Multifragmentation of non-spherical nuclei : Analysis of central Xe + Sn collisions at 50 MeV/nucl

The influence of shape of expanding and rotating source on various characteristics of the multifragmentation process is studied. The analysis is based on the extension of the statistical microcanonical multifragmentation model. The comparison with the data is done for central Xe+Sn collisions at 50 A MeV as measured by INDRA Collaboration.Comment: 9 pages, 2 figures; Talk given at the XXVII International Workshop on Gross Properties of Nuclei and Nuclear Excitation, Hirschegg (Austria), January 17 - 23, 199

A search for gravitational lensing in 38 X-ray selected clusters of galaxies

We present the results of a CCD imaging survey for gravitational lensing in a sample of 38 X-ray-selected clusters of galaxies. Our sample consists of the most X-ray luminous (Lx>= 2x10^{44} erg s^{-1}) clusters selected from the Einstein Observatory Extended Medium Sensitivity Survey (EMSS) that are observable from Mauna Kea (dec > -40deg). The sample spans a redshift range of 0.15 0.5. CCD images of the clusters were obtained in excellent seeing. There is evidence of strong gravitational lensing in the form of giant arcs (length l > 8'', axis ratio l/w > 10) in 8 of the 38 clusters. Two additional clusters contain shorter arclets, and 6 more clusters contain candidate arcs that require follow-up observations to confirm their lensing origin. Since the survey does not have a uniform surface brightness limit we do not draw any conclusion based on the statistics of the arcs found. We note, however, that 60% (3 of 5) of the clusters with Lx > 10^{45} erg s^{-1}, and none of the 15 clusters with Lx < 4x10^{44} erg s^{-1} contain giant arcs, thereby confirming that high X-ray luminosity does identify the most massive systems, and thus X-ray selection is the preferred method for finding true, rich clusters at intermediate and high redshifts. The observed geometry of the arcs, most of which are thin, have large axis ratios (l/w > 10), and are aligned orthogonal to the optical major axes of the clusters, indicate the cluster core mass density profiles must be compact (steeper than isothermal). In several cases, however, there is also some evidence, in the form of possible radial arcs, for density profiles with finite core radii.Comment: Latex file, 17 pages, 7 jpeg figures, to be published in Astronomy and Astrophysics Supplement

Étude de la multifragmentation : apport des corrélations en vitesse réduite entre particules et fragments

This work is focused on the study of fragment and light particle production mechanisms in the multifragmentation process of hot nuclei, which are formed in the central collisions of Xe+Sn at 50 MeV/u. The experiment has been performed with the INDRA multidetector. The central collision events, selected via the flow angle variable, exhibit the presence of a heavy (Z=90) and highly excited (E*=12.5 MeV/u) isotropic emission source. The comparison of the data with a statistical multifragmentation model (MMMC) and a dynamical model (BNV) makes us conclude that the multifragmentation can only be explained in the frame of a relatively cold process, around 6 MeV/u of thermal excitation energy, preceded by a primary emission stage of the expanding source, during which nearly one third of the excitation energy is dissipated. In addition, it appears that the fragment energy spectra are not explained by a purely thermal process, and that one has to put forward an expansion collective motion, of 2 MeV/u of energy, following the compression of the compound system. In order to precise the existence of a two-step particle emission (primary and secondary), we have developed and applied an original method of reduced velocity correlations between particles and fragments. It has allowed us to underline two distinct origins for the particle production: one corresponding to secondary emissions, coming from the fragments, and the other one, associated with emissions which occur prior to the fragment production. At last, it has allowed us, also to bring out a hierarchy in the emission time in the decay process, with respect to the particle type.Ce travail est centré sur l'étude des mécanismes de production des fragments et des particules légères dans la multifragmentation des noyaux chauds formés dans les collisions centrales Xe + Sn à 50 A.MeV. L'expérience a été effectuée avec le multidétecteur INDRA. Les événements de collision centrale, sélectionnés par l'angle de flot, font apparaître la présence d'une source d'émission isotrope, massive (Z = 90), et très excitée (E* = 12,5 A.MeV). La comparaison des données avec un modèle de multifragmentation statistique (MMMC) et un modèle dynamique (BNV) nous permet de conclure que la multifragmentation ne peut être expliquée que dans le cadre d'un processus relativement froid, autour de 6 A.MeV d'énergie d'excitation thermique, précédé d'une phase d'émission primaire par la source en expansion, au cours de laquelle près du tiers de l'énergie d'excitation est dissipé. De plus, il apparaît que les spectres en énergie des fragments ne sont pas expliqués par un processus purement thermique, et qu'il faut invoquer un mouvement collectif d'expansion, de 2 A.MeV d'énergie, consécutif à la compression du système composé. De manière à préciser l'existence d'une émission de particules en deux étapes (primaire et secondaire), nous avons développé et appliqué une méthode originale de corrélations en vitesse réduite entre particules et fragments. Elle nous a permis de mettre en évidence deux origines distinctes pour la production des particules : une correspondant aux émissions secondaires, issues des multifragments, et une associée à une émission antérieure à la formation de ces derniers. Enfin, cela nous a aussi permis de faire ressortir une hiérarchie en temps d'émission des différents types de particules dans le processus de désintégration du système

Modélisation jusqu'à 45 GHz des couplages entre microvias et cavités en technologie PCB multicouches quelles que soient les frontières

National audienceNous développons ici une modélisation simple du couplage entre les microvias et les cavités créées par les plans métalliques au sein des PCB multicouches. Ce modèle, basé sur la notion d'impédance des plans parallèles permet la prise en compte de frontières des cavités de natures différentes, ouvertes ou réalisées à l'aide de trous métallisés. Le modèle circuit est validé par comparaison avec des simulations électromagnétiques dans la bande 0-45 GHz et cela pour différents cas de figure des frontières de la cavité, en circuit ouvert ou court-circuit réalisé à l'aide de trous perforants métallisés. 1. Introduction Les technologies multicouches telles que le LTCC ou bien encore les PCB multicouches permettent une réduction des dimensions des circuits en utilisant la 3 ième dimension pour réduire les distances. Pour accéder à cette 3 ième dimension il est nécessaire d'utiliser des vias ou des microvias permettant de relier les différentes couches métalliques entre elles. Ces vias et microvias, traversant plusieurs couches métalliques, entraînent des couplages avec les cavités qu'ils traversent ce qui se traduit par des dégradations des signaux[1][2]. Afin de prévoir et de limiter ces effets, il est nécessaire d'avoir une méthode analytique rendant compte rapidement de ces phénomènes de couplage. Nous présentons dans cette communication une méthode basée sur la détermination des modes de cavités [2] tenant compte des frontières des cavités créées par les différents plans métalliques. Ces cavités peuvent être ouvertes ou fermées à l'aide de trous métallisés, les frontières pouvant être de natures différentes suivant le bord de la cavité considéré. L'approche circuit que nous proposons est validée à l'aide de simulations électromagnétiques et de mesures jusqu'à 45 GHz. 2. Modélisation du couplage

Angle-resolved photoemission study of the role of nesting and orbital orderings in the antiferromagnetic phase of BaFe2As2

We present a detailed comparison of the electronic structure of BaFe2As2 in its paramagnetic and antiferromagnetic (AFM) phases, through angle-resolved photoemission studies. Using different experimental geometries, we resolve the full elliptic shape of the electron pockets, including parts of dxy symmetry along its major axis that are usually missing. This allows us to define precisely how the hole and electron pockets are nested and how the different orbitals evolve at the transition. We conclude that the imperfect nesting between hole and electron pockets explains rather well the formation of gaps and residual metallic droplets in the AFM phase, provided the relative parity of the different bands is taken into account. Beyond this nesting picture, we observe shifts and splittings of numerous bands at the transition. We show that the splittings are surface sensitive and probably not a reliable signature of the magnetic order. On the other hand, the shifts indicate a significant redistribution of the orbital occupations at the transition, especially within the dxz/dyz system, which we discuss

Coherent and incoherent bands in La and Rh doped Sr3Ir2O7

In Sr2IrO4 and Sr3Ir2O7, correlations, magnetism and spin-orbit coupling compete on similar energy scales, creating a new context to study metal-insulator transitions (MIT). We use here Angle-Resolved photoemission to investigate the MIT as a function of hole and electron doping in Sr3Ir2O7, obtained respectively by Ir/Rh and Sr/La substitutions. We show that there is a clear reduction as a function of doping of the gap between a lower and upper band on both sides of the Fermi level, from 0.2eV to 0.05eV. Although these two bands have a counterpart in band structure calculations, they are characterized by a very different degree of coherence. The upper band exhibits clear quasiparticle peaks, while the lower band is very broad and loses weight as a function of doping. Moreover, their ARPES spectral weights obey different periodicities, reinforcing the idea of their different nature. We argue that a very similar situation occurs in Sr2IrO4 and conclude that the physics of the two families is essentially the same

Flow probe of symmetry energy in relativistic heavy-ion reactions

Flow observables in heavy-ion reactions at incident energies up to about 1 GeV per nucleon have been shown to be very useful for investigating the reaction dynamics and for determining the parameters of reaction models based on transport theory. In particular, the elliptic flow in collisions of neutron-rich heavy-ion systems emerges as an observable sensitive to the strength of the symmetry energy at supra-saturation densities. The comparison of ratios or differences of neutron and proton flows or neutron and hydrogen flows with predictions of transport models favors an approximately linear density dependence, consistent with ab-initio nuclear-matter theories. Extensive parameter searches have shown that the model dependence is comparable to the uncertainties of existing experimental data. Comprehensive new flow data of high accuracy, partly also through providing stronger constraints on model parameters, can thus be expected to improve our knowledge of the equation of state of asymmetric nuclear matter.Comment: 20 pages, 24 figures, review to appear in EPJA special volume on nuclear symmetry energ

The VIMOS-VLT Deep Survey: Dependence of Galaxy Clustering on Luminosity

We have investigated the dependence of galaxy clustering on their intrinsic luminosities at z ~ 1, using the data from the First Epoch VIMOS-VLT Deep Survey (VVDS). We have measured the projected two-point correlation function of galaxies, w_p(r_p), for a set of volume-limited samples at an effective redshift =0.9 and median absolute magnitude -19.6< M_B < -21.3. We find that the clustering strength is rising around M_B^*, apparently with a sharper turn than observed at low redshifts. The slope of the correlation function is observed to steepen significantly from \gamma=1.6^{+0.1}_{-0.1} to \gamma=2.4^{+0.4}_{-0.2}. This is due to a significant change in the shape of w_p(r_p), increasingly deviating from a power-law for the most luminous samples, with a strong upturn at small (< 1-2 h^{-1} Mpc) scales. This trend, not observed locally, also results in a strong scale dependence of the relative bias, b/b* and possibly imply a significant change in the way luminous galaxies trace dark-matter halos at z ~ 1 with respect to z ~ 0