Simulated solvation of organic ions II: Study of linear alkylated

International audienceWe investigated the solvation of carboxylate ions from formate to hexanoate, in droplets of 50 to 1000 water molecules and neat water, by computations using standard molecular dynamics and sophisticated polarizable models. The carboxylate ions from methanoate to hexanoate show strong propensity for the air/water interface in small droplets. Only the ions larger than propanoate retain propensity for the interface in larger droplets, where their enthalpic stabilization by ion/water dispersion is reduced there by 3 kcal mol–1 per CH2 group. This is compensated by entropy effects over +3.3 cal mol–1 K–1 per CH2 group. On the surface, the anionic headgroups are strongly oriented toward the aqueous core, while the hydrophobic alkyl chains are repelled into air and lose their structure-making effects. These results reproduce the structure-making effects of alkyl groups in solution, and suggest that the hydrocarbon chains of ionic headgroups and alkyl substituents solvate independently. Extrapolation to bulk solution using standard extrapolation schemes yields absolute carboxylate solvation energies. The results for formate and acetate yield a proton solvation enthalpy of about 270 kcal mol–1, close to the experiment-based value. The largest carboxylate ions yield a value smaller by about 10 kcal mol–1, which requires studies in much larger droplets

Intense Source of Slow Positrons

We describe a novel design for an intense source of slow positrons based on pair production with a beam of electrons from a 10 MeV accelerator hitting a thin target at a low incidence angle. The positrons are collected with a set of coils adapted to the large production angle. The collection system is designed to inject the positrons in a Greaves-Surko trap [1]. Such a source could be the basis for a series of experiments in fundamental and applied research and would also be a prototype source for industrial applications which concern the field of defect characterization in the nanometer scale.Comment: submitted to N.I.M.

Zgoubi users guide

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Spin dynamic tool developments and study regarding the Super-B project

International audience

Simulation of radiation damping in rings using stepwise ray-tracing methods

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Etude de la réaction de capture neutronique radiative pour le noyau instable du Lu par méthode directe et par réaction de substitution

L objectif de ce document est de présenter une étude sur la réaction de capture neutronique radiative du noyau instable du Lu a n de déterminer sa section efficace (n,g). Si globalement, pour les noyaux stables de nombreuses informations sont à disposition, il reste un véritable manque de données pour les noyaux radioactifs. La première partie de cette thèse se consacre à la présentation des différents formalismes impliqués dans le calcul d une section efficace ainsi qu à l utilisation du code TALYS basé sur ces différents modèles. TALYS nous a permis d évaluer la section efficace (n,g) sur le Lu en s appuyant sur la connaissance de la réaction de capture sur le Lu. Dans un deuxième temps, la section efficace (n,g) sur le Lu a été mesurée sur l installation LANSCE avec le détecteur 4p DANCE. Cette expérience s est révélée être un véritable dé tant pour la fabrication de la cible de Lu que pour l obtention de données dû à la grande radioactivité de cet isotope. Nous avons pu extraire des informations intéressantes comme le taux de capture total, identi er et caractériser de nouvelles résonances, déterminer des paramètres comme l espacement moyen, les largeurs neutroniques et les largeurs g ainsi que la valeur de la fonction densité. Tous ces nouveaux renseignements nous ont permis de reconstruire la section efficace (n,g) sur le Lu jusqu à 200 eV. Grâce à ces différentes informations, nous avons estimé une correction à appliquer sur la section efficace évaluée par TALYS permettant in ne d obtenir une nouvelle évaluation de cette quantité. La troisième partie de cette thèse est consacrée à apporter des informations supplémentaires sur la réaction Lu(n,g) Lu en utilisant la méthode de substitution. En premier lieu, nous avons testé la validité de cette méthode sur une réaction connue : Lu(n,g) Lu avec la réaction Yb( He,p) Lu puis dans un second temps nous avons regardé la réaction Lu(n,g) Lu avec la réaction Yb( He,t) Lu. Pour cela, la probabilité d émission g dans ces deux voies a été mesurée et comparée à un calcul TALYS. Cette comparaison a révélé de grandes disparités entre les réactions de transfert utilisées et les réactions induites par des neutrons. Notre investigation s est alors orientée à regarder de plus près la distribution de spins du noyau composé formé. Celle-ci a pu être extraite grâce à un ajustement de la probabilité d émission g mesurée. Cette distribution montre que les réaction ( He,X) peuplent en réalité des spins beaucoup plus grands que ceux issus de la réaction (n,g). La mesure des rapports des intensités de transitions g corroborent aussi ce résultat même si les spins peuplés semblent être plus faibles. Cette expérience a mis clairement en évidence qu une réaction induite par un faisceau d He ne pouvait pas se substituer à une réaction (n,g). Tout au long de ce document, nous avons montré par différents aspects, à quel point il peut être difficile d obtenir des informations sur les noyaux radioactifs tels que le Lu et que pour le futur de nombreux dé s tant expérimentaux que théoriques sont encore à relever.This PhD report presents a study of radiative neutron capture reaction of the Lu to determine the (n,g) cross section. Contrary to stable nuclei for which a huge amount of data is available, there is a real lack of information for radioactive nuclei. The rst part of this PhD focuses on the presentation of different formalisms involved in cross section calculations and on the use of TALYS code based on these models. TALYS allowed us to evaluate the (n,g) cross section on Lu with data from the capture reaction on the Lu. In a second step, the (n,g) cross section on Lu was measured at the LANSCE facility with the 4p detector DANCE. This experience turned out to be a real challenge for both the target production of Lu and for obtaining data due to the high radioactivity of this isotope. Interesting informations have been extracted such as the total capture yield, the identi cation and the characterization of new resonances, the determination of parameters like average spacing, neutron and g widths and the value of the density function. All these new informations enabled us to reconstruct the (n,g) cross section on Lu up to 200 eV. We could then estimate a correction for the cross section evaluated by TALYS in order to obtain a new evaluation. The third part of this PhD deals with getting additionnal information on the Lu(n,g) Lu reaction by using the surrogate method. Firstly, this technique has been tested on a well-known reaction : Lu(n,g) Lu with the reaction Yb( He,p) Lu. Secondly, we studied the Lu(n,g) Lu reaction with the Yb( He,t) Lu reaction. The g decay probability in different ways have been measured and compared with a TALYS calculation. This comparison showed large discrepancies between the transfer reactions and the reactions induced by neutrons. Then our investigation was focused on the spin distribution of the formed compound nucleus. It has been extracted thanks to a t of the measured g decay probability. This distribution has shown ( He,X) reactions populate much higher spins than observed in (n,g) reactions. The measurement of the g transitions intensities ratios also corroborates this result even if the mean value of the populated spins seem to be lower. This experiment clearly demonstrated a reaction induced by a He beam could not to be substituted to a (n,g) reaction. Throughout this document, we have shown by different aspects on one hand, how it can be difficult to extract information on unstable nuclei such as Lu and on the other hand that both experimental and theoretical challenges still remain for the future.PARIS11-SCD-Bib. électronique (914719901) / SudocSudocFranceF

The Effect of Age of Acquisition in Visual Word Processing: Further Evidence for the Semantic Hypothesis

The authors investigated whether the meaning of visually presented words is activated faster for early-acquired words than for late-acquired words. They addressed the issue using the semantic Simon paradigm. In this paradigm, participants are instructed to decide whether a stimulus word is printed in uppercase or lowercase letters. However, they have to respond with a verbal label ("living" or "nonliving") that is either congruent with the meaning of the word (e.g., saying "living" to the stimulus DOG) or incongruent (e.g., saying "nonliving" to the stimulus dog). Results showed a significant congruency effect that was stronger for early-acquired words than for late-acquired words. The authors conclude that the age of acquisition is an important variable in the activation of the meaning of visually presented words. In the past decade, researchers have revived the question regarding the extent to which the frequency effect in visual word recognition is a confound of the age at which words have been acquired (hence called age of acquisition One hypothesis is that part of the AoA effect originates from the semantic system. According to this explanation, the order of acquisition has a lasting effect on the time needed to activate the meanings of words. Empirical evidence for this idea comes from the word-associate generation task (Brysbaert, Theoretical support for a semantic involvement in the AoA effect comes from simulations with models based on both distributed and localist representations. The distributed account attributes the AoA effect to differences in the connection weights between the units of the orthographic and the semantic layers; the localist account attributes it to the organization of the semantic system

Electronic Structures and Bonding of Oxygen on Plutonium Layers

Oxygen adsorption on delta-Pu (100) and (111) surfaces have been studied at both non-spin-polarized and spin-polarized levels using the generalized gradient approximation of density functional theory (GGA-DFT)with Perdew and Wang functionals. The center position of the (100) surface is found to be the most favorable site with chemisorption energies of 7.386 eV and 7.080 eV at the two levels of theory. The distances of the oxygen adatom from the Pu surface are found to be 0.92A and 1.02A, respectively. For the (111) surface non-spin-polarized calculations, the center position is also the preferred site with a chemisorption energy of 7.070 eV and the distance of the adatom being 1.31A, but for spin-polarized calculations the bridge and the center sites are found to be basically degenerate, the difference in chemisorption energies being only 0.021 eV. In general, due to the adsorption of oxygen, plutonium 5f orbitals are pushed further below the Fermi energy, compared to the bare plutonium layers. The work function, in general, increases due to oxygen adsorption on plutonium surfaces.Comment: Spin-polarization is considered, and the paper is revised accordingl