39 research outputs found
A sextupole ion beam guide to improve the efficiency and beam quality at IGISOL
The laser ion source project at the IGISOL facility, Jyvaskyla, has motivated
the development and construction of an rf sextupole ion beam guide (SPIG) to
replace the original skimmer electrode. The SPIG has been tested both off-line
and on-line in proton-induced fission, light-ion and heavy-ion induced
fusion-evaporation reactions and, in each case, has been directly compared to
the skimmer system. For both fission and light-ion induced fusion, the SPIG has
improved the mass-separated ion yields by a factor of typically 4 to 8.
Correspondingly, the transmission efficiency of both systems has been studied
in simulations with and without space charge effects. The transport capacity of
the SPIG has been experimentally determined to be 10^12 ions/s before space
charge effects start to take effect. A direct comparison with the simulation
has been made using data obtained via light-ion fusion evaporation. Both
experiment and simulation show an encouraging agreement as a function of
current extracted from the ion guide.Comment: Latex formatted, submitted to NIM B, 17 pages with 22 .eps figure
Event categories in the EDELWEISS WIMP search experiment
Four categories of events have been identified in the EDELWEISS-I dark matter
experiment using germanium cryogenic detectors measuring simultaneously charge
and heat signals. These categories of events are interpreted as electron and
nuclear interactions occurring in the volume of the detector, and electron and
nuclear interactions occurring close to the surface of the detectors(10-20 mu-m
of the surface). We discuss the hypothesis that low energy surface nuclear
recoils,which seem to have been unnoticed by previous WIMP searches, may
provide an interpretation of the anomalous events recorded by the UKDMC and
Saclay NaI experiments. The present analysis points to the necessity of taking
into account surface nuclear and electron recoil interactions for a reliable
estimate of background rejection factors.Comment: 11 pages, submitted to Phys. Lett.
Bayesian compositional regression with microbiome features via variational inference
The microbiome plays a key role in the health of the human body. Interest often lies in finding features of the microbiome, alongside other covariates, which are associated with a phenotype of interest. One important property of microbiome data, which is often overlooked, is its compositionality as it can only provide information about the relative abundance of its constituting components. Typically, these proportions vary by several orders of magnitude in datasets of high dimensions. To address these challenges we develop a Bayesian hierarchical linear log-contrast model which is estimated by mean field Monte-Carlo co-ordinate ascent variational inference (CAVI-MC) and easily scales to high dimensional data. We use novel priors which account for the large differences in scale and constrained parameter space associated with the compositional covariates. A reversible jump Monte Carlo Markov chain guided by the data through univariate approximations of the variational posterior probability of inclusion, with proposal parameters informed by approximating variational densities via auxiliary parameters, is used to estimate intractable marginal expectations. We demonstrate that our proposed Bayesian method performs favourably against existing frequentist state of the art compositional data analysis methods. We then apply the CAVI-MC to the analysis of real data exploring the relationship of the gut microbiome to body mass index
Bayesian compositional regression with microbiome features via variational inference.
The microbiome plays a key role in the health of the human body. Interest often lies in finding features of the microbiome, alongside other covariates, which are associated with a phenotype of interest. One important property of microbiome data, which is often overlooked, is its compositionality as it can only provide information about the relative abundance of its constituting components. Typically, these proportions vary by several orders of magnitude in datasets of high dimensions. To address these challenges we develop a Bayesian hierarchical linear log-contrast model which is estimated by mean field Monte-Carlo co-ordinate ascent variational inference (CAVI-MC) and easily scales to high dimensional data. We use novel priors which account for the large differences in scale and constrained parameter space associated with the compositional covariates. A reversible jump Monte Carlo Markov chain guided by the data through univariate approximations of the variational posterior probability of inclusion, with proposal parameters informed by approximating variational densities via auxiliary parameters, is used to estimate intractable marginal expectations. We demonstrate that our proposed Bayesian method performs favourably against existing frequentist state of the art compositional data analysis methods. We then apply the CAVI-MC to the analysis of real data exploring the relationship of the gut microbiome to body mass index
The ANTARES Optical Beacon System
ANTARES is a neutrino telescope being deployed in the Mediterranean Sea. It
consists of a three dimensional array of photomultiplier tubes that can detect
the Cherenkov light induced by charged particles produced in the interactions
of neutrinos with the surrounding medium. High angular resolution can be
achieved, in particular when a muon is produced, provided that the Cherenkov
photons are detected with sufficient timing precision. Considerations of the
intrinsic time uncertainties stemming from the transit time spread in the
photomultiplier tubes and the mechanism of transmission of light in sea water
lead to the conclusion that a relative time accuracy of the order of 0.5 ns is
desirable. Accordingly, different time calibration systems have been developed
for the ANTARES telescope. In this article, a system based on Optical Beacons,
a set of external and well-controlled pulsed light sources located throughout
the detector, is described. This calibration system takes into account the
optical properties of sea water, which is used as the detection volume of the
ANTARES telescope. The design, tests, construction and first results of the two
types of beacons, LED and laser-based, are presented.Comment: 21 pages, 18 figures, submitted to Nucl. Instr. and Meth. Phys. Res.
First results of the Instrumentation Line for the deep-sea ANTARES neutrino telescope
In 2005, the ANTARES Collaboration deployed and operated at a depth of 2500 m a so-called Mini Instrumentation Line equipped with Optical Modules (MILOM) at the ANTARES site. The various data acquired during the continuous operation from April to December 2005 of the MILOM confirm the satisfactory performance of the Optical Modules, their front-end electronics and readout system. as well as the calibration devices of the detector. The in situ measurement of the Optical Module time response yields a resolution better than 0.5 ns. The performance of the acoustic positioning system, which enables the spatial reconstruction of the ANTARES detector with a precision of about 10 cm, is verified. These results demonstrate that with the full ANTARES neutrino telescope the design angular resolution of better than 0.3 degrees can be realistically achieved
The data acquisition system for the ANTARES neutrino telescope
The ANTARES neutrino telescope is being constructed in the Mediterranean Sea.
It consists of a large three-dimensional array of photo-multiplier tubes. The
data acquisition system of the detector takes care of the digitisation of the
photo-multiplier tube signals, data transport, data filtering, and data
storage. The detector is operated using a control program interfaced with all
elements. The design and the implementation of the data acquisition system are
described.Comment: 20 pages, 6 figures, accepted for publication in Nucl. Instrum. Meth.
Radar scattering of the ocean surface and sea-surface roughness properties : a combined analysis from dual-polarizations airborne radar observations and models in C-Band
International audienceAn analysis of radar observations in C band combined with models is proposed to study some of the ocean surface properties and their relation with the sea surface backscatter. The electromagnetic part of the models is of different kinds: composite Bragg model with or without including effect of wave breaking zones on the normalized radar cross-section (NRCS), geometrical optics approximation and small-slope approximation model. The surface description is based on the wave spectrum proposed by Kudryavtsev et al. (2003), but tests with the spectrum of Elfouhaily et al. (1997) are also discussed to assess our conclusions. The originality is to use not only the NRCS in HH and VV polarizations, but also their difference in linear units. First, we show that the upwind-to-downwind anisotropy of the radar signal cannot be explained entirely by the modulation of Bragg waves by longer surface waves, but that an additional nonpolarized contribution must be invoked to explain it, consistently with scattering from zones of enhanced roughness associated with breaking waves. Then, combining a composite model and observations in the two polarizations, we assess the contribution of the nonpolarized backscatter on the total NRCS. Finally, the proposed full model, which takes into account the nonpolarized contribution over breaking zones, gives good agreement with the observed polarization ratio and with the NRCS in each polarization. Citation: Mouche, A. A., D. Hauser, and V. Kudryavtsev (2006), Radar scattering of the ocean surface and sea-roughness properties: A combined analysis from dual-polarizations airborne radar observations and models in C band
Use of Dual Polarization Radar Measurements to understand the azimuth behavior of the sea surface backscattered signal
International audienc