1,048 research outputs found
The X-ray Telescope of CAST
The Cern Axion Solar Telescope (CAST) is in operation and taking data since
2003. The main objective of the CAST experiment is to search for a hypothetical
pseudoscalar boson, the axion, which might be produced in the core of the sun.
The basic physics process CAST is based on is the time inverted Primakoff
effect, by which an axion can be converted into a detectable photon in an
external electromagnetic field. The resulting X-ray photons are expected to be
thermally distributed between 1 and 7 keV. The most sensitive detector system
of CAST is a pn-CCD detector combined with a Wolter I type X-ray mirror system.
With the X-ray telescope of CAST a background reduction of more than 2 orders
off magnitude is achieved, such that for the first time the axion photon
coupling constant g_agg can be probed beyond the best astrophysical constraints
g_agg < 1 x 10^-10 GeV^-1.Comment: 19 pages, 25 figures and images, replaced by the revised version
accepted for publication in New Journal of Physic
Origins and composition of fine atmospheric carbonaceous aerosol in the Sierra Nevada Mountains, California
In this paper we report chemically resolved measurements
of organic aerosol (OA) and related tracers during the Biosphere Effects on Aerosols and Photochemistry Experiment (BEARPEX) at the Blodgett Forest Research Station, California from 15 August–10 October 2007. OA contributed the majority of the mass to the fine atmospheric particles and was predominately oxygenated (OOA). The highest concentrations of OA were during sporadic wildfire influence when aged plumes were impacting the site. In
situ measurements of particle phase molecular markers were dominated by secondary compounds and along with gas phase compounds could be categorized into six factors or sources: (1) aged biomass burning emissions and oxidized urban emissions, (2) oxidized urban emissions (3) oxidation products of monoterpene emissions, (4) monoterpene emissions, (5) anthropogenic emissions and (6) local
methyl chavicol emissions and oxidation products. There were multiple biogenic components that contributed to OA at this site whose contributions varied diurnally, seasonally and in response to changing meteorological conditions, e.g. temperature and precipitation events. Concentrations of isoprene oxidation products were larger when temperatures were higher during the first half of the campaign (15 August–12 September) due to more substantial emissions of isoprene and enhanced photochemistry. The oxidation of methyl chavicol, an oxygenated terpene emitted by
ponderosa pine trees, contributed similarly to OA throughout the campaign. In contrast, the abundances of monoterpene oxidation products in the particle phase were greater during the cooler conditions in the latter half of the campaign (13 September–10 October), even though emissions of the precursors were lower, although the mechanism is not known. OA was correlated with the anthropogenic tracers 2-propyl nitrate and carbon monoxide (CO), consistent with previous observations, while being comprised of mostly non-fossil carbon (>75%). The correlation between OA and an anthropogenic tracer does not necessarily identify the source of the carbon as being anthropogenic but instead suggests a coupling between the anthropogenic and biogenic components in the air mass that might be related to the source of the oxidant and/or the aerosol sulfate. Observations of organosulfates of isoprene and α-pinene provided evidence for the likely importance of aerosol sulfate in spite of neutralized aerosol although acidic plumes might have played a role upwind of the site. This is in contrast to laboratory studies where strongly acidic seed aerosols were needed in order to form these compounds. These compounds together represented only a minor fraction (<1%) of the total OA mass, which may be the result of the neutralized aerosol at the site or because only a small number of organosulfates were quantified. The low contribution of organosulfates to total OA suggests that other mechanisms, e.g. NO_x enhancement of oxidant levels, are likely responsible for the majority of the anthropogenic enhancement of biogenic secondary organic aerosol observed at this site
Background studies and shielding effects for the TPC detector of the CAST experiment
Sunset solar axions traversing the intense magnetic field of the CERN Axion
Solar Telescope (CAST) experiment may be detected in a Time Projection Chamber
(TPC) detector, as X-rays signals. These signals could be masked, however, by
the inhomogeneous background of materials in the experimental site. A detailed
analysis, based on the detector characteristics, the background radiation at
the CAST site, simulations and experimental results, has allowed us to design a
shielding which reduces the background level by a factor of ~4 compared to the
detector without shielding, depending on its position, in the energy range
between 1 and 10 keV. Moreover, this shielding has improved the homogeneity of
background measured by the TPC.Comment: 14 pages, 5 figures, accepted in New Journal of Physic
Background study for the pn-CCD detector of CERN Axion Solar Telescope
The CERN Axion Solar Telescope (CAST) experiment searches for axions from the
Sun converted into photons with energies up to around 10 keV via the inverse
Primakoff effect in the high magnetic field of a superconducting Large Hadron
Collider (LHC) prototype magnet. A backside illuminated pn-CCD detector in
conjunction with an X-ray mirror optics is one of the three detectors used in
CAST to register the expected photon signal. Since this signal is very rare and
different background components (environmental gamma radiation, cosmic rays,
intrinsic radioactive impurities in the set-up, ...) entangle it, a detailed
study of the detector background has been undertaken with the aim to understand
and further reduce the background level of the detector. The analysis is based
on measured data taken during the Phase I of CAST and on Monte Carlo
simulations of different background components. This study will show that the
observed background level (at a rate of (8.00+-0.07)10^-5 counts/cm^2/s/keV
between 1 and 7 keV) seems to be dominated by the external gamma background due
to usual activities at the experimental site, while radioactive impurities in
the detector itself and cosmic neutrons could make just smaller contribution.Comment: Comments: 10 pages, 9 figures and images, submitted to Astroparticle
Physic
A hydrodynamically optimized nano-electrospray ionization source and vacuum interface
The coupling of atmospheric pressure ionization (API) sources like electrospray ionization (ESI) to vacuum based applications like mass spectrometry (MS) or ion beam deposition (IBD) is done by differential pumping, starting with a capillary or pinhole inlet. Because of its low ion transfer efficiency the inlet represents a major bottleneck for these applications. Here we present a nano-ESI vacuum interface optimized to exploit the hydrodynamic drag of the background gas for collimation and the reduction of space charge repulsion. Up to a space charge limit of 40 nA we observe 100% current transmission through a capillary with an inlet and show by MS and IBD experiments that the transmitted ion beams are well defined and free of additional contamination compared to a conventional interface. Based on computational fluid dynamics modelling and ion transport simulations, we show how the specific shape enhances the collimation of the ion cloud. Mass selected ion currents in the nanoampere range available further downstream in high vacuum open many perspectives for the efficient use of electrospray ion beam deposition (ES-IBD) as a surface coating method
CAST constraints on the axion-electron coupling
In non-hadronic axion models, which have a tree-level axion-electron
interaction, the Sun produces a strong axion flux by bremsstrahlung, Compton
scattering, and axio-recombination, the "BCA processes." Based on a new
calculation of this flux, including for the first time axio-recombination, we
derive limits on the axion-electron Yukawa coupling g_ae and axion-photon
interaction strength g_ag using the CAST phase-I data (vacuum phase). For m_a <
10 meV/c2 we find g_ag x g_ae< 8.1 x 10^-23 GeV^-1 at 95% CL. We stress that a
next-generation axion helioscope such as the proposed IAXO could push this
sensitivity into a range beyond stellar energy-loss limits and test the
hypothesis that white-dwarf cooling is dominated by axion emission
Atrial fibrillation and treatment changes in cryptogenic stroke patients with an implantable loop recorder for continuous cardiac rhythm monitoring
Introduction: This interim analysis evaluates the risk profile and incidence of atrial fibrillation (AF) in patients who underwent continuous monitoring with an implantable loop recorder (ILR) for cryptogenic (unexplained) stroke or transient ischemic attack (TIA). Methods: The observational INSIGHT XT study prospectively enrolled patients who received an ILR with dedicated diagnostics for atrial fibrillation, irrespective of the clinical indication. Of 1002 patients enrolled in the study between Aug 2008 and Jan 2012, 121 received the ILR to evaluate cryptogenic stroke or TIA. The definition of cryptogenic stroke/TIA was at the investigators' appraisal and no unified approach to patient work-up was required. This analysis includes 74 patients with cryptogenic stroke or TIA for whom at least one follow-up visit was available at the time of interim analysis. Results: The mean age was 63±12 (50% female). Stroke was the index event in 46 of 74 (62%) of patients. 61% had hypertension, 14% diabetes, and none had heart failure. The mean CHADS2 score was 3.0±0.8 and the mean CHA2DS2VASc score 4.0±1.2. Most patients (72%) had no prior symptoms or cardiac rhythm disturbances, whereas 18% had a history of prior palpitations. Sixty-seven patients were taking antiplatelet medication and four were on oral anticoagulation (OAC) at enrollment. During a median follow up of 12 months (IQR 7 to 18) AF was reported in 17 patients (23%) and two patients were started on OAC and 10 patients were converted from antiplatelets to OAC. Five patients experienced a stroke or TIA (median time to event 1.2 months), of which one patient died. Three of the patients with stroke or TIA had AF detected prior to the recurrent event. Conclusion: Continuous monitoring with an ILR in patients with cryptogenic stroke of TIA detects a high proportion of AF; this can be attributed to longer continuous monitoring in this study. These patients have high CHADS2/CHA2DS2VASc scores; documenting AF in these cases may therefore be clinically relevant in order to decide appropriate treatment
Emission factor ratios, SOA mass yields, and the impact of vehicular emissions on SOA formation
The underprediction of ambient secondary organic aerosol (SOA) levels by current atmospheric models in urban areas is well established, yet the cause of this underprediction remains elusive. Likewise, the relative contribution of emissions from gasoline- and diesel-fueled vehicles to the formation of SOA is generally unresolved. We investigate the source of these two discrepancies using data from the 2010 CalNex experiment carried out in the Los Angeles Basin (Ryerson et al., 2013). Specifically, we use gas-phase organic mass (GPOM) and CO emission factors in conjunction with measured enhancements in oxygenated organic aerosol (OOA) relative to CO to quantify the significant lack of closure between expected and observed organic aerosol concentrations attributable to fossil-fuel emissions. Two possible conclusions emerge from the analysis to yield consistency with the ambient data: (1) vehicular emissions are not a dominant source of anthropogenic fossil SOA in the Los Angeles Basin, or (2) the ambient SOA mass yields used to determine the SOA formation potential of vehicular emissions are substantially higher than those derived from laboratory chamber studies
Search for solar axion emission from 7Li and D(p,gamma)3He nuclear decays with the CAST gamma-ray calorimeter
We present the results of a search for a high-energy axion emission signal
from 7Li (0.478 MeV) and D(p,gamma)3He (5.5 MeV) nuclear transitions using a
low-background gamma-ray calorimeter during Phase I of the CAST experiment.
These so-called "hadronic axions" could provide a solution to the long-standing
strong-CP problem and can be emitted from the solar core from nuclear M1
transitions. This is the first such search for high-energy pseudoscalar bosons
with couplings to nucleons conducted using a helioscope approach. No excess
signal above background was found.Comment: 20 pages, 8 figures, final version to be published in JCA
The CAST Time Projection Chamber
One of the three X-ray detectors of the CAST experiment searching for solar
axions is a Time Projection Chamber (TPC) with a multi-wire proportional
counter (MWPC) as a readout structure. Its design has been optimized to provide
high sensitivity to the detection of the low intensity X-ray signal expected in
the CAST experiment. A low hardware threshold of 0.8 keV is safely set during
normal data taking periods, and the overall efficiency for the detection of
photons coming from conversion of solar axions is 62 %. Shielding has been
installed around the detector, lowering the background level to 4.10 x 10^-5
counts/cm^2/s/keV between 1 and 10 keV. During phase I of the CAST experiment
the TPC has provided robust and stable operation, thus contributing with a
competitive result to the overall CAST limit on axion-photon coupling and mass.Comment: 19 pages, 11 figures and images, submitted to New Journal of Physic
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