8,152 research outputs found
Stellar ArAr reactions and their effect on light neutron-rich nuclide synthesis
The ArAr ( = 35 d) and
ArAr (269 y) reactions were studied for the first time
with a quasi-Maxwellian ( keV) neutron flux for Maxwellian Average
Cross Section (MACS) measurements at stellar energies. Gas samples were
irradiated at the high-intensity Soreq applied research accelerator
facility-liquid-lithium target neutron source and the Ar/Ar and
Ar/Ar ratios in the activated samples were determined by
accelerator mass spectrometry at the ATLAS facility (Argonne National
Laboratory). The Ar activity was also measured by low-level counting at
the University of Bern. Experimental MACS of Ar and Ar, corrected
to the standard 30 keV thermal energy, are 1.9(3) mb and 1.3(2) mb,
respectively, differing from the theoretical and evaluated values published to
date by up to an order of magnitude. The neutron capture cross sections of
Ar are relevant to the stellar nucleosynthesis of light neutron-rich
nuclides; the two experimental values are shown to affect the calculated mass
fraction of nuclides in the region A=36-48 during the weak -process. The new
production cross sections have implications also for the use of Ar and
Ar as environmental tracers in the atmosphere and hydrosphere.Comment: 18 pages + Supp. Mat. (13 pages) Accepted for publication in Phys.
Rev. Let
Approximating nonequilibrium processes using a collection of surrogate diffusion models
The surrogate process approximation (SPA) is applied to model the
nonequilibrium dynamics of a reaction coordinate (RC) associated with the
unfolding and refolding processes of a deca-alanine peptide at 300 K. The RC
dynamics, which correspond to the evolution of the end-to-end distance of the
polypeptide, are produced by steered molecular dynamics (SMD) simulations and
approximated using overdamped diffusion models. We show that the collection of
(estimated) SPA models contain structural information "orthogonal" to the RC
monitored in this study. Functional data analysis ideas are used to correlate
functions associated with the fitted SPA models with the work done on the
system in SMD simulations. It is demonstrated that the shape of the
nonequilibrium work distributions for the unfolding and refolding processes of
deca-alanine can be predicted with functional data analysis ideas using a
relatively small number of simulated SMD paths for calibrating the SPA
diffusion models.Comment: 13 pages, 7 figure
Surface characterization of Ti-Si-C-ON coatings for orthopedic devices : XPS and Raman spectroscopy
Ti–Si–C–ON films were deposited by DC reactive magnetron sputtering and their chemical properties, biofilm formation and toxicity were characterized. Based on the films composition three different growth regimes were identified on the films; (I) N/Ti = 2.11 (high atomic ratio) and low oxygen content; (II) 0.77 ≤ N/Ti ≤ 1.86 (intermediate atomic ratio) and (III) N/Ti ≤ 0.12 (low ratio) and high oxygen content. The phase composition varied from mainly TiN on regime I to TiCN on regime 2 and titanium oxides on regime III. Taking into account the results of biological characterization (biofilm formation and cytotoxicity), it was possible to conclude that samples with a high TiN content (regime I) presented more favorable biocompatibility, since it was less prone to microbial colonization and also displayed a low cytotoxicity.The authors are grateful to Dr. Alicia Andres, Institut de Ciencia de Materiales de Madrid (ICMM-CSIC), for his assistance in carrying out the Raman spectroscopic analysis. The work was financially supported by the CRUP Institution (project "Accao No E-1007/08), and the Spanish Ministry of Science and Innovation (projects FUN-COAT CSD2008-00023, MAT2008-06618-C02 and Integrated action HP016-2007). This research is partially sponsored by FEDER funds through the program COMPETE- Programa Operacional Factores de Competitividade and by Portuguese national funds through FCT-Fundacao para a Ciencia e a Tecnologia, under the project PTDC/CTM/102853/2008
The first direct measurement of ¹²C (¹²C,n) ²³Mg at stellar energies
Neutrons produced by the carbon fusion reaction ¹²C(¹²C,n)²³Mg play an important role in stellar nucleosynthesis. However, past studies have shown large discrepancies between experimental data and theory, leading to an uncertain cross section extrapolation at astrophysical energies. We present the first direct measurement that extends deep into the astrophysical energy range along with a new and improved extrapolation technique based on experimental data from the mirror reaction ¹²C(¹²C,p)²³Na. The new reaction rate has been determined with a well-defined uncertainty that exceeds the precision required by astrophysics models. Using our constrained rate, we find that ¹²C(¹²C,n)²³Mg is crucial to the production of Na and Al in Pop-III Pair Instability Supernovae. It also plays a non-negligible role in the production of weak s-process elements as well as in the production of the important galacti
Influence of the surface morphology and microstructure on the biological properties of Ti-Si-C-N-O coatings
Detailed structural, microstructural, biofilm formation and cytotoxicity studies were performed on Ti-Si-C-ON hard coatings prepared by dc reactive magnetron sputtering, in order to evaluate the relation among these properties. Compositional analysis showed the existence of two distinct regimens; regime I: high C/Si atomic ratio (C/Si ≥1,42) and intermediate N/Ti atomic ratio; regime II: low C/Si atomic ratio (C/Si≤0, 49) and low N/Ti atomic ratio. The structural analysis revealed that, in regime I, films crystallized in a B1-NaCl crystal structure typical of TiC0.2 N0.8. In regime II, the decrease of C/Si and increase in silicon concentration led to the formation of Ti-Si-CON along with a reduction of grain size in the films. Atomic Force Microscopy observations showed that the surface morphology of these Ti-Si-C-ON films became smoother when the silicon content increased and the nitrogen content decreased, which is consistent with the formation of nanosized clusters. Concerning biological properties, it was observed that cytotoxicity could be related with the titanium concentration while biofilm formation ability was found to be related with the surface morphology of the films.The authors are grateful to Dr. Alicia Andres, Instituto de Ciencia de Materiales de Madrid (ICMM-CSIC), for her assistance in carrying out the Raman spectroscopic analysis. The project was financially supported by the CRUP Institution (project "Accao N0 E-1007/08"), and the Spanish Ministry of Science and Innovation (projects FUNCOAT CSD2008-00023 and HP2007-0116)
Gamow Shell Model Description of Weakly Bound Nuclei and Unbound Nuclear States
We present the study of weakly bound, neutron-rich nuclei using the nuclear
shell model employing the complex Berggren ensemble representing the bound
single-particle states, unbound Gamow states, and the non-resonant continuum.
In the proposed Gamow Shell Model, the Hamiltonian consists of a one-body
finite depth (Woods-Saxon) potential and a residual two-body interaction. We
discuss the basic ingredients of the Gamow Shell Model. The formalism is
illustrated by calculations involving {\it several} valence neutrons outside
the double-magic core: He and O.Comment: 19 pages, 20 encapsulated PostScript figure
Independent measurement of the Hoyle state feeding from 12B using Gammasphere
Using an array of high-purity Compton-suppressed germanium detectors, we
performed an independent measurement of the -decay branching ratio from
to the second-excited (Hoyle) state in . Our
result is , which is a factor smaller than the previously
established literature value, but is in agreement with another recent
measurement. This could indicate that the Hoyle state is more clustered than
previously believed. The angular correlation of the Hoyle state
cascade has also been measured for the first time. It is consistent with
theoretical predictions
The critical role of logarithmic transformation in Nernstian equilibrium potential calculations
The membrane potential, arising from uneven distribution of ions across cell membranes containing selectively permeable ion channels, is of fundamental importance to cell signaling. The necessity of maintaining the membrane potential may be appreciated by expressing Ohm’s law as current = voltage/resistance and recognizing that no current flows when voltage = 0, i.e., transmembrane voltage gradients, created by uneven transmembrane ion concentrations, are an absolute requirement for the generation of currents that precipitate the action and synaptic potentials that consume >80% of the brain’s energy budget and underlie the electrical activity that defines brain function. The concept of the equilibrium potential is vital to understanding the origins of the membrane potential. The equilibrium potential defines a potential at which there is no net transmembrane ion flux, where the work created by the concentration gradient is balanced by the transmembrane voltage difference, and derives from a relationship describing the work done by the diffusion of ions down a concentration gradient. The Nernst equation predicts the equilibrium potential and, as such, is fundamental to understanding the interplay between transmembrane ion concentrations and equilibrium potentials. Logarithmic transformation of the ratio of internal and external ion concentrations lies at the heart of the Nernst equation, but most undergraduate neuroscience students have little understanding of the logarithmic function. To compound this, no current undergraduate neuroscience textbooks describe the effect of logarithmic transformation in appreciable detail, leaving the majority of students with little insight into how ion concentrations determine, or how ion perturbations alter, the membrane potential
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