2,612 research outputs found
Crack detection in a rotating shaft using artificial neural networks and PSD characterisation
Peer reviewedPostprin
A Multi-Moded RF Delay Line Distribution System for the Next Linear Collider
The Delay Line Distribution System (DLDS) is an alternative to conventional
pulse compression, which enhances the peak power of rf sources while matching
the long pulse of those sources to the shorter filling time of accelerator
structures. We present an implementation of this scheme that combines pairs of
parallel delay lines of the system into single lines. The power of several
sources is combined into a single waveguide delay line using a multi-mode
launcher. The output mode of the launcher is determined by the phase coding of
the input signals. The combined power is extracted from the delay line using
mode-selective extractors, each of which extracts a single mode. Hence, the
phase coding of the sources controls the output port of the combined power. The
power is then fed to the local accelerator structures. We present a detailed
design of such a system, including several implementation methods for the
launchers, extractors, and ancillary high power rf components. The system is
designed so that it can handle the 600 MW peak power required by the NLC design
while maintaining high efficiency.Comment: 25 pages, 11 figure
Cepheid limb darkening, angular diameter corrections, and projection factor from static spherical model stellar atmospheres
Context. One challenge for measuring the Hubble constant using Classical
Cepheids is the calibration of the Leavitt Law or period-luminosity
relationship. The Baade-Wesselink method for distance determination to Cepheids
relies on the ratio of the measured radial velocity and pulsation velocity, the
so-called projection factor and the ability to measure the stellar angular
diameters. Aims. We use spherically-symmetric model stellar atmospheres to
explore the dependence of the p-factor and angular diameter corrections as a
function of pulsation period. Methods. Intensity profiles are computed from a
grid of plane-parallel and spherically-symmetric model stellar atmospheres
using the SAtlas code. Projection factors and angular diameter corrections are
determined from these intensity profiles and compared to previous results.
Results. Our predicted geometric period-projection factor relation including
previously published state-of-the-art hydrodynamical predictions is not with
recent observational constraints. We suggest a number of potential resolutions
to this discrepancy. The model atmosphere geometry also affects predictions for
angular diameter corrections used to interpret interferometric observations,
suggesting corrections used in the past underestimated Cepheid angular
diameters by 3 - 5%. Conclusions. While spherically-symmetric hydrostatic model
atmospheres cannot resolve differences between projection factors from theory
and observations, they do help constrain underlying physics that must be
included, including chromospheres and mass loss. The models also predict more
physically-based limb-darkening corrections for interferometric observations.Comment: 8 pages, 6 figures, 2 tables, accepted for publication in A&
Understanding complex magnetic order in disordered cobalt hydroxides through analysis of the local structure
In many ostensibly crystalline materials, unit-cell-based descriptions do not
always capture the complete physics of the system due to disruption in
long-range order. In the series of cobalt hydroxides studied here,
Co(OH)(Cl)(HO), magnetic Bragg diffraction reveals a
fully compensated N\'eel state, yet the materials show significant and open
magnetization loops. A detailed analysis of the local structure defines the
aperiodic arrangement of cobalt coordination polyhedra. Representation of the
structure as a combination of distinct polyhedral motifs explains the existence
of locally uncompensated moments and provides a quantitative agreement with
bulk magnetic measurements and magnetic Bragg diffraction
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Fixation of aqueous tritiated waste in polymer impregnated concrete and in polyacetylene
Tritiated aqueous waste can be used to hydrate hydraulic cement producing a concrete that contains a network of interconnected porosity. Such a product is subject to water intrusion and subsequent tritium loss by leaching and exchange. Techniques have been developed to impregnate this porosity with styrene monomer which is then polymerized in situ, resulting in a concrete that is essentially impermeable while its strength, durability, and resistance to chemical attack are significantly improved. Tritium bulk leach rates as low as 1.68 x 10 g/(cm-day) have been measured for polymer-impregnated concrete in water. An adsorbent additive can be incorporated to increase the specific tritiated waste loading of the concrete. Depending upon the formulations, these composites incorporate 5.8 to 8.3 liters HTO per cubic foot. Tritiated aqueous waste can also be fixed in polymeric form as polyacetylene through reaction with calcium carbide to form tritiated acetylene which is subsequently polymerized. Acetylene polymerization is accomplished by either Co gamma irradiation or thermal catalysis over cupric oxide. The tritiated polyacetylene produced can contain up to 50 atomic percent tritium and is essentially non-leachable, insoluble, thermally stable to 325C, and chemically inert. The bulk leach rate of tritiated polyacetylene was determined to be of the order of 1.8 x 10 g/(cm-day). Radiolysis by Co gamma irradiation results in a G value for hydrogen production of 0.15 molecules/100 eV. Tritiated polyacetylene can be incorporated as an aggregate in concrete, polymer concrete, or polymer impregnated concrete to form a monolithic solid. (auth
The long-period Galactic Cepheid RS Puppis - II. 3D structure and mass of the nebula from VLT/FORS polarimetry
The long-period Cepheid RS Pup is surrounded by a large dusty nebula
reflecting the light from the central star. Due to the changing luminosity of
the central source, light echoes propagate into the nebula. This remarkable
phenomenon was the subject of Paper I.The origin and physical properties of the
nebula are however uncertain: it may have been created through mass loss from
the star itself, or it could be the remnant of a pre-existing interstellar
cloud. Our goal is to determine the 3D structure of the nebula, and estimate
its mass. Knowing the geometrical shape of the nebula will also allow us to
retrieve the distance of RS Pup in an unambiguous manner using a model of its
light echoes (in a forthcoming work). The scattering angle of the Cepheid light
in the circumstellar nebula can be recovered from its degree of linear
polarization. We thus observed the nebula surrounding RS Pup using the
polarimetric imaging mode of the VLT/FORS instrument, and obtained a map of the
degree and position angle of linear polarization. From our FORS observations,
we derive a 3D map of the distribution of the dust, whose overall geometry is
an irregular and thin layer. The nebula does not present a well-defined
symmetry. Using a simple model, we derive a total dust mass of M(dust) = 2.9
+/- 0.9 Msun for the dust within 1.8 arcmin of the Cepheid. This translates
into a total mass of M(gas+dust) = 290 +/- 120 Msun, assuming a dust-to-gas
ratio of 1.0 +/- 0.3 %. The high mass of the dusty nebula excludes that it was
created by mass-loss from the star. However, the thinness nebula is an
indication that the Cepheid participated to its shaping, e.g. through its
radiation pressure or stellar wind. RS Pup therefore appears as a regular
long-period Cepheid located in an exceptionally dense interstellar environment.Comment: 14 pages, 21 figures. Accepted for publication in A&
Liquid Xenon Detectors for Positron Emission Tomography
PET is a functional imaging technique based on detection of annihilation
photons following beta decay producing positrons. In this paper, we present the
concept of a new PET system for preclinical applications consisting of a ring
of twelve time projection chambers filled with liquid xenon viewed by avalanche
photodiodes. Simultaneous measurement of ionization charge and scintillation
light leads to a significant improvement to spatial resolution, image quality,
and sensitivity. Simulated performance shows that an energy resolution of <10%
(FWHM) and a sensitivity of 15% are achievable. First tests with a prototype
TPC indicate position resolution <1 mm (FWHM).Comment: Paper presented at the International Nuclear Physics Conference,
Vancouver, Canada, 201
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