1,268 research outputs found
Finite universe of discourse. The systems biology of Walter Elsasser (1904-1991).
No full textWalter Elsasser (1904-1991), an eminent quantum physicist and geophysicist, was also active in theoretical biology over a 35-year period from the early 1950s to the late 1980s. Although increasingly estranged from the biological establishment during the last fifteen years of his life, Elsasser's central concern with complexity has resulted in a revival of interest in his theories over the last decade, particularly among those who see biology from a systems holist rather than a molecular reductionist viewpoint. This article reviews the development of Elsasser's thought from his early opposition to genetic determinism, through the radical epistemology of his middle period, to his later more broadly philosophical ideas. After a summary of existing responses to Elsasser in the literature, a fresh critique and assessment of his work is presented, with particular attention to the implications for systems biology. It is concluded that although Elsasser drew some conclusions from his epistemology that are not justifiable in the light of subsequent research, his insistence on the existence of biotonic phenomena in biology, irreducible (either at present, or in principle) to physics, is correct. Ironically, the most significant biotonic principle is one which Elsasser largely ignored in his own work, that of Natural Selection
First-principles analysis of cation segregation at grain boundaries in alpha-Al2O3
No full textThe modifications in atomistic structure, chemical bonding, and energetics induced by substitutional cation impurities isolated in the bulk volume and segregated at grain boundaries of alpha-Al2O3 were investigated by combining empirical ionic-model and first-principles electronic-structure calculations. The dependency of these modifications on the boundary type, species and concentration of impurities, was studied by selecting the following variety of systems: three twin boundaries (the prismatic Sigma3 (10 (1) over bar0), the rhombohedral Sigma7 (10 (1) over bar2), and the pyramidal Sigma13 (10 (1) over bar4) twins), three impurities X (X = Se, Y, and La), and two concentrations for the segregant ( approximate to3 and approximate to6 atoms/nm(2)). A partial covalent character is found to be a distinctive feature of the X-O bonds in both bulk and interfacial atomic environments, and to drive the structural distortions of the octahedral XO6 clusters. The energetics of segregation reveals a linear relationship between segregation energy and impurity size. This is interpreted as resulting from a stress field localized at the interface. (C) 2002 Published by Elsevier Science Ltd on behalf of Acta Materialia Inc
Sigma 13 (10(1)over-bar4) twin in alpha-Al2O3: A model for a general grain boundary
No full textThe atomistic structure and energetics of the Sigma13 (10(1) over bar4)[1(2) over bar 10] symmetrical tilt grain boundary in alpha-Al2O3 are studied by first-principles calculations based on the local-density-functional theory with a mixed-basis pseudopotential method. Three configurations, stable with respect to intergranular cleavage, are identified: one Al-terminated glide-mirror twin boundary and two O-terminated twin boundaries, with glide-mirror and twofold screw-rotation symmetries, respectively. Their relative energetics as a function of axial grain separation are described, and the local electronic structure and bonding are analyzed. The Al-terminated variant is predicted to be the most stable one, confirming previous empirical calculations, but in contrast with high-resolution transmission electron microscopy observations on high-purity diffusion-bonded bicrystals. which resulted in an O-terminated structure. An explanation of this discrepancy is proposed based on the different relative energetics of the internal interfaces with respect to the free surfaces
Prismatic Sigma 3 (10(1)over-bar-0) twin boundary in alpha-Al2O3 investigated by density functional theory and transmission electron microscopy
No full textThe microscopic structure of a prismatic Sigma3 (10 (1) over bar0) twin boundary in alpha-Al2O3 is characterized by combining ab initio local-density-functional theory, electron energy-loss spectroscopy measuring energy-loss near-edge structures (ELNES) of the oxygen K-ionization edge, and high-resolution transmission electron microscopy (HRTEM). Theoretically, two distinct microscopic boundary variants with very low interface energies are derived and analyzed. The interface-projected densities of states (PDOS) calculated for the two variants agree equally well with ELNES, therefore the comparison between experimental ELNES and theoretical PDOS cannot discriminate the one or the other boundary structure. The analysis reveals that the distinction between the metastable interfaces from ELNES is limited by the spatial resolution of the scanning transmission electron microscope used to measure ELNES, not by its energetical resolution. The quantitative analysis of experimental HRTEM images obtained with an atomic-resolution microscope yields that the experimentally observed interface corresponds to the boundary variant with the lowest energy
On the Statistics of Elsasser Increments in Solar Wind and Magnetohydrodynamic Turbulence
No full textWe investigate the dependency with scale of the empirical probability
distribution functions (PDF) of Elsasser increments using large sets of WIND
data (collected between 1995 and 2017) near 1 au. The empirical PDF are
compared to the ones obtained from high-resolution numerical simulations of
steadily driven, homogeneous Reduced MHD turbulence on a rectangular
mesh. A large statistical sample of Alfv\'enic increments is obtained by using
conditional analysis based on the solar wind average properties. The PDF tails
obtained from observations and numerical simulations are found to have
exponential behavior in the inertial range, with an exponential decrement that
satisfies power-laws of the form , where the
scale size, with around 0.2 for observations and 0.4 for simulations. PDF
tails were extrapolated assuming their exponential behavior extends to
arbitrarily large increments in order to determine structure function scaling
laws at very high orders. Our results points to potentially universal scaling
laws governing the PDF of Elsasser increments and to an alternative methodology
to investigate high-order statistics in solar wind observations.Comment: 7 pages, 4 figures. Accepted for publication in the Astrophysical
Journal Letter
Measurement of the ratio of prompt χ c to J / ψ production in pp collisions at √s = 7 TeV
No full textThe prompt production of charmonium χ c and J / ψ states is studied in proton-proton collisions at a centre-of-mass energy of √s = 7 TeV at the Large Hadron Collider. The χ c and J / ψ mesons are identified through their decays χ c → J / ψ γ and J / ψ → μ + μ - using 36 pb - 1 of data collected by the LHCb detector in 2010. The ratio of the prompt production cross-sections for χ c and J / ψ, σ (χ c → J / ψ γ) / σ (J / ψ), is determined as a function of the J / ψ transverse momentum in the range 2 < p T J / ψ < 15 GeV / c. The results are in excellent agreement with next-to-leading order non-relativistic expectations and show a significant discrepancy compared with the colour singlet model prediction at leading order, especially in the low p T J / ψ region
First Evidence for the Decay B-s(0) -> mu(+) mu(-)
No full textA search for the rare decays Bs0→μ+μ- and B0→μ+μ- is performed with data collected in 2011 and 2012 with the LHCb experiment at the Large Hadron Collider. The data samples comprise 1.1 fb-1 of proton-proton collisions at √s=8 TeV and 1.0 fb-1 at √s=7 TeV. We observe an excess of Bs0→μ+μ- candidates with respect to the background expectation. The probability that the background could produce such an excess or larger is 5.3×10-4 corresponding to a signal significance of 3.5 standard deviations. A maximum-likelihood fit gives a branching fraction of B(Bs0→μ+μ-)=(3.2-1.2+1.5)×10-9, where the statistical uncertainty is 95% of the total uncertainty. This result is in agreement with the standard model expectation. The observed number of B0→μ+μ- candidates is consistent with the background expectation, giving an upper limit of B(B0→μ+μ-)<9.4×10-10 at 95% confidence level
Systematic analysis of RBE and related quantities using a database of cell survival experiments with ion beam irradiation
No full textFor tumor therapy with light ions and for experimental aspects in particle radiobiology the relative biological effectiveness (RBE) is an important quantity to describe the increased effectiveness of particle radiation. By establishing and analysing a database of ion and photon cell survival data, some remarkable properties of RBE-related quantities were observed. The database consists of 855 in vitro cell survival experiments after ion and photon irradiation. The experiments comprise curves obtained in different labs, using different ion species, different irradiation modalities, the whole range of accessible energies and linear energy transfers (LETs) and various cell types. Each survival curve has been parameterized using the linear-quadratic (LQ) model. The photon parameters, α and β, appear to be slightly anti-correlated, which might point toward an underlying biological mechanism. The RBE values derived from the survival curves support the known dependence of RBE on LET, on particle species and dose. A positive correlation of RBE with the ratio α/β of the photon LQ parameters is found at low doses, which unexpectedly changes to a negative correlation at high doses. Furthermore, we investigated the course of the β coefficient of the LQ model with increasing LET, finding typically a slight initial increase and a final falloff to zero. The observed fluctuations in RBE values of comparable experiments resemble overall RBE uncertainties, which is of relevance for treatment planning. The database can also be used for extensive testing of RBE models. We thus compare simulations with the local effect model to achieve this goal. © 2012 The Author 2012
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