Model of random packings of different size balls

We develop a model to describe the properties of random assemblies of polydisperse hard spheres. We show that the key features to describe the system are (i) the dependence between the free volume of a sphere and the various coordination numbers between the species, and (ii) the dependence of the coordination numbers with the concentration of species; quantities that are calculated analytically. The model predicts the density of random close packing and random loose packing of polydisperse systems for a given distribution of ball size and describes packings for any interparticle friction coefficient. The formalism allows to determine the optimal packing over different distributions and may help to treat packing problems of non-spherical particles which are notoriously difficult to solve.Comment: 6 pages, 6 figure

Scintillation light detection in XENON1T: Photosensor tests and light collection simulations

The experiment XENON1T aims at directly detecting dark matter via interactions of weakly interacting massive particles (WIMPs) with xenon nuclei. The goal of XENON1T is to improve the sensitivity compared to its predecessor experiment XENON100 by 2 orders of magnitude by using a larger xenon volume and by reducing the amount of background events. The XENON1T inner detector, which contains 2:2 t of liquid xenon, will be equipped with 248 photomultiplier tubes (PMTs) to detect the scintillation light induced by a possible WIMP interaction. For this purpose 3 00 R11410-21 PMTs with low intrinsic radioactivity and high quantum eciency are being manufactured by Hamamatsu. One part of this thesis describes the tests of the rst 90 R11410-21 PMTs. Important features like the dark count rate and the gain were measured at room- and liquid xenon temperature. The second part of this thesis is dedicated to investigating the scintillation light propagation and the light collection eciency (LCE) in a XENON1T-like detector. A Monte Carlo simulation based on GEANT4 was developed to test how dierent assignments of quantum eciency (QE) values to the PMT positions aect the LCE. In a rst approach, it is found that optimizing the PMT arrangement, considering the QE values, can enhance the LCE value by a factor of 7% at most, which would result in a 7% higher light yield for XENON1T

Mining bipartite graphs to improve semantic pedophile activity detection

International audiencePeer-to-peer (P2P) networks are popular to exchange large volumes of data through the Internet. Paedophile activity is a very important topic for our society and some works have recently attempted to gauge the extent of paedophile exchanges on P2P networks. A key issue is to obtain an efficient detection tool, which may decide if a sequence of keywords is related to the topic or not. We propose to use social network analysis in a large dataset from a P2P network to improve a state-of-the-art filter for paedophile queries. We obtain queries and thus combinations of words which are not tagged by the filter but should be. We also perform some experiments to explore if the original four categories of paedophile queries were to be found by topological measures only

Une approche à base de proximité pour la détection de communautés egocentrées

International audienceNous proposons ici une approche performante pour déplier la structure communautaire egocentrée sur un sommet d'un gaphe. Nous montrons que, bien que chaque sommet d'un réseau appartienne en général à plusieurs communautés, il est souvent possible d'identifier une communauté unique si l'on considère deux sommets bien choisis. La méthodologie que nous proposons repose sur cette notion de communauté multi-egocentrée ainsi que sur l'utilisation d'une mesure de proximité dérivée de techniques de dynamique d'opinion, la carryover opinion. Cette approche pallie les limites des fonctions de qualité traditionnellement utilisées pour la détection de communautés egocentrées, et consiste à étudier les irrégularités dans la décroissance de cette mesure de proximité

Multi-ego-centered communities in practice

International audienceWe propose here a framework to unfold the ego-centered community structure of a given node in a network. The framework is not based on the optimization of a quality function, but on the study of the irregularity of the decrease of a proximity measure. It is a practical use of the notion of multi-ego-centered community and we validate the pertinence of the approach on benchmarks and a real-world network of wikipedia pages

Non-invasive, multichromatic eye oximeter Final report

Optical eye oximeter for measuring oxygen of choroidal blood for monitoring brain oxygen suppl

Calculation of the Voronoi boundary for lens-shaped particles and spherocylinders

We have recently developed a mean-field theory to estimate the packing fraction of non-spherical particles [A. Baule et al., Nature Commun. (2013)]. The central quantity in this framework is the Voronoi excluded volume, which generalizes the standard hard-core excluded volume appearing in Onsager's theory. The Voronoi excluded volume is defined from an exclusion condition for the Voronoi boundary between two particles, which is usually not tractable analytically. Here, we show how the technical difficulties in calculating the Voronoi boundary can be overcome for lens-shaped particles and spherocylinders, two standard prolate and oblate shapes with rotational symmetry. By decomposing these shapes into unions and intersections of spheres analytical expressions can be obtained.Comment: 19 pages, 8 figure

Qualification Tests of the R11410-21 Photomultiplier Tubes for the XENON1T Detector

The Hamamatsu R11410-21 photomultiplier tube is the photodetector of choice for the XENON1T dual-phase time projection chamber. The device has been optimized for a very low intrinsic radioactivity, a high quantum efficiency and a high sensitivity to single photon detection. A total of 248 tubes are currently operated in XENON1T, selected out of 321 tested units. In this article the procedures implemented to evaluate the large number of tubes prior to their installation in XENON1T are described. The parameter distributions for all tested tubes are shown, with an emphasis on those selected for XENON1T, of which the impact on the detector performance is discussed. All photomultipliers have been tested in a nitrogen atmosphere at cryogenic temperatures, with a subset of the tubes being tested in gaseous and liquid xenon, simulating their operating conditions in the dark matter detector. The performance and evaluation of the tubes in the different environments is reported and the criteria for rejection of PMTs are outlined and quantified.Comment: 24 pages, 16 figure

Déplier la structure communautaire d’un réseau en mesurant la proximité aux représentants de communauté

International audienceHow to find all overlapping communities in a complex network? That is, how to find all relevant groups of nodes in a linked dataset? No entirely satisfying solution to that important problem exists, having a criterion to decide which group is relevant and finding quickly these groups in large networks are bottlenecks. We found that in many networks the number of these groups is limited and that there exist, for each group, at least one node that can characterize it by itself: a node belonging only to that group and important within it. We call such a node a community representative. We develop an algorithm to find these overlapping communities. The community detection is done through measuring the proximities of all nodes to the representatives and then finding irregularities in the decrease of these values reflecting the presence of relevant groups. We show that our approach handles very large real-world networks and have comparable or even better performances compared to state of the art methods.Nous proposons un algorithme pour déplier la structure communautaire des grands graphes de terrain. L'algorithme est basé sur la détection de la communauté de chaque représentant communautaire : noeud contenu dans une seule communauté et important en son sein. Cette détection est faite avec une approche à base de mesure de proximité développée récemment. Par comparaison avec d'autres méthodes de l'état de l'art nous montrons que notre algorithme a des performances équivalentes voire meilleures et est capable de traiter les plus grands graphes de terrain