40 research outputs found
A low background Micromegas detector for the CAST experiment
A low background Micromegas detector has been operating on the CAST
experiment at CERN for the search of solar axions during the first phase of the
experiment (2002-2004). The detector operated efficiently and achieved a very
low level of background rejection ( counts
keVcms) thanks to its good spatial and energy resolution
as well as the low radioactivity materials used in the construction of the
detector. For the second phase of the experiment (2005-2007), the detector will
be upgraded by adding a shielding and including focusing optics. These
improvements should allow for a background rejection better than two orders of
magnitude.Comment: 6 pages, 3 figures To appear on the proceedings of the 9th ICATPP
Conference on AStroparticle, Particle, Space Physics, Detectors and Medical
Physics Application
Solar axion search with the CAST experiment
The CAST (CERN Axion Solar Telescope) experiment is searching for solar
axions by their conversion into photons inside the magnet pipe of an LHC
dipole. The analysis of the data recorded during the first phase of the
experiment with vacuum in the magnet pipes has resulted in the most restrictive
experimental limit on the coupling constant of axions to photons. In the second
phase, CAST is operating with a buffer gas inside the magnet pipes in order to
extent the sensitivity of the experiment to higher axion masses. We will
present the first results on the data taking as well as the
system upgrades that have been operated in the last year in order to adapt the
experiment for the data taking. Expected sensitivities on the
coupling constant of axions to photons will be given for the recent run just started in March 2008.Comment: Proceedings of the ICHEP 2008 conferenc
Search for low Energy solar Axions with CAST
We have started the development of a detector system, sensitive to single
photons in the eV energy range, to be suitably coupled to one of the CAST
magnet ports. This system should open to CAST a window on possible detection of
low energy Axion Like Particles emitted by the sun. Preliminary tests have
involved a cooled photomultiplier tube coupled to the CAST magnet via a
Galileian telescope and a switched 40 m long optical fiber. This system has
reached the limit background level of the detector alone in ideal conditions,
and two solar tracking runs have been performed with it at CAST. Such a
measurement has never been done before with an axion helioscope. We will
present results from these runs and briefly discuss future detector
developments.Comment: Paper submitted to the proceedings of the "4th Patras Workshop on
Axions, WIMPs and WISPs", DESY, Hamburg Site - Germany, 18-21 June 2008.
Author affiliations are reported on the title page of the paper. In version
2: 1 affiliation change, 3 references adde
NF-κB2 signalling in enteroids modulates enterocyte responses to secreted factors from bone marrow-derived dendritic cells
Alternative pathway NF-κB signalling regulates susceptibility towards developing inflammatory bowel disease (IBD), colitis-associated cancer and sepsis-associated intestinal epithelial cell apoptosis and shedding. However, the cell populations responsible for the perturbed alternative pathway NF-κB signalling in intestinal mucosal pathology remain unclear. In order to investigate the contribution of the epithelial compartment, we have tested whether NF-κB2 regulated transcription in intestinal epithelial cells controls the intestinal epithelial response to cytokines that are known to disrupt intestinal barrier permeability. Enteroids were generated from the proximal, middle and distal regions of small intestine (SI) from C57BL/6J wild-type mice and displayed region-specific morphology that was maintained during sub-culture. Enteroids treated with 100 ng/mL TNF were compared with corresponding regions of SI from C57BL/6J mice treated systemically with 0.33 mg/kg TNF for 1.5 h. TNF-induced apoptosis in all regions of the intestine in vitro and in vivo but resulted in Paneth cell degranulation only in proximal tissue-derived SI and enteroids. TNF also resulted in increased enteroid sphericity (quantified as circularity from two-dimensional bright field images). This response was dose and time-dependent and correlated with active caspase-3 immunopositivity. Proximal tissue-derived enteroids generated from Nfκb2−/− mice showed a significantly blunted circularity response following the addition of TNF, IFNγ, lipopolysaccharide (LPS) activated C57BL/6J-derived bone marrow-derived dendritic cells (BMDC) and secreted factors from LPS-activated BMDCs. However, Nfκb1−/− mouse-derived enteroids showed no significant changes in response to these stimuli. In conclusion, the selection of SI region is important when designing enteroid studies as region-specific identity and response to stimuli such as TNF are maintained in culture. Intestinal epithelial cells are at least partially responsible for regulating their own fate by modulating NF-κB2 signalling in response to stimuli known to be involved in multiple intestinal and systemic diseases. Future studies are warranted to investigate the therapeutic potential of intestinal epithelial NF-κB2 inhibition
Probing eV-scale axions with CAST
We have searched for solar axions or other pseudoscalar particles that couple
to two photons by using the CERN Axion Solar Telescope (CAST) setup. Whereas we
previously have reported results from CAST with evacuated magnet bores (Phase
I), setting limits on lower mass axions, here we report results from CAST where
the magnet bores were filled with \hefour gas (Phase II) of variable pressure.
The introduction of gas generated a refractive photon mass , thereby
achieving the maximum possible conversion rate for those axion masses \ma that
match . With 160 different pressure settings we have scanned \ma up
to about 0.4 eV, taking approximately 2 h of data for each setting. From the
absence of excess X-rays when the magnet was pointing to the Sun, we set a
typical upper limit on the axion-photon coupling of \gag\lesssim 2.17\times
10^{-10} {\rm GeV}^{-1} at 95% CL for \ma \lesssim 0.4 eV, the exact result
depending on the pressure setting. The excluded parameter range covers
realistic axion models with a Peccei-Quinn scale in the neighborhood of GeV. Currently in the second part of CAST Phase II, we are
searching for axions with masses up to about 1.2 eV using \hethree as a buffer
gas.Comment: 18 pages, 7 figures. Revised version of the paper after referee's
comments. Main changes on the gas sectio
Soft x-ray optical constants of sputtered chromium thin films with improved accuracy in the L and M absorption edge regions
In this study, we determine with improved accuracy the complex index of refraction n = 1 - δ + iβ of sputtered chromium thin films for photon energies ranging from 25 eV to 813 eV. These data include the first absolute measurements of the absorption fine structure near the Cr-L edge. First, we verified by combining Rutherford Backscattering Spectrometry and grazing-incidence x-ray reflectometry that the sputtered thin films were pure Cr with a density consistent with tabulated values. Then, we demonstrated that the Cr surface oxide layer remains stable when the samples are exposed to air for up to 4 years. The Cr absorption coefficient β was determined from the transmittance of freestanding Cr thin films with various thicknesses, measured at the ALS synchrotron radiation source. A model is proposed to correct the transmittance data from the spectral contamination of the source. Finally, we used the new β values, combined with theoretical and tabulated data from the literature, in order to calculate the δ values by the Kramers-Kronig relation. The improvement in the accuracy of β values is demonstrated by the f-sum rule. An additional validation of the new Cr optical constants (δ, β) is performed by comparing the simulated and experimental reflectance of a Cr/B4C multilayer mirror near the Cr-L2,3 edge
Coherence Properties of Individual Femtosecond Pulses of an X-ray Free-Electron Laser
Measurements of the spatial and temporal coherence of single, femtosecond
x-ray pulses generated by the first hard x-ray free-electron laser (FEL), the
Linac Coherent Light Source (LCLS), are presented. Single shot measurements
were performed at 780 eV x-ray photon energy using apertures containing double
pinholes in "diffract and destroy" mode. We determined a coherence length of 17
micrometers in the vertical direction, which is approximately the size of the
focused LCLS beam in the same direction. The analysis of the diffraction
patterns produced by the pinholes with the largest separation yields an
estimate of the temporal coherence time of 0.6 fs. We find that the total
degree of transverse coherence is 56% and that the x-ray pulses are adequately
described by two transverse coherent modes in each direction. This leads us to
the conclusion that 78% of the total power is contained in the dominant mode.Comment: 6 pages, 4 figure