155 research outputs found
MPGD's spatial and energy resolution studies with an adjustable point-like electron source
11th Vienna Conference on Instrumentation (February 2007) , to appear in the Proceedings (NIM A)International audienceMicropattern Gaseous Detectors (MPGD), like Micromegas or GEM, are used or foreseen in particle physics experiments for which a very good spatial resolution is required. We have developed an experimental method to separate the contribution of the transverse diffusion and the multiplication process by varying the number of primary electrons generated by a point-like source. A pulsed nitrogen laser is focused by an optical set-up on the drift electrode which is made of a thin metal layer deposited on a quartz lamina. The number of primary electrons can be adjusted from a few to several thousands on a spot which transverse size is less than RMS. The detector can be positioned with an accuracy of by a motorized three dimensional system. This method was applied to a small Micromegas detector with a gain set between and and an injection of 60 to 2000 photoelectrons. Spatial resolutions as small as were measured with 2000 primary electrons. An estimation of the upper limit of the relative gain variance can be obtained from the measurements
Sub-Riemannian Fast Marching in SE(2)
We propose a Fast Marching based implementation for computing sub-Riemanninan
(SR) geodesics in the roto-translation group SE(2), with a metric depending on
a cost induced by the image data. The key ingredient is a Riemannian
approximation of the SR-metric. Then, a state of the art Fast Marching solver
that is able to deal with extreme anisotropies is used to compute a SR-distance
map as the solution of a corresponding eikonal equation. Subsequent
backtracking on the distance map gives the geodesics. To validate the method,
we consider the uniform cost case in which exact formulas for SR-geodesics are
known and we show remarkable accuracy of the numerically computed SR-spheres.
We also show a dramatic decrease in computational time with respect to a
previous PDE-based iterative approach. Regarding image analysis applications,
we show the potential of considering these data adaptive geodesics for a fully
automated retinal vessel tree segmentation.Comment: CIARP 201
Generalized Forward-Backward Splitting
This paper introduces the generalized forward-backward splitting algorithm
for minimizing convex functions of the form , where
has a Lipschitz-continuous gradient and the 's are simple in the sense
that their Moreau proximity operators are easy to compute. While the
forward-backward algorithm cannot deal with more than non-smooth
function, our method generalizes it to the case of arbitrary . Our method
makes an explicit use of the regularity of in the forward step, and the
proximity operators of the 's are applied in parallel in the backward
step. This allows the generalized forward backward to efficiently address an
important class of convex problems. We prove its convergence in infinite
dimension, and its robustness to errors on the computation of the proximity
operators and of the gradient of . Examples on inverse problems in imaging
demonstrate the advantage of the proposed methods in comparison to other
splitting algorithms.Comment: 24 pages, 4 figure
FLOT: Scene Flow on Point Clouds Guided by Optimal Transport
We propose and study a method called FLOT that estimates scene flow on point
clouds. We start the design of FLOT by noticing that scene flow estimation on
point clouds reduces to estimating a permutation matrix in a perfect world.
Inspired by recent works on graph matching, we build a method to find these
correspondences by borrowing tools from optimal transport. Then, we relax the
transport constraints to take into account real-world imperfections. The
transport cost between two points is given by the pairwise similarity between
deep features extracted by a neural network trained under full supervision
using synthetic datasets. Our main finding is that FLOT can perform as well as
the best existing methods on synthetic and real-world datasets while requiring
much less parameters and without using multiscale analysis. Our second finding
is that, on the training datasets considered, most of the performance can be
explained by the learned transport cost. This yields a simpler method,
FLOT, which is obtained using a particular choice of optimal transport
parameters and performs nearly as well as FLOT.Comment: Accepted at ECCV2
Signification géodynamique des calcaires de plate-forme en cours de subduction sous l'arc des Nouvelles-Hébrides (Sud-Ouest de l'océan Pacifique)
Note présentée par Jean DercourtInternational audienceThe analysis of carbonates from New Hébrides Trench shows that three main épisodes of shallow water carbonate déposition occurred during Late Eocene,Late Oligocene-Early Miocène,Mio-Pliocene-Quaternary, controlled by eustatism and tectonic.L'analyse de carbonates issus de la fosse des Nouvelles-Hébrides a permis de reconnaître trois périodes favorables au développement de plates-formes(Éocène supérieur,Oligocène supérieur-Miocène inférieur,Mio-Pliocène-Quaternaire)contrôlé par l'eustatisme et la tectonique
Proceedings of the second "international Traveling Workshop on Interactions between Sparse models and Technology" (iTWIST'14)
The implicit objective of the biennial "international - Traveling Workshop on
Interactions between Sparse models and Technology" (iTWIST) is to foster
collaboration between international scientific teams by disseminating ideas
through both specific oral/poster presentations and free discussions. For its
second edition, the iTWIST workshop took place in the medieval and picturesque
town of Namur in Belgium, from Wednesday August 27th till Friday August 29th,
2014. The workshop was conveniently located in "The Arsenal" building within
walking distance of both hotels and town center. iTWIST'14 has gathered about
70 international participants and has featured 9 invited talks, 10 oral
presentations, and 14 posters on the following themes, all related to the
theory, application and generalization of the "sparsity paradigm":
Sparsity-driven data sensing and processing; Union of low dimensional
subspaces; Beyond linear and convex inverse problem; Matrix/manifold/graph
sensing/processing; Blind inverse problems and dictionary learning; Sparsity
and computational neuroscience; Information theory, geometry and randomness;
Complexity/accuracy tradeoffs in numerical methods; Sparsity? What's next?;
Sparse machine learning and inference.Comment: 69 pages, 24 extended abstracts, iTWIST'14 website:
http://sites.google.com/site/itwist1
Single electron response and energy resolution of a Micromegas detector
Micro-Pattern Gaseous Detectors (MPGDs) such as Micromegas or GEM are used in
particle physics experiments for their capabilities in particle tracking at
high rates. Their excellent position resolutions are well known but their
energy characteristics have been less studied. The energy resolution is mainly
affected by the ionisation processes and detector gain fluctuations. This paper
presents a method to separetely measure those two contributions to the energy
resolution of a Micromegas detector. The method relies on the injection of a
controlled number of electrons. The Micromegas has a 1.6-mm drift zone and a
160-m amplification gap. It is operated in Ne
95%-iCH 5% at atmospheric pressure. The electrons
are generated by non-linear photoelectric emission issued from the photons of a
pulsed 337-nm wavelength laser coupled to a focusing system. The single
electron response has been measured at different gains (3.7 10,
5.0 10 and 7.0 10) and is fitted with a good
agreement by a Polya distribution. From those fits, a relative gain variance of
0.310.02 is deduced. The setup has also been characterised at several
voltages by fitting the energy resolution measured as a function of the number
of primary electrons, ranging from 5 up to 210. A maximum value of the Fano
factor (0.37) has been estimated for a 5.9 keV X-rays interacting in the Ne
95%-iCH 5% gas mixture.Comment: Preprint submitted to Nuclear Instrumentation and Methods in Physics
Research Section A: Accelerators, Spectrometers, Detectors and Associated
Equipment; Nuclear Instruments and Methods in Physics Research Section A:
Accelerators, Spectrometers, Detectors and Associated Equipment in press
(2009
PMm2: large photomultipliers and innovative electronics for the next-generation neutrino experiments
The next generation of proton decay and neutrino experiments, the
post-SuperKamiokande detectors as those that will take place in megaton size
water tanks, will require very large surfaces of photodetection and a large
volume of data. Even with large hemispherical photomultiplier tubes, the
expected number of channels should reach hundreds of thousands. A funded R&D
program to implement a solution is presented here. The very large surface of
photodetection is segmented in macro pixels made of 16 hemispherical (12
inches) photomultiplier tubes connected to an autonomous front-end which works
on a triggerless data acquisition mode. The expected data transmission rate is
5 Mb/s per cable, which can be achieved with existing techniques. This
architecture allows to reduce considerably the cost and facilitate the
industrialization. This document presents the simulations and measurements
which define the requirements for the photomultipliers and the electronics. A
proto-type of front-end electronics was successfully tested with 16
photomultiplier tubes supplied by a single high voltage, validating the
built-in gain adjustment and the calibration principle. The first tests and
calculations on the photomultiplier glass led to the study of a new package
optimized for a 10 bar pressure in order to sustain the high underwater
pressure.Comment: 1 pdf file, 4 pages, 4 figures, NDIP08, submitted to Nucl. Instr. and
Meth. Phys. Res.
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