2,775 research outputs found
Experimental study of a liquid Xenon PET prototype module
A detector using liquid Xenon in the scintillation mode is studied for
Positron Emission Tomography (PET). The specific design aims at taking full
advantage of the liquid Xenon properties. It does feature a promising
insensitive to any parallax effect. This work reports on the performances of
the first LXe prototype module, equipped with a position sensitive PMT
operating in the VUV range (178 nm).Comment: Proc. of the 7th International Workshops on Radiation Imaging
Detectors (IWORID-7), Grenoble, France 4-7 July 200
Do Lognormal Column-Density Distributions in Molecular Clouds Imply Supersonic Turbulence?
Recent observations of column densities in molecular clouds find lognormal
distributions with power-law high-density tails. These results are often
interpreted as indications that supersonic turbulence dominates the dynamics of
the observed clouds. We calculate and present the column-density distributions
of three clouds, modeled with very different techniques, none of which is
dominated by supersonic turbulence. The first star-forming cloud is simulated
using smoothed particle hydrodynamics (SPH); in this case gravity, opposed only
by thermal-pressure forces, drives the evolution. The second cloud is
magnetically subcritical with subsonic turbulence, simulated using nonideal
MHD; in this case the evolution is due to gravitationally-driven ambipolar
diffusion. The third cloud is isothermal, self-gravitating, and has a smooth
density distribution analytically approximated with a uniform inner region and
an r^-2 profile at larger radii. We show that in all three cases the
column-density distributions are lognormal. Power-law tails develop only at
late times (or, in the case of the smooth analytic profile, for strongly
centrally concentrated configurations), when gravity dominates all opposing
forces. It therefore follows that lognormal column-density distributions are
generic features of diverse model clouds, and should not be interpreted as
being a consequence of supersonic turbulence.Comment: 6 pages, 6 figures, accepted for publication in MNRA
Global well-posedness of the KP-I initial-value problem in the energy space
We prove that the KP-I initial value problem is globally well-posed in the
natural energy space of the equation
Design of High Dynamic Range Digital to Analog Converters for the Calibration of the CALICE Si-W Ecal readout electronics
The ILC ECAL front-end chip will integrate many functions of the readout electronics including a DAC dedicated to calibration. We present two versions of DAC with respectively 12 and 14 bits, designed in a CMOS 0.35μm process. Both are based on segmented arrays of switched capacitors controlled by a Dynamic Element Matching (DEM) algorithm. A full differential architecture is used, and the amplifiers can be turned into a standby mode reducing the power dissipation. The 12 bit DAC features an INL lower than 0.3 LSB at 5MHz, and dissipates less than 7mW. The 14 bit DAC is an improved version of the 12 bit design
Finite size effects near the onset of the oscillatory instability
A system of two complex Ginzburg - Landau equations is considered that applies at the onset of the oscillatory instability in spatial domains whose size is large (but finite) in one direction; the dependent variables are the slowly modulated complex amplitudes of two counterpropagating wavetrains. In order to obtain a well posed problem, four boundary conditions must be imposed at the boundaries. Two of them were already known, and the other two are first derived in this paper. In the generic case when the group velocity is of order unity, the resulting problem has terms that are not of the same order of magnitude. This fact allows us to consider two distinguished limits and to derive two associated (simpler) sub-models, that are briefly discussed. Our results predict quite a rich variety of complex dynamics that is due to both the modulational instability and finite size effects
Experimental results and first 22Na source image reconstruction by two prototype modules in coincidence of a liquid xenon positron emission tomograph for small animal imaging
International audienceA detector with a very specific design using liquid Xenon (LXe) in the scintillation mode is studied for Positron Emission Tomography (PET) of small animals. Two prototype modules equipped with Position Sensitive Photo Multiplier Tubes (PSPMTs) operating in the VUV range (178 nm) and at 165 K were built and studied in coincidence. This paper reports on energy, time and spatial resolution capabilities of this experimental test bench. Furthermore, these experimental results were used to perform the first image reconstruction of a 22Na source placed in the experimental setup
Polarization of Broad Absorption Line QSOs I. A Spectropolarimetric Atlas
We present a spectropolarimetric survey of 36 broad absorption line
quasi-stellar objects (BAL QSOs). The continuum, absorption trough, and
emission line polarization of BAL QSOs yield clues about their structure. We
confirm that BAL QSOs are in general more highly polarized than non-BAL QSOs,
consistent with a more equatorial viewing direction for the former than the
latter. We have identified two new highly-polarized QSOs in our sample
(1232+1325 and 1333+2840). The polarization rises weakly to the blue in most
objects, perhaps due to scattering and absorption by dust particles. We find
that a polarization increase in the BAL troughs is a general property of
polarized BAL QSOs, indicating an excess of scattered light relative to direct
light, and consistent with the unification of BAL QSOs and non-BAL QSOs. We
have also discovered evidence of resonantly scattered photons in the red wing
of the C IV broad emission lines of a few objects. In most cases, the broad
emission lines have lower polarization and a different position angle than the
continuum. The polarization characteristics of low-ionization BAL QSOs are
similar to those of high-ionization BAL QSOs, suggesting a similar BAL wind
geometry.Comment: 39 pages, 6 figures (20 .gif files), accepted for publication in The
Astrophysical Journal Supplement
Gravitational radiation reaction in compact binary systems: Contribution of the quadrupole-monopole interaction
The radiation reaction in compact spinning binaries on eccentric orbits due
to the quadrupole-monopole interaction is studied. This contribution is of
second post-Newtonian order. As result of the precession of spins the magnitude
of the orbital angular momentum is not conserved. Therefore a proper
characterization of the perturbed radial motion is provided by the energy
and angular average . As powerful computing tools, the generalized
true and eccentric anomaly parametrizations are introduced. Then the secular
losses in energy and magnitude of orbital angular momentum together with the
secular evolution of the relative orientations of the orbital angular momentum
and spins are found for eccentric orbits by use of the residue theorem. The
circular orbit limit of the energy loss agrees with Poisson's earlier result.Comment: accepted for publication in Phys. Rev.
Valence band offset of the ZnO/AlN heterojunction determined by X-ray photoemission spectroscopy
The valence band offset of ZnO/AlN heterojunctions is determined by high resolution x-ray photoemission spectroscopy. The valence band of ZnO is found to be 0.43±0.17 eV below that of AlN. Together with the resulting conduction band offset of 3.29±0.20 eV, this indicates that a type-II (staggered) band line up exists at the ZnO/AlN heterojunction. Using the III-nitride band offsets and the transitivity rule, the valence band offsets for ZnO/GaN and ZnO/InN heterojunctions are derived as 1.37 and 1.95 eV, respectively, significantly higher than the previously determined values
Instability of two interacting, quasi-monochromatic waves in shallow water
We study the nonlinear interactions of waves with a doubled-peaked power
spectrum in shallow water. The starting point is the prototypical equation for
nonlinear uni-directional waves in shallow water, i.e. the Korteweg de Vries
equation. Using a multiple-scale technique two defocusing coupled Nonlinear
Schr\"odinger equations are derived. We show analytically that plane wave
solutions of such a system can be unstable to small perturbations. This
surprising result suggests the existence of a new energy exchange mechanism
which could influence the behaviour of ocean waves in shallow water.Comment: 4 pages, 2 figure
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