations protects against ischemic brain damage, neuroinflammation, and synapse downregulation after MCAO
Cherenkov radiation emitted by ultrafast laser pulses and the generation of coherent polaritons
We report on the generation of coherent phonon polaritons in ZnTe, GaP and
LiTaO using ultrafast optical pulses. These polaritons are coupled modes
consisting of mostly far-infrared radiation and a small phonon component, which
are excited through nonlinear optical processes involving the Raman and the
second-order susceptibilities (difference frequency generation). We probe their
associated hybrid vibrational-electric field, in the THz range, by
electro-optic sampling methods. The measured field patterns agree very well
with calculations for the field due to a distribution of dipoles that follows
the shape and moves with the group velocity of the optical pulses. For a
tightly focused pulse, the pattern is identical to that of classical Cherenkov
radiation by a moving dipole. Results for other shapes and, in particular, for
the planar and transient-grating geometries, are accounted for by a convolution
of the Cherenkov field due to a point dipole with the function describing the
slowly-varying intensity of the pulse. Hence, polariton fields resulting from
pulses of arbitrary shape can be described quantitatively in terms of
expressions for the Cherenkov radiation emitted by an extended source. Using
the Cherenkov approach, we recover the phase-matching conditions that lead to
the selection of specific polariton wavevectors in the planar and transient
grating geometry as well as the Cherenkov angle itself. The formalism can be
easily extended to media exhibiting dispersion in the THz range. Calculations
and experimental data for point-like and planar sources reveal significant
differences between the so-called superluminal and subluminal cases where the
group velocity of the optical pulses is, respectively, above and below the
highest phase velocity in the infrared.Comment: 13 pages, 11 figure
Big GABA II: Water-referenced edited MR spectroscopy at 25 research sites
Accurate and reliable quantification of brain metabolites measured in vivo using 1H magnetic resonance spectroscopy (MRS) is a topic of continued interest. Aside from differences in the basic approach to quantification, the quantification of metabolite data acquired at different sites and on different platforms poses an additional methodological challenge. In this study, spectrally edited γ-aminobutyric acid (GABA) MRS data were analyzed and GABA levels were quantified relative to an internal tissue water reference. Data from 284 volunteers scanned across 25 research sites were collected using GABA+ (GABA + co-edited macromolecules (MM)) and MM-suppressed GABA editing. The unsuppressed water signal from the volume of interest was acquired for concentration referencing. Whole-brain T1-weighted structural images were acquired and segmented to determine gray matter, white matter and cerebrospinal fluid voxel tissue fractions. Water-referenced GABA measurements were fully corrected for tissue-dependent signal relaxation and water visibility effects. The cohort-wide coefficient of variation was 17% for the GABA + data and 29% for the MM-suppressed GABA data. The mean within-site coefficient of variation was 10% for the GABA + data and 19% for the MM-suppressed GABA data. Vendor differences contributed 53% to the total variance in the GABA + data, while the remaining variance was attributed to site- (11%) and participant-level (36%) effects. For the MM-suppressed data, 54% of the variance was attributed to site differences, while the remaining 46% was attributed to participant differences. Results from an exploratory analysis suggested that the vendor differences were related to the unsuppressed water signal acquisition. Discounting the observed vendor-specific effects, water-referenced GABA measurements exhibit similar levels of variance to creatine-referenced GABA measurements. It is concluded that quantification using internal tissue water referencing is a viable and reliable method for the quantification of in vivo GABA levels
The burden of unintentional drowning: Global, regional and national estimates of mortality from the Global Burden of Disease 2017 Study
__Background:__ Drowning is a leading cause of injury-related mortality globally. Unintentional drowning (International Classification of Diseases (ICD) 10 codes W65-74 and ICD9 E910) is one of the 30 mutually exclusive and collectively exhaustive causes of injury-related mortality in the Global Burden of Disease (GBD) study. This study's objective is to describe unintentional drowning using GBD estimates from 1990 to 2017.
__Methods:__ Unintentional drowning from GBD 2017 was estimated for cause-specific mortality and years of life lost (YLLs), age, sex, country, region, Socio-demographic Index (SDI) quintile, and trends from 1990 to 2017. GBD 2017 used standard GBD methods for estimating mortality from drowning.
__Results:__ Globally, unintentional drowning mortality decreased by 44.5% between 1990 and 2017, from 531 956 (uncertainty interval (UI): 484 107 to 572 854) to 295 210 (284 493 to 306 187) deaths. Global age-standardised mortality rates decreased 57.4%, from 9.3 (8.5 to 10.0) in 1990 to 4.0 (3.8 to 4.1) per 100 000 per annum in 2017. Unintentional drowning-associated mortality was generally higher in children, males and in low-SDI to middle-SDI countries. China, India, Pakistan and Bangladesh accounted for 51.2% of all drowning deaths in 2017. Oceania was the region with the highest rate of age-standardised YLLs in 2017, with 45 434 (40 850 to 50 539) YLLs per 100 000 across both sexes.
__Conclusions:__ There has been a decline in global drowning rates. This study shows that the decline was not consistent across countries. The results reinforce the need for continued and improved policy, prevention and research efforts, with a focus on low-and middle-income countries
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