950 research outputs found
Rank-(n – 1) convexity and quasiconvexity for divergence free fields
The CAST experiment at CERN (European Organization of Nuclear Research)
searches for axions from the sun. The axion is a pseudoscalar particle that was
motivated by theory thirty years ago, with the intention to solve the strong CP
problem. Together with the neutralino, the axion is one of the most promising
dark matter candidates. The CAST experiment has been taking data during the
last two years, setting an upper limit on the coupling of axions to photons
more restrictive than from any other solar axion search in the mass range below
0.1 eV. In 2005 CAST will enter a new experimental phase extending the
sensitivity of the experiment to higher axion masses. The CAST experiment
strongly profits from technology developed for high energy physics and for
X-ray astronomy: A superconducting prototype LHC magnet is used to convert
potential axions to detectable X-rays in the 1-10 keV range via the inverse
Primakoff effect. The most sensitive detector system of CAST is a spin-off from
space technology, a Wolter I type X-ray optics in combination with a prototype
pn-CCD developed for ESA's XMM-Newton mission. As in other rare event searches,
background suppression and a thorough shielding concept is essential to improve
the sensitivity of the experiment to the best possible. In this context CAST
offers the opportunity to study the background of pn-CCDs and its long term
behavior in a terrestrial environment with possible implications for future
space applications. We will present a systematic study of the detector
background of the pn-CCD of CAST based on the data acquired since 2002
including preliminary results of our background simulations.Comment: 11 pages, 8 figures, to appear in Proc. SPIE 5898, UV, X-Ray, and
Gamma-Ray Space Instrumentation for Astronomy XI
Recommended from our members
Use of the ECL-CAMAC trigger processor system for recoil missing mass triggers at the Tagged Photon Spectrometer at Fermilab
A trigger processor in operation since May 1980 at the Tagged Photon Spectrometer at Fermilab will be described. The processor, based on the Fermilab ECL-CAMAC system, allows fast selection of high mass diffractive events from the total hadronic cross section. Data from a recoil detector, consisting of 3 wire chambers and 4 layers of scintillator concentric about a 1.5 m liquid hydrogen target, is digitized and presented to the processor within 3 sec. From the chamber data are found the vertices and angles of all recoiling tracks
First Light Measurements of Capella with the Low Energy Transmission Grating Spectrometer aboard the Chandra X-ray Observatory
We present the first X-ray spectrum obtained by the Low Energy Transmission
Grating Spectrometer (LETGS) aboard the Chandra X-ray Observatory. The spectrum
is of Capella and covers a wavelength range of 5-175 A (2.5-0.07 keV). The
measured wavelength resolution, which is in good agreement with ground
calibration, is 0.06 A (FWHM). Although in-flight
calibration of the LETGS is in progress, the high spectral resolution and
unique wavelength coverage of the LETGS are well demonstrated by the results
from Capella, a coronal source rich in spectral emission lines. While the
primary purpose of this letter is to demonstrate the spectroscopic potential of
the LETGS, we also briefly present some preliminary astrophysical results. We
discuss plasma parameters derived from line ratios in narrow spectral bands,
such as the electron density diagnostics of the He-like triplets of carbon,
nitrogen, and oxygen, as well as resonance scattering of the strong Fe XVII
line at 15.014 A.Comment: 4 pages (ApJ letter LaTeX), 2 PostScript figures, accepted for
publication in ApJ Letters, 200
Analysis of the NGXO Telescope X-Ray Hartmann Data
Next Generation X-Ray Optics (NGXO) team at the Goddard Space Flight Center (GSFC) has been developing a new silicon-based grazing incidence mirror technology for future high resolution x-ray astronomical missions. Recently, the GSFC team completed the construction of first few mirror modules that contain one pair of mirrors. One of the mirror pairs was tested in GSFC 600-m long beamline facility and Panter (Neuried, Germay) 120-m long x-ray beamline facility. Both full aperture x-ray tests, Hartmann tests, and focal plane sweeps were completed. In this paper we present the data analysis process and compare the results from our models to measured x-ray centroid data, x-ray performance data, and out of focus images of the mirror pair
Improving XMM-Newton EPIC pn data at low energies: method and application to the Vela SNR
High quantum efficiency over a broad spectral range is one of the main
properties of the EPIC pn camera on-board XMM-Newton. The quantum efficiency
rises from ~75% at 0.2 keV to ~100% at 1 keV, stays close to 100% until 8 keV,
and is still ~90% at 10 keV. The EPIC pn camera is attached to an X-ray
telescope which has the highest collecting area currently available, in
particular at low energies (more than 1400 cm2 between 0.1 and 2.0 keV). Thus,
this instrument is very sensitive to the low-energy X-ray emission. However,
X-ray data at energies below ~0.2 keV are considerably affected by detector
effects, which become more and more important towards the lowest transmitted
energies. In addition to that, pixels which have received incorrect offsets
during the calculation of the offset map at the beginning of each observation,
show up as bright patches in low-energy images. Here we describe a method which
is not only capable of suppressing the contaminations found at low energies,
but which also improves the data quality throughout the whole EPIC pn spectral
range. This method is then applied to data from the Vela supernova remnant.Comment: Proc. SPIE Vol. 5488: Astronomical Telescopes and Instrumentation, UV
- Gamma-Ray Space Telescope Systems, Eds. Guenther Hasinger and Martin J.
Turner, 22-24 June 2004, Glasgow, Scotland United Kingdo
Toward volume manufacturing of high-performance soft x-ray critical-angle transmission gratings
High-resolution () x-ray absorption and
emission line spectroscopy in the soft x-ray band is a crucial diagnostic for
the exploration of the properties of ubiquitous warm and hot plasmas and their
dynamics in the cosmic web, galaxy clusters, galaxy halos, intragalactic space,
and star atmospheres. Soft x-ray grating spectroscopy with has
been demonstrated with critical-angle transmission (CAT) gratings. CAT gratings
combine the relaxed alignment and temperature tolerances and low mass of
transmission gratings with high diffraction efficiency blazed in high orders.
They are an enabling technology for the proposed Arcus grating explorer and
were selected for the Lynx design reference mission grating spectrometer
instrument. Both Arcus and Lynx require the manufacture of hundreds to perhaps
large-area CAT gratings. We are developing new patterning and
fabrication process sequences that are conducive to large-format volume
processing on state-of-the-art 200 mm wafer tools. Recent x-ray tests on 200
nm-period gratings patterned using e-beam-written masks and 4x projection
lithography in conjunction with silicon pore focusing optics demonstrated at 1.49 keV. Extending the grating depth from 4 m to 6
m is predicted to lead to significant improvements in diffraction
efficiency and is part of our current efforts using a combination of deep
reactive-ion etching and wet etching in KOH solution. We describe our recent
progress in grating fabrication and report our latest diffraction efficiency
and modeling results.Comment: 11 pages, 8 figures, submitted to Proc. SPIE 1144
Measurement of inclusive D*+- and associated dijet cross sections in photoproduction at HERA
Inclusive photoproduction of D*+- mesons has been measured for photon-proton
centre-of-mass energies in the range 130 < W < 280 GeV and a photon virtuality
Q^2 < 1 GeV^2. The data sample used corresponds to an integrated luminosity of
37 pb^-1. Total and differential cross sections as functions of the D*
transverse momentum and pseudorapidity are presented in restricted kinematical
regions and the data are compared with next-to-leading order (NLO) perturbative
QCD calculations using the "massive charm" and "massless charm" schemes. The
measured cross sections are generally above the NLO calculations, in particular
in the forward (proton) direction. The large data sample also allows the study
of dijet production associated with charm. A significant resolved as well as a
direct photon component contribute to the cross section. Leading order QCD
Monte Carlo calculations indicate that the resolved contribution arises from a
significant charm component in the photon. A massive charm NLO parton level
calculation yields lower cross sections compared to the measured results in a
kinematic region where the resolved photon contribution is significant.Comment: 32 pages including 6 figure
Measurement of neutral current e+/-p cross sections at high Bjorken x with the ZEUS detector
The neutral current e+/-p cross section has been measured up to values of
Bjorken x of approximately 1 with the ZEUS detector at HERA using an integrated
luminosity of 187 inv. pb of e-p and 142 inv. pb of e+p collisions at sqrt(s) =
318GeV. Differential cross sections in x and Q2, the exchanged boson
virtuality, are presented for Q2 geq 725GeV2. An improved reconstruction method
and greatly increased amount of data allows a finer binning in the high-x
region of the neutral current cross section and leads to a measurement with
much improved precision compared to a similar earlier analysis. The
measurements are compared to Standard Model expectations based on a variety of
recent parton distribution functions.Comment: 39 pages, 9 figure
Measurement of Jet Shapes in Photoproduction at HERA
The shape of jets produced in quasi-real photon-proton collisions at
centre-of-mass energies in the range GeV has been measured using the
hadronic energy flow. The measurement was done with the ZEUS detector at HERA.
Jets are identified using a cone algorithm in the plane with a
cone radius of one unit. Measured jet shapes both in inclusive jet and dijet
production with transverse energies GeV are presented. The jet
shape broadens as the jet pseudorapidity () increases and narrows
as increases. In dijet photoproduction, the jet shapes have been
measured separately for samples dominated by resolved and by direct processes.
Leading-logarithm parton-shower Monte Carlo calculations of resolved and direct
processes describe well the measured jet shapes except for the inclusive
production of jets with high and low . The observed
broadening of the jet shape as increases is consistent with the
predicted increase in the fraction of final state gluon jets.Comment: 29 pages including 9 figure
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