Crater formation by fast ions: comparison of experiment with Molecular Dynamics simulations

An incident fast ion in the electronic stopping regime produces a track of excitations which can lead to particle ejection and cratering. Molecular Dynamics simulations of the evolution of the deposited energy were used to study the resulting crater morphology as a function of the excitation density in a cylindrical track for large angle of incidence with respect to the surface normal. Surprisingly, the overall behavior is shown to be similar to that seen in the experimental data for crater formation in polymers. However, the simulations give greater insight into the cratering process. The threshold for crater formation occurs when the excitation density approaches the cohesive energy density, and a crater rim is formed at about six times that energy density. The crater length scales roughly as the square root of the electronic stopping power, and the crater width and depth seem to saturate for the largest energy densities considered here. The number of ejected particles, the sputtering yield, is shown to be much smaller than simple estimates based on crater size unless the full crater morphology is considered. Therefore, crater size can not easily be used to estimate the sputtering yield.Comment: LaTeX, 7 pages, 5 EPS figures. For related figures/movies, see: http://dirac.ms.virginia.edu/~emb3t/craters/craters.html New version uploaded 5/16/01, with minor text changes + new figure

Origin of atomic clusters during ion sputtering

Previous studies have shown that the size distributions of small clusters ( n<=40 n = number of atoms/cluster) generated by sputtering obey an inverse power law with an exponent between -8 and -4. Here we report electron microscopy studies of the size distributions of larger clusters ( n>=500) sputtered by high-energy ion impacts. These new measurements also yield an inverse power law, but one with an exponent of -2 and one independent of sputtering yield, indicating that the large clusters are produced when shock waves, generated by subsurface displacement cascades, ablate the surface

Molecular mechanisms of intercellular communication: transmembrane signaling

This short discussion of transmembrane signaling depicts a particular class of signaling devices whose functional characteristics may well be representative of broader classes of membrane switches. These multicomponent aggregates are characterized by tight organization of interacting components which function by conformational interactions to provide sensitive, amplified, rapid, and modulated responses. It is clear that the essential role of such switches in cell-cell interactions necessitated their appearance early in the history of the development of multicellular organisms. It also seems clear that once such devices made their appearance, the conformationally interactive moieties were firmly locked into a regulatory relationship. Since modification of interacting components could perturb or interfere with the functional integrity of the whole switch, genetic drift was only permitted at the input and outflow extremes. However, the GTP binding moiety and its interacting protein domains on contiguous portions of the receptor and readout components were highly conserved. The observed stringent evolutionary conservation of the molecular features of these membrane switches thus applies primarily to the central (GTP binding) elements. An extraordinary degree of variation was permitted within the domains of signal recognition and enzymatic output. Thus, time and evolution have adapted the central logic of the regulatory algorithm to serve a great variety of cellular purposes and to recognize a great variety of chemical and physical signals. This is exemplified by the richness of the hormonal and cellular dialogues found in primates such as man. Here the wealth of intercellular communiation can support the composition and performance of symphonies and the study of cellular immunology

Vitamin K Supplementation in Postmenopausal Women with Osteopenia (ECKO Trial): A Randomized Controlled Trial

Angela Cheung and colleagues investigate whether vitamin K1 can prevent bone loss among postmenopausal women with osteopenia

Overview of the assessment practices of occupational therapists working in Quebec

Description. L'évaluation du client est primordiale dans la pratique de l'ergothérapie, notamment pour établir les priorités de traitement et pour vérifier l'efficacité des interventions. Les pratiques évaluatives des ergothérapeutes sont cependant peu documentées. But. Dresser un portrait des pratiques évaluatives des ergothérapeutes du Québec selon les dimensions personne-environnement-occupation et les clientèles. Méthodologie. Une enquête transversale a été réalisée à l'aide d'un sondage en ligne envoyé aux ergothérapeutes du Québec. Résultats. En pédiatrie, les ergothérapeutes utilisent majoritairement des outils standardisés des aptitudes physiques et neurologiques. L'évaluation d'adultes cible principalement les aptitudes physiques et la productivité. Auprès des aînés, l'évaluation repose surtout sur les aspects fonctionnels, particulièrement les aptitudes physiques, les soins personnels et la sécurité à domicile, et sur le dépistage des difficultés cognitives. Implications pour la pratique. L'évaluation ergothérapique serait généralement axée sur les aptitudes physiques. Pour assurer une approche holistique, l'occupation et l'environnement devraient être davantage considérés dans l'évaluation.Abstract : Background. In occupational therapy practice, client assessments are essential for establishing treatment priorities and determining the effectiveness of interventions. However, occupational therapists' assessment practices are not well documented. Purpose. This work aimed to provide an overview of the assessment practices of Quebec occupational therapists based on the person–environment–occupation components and clienteles. Method. A cross-sectional survey was conducted using an online survey that was sent to occupational therapists in Quebec. Findings. In paediatrics, occupational therapists tend to use standardized tools to assess physical and neurological abilities. Adult assessment focuses mainly on physical abilities and productivity. For seniors, assessment focuses mainly on functional aspects (physical abilities, personal care, and home safety) and screening for cognitive difficulties. Implications. Occupational therapy assessment mostly focuses on physicial abilities. To ensure a holistic approach, more occupational and environmental components should be included in the assessment practices

Safe extension of red blood cell storage life at 4{degree}C

The project sought to develop methods to extend the storage life of red blood cells. Extended storage would allow donor to self or autologous transfusion, expand and stabilize the blood supply, reduce the cost of medical care and eliminate the risk of transfusion related infections, including a spectrum of hepatitides (A, B and C) and HIV. The putative cause of red blood cell spoilage at 4 C has been identified as oxidative membrane damage resulting from deoxyhemoglobin and its denaturation products including hemichrome, hemin and Fe{sup 3+}. Trials with carbon monoxide, which is a stabilizer of hemoglobin, have produced striking improvement of red blood cell diagnostics for cells stored at 4 C. Carbonmonoxy hemoglobin is readily converted to oxyhemoglobin by light in the presence of oxygen. These findings have generated a working model and an approach to identify the best protocols for optimal red cell storage and hemoglobin regeneration