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Density Threshold (density + threshold)
Selected AbstractsInfluence of light energy and power density on the microhardness of two nanohybrid compositesEUROPEAN JOURNAL OF ORAL SCIENCES, Issue 1 2008Kerstin Gritsch The purpose of this study was to investigate the role of light parameters on nanohybrid composite curing. Two nanohybrid resins were cured by two light-emitting diode (LED) devices and by one quartz-tungsten-halogen (QTH) device using different combinations of energy density and power density (8 J cm,2 and 400 mW cm,2; 8 J cm,2 and 1,000 mW cm,2; 16 J cm,2 and 400 mW cm,2; and 16 J cm,2,1,000 mW cm,2). The effects of these combinations on polymerization were assessed by measuring the Vickers microhardness. Data differed for the two composites and varied according to the light parameters and the nature of the curing device. For both resins, an energy density of 16 J cm,2 yielded the best microhardness values at both the top and the bottom of the sample, independently of the power density. When using a lower energy density of 8 J cm,2, a modulated power density was required to achieve proper curing at the bottom of the sample: 8 J cm,2 and 400 mW cm,2 induced greater values at the bottom surface. At an energy density of 16 J cm,2, the power density was not relevant (no significant differences were found between 400 and 1,000 mW cm,2), except when the emission spectra of the light-curing units (LCUs) did not match exactly with the absorption spectra of the photoinitators included in the resins (greatest values with 16 J cm,2 and1,000 mW cm,2). These results suggest that above a certain energy density threshold, the power density may not significantly influence the polymerization kinetics. [source] The environmental dependence of radio-loud AGN activity and star formation in the 2dFGRSMONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 1 2004P. N. Best ABSTRACT By combining the 2-degree Field Galaxy Redshift Survey with the NRAO VLA Sky Survey at 1.4 GHz, the environments of radio-loud active galactic nuclei (AGN) in the nearby Universe are investigated using both local projected galaxy densities and a friends-of-friends group-finding algorithm. Radio-loud AGN are preferentially located in galaxy groups and poor-to-moderate richness galaxy clusters. The AGN fraction appears to depend more strongly on the large-scale environment (group, cluster, etc.) in which a galaxy is located than on its more local environment, except at the lowest galaxy surface densities where practically no radio-loud AGN are found. The ratio of absorption-line to emission-line AGN changes dramatically with environment, with essentially all radio-loud AGN in rich environments showing no emission lines. This result could be connected with the lack of cool gas in cluster galaxies, and may have important consequences for analyses of optically selected AGN, which are invariably selected on emission-line properties. The local galaxy surface density of the absorption-line AGN is strongly correlated with radio luminosity, implying that the radio luminosities may be significantly boosted in dense environments due to confinement by the hot intracluster gas. The environments of a radio-selected sample of star-forming galaxies are also investigated to provide an independent test of optical studies. In line with those studies, the fraction of star-forming galaxies is found to decrease strongly with increasing local galaxy surface density; this correlation extends across the whole range of galaxy surface densities, with no evidence for the density threshold found in some optical studies. [source] Scaling laws in gravitational clustering for counts-in-cells and mass functionsMONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 2 2000P. Valageas We present an analysis of some of the properties of the density field realized in numerical simulations for power-law initial power spectra in the case of a critical density universe. We study the non-linear regime, which is the most difficult to handle analytically, and we compare our numerical results with the predictions of a specific hierarchical clustering scaling model that have been made recently, focusing specifically on its much wider range of applicability, which is one of its main advantages over the standard Press,Schechter approximation. We first check that the two-point correlation functions, measured from both counts-in-cells and neighbour counts, agree with the known analytically exact scaling requirement (i.e., depend only on ,2), and we also find the stable-clustering hypothesis to hold. Next, we show that the statistics of the counts-in-cells obey the scaling law predicted by the above scaling model. Then we turn to mass functions of overdense and underdense regions, which we obtain numerically from ,spherical overdensity' and ,friends-of-friends' algorithms. We first consider the mass function of ,just-collapsed' objects defined by a density threshold ,=177, and we note, as was found by previous studies, that the usual Press,Schechter prescription agrees reasonably well with the simulations (although there are some discrepancies). On the other hand, the numerical results are also consistent with the predictions of the scaling model. Next, we consider more general mass functions (needed to describe for instance galaxies or Lyman- , absorbers) defined by different density thresholds, which can even be negative. The scaling model is especially suited to account for such cases, which are out of reach of the Press,Schechter approach, and it still shows reasonably good agreement with the numerical results. Finally, we show that mass functions defined by a condition on the radius of the objects also satisfy the theoretical scaling predictions. Thus we find that the scaling model provides a reasonable description of the density field in the highly non-linear regime, for the cosmologies we have considered, for both the counts-in-cells statistics and the mass functions. The advantages of this approach are that it clarifies the links between several statistical tools and it allows one to study many different classes of objects, for any density threshold, provided one is in the fully non-linear regime. [source] JOINTLY-DETERMINED ECOLOGICAL THRESHOLDS AND ECONOMIC TRADE-OFFS IN WILDLIFE DISEASE MANAGEMENTNATURAL RESOURCE MODELING, Issue 4 2007ELI P. FENICHEL ABSTRACT. We investigate wildlife disease management, in a bioeconomic framework, when the wildlife host is valuable and disease transmission is density-dependent. Disease prevalence is reduced in density-dependent models whenever the population is harvested below a host-density threshold a threshold population density below which disease prevalence declines and above which a disease becomes epidemic. In conventional models, the threshold is an exogenous function of disease parameters. We consider this case and find a steady state with positive disease prevalence to be optimal. Next, we consider a case in which disease dynamics are affected by both population controls and changes in human-environmental interactions. The host-density threshold is endogenous in this case. That is, the manager does not simply manage the population relative to the threshold, but rather manages both the population and the threshold. The optimal threshold depends on the economic and ecological trade-offs arising from the jointly-determined system. Accounting for this endogene-ity can lead to reduced disease prevalence rates and higher population levels. Additionally, we show that ecological parameters that may be unimportant in conventional models that do not account for the endogeneity of the host-density threshold are potentially important when host density threshold is recognized as endogenous. [source] Gain-narrowed emissions of thiophene/phenylene co-oligomer single crystalsPHYSICA STATUS SOLIDI (C) - CURRENT TOPICS IN SOLID STATE PHYSICS, Issue 1 2009Toru Hiramatsu Abstract In order to elucidate the origin of gain-narrowed photoluminescence spectra of 5,5,-bis(4-biphenylyl)-2,2,:5,,2,-terthiophene (BP3T) single crystals, the dependence of excitation energy threshold on excitation lengths was studied at 12 K. The emission peak of the 0-1 vibrational band was narrowed at a constant excitation density irrespective of the excitation length, indicating that the gain-narrowing followed a stimulated emission process. On the other hand, the excitation density threshold for the 0-2 peak decreased with increasing excitation length. The latter behaviour is suggestive of a cooperative interaction among uniaxially oriented transition dipoles in the BP3T crystal. (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Current density threshold for the stimulation of neurons in the motor cortex areaBIOELECTROMAGNETICS, Issue 6 2002T. Kowalski Abstract The aim of this study was to determine a current density threshold for exciting the motor cortex area of the brain. The current density threshold for excitation of nerve fibres (20 ,m in diameter) found in the literature is approximately 1 A/m2 at frequencies lower than 1 kHz. In consideration of a safety factor of 100, the International Commission on Non-Ionizing Radiation Protection (ICNIRP) recommends to restrict the exposure to 0.01 A/m2. The electromagnetic stimulation of neurons in the motor cortex is used in the clinical diagnosis of nerve lesions and neuropathy by means of magnetic or electrical transcranial stimulation. Combining medical data from clinical studies and technical specifications of the Magstim® Model 200 stimulator, we were able to compute the current density threshold for the excitation of the human motor cortex by means of the finite element method (FEM). A 3D-CAD head model was built on the basis of magnetic resonance imaging (MRI) slices and segmented into four anatomical structures (scalp, skull, brain, and ventricular system) with different conductivities. A current density threshold for the stimulation of the motor cortex area of the upper limbs of 6 and 2.5 A/m2 at 2.44 kHz and 50 Hz, respectively, was calculated. As these values lie above the recommended ICNIRP values by two orders of magnitude there is no need for lower safety standards with regard to stimulation of the brain. Bioelectromagnetics 23:421,428, 2002. © 2002 Wiley-Liss, Inc. [source] Scaling laws in gravitational clustering for counts-in-cells and mass functionsMONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 2 2000P. Valageas We present an analysis of some of the properties of the density field realized in numerical simulations for power-law initial power spectra in the case of a critical density universe. We study the non-linear regime, which is the most difficult to handle analytically, and we compare our numerical results with the predictions of a specific hierarchical clustering scaling model that have been made recently, focusing specifically on its much wider range of applicability, which is one of its main advantages over the standard Press,Schechter approximation. We first check that the two-point correlation functions, measured from both counts-in-cells and neighbour counts, agree with the known analytically exact scaling requirement (i.e., depend only on ,2), and we also find the stable-clustering hypothesis to hold. Next, we show that the statistics of the counts-in-cells obey the scaling law predicted by the above scaling model. Then we turn to mass functions of overdense and underdense regions, which we obtain numerically from ,spherical overdensity' and ,friends-of-friends' algorithms. We first consider the mass function of ,just-collapsed' objects defined by a density threshold ,=177, and we note, as was found by previous studies, that the usual Press,Schechter prescription agrees reasonably well with the simulations (although there are some discrepancies). On the other hand, the numerical results are also consistent with the predictions of the scaling model. Next, we consider more general mass functions (needed to describe for instance galaxies or Lyman- , absorbers) defined by different density thresholds, which can even be negative. The scaling model is especially suited to account for such cases, which are out of reach of the Press,Schechter approach, and it still shows reasonably good agreement with the numerical results. Finally, we show that mass functions defined by a condition on the radius of the objects also satisfy the theoretical scaling predictions. Thus we find that the scaling model provides a reasonable description of the density field in the highly non-linear regime, for the cosmologies we have considered, for both the counts-in-cells statistics and the mass functions. The advantages of this approach are that it clarifies the links between several statistical tools and it allows one to study many different classes of objects, for any density threshold, provided one is in the fully non-linear regime. [source] Responses of two species of heathland rodents to habitat manipulation: Vegetation density thresholds and the habitat accommodation modelAUSTRAL ECOLOGY, Issue 3 2010VAUGHAN MONAMY Abstract The abundance of two native rodent species, Rattus lutreolus and Pseudomys gracilicaudatus, has been shown to correlate with vegetation density in coastal wet heath. Fox's habitat accommodation model relates relative abundances of such small mammal species to heathland vegetation regeneration following disturbance. Implicit in the model is recognition that it is successional changes in vegetation, not time per se, that drives the responses of small mammal species along a regeneration axis. Using a brush-cutter we deliberately removed approximately 85% of vegetation around trapping stations and recorded significant reductions in the abundance of both P. gracilicaudatus (an earlier-stage colonizing species) and R. lutreolus (a late seral-stage species). A significant decrease in the abundance of only the latter had been demonstrated previously when 60,70% of the vegetation had been removed. Following the brush-cutting both species re-entered the mammalian secondary succession at different times, first P. gracilicaudatus followed by R. lutreolus after the vegetation cover thresholds of each species had been reached. The impact of this habitat manipulation experiment was to produce a retrogression of the small mammal succession, experimentally demonstrating causality between changes in vegetation density and subsequent small mammal habitat use. [source] | |||||||||||||