Determining the phonon density of states from specific heat measurements via maximum entropy methods

The maximum entropy and reverse Monte Carlo methods are applied to the computation of the phonon density of states (DOS) from heat capacity data. The approach is introduced and the formalism is described. Simulated data are used to test the method, and its sensitivity to noise. Heat capacity measurements from diamond are used to demonstrate the use of the method with experimental data. Comparison between maximum entropy and reverse Monte Carlo results shows that the form of the entropy used here is correct, and that results are stable and reliable. Major features of the DOS are picked out, and acoustic and optical phonons can be treated with the same approach. The treatment set out in this paper provides a cost-effective and reliable method for studies of the phonon properties of materials

Choosing best practices for managing impacts of trawl fishing on seabed habitats and biota

Bottom trawling accounts for almost one quarter of global fish landings but may also have significant and unwanted impacts on seabed habitats and biota. Management measures and voluntary industry actions can reduce these impacts, helping to meet sustainability objectives for fisheries, conservation and environmental management. These include changes in gear design and operation of trawls, spatial controls, impact quotas and effort controls. We review nine different measures and actions and use published studies anda simple conceptual model to evaluate and compare their performance. The risks and benefits of these management measures depend on the extent to which the fishery is already achieving management objectives for target stocks and the characteristics of the management system that is already in place. We offer guidance on identifying best practices for trawl-fisheries management and show that best practices and their likelihood of reducing trawling impacts depend on local, national and regional management objectives and priorities, societal values and resources for implementation. There is no universalbest practice, and multiple management measures and industry actions are required to meet sustainability objectives and improve trade-offs between food production and environmental protection

Search for 22^{22}Na in novae supported by a novel method for measuring femtosecond nuclear lifetimes

Classical novae are thermonuclear explosions in stellar binary systems, and important sources of $^{26}$Al and $^{22}$Na. While gamma rays from the decay of the former radioisotope have been observed throughout the Galaxy, $^{22}$Na remains untraceable. The half-life of $^{22}$Na (2.6 yr) would allow the observation of its 1.275 MeV gamma-ray line from a cosmic source. However, the prediction of such an observation requires good knowledge of the nuclear reactions involved in the production and destruction of this nucleus. The $^{22}$Na($p,\gamma$)$^{23}$Mg reaction remains the only source of large uncertainty about the amount of $^{22}$Na ejected. Its rate is dominated by a single resonance on the short-lived state at 7785.0(7) keV in $^{23}$Mg. In the present work, a combined analysis of particle-particle correlations and velocity-difference profiles is proposed to measure femtosecond nuclear lifetimes. The application of this novel method to the study of the $^{23}$Mg states, combining magnetic and highly-segmented tracking gamma-ray spectrometers, places strong limits on the amount of $^{22}$Na produced in novae, explains its non-observation to date in gamma rays (flux < 2.5x$10^{-4}$ ph/(cm$^2$s)), and constrains its detectability with future space-borne observatories.Comment: 18 pages, 3 figures, 1 tabl

Narrow resonances in the continuum of the unbound nucleus 15^{15}F

The structure of the unbound $^{15}$F nucleus is investigated using the inverse kinematics resonant scattering of a radioactive $^{14}$O beam impinging on a CH$_2$ target. The analysis of $^{1}$H($^{14}$O,p)$^{14}$O and $^{1}$H($^{14}$O,2p)$^{13}$N reactions allowed the confirmation of the previously observed narrow $1/2^{-}$ resonance, near the two-proton decay threshold, and the identification of two new narrow 5/2$^{-}$ and 3/2$^{-}$ resonances. The newly observed levels decay by 1p emission to the ground of $^{14}$O, and by sequential 2p emission to the ground state (g.s.) of $^{13}$N via the $1^-$ resonance of $^{14}$O. Gamow shell model (GSM) analysis of the experimental data suggests that the wave functions of the 5/2$^{-}$ and 3/2$^{-}$ resonances may be collectivized by the continuum coupling to nearby 2p- and 1p- decay channels. The observed excitation function $^{1}$H($^{14}$O,p)$^{14}$O and resonance spectrum in $^{15}$F are well reproduced in the unified framework of the GSM

Origine microstructurale de l'amorçage des fissures intergranulaires dans les alliages Al-Li 8090

L'objectif de ce travail était de comprendre et de modéliser l'amorçage des fissures dans les alliages Al-Li 8090 sollicités en traction monotone. Une attention particulière a été portée sur les fissures intergranulaires — ou de délaminage —. Nous avons montré que la forte localisation de la déformation sur un seul système de glissement, ainsi que l'apparition d'un glissement « sinueux » et intense résultant probablement des incompatibilités de déformation de grain à grain, sont à l'origine de l'amorçage des fissures, et cela quelles que soient les conditions de revenu. Nous avons montré aussi que la précipitation intergranulaire augmente le risque de délaminage. Mais l'interaction glissement-joints de grains semble tout de même être responsable de l'amorçage intergranulaire. Enfin, nous avons développé pour les états sous revenus une analyse quantitative basée sur un modèle d'empilements de dislocations contre les joints de grains qui est en accord avec les résultats expérimentaux

Origine microstructurale de l'amorçage des fissures intergranulaires dans les alliages Al-Li 8090

This study is concerned with crack origin of 8090 Al-Li alloys tensile tested. Particular attention has been given on grain boundary crack or delamination. It has been shown that the origin of the fracture is due to a strong slip localisation on a one single slip system and to the appearance of a sinuous and coarse slip probably resulting from incompatible strains between adjacent grains, whatever the ageing conditions. Also, it has been shown that the intergranular precipitation increases the risk of intergranular cracks. But the slip localisation seems to be responsible for the delamination initiation. In the case of underaged condition a quantitative analysis from a model of dislocations piled up against grain boundary has been developed. This approach agrees with experimental results.L'objectif de ce travail était de comprendre et de modéliser l'amorçage des fissures dans les alliages Al-Li 8090 sollicités en traction monotone. Une attention particulière a été portée sur les fissures intergranulaires — ou de délaminage —. Nous avons montré que la forte localisation de la déformation sur un seul système de glissement, ainsi que l'apparition d'un glissement « sinueux » et intense résultant probablement des incompatibilités de déformation de grain à grain, sont à l'origine de l'amorçage des fissures, et cela quelles que soient les conditions de revenu. Nous avons montré aussi que la précipitation intergranulaire augmente le risque de délaminage. Mais l'interaction glissement-joints de grains semble tout de même être responsable de l'amorçage intergranulaire. Enfin, nous avons développé pour les états sous revenus une analyse quantitative basée sur un modèle d'empilements de dislocations contre les joints de grains qui est en accord avec les résultats expérimentaux

Etude par émission acoustique de la dynamique des dislocations pendant la déformation cyclique de polycristaux d'aluminium

The cyclic plasticity of 5N polycrystalline aluminium have been studied at room temperature by measuring the continuous acoustic emission (A.E.) due to dislocations movements in the metal. In this study, original data have been obtained in the understanding of continuous A.E. sources. In comparison with classical interpretation given in the literature, the fact that dislocations are arranged according to a dislocation cell structure from the first cycle has been included in the analysis of the results. From this, it has been shown that the amplitude of the A.E. signal is not directly connected with the plastic strain rate prescribed to the fatigue sample and that the probability density function of dislocation loops created during the cycling can be determined.La plasticité cyclique de l'AI 5N polycristallin a été étudiée à la température ambiante à partir de mesures d'émission acoustique continue (E.A.). L'application de la technique de l'E.A. nous a permis d'obtenir des données originales quant aux mécanismes sources d'E.A. Par rapport aux interprétations classiques de la littérature, nous avons fait intervenir le fait que, dès les premiers cycles, une structure cellulaire de dislocations est établie. Nous montrons que l'amplitude du signal d'E.A. n'est plus liée directement à la vitesse de déformation plastique macroscopique. A partir de cette donnée, l'analyse des résultats d'E.A. permet d'obtenir des informations sur la fonction distribution des boucles de dislocations créées au cours de la déformation cyclique