74 research outputs found
Ionization Quenching Factor Measurement of Helium 4
The ionization quenching factor (IQF) is defined as the fraction of energy released by a recoil in a medium through ionization compared with its total kinetic energy. At low energies, in the range of a few keV, the ionization produced in a medium falls rapidly and systematic measurements are needed. We report measurements carried out at such low energies as a function of the pressure in He4 at 350, 700, 1000 and 1300 mbar. In order to produce a nucleus moving with a controlled energy in the detection volume, we have developed an Electron Cyclotron Resonance Ion Source (ECRIS) coupled to an ionization chamber by a differential pumping. The quenching factor of He4 has been measured for the first time down to 1 keV recoil energies. An important deviation with respect to the phenomenological calculations has been found allowing an estimation of the scintillation produced in He4 as a function of pressure. The variation of the IQF as a function of the percentage of isobutane, used as quencher, is also presented
Amyloid Precursor-Like Protein 2 deletion-induced retinal synaptopathy related to congenital stationary night blindness: structural, functional and molecular characteristics.
Amyloid precursor protein knockout mice (APP-KO) have impaired differentiation of amacrine and horizontal cells. APP is part of a gene family and its paralogue amyloid precursor-like protein 2 (APLP2) has both shared as well as distinct expression patterns to APP, including in the retina. Given the impact of APP in the retina we investigated how APLP2 expression affected the retina using APLP2 knockout mice (APLP2-KO).
Using histology, morphometric analysis with noninvasive imaging technique and electron microscopy, we showed that APLP2-KO retina displayed abnormal formation of the outer synaptic layer, accompanied with greatly impaired photoreceptor ribbon synapses in adults. Moreover, APLP2-KO displayed a significant decease in ON-bipolar, rod bipolar and type 2 OFF-cone bipolar cells (36, 21 and 63 %, respectively). Reduction of the number of bipolar cells was accompanied with disrupted dendrites, reduced expression of metabotropic glutamate receptor 6 at the dendritic tips and alteration of axon terminals in the OFF laminae of the inner plexiform layer. In contrast, the APP-KO photoreceptor ribbon synapses and bipolar cells were intact. The APLP2-KO retina displayed numerous phenotypic similarities with the congenital stationary night blindness, a non-progressive retinal degeneration disease characterized by the loss of night vision. The pathological phenotypes in the APLP2-KO mouse correlated to altered transcription of genes involved in pre- and postsynatic structure/function, including CACNA1F, GRM6, TRMP1 and Gα0, and a normal scotopic a-wave electroretinogram amplitude, markedly reduced scotopic electroretinogram b-wave and modestly reduced photopic cone response. This confirmed the impaired function of the photoreceptor ribbon synapses and retinal bipolar cells, as is also observed in congenital stationary night blindness. Since congenital stationary night blindness present at birth, we extended our analysis to retinal differentiation and showed impaired differentiation of different bipolar cell subtypes and an altered temporal sequence of development from OFF to ON laminae in the inner plexiform layer. This was associated with the altered expression patterns of bipolar cell generation and differentiation factors, including MATH3, CHX10, VSX1 and OTX2.
These findings demonstrate that APLP2 couples retina development and synaptic genes and present the first evidence that APLP2 expression may be linked to synaptic disease
Orally Active Multi-Functional Antioxidants Are Neuroprotective in a Rat Model of Light-Induced Retinal Damage
Progression of age-related macular degeneration has been linked to iron dysregulation and oxidative stress that induce apoptosis of neural retinal cells. Since both antioxidants and chelating agents have been reported to reduce the progression of retinal lesions associated with AMD in experimental animals, the present study evaluates the ability of multi-functional antioxidants containing functional groups that can independently chelate redox metals and quench free radicals to protect the retina against light-induced retinal degeneration, a rat model of dry atrophic AMD.Proof of concept studies were conducted to evaluate the ability of 4-(5-hydroxypyrimidin-2-yl)-N,N-dimethyl-3,5-dioxopiperazine-1-sulfonamide (compound 4) and 4-(5-hydroxy-4,6-dimethoxypyrimidin-2-yl)-N,N-dimethyl-3,5-dioxopiperazine-1-sulfonamide (compound 8) to reduce retinal damage in 2-week dark adapted Wistar rats exposed to 1000 lx of light for 3 hours. Assessment of the oxidative stress markers 4- hydroxynonenal and nitrotyrosine modified proteins and Thioredoxin by ELISA and Western blots indicated that these compounds reduced the oxidative insult caused by light exposure. The beneficial antioxidant effects of these compounds in providing significant functional and structural protection were confirmed by electroretinography and quantitative histology of the retina.The present study suggests that multi-functional compounds may be effective candidates for preventive therapy of AMD
Calibration of the surface array of the Pierre Auger Observatory
The Pierre Auger Observatory is designed to study cosmic rays of the highest energies (>eV). The ground array of the Observatory will consist of 1600 water Cherenkov detectors deployed over 3000 km. The remoteness and large number of detectors require a robust, automatic self-calibration procedure. It relies on the measurement of the average charge collected by a photomultiplier tube from the Cherenkov light produced by a vertical and central through-going muon, determined to 5–10% at the detector via a novel rate-based technique and to 3% precision through analysis of histograms of the charge distribution. The parameters needed for the calibration are measured every minute, allowing for an accurate determination of the signals recorded from extensive air showers produced by primary cosmic rays. The method also enables stable and uniform triggering conditions to be achieved
Calibration of the surface array of the Pierre Auger Observatory
The Pierre Auger Observatory is designed to study cosmic rays of the highest energies ð41019 eVÞ. The ground array of the Observatory will consist of 1600 water Cherenkov detectors deployed over 3000 km2. The remoteness and large number of detectors require a robust, automatic self-calibration procedure. It relies on the measurement of the average charge collected by a photomultiplier tube from the Cherenkov light produced by a vertical and central through-going muon, determined to 5–10% at the detector via a novel rate-based technique and to 3% precision through analysis of histograms of the charge distribution. The parameters needed for the calibration are measured every minute, allowing for an accurate determination of the signals recorded from extensive air showers produced by primary cosmic rays. The method also enables stable and uniform triggering conditions to be achieved.Instituto de Física La Plat
Upper Limits from HESS AGN Observations in 2005-2007
AIMS: Very high energy (VHE; E>100 GeV) gamma-ray studies were performed for
18 active galactic nuclei (AGN) from a variety of AGN classes.
METHODS: VHE observations of a sample of 14 AGN, considered candidate VHE
emitters, were made with the High Energy Stereoscopic System (HESS) between
January 2005 and July 2007. Large-zenith-angle observations of three northern
AGN (Mkn 421, Mkn 501, 1ES 1218+304), known to emit VHE gamma rays, were also
performed in order to sample their spectral energy distributions (SEDs) above 1
TeV. In addition, the VHE flux from 1ES 1101-232, previously detected by HESS
in 2004-2005, was monitored during 2006 and 2007.
RESULTS: As significant detections from the HESS observation program are
reported elsewhere, the results reported here are primarily integral flux upper
limits. The average exposure for each of the 14 VHE-candidate AGN is ~7 h live
time, and the observations have an average energy threshold between 230 GeV and
590 GeV. Upper limits for these 14 AGN range from <0.9% to <4.9% of the Crab
Nebula flux, and eight of these are the most constraining ever reported for the
object. The brief (<2.2 h each) large-zenith-angle observations yield upper
limits for Mkn 501 (<20% Crab above 2.5 TeV) and 1ES 1218+304 (<17% Crab above
1.0 TeV), and a marginal detection (3.5 sigma) of Mkn 421 (50% Crab above 2.1
TeV). 1ES 1101-232 was marginally detected (3.6 sigma, 1.7% Crab above 260 GeV)
during the 2006 (13.7 h live time) observations, but not in the 2007 (4.6 h
live time) data. The upper limit in 2007 (<1.9% Crab above 260 GeV) is below
the average flux measured by HESS from 2004-2006.Comment: 8 Pages, 2 Figures; Accepted on Nov 6, 2007 for publication in
Astronomy & Astrophysic
HESS VHE Gamma-Ray Sources Without Identified Counterparts
The detection of gamma rays in the very-high-energy (VHE) energy range (100
GeV--100 TeV) provides a direct view of the parent population of
ultra-relativistic particles found in astrophysical sources. For this reason,
VHE gamma rays are useful for understanding the underlying astrophysical
processes in non-thermal sources. We investigate unidentified VHE gamma-ray
sources that have been discovered with HESS in the most sensitive blind survey
of the Galactic plane at VHE energies conducted so far. The HESS array of
imaging atmospheric Cherenkov telescopes (IACTs) has a high sensitivity
compared with previous instruments(~ 0.01 Crab) in 25 hours observation time
for a 5 sigma point-source detection), and with its large field of view, is
well suited for scan-based observations. The on-going HESS survey of the inner
Galaxy has revealed a large number of new VHE sources, and for each we attempt
to associate the VHE emission with multi-wavelength data in the radio through
X-ray wavebands. For each of the eight unidentified VHE sources considered
here, we present the energy spectra and sky maps of the sources and their
environment. The VHE morphology is compared with available multi-wavelength
data (mainly radio and X-rays). No plausible counterparts are found
Discovery of very high energy gamma-ray emission coincident with molecular clouds in the W28 (G6.4-0.1) field
We observed the W28 field (for ~40 h) at Very High Energy (VHE) gamma-ray
energies (E>0.1 TeV) with the H.E.S.S. Cherenkov telescopes. A reanalysis of
EGRET E>100 MeV data was also undertaken. Results from the NANTEN 4m telescope
Galactic plane survey and other CO observations have been used to study
molecular clouds. We have discovered VHE gamma-ray emission (HESSJ1801-233)
coincident with the northeastern boundary of W28, and a complex of sources
(HESSJ1800-240A, B and C) ~0.5 deg south of W28, in the Galactic disc. The VHE
differential photon spectra are well fit by pure power laws with indices
Gamma~2.3 to 2.7. The NANTEN ^{12}CO(J=1-0) data reveal molecular clouds
positionally associating with the VHE emission, spanning a ~15 km s^{-1} range
in local standard of rest velocity. The VHE/molecular cloud association could
indicate a hadronic origin for HESSJ1801-233 and HESSJ1800-240, and several
cloud components in projection may contribute to the VHE emission. The clouds
have components covering a broad velocity range encompassing the distance
estimates for W28 (~2 kpc), and extending up to ~4 kpc. Assuming a hadronic
origin, and distances of 2 and 4 kpc for cloud components, the required cosmic
ray density enhancement factors (with respect to the solar value) are in the
range ~10 to ~30. If situated at 2 kpc distance, such cosmic ray densities may
be supplied by a SNR like W28. Additionally and/or alternatively, particle
acceleration may come from several catalogued SNRs and SNR candidates, the
energetic ultra compact HII region W28A2, and the HII regions M8 and M20 along
with their associated open clusters. Further sub-mm observations would be
recommended to probe in detail the dynamics of the molecular clouds at
velocites >10 km s^{-1}, and their possible connection to W28.Comment: 10 pages, 4 figures. Accepted for publication in Astronomy &
Astrophysics. (Abstract shortened
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