Spontaneous Branching Research Articles

Modeling development in retinal afferents: retinotopy, segregation, and ephrinA/EphA mutants.

During neural development, neurons extend axons to target areas of the brain. Through processes of growth, branching and retraction these axons establish stereotypic patterns of connectivity. In the visual system, these patterns include retinotopic organization and the segregation of individual axons onto different subsets of target neurons based on the eye of origin (ocular dominance) or receptive field type (ON or OFF). Characteristic disruptions to these patterns occur when neural activity or guidance molecule expression is perturbed. In this paper we present a model that explains how these developmental patterns might emerge as a result of the coordinated growth and retraction of individual axons and synapses responding to position-specific markers, trophic factors and spontaneous neural activity. This model derives from one presented earlier (Godfrey et al., 2009) but which is here extended to account for a wider range of phenomena than previously described. These include ocular dominance and ON-OFF segregation and the results of altered ephrinA and EphA guidance molecule expression. The model takes into account molecular guidance factors, realistic patterns of spontaneous retinal wave activity, trophic molecules, homeostatic mechanisms, axon branching and retraction rules and intra-axonal signaling mechanisms that contribute to the survival of nearby synapses on an axon. We show that, collectively, these mechanisms can account for a wider range of phenomena than previous models of retino-tectal development.

2.7 μm Emission properties of Er3+ doped fluorozirconate glass

Fluorozirconate glass, considered as the most stable heavy metal fluoride glass and excellent host for rare earth ions, has been extensively used for 2.7 μm laser materials due to its low intrinsic loss, wide transparency window, and small phonon energy. In this paper, a novel erbium ion doped fluorozirconate glass was prepared, 2.7 μm emission properties, thermal stability and Raman spectra were investigated and the spontaneous transition probability and branching ratio were calculated and analyzed. It was found that the thermal stability was greatly improved due to the ΔT increased from 59 to 89°C after introducing YF3 on the basis of ZBLA(ZrF4-BaF2-LaF3-AlF3). In addition, intensive 2.7 μm emission was observed and the present sample with the addition of Y3+ possessed larger 2.7 μm emission cross section (1.12 × 10−20 cm2) which indicated that this kind of fluoride glasses could potentially be used as an efficient gain media in the 2.7 μm fiber laser system.

Effects of numerical and physical anisotropic diffusion on branching phenomena of negative-streamer dynamics

This paper is a contribution to the fluid modelling and simulation of the spontaneous branching of an initial mono-filamentary negative streamer propagating in molecular nitrogen at atmospheric pressure. The effects of both numerical diffusion and physical anisotropic diffusion on the branching structure are studied. We used MUSCL-type flux limiters where an artificial amount of numerical diffusion can be introduced through the choice of the value of a characteristic slope parameter. It was shown that a small amount of numerical diffusion can inhibit the spontaneous streamer branching. This means that the use of a high-order numerical scheme preventing the numerical diffusion and dispersion is a major parameter that must be taken into account in the interpretation of the simulated streamer development and splitting. This paper also clearly shows that the consideration of the anisotropy of electron diffusion affects the streamer head structure in comparison with the isotropic diffusion case. This especially occurs for electrons in gases presenting a large difference between the longitudinal and transversal diffusion coefficients as in N2 or in air.

Spectroscopic properties and Judd-Ofelt theory analysis of La, Nd codoped Y2O3 high refractivity transparent ceramics

Nd，La:Y2O3 nanocrystalline powder was prepared by co-precipitation method. The transparent ceramic was fabricated by vacuum sintering at 1700℃ for 4 h in vacuum. Transparent polycrystalline ceramics with uniform lanthanum distribution was obtained. The grain size was around 22 μm. No pores were found in the grain and grain boundary. The absorption spectrum and the fluorescence spectrum of the sample were measured. The highest absorption peak is centered at 821 nm and the absorption cross section is 43×10-24 m2 The main emission peak is at 1078 nm, and the fluorescence lifetime is 0287 ms. The Judd-Ofelt intensity parameter Ωλ（λ=2，4，6）, spontaneous transition probability, radiative lifetime and fluorescence branching ratio of Nd3+ were calculated by Judd-Ofelt theory. The calculated emission cross section of 4F3/2→4I11/2 transition is 20×10-24 m2 It was found that the addition of La ion can change the crystalline field of yttria, which helps to obtain suitable solid laser materials.

The Systematic Behavior of the Spontaneous Fission Branching Ratios of Even-Z Isotopes

The spontaneous fission branching ratios (B) of even-Z, 90 ≤ Z ≤ 100, nuclei were found to be correlated to log B = 1.6247 × Z – 1.0361(2Z – N) – 114.66. Based on this correlation, it is predicted that the isotopes 226Ra and 228Th are spontaneous fission isotopes.

Preparation and performance of Cr3+/Tm3+/Ho3+ co-doped fluorophosphate glasses

Fluorophosphate glasses with various Al(PO3)3 contents were prepared，and their structure,thermal properties and spectral properties were studied.The emission properties at around 2.0μm of Cr3+/Tm3+/Ho3+ co-doped fluorophosphate glasses with different mole fractions of Tm3+ and Ho3+were studied. The spectral strength parameters Ωt(t=2,4,6) of those glasses have been calculated. Meanwhile,the spectral parameters, including the oscillator strength,spontaneous emission probability and branching ratios of Ho3+, have been calculated. The results show that the 2.0μm emission intensity gradually grows higher with the increase of the mole fraction of Tm3+. The Tm3+→(3F4) →Ho3+(5I7) energy transfer in (Tm3+/Ho3+) co-doped fluorophosphate glasses is very efficient, and it is dependent on the doping concentration.

Plant–fungal symbiosis en gros and en détail

Plant–fungal symbiosis <i>en gros</i> and <i>en détail</i>

Judd–Ofelt analysis and upconversion emission of Er3+–Yb3+ co-doped oxyfluoride glass ceramics containing LaF3 nanocrystals

Er3+–Yb3+ co-doped glasses and glass ceramics containing LaF3 nanocrystals were prepared and their absorption spectra obtained. The Judd–Ofelt parameters Ω t (t = 2, 4, 6) for f–f transition of Er3+, as well as spontaneous emission probabilities, branching ratios and radiative lifetimes for stimulated emission of each band were determined. Addition of Er3+ and Yb3+ ions into low-phonon energy LaF3 nanocrystals makes the upconversion emission of Er3+–Yb3+ co-doped glass ceramic much stronger than that of Er3+–Yb3+ co-doped glass.

Investigations of pre-breakdown phenomena: streamer discharges

Physical principles of electrical breakdown are reviewed in this article. The phenomena of streamer formation and spontaneous branching are studied in terms of a fluid description based on kinetic theory. Particular attention is paid to a minimal model which is suitable for non-attaching gases. The evidence that anode directed fronts can branch due to a Laplacian instability is shown. Finally an electric shielding factor is introduced which allows us to extend previous results to curved geometries.

Dendritic Flux Avalanches and Nonlocal Electrodynamics in Thin Superconducting Films

We report a mechanism of nonisothermal dendritic flux penetration in superconducting films. Our numerical and analytical analysis of coupled nonlinear Maxwell and thermal diffusion equations shows that dendritic flux pattern formation results from spontaneous branching of propagating flux filaments due to nonlocal magnetic flux diffusion and positive feedback between flux motion and Joule heating. The branching is triggered by a thermomagnetic edge instability, which causes stratification of the critical state. The resulting distribution of thermomagnetic microavalanches is not universal, because it depends on a spatial distribution of defects. Our results are in good agreement with experiments on Nb films.

On negative streamers: A deterministic approach

The formation of streamers and spontaneous branching is commonly observed in dielectric breakdown phenomena. We discuss a deterministic streamer model that explains branching phenomena in terms of a Laplacian instability similar to that found in viscous fingering. Particular attention is paid to the physical interpretation of the model.

Streamer branching rationalized by conformal mapping techniques.

Spontaneous branching of discharge channels is frequently observed, but not well understood. We recently proposed a new branching mechanism based on simulations of a simple continuous discharge model in high fields. We here present analytical results for such streamers in the Lozansky-Firsov limit where they can be modeled as moving equipotential ionization fronts. This model can be analyzed by conformal mapping techniques which allow the reduction of the dynamical problem to finite sets of nonlinear ordinary differential equations. Our solutions illustrate that branching is generic for the intricate head dynamics of streamers in the Lozansky-Firsov limit.

Polyp oriented modelling of coral growth

The morphogenesis of colonial stony corals is the result of the collective behaviour of many coral polyps depositing coral skeleton on top of the old skeleton on which they live. Yet, models of coral growth often consider the polyps as a single continuous surface. In the present work, the polyps are modelled individually. Each polyp takes up resources, deposits skeleton, buds off new polyps and dies. In this polyp oriented model, spontaneous branching occurs. We argue that branching is caused by a so called “polyp fanning effect” by which polyps on a convex surface have a competitive advantage relative to polyps on a flat or concave surface. The fanning effect generates a more potent branching mechanism than the Laplacian growth mechanism that we have studied previously (J. Theor. Biol. 224 (2003) 153). We discuss the application of the polyp oriented model to the study of environmentally driven morphological plasticity in stony corals. In a few examples we show how the properties of the individual polyps influence the whole colony morphology. In our model, the spacing of polyps influences the thickness of coral branches and the overall compactness of the colony. Density variations in the coral skeleton may also be important for the whole colony morphology, which we address by studying two variants of the model. Finally, we discuss the importance of small scale resource translocation in the coral colony and its effects on the morphology of the colony.

Stability of negative ionization fronts: Regularization by electric screening?

We recently have proposed that a reduced interfacial model for streamer propagation is able to explain spontaneous branching. Such models require regularization. In the present paper we investigate how transversal Fourier modes of a planar ionization front are regularized by the electric screening length. For a fixed value of the electric field ahead of the front we calculate the dispersion relation numerically. These results guide the derivation of analytical asymptotes for arbitrary fields: for small wave-vector k, the growth rate s(k) grows linearly with k, for large k, it saturates at some positive plateau value. We give a physical interpretation of these results.

Models of coral growth: spontaneous branching, compactification and the Laplacian growth assumption

In stony corals it is often observed that specimens collected from a sheltered growth site have more open and more thinly branched growth forms than specimens of the same species from more exposed growth sites, where stronger water currents are found. This observation was explained using an abiotic computational model inspired by coral growth, in which the growth velocity depended locally on the absorption of a resource dispersed by advection and diffusion (Kaandorp and Sloot, J. Theor. Biol 209 (2001) 257). In that model a morphological range was found; as the Péclet-number (indicating the relative importance of advective and diffusive nutrient transport) was increased, more compact and spherical growth forms were found. Two unsatisfactory items have remained in this model, which we address in the present paper. First, an explicit curvature rule was responsible for branching. In this work we show that the curvature rule is not needed: the model exhibits spontaneous branching, provided that the resource field is computed with enough precision. Second, previously no explanation was given for the morphological range found in the simulations. Here we show that such an explanation is given by the conditions under which spontaneous branching occurs in our model, in which the compactness of the growth forms depends on the ratio of the rates of growth and nutrient transport. We did not find an effect of flow. This suggests that the computational evidence that hydrodynamics influences the compactness of corals in laminar flows may not be conclusive. The applicability of the Laplacian growth paradigm to understand coral growth is discussed.

Spontaneous branching of anode-directed streamers between planar electrodes.

Nonionized media subject to strong fields can become locally ionized by penetration of finger-shaped streamers. We study negative streamers between planar electrodes in a simple deterministic continuum approximation. We observe that, for sufficiently large fields, the streamer tip can split. This happens close to the limit of "ideal conductivity." Qualitatively, the tip splitting is due to a Laplacian instability quite like that in viscous fingering. For future quantitative analytical progress, our stability analysis of planar fronts identifies the screening length as a regularization mechanism.

Measurement of alpha-decay constant to fission cross section ratio for 237Np, 241Am and alpha to spontaneous fission branching ratio for 243Am using solid state nuclear track detectors

The alpha-decay constant to fission cross section ratio for epi-cadmium neutron induced fission of 237Np, 241Am have been measured using solid state nuclear track detectors (SSNTDs). The measured values were (0.63±0.14).109 s-1.cm-2, (2.31±0.46).1012 s-1.cm-2 for 237Np and 241Am, respectively. The alpha to spontaneous fission branching ratio for 243Am was also measured to be (2.66±0.50).1010 using SSNTDs. These ratios are useful as signatures for the trace level identification of these actinides.

A critical evaluation of cohesive zone models of dynamic fractur

Finite element calculations of dynamic fracture based on embedding cohesive surfaces in a continuum indicate that the predictions are sensitive to the cohesive law used. Simulations were performed on a square block in plane strain with an initial edge crack loaded at a constant rate of strain, Cohesive laws that have an initial elastic response were observed to produce spontaneous branching at high velocity, but to modify the linear elastic properties of the body As a consequence the cohesive surface spacing cannot be refined arbitrarily and becomes an important length scale in the simulations. Cohesive laws that are initially rigid do not alter the linear elastic response of the body. However, crack branching behavior was not observed when such a cohesive relation was implemented using a regular finite element mesh.

Measurement of alpha to spontaneous fission branching ratios of238Pu and240Pu by solid state nuclear track detectors

Alpha to spontaneous fission branching ratios of heavy actinides such as238Pu and240Pu in solution have been determined by solid state nuclear track detectors (SSNTDs). The detectors used in the present studies are LR-115, CR-39 (DOP), Lexan polycarbonate and Tuffak polycarbonate. Before using Tuffak polycarbonate, a detailed and systematic study on the registration and chemical etching characteristics of fission fragment tracks in this detector was also carried out. The experimentally measured values of alpha to spontaneous fission ratios by this method are found to agree well with the expected values.

Optical Properties and Characterization of Eu3+ Activated Pmma:Paac Copolymer Samples: Eu Concentration Effects

ABSTRACTSamples of Eu3+ -activated PMMA:PAAc copolymers were prepared in order to investigate their spectroscopic properties. The densities, refractive indices, UV-Visible absorption and emission spectra and fluorescence decay times were obtained experimentally. These data were used in conjunction with Judd-Ofelt theory to calculate the spontaneous emission probabilities and branching ratios, as a function of the europium content. The range of the concentration was 0.1 to 2.5 % mol. Our results are combined with previous information on the optical properties of rare-earth activated polymers in order to get a better understanding of the role played by the Eu3+ ions on the polymerization process in our samples.