Density-dependent effects

Annihilation at positron energies below the threshold for Ps formation, which used to be measured for a variety of high-density gases, can now be measured by introducing low-pressure gas in the presence of a cloud of trapped positrons, thus avoiding the density-dependent effects. It has become possible even to use a positron beam to determine the annihilation cross section as a function of the positron energy. 

Linke-Gamenick I, Forbes VE, Sibly RM. 1999. Density-dependent effects of a toxicant on life-history traits and population dynamics of a capitellid polychaete. Mar Ecol Prog Ser 184 139-148. 

Quantitative phototaxis tests use known densities of D. discoi-dmm amoebae inoculated onto charcoal agar plates. This allows proper account to be taken of cell-density-dependent effects on slug behaviour and ensures that differences caused by mutational defects or other treatments, such as the addition of pharmacological agents, are not due simply to different cell densities. 

The inclusion of a range of densities in bioassays can also be very useful. Under the right experimental conditions density-dependent effects may be used to identify potential inhibitory allelopathic interactions and distinguish inhibitory plant-plant allelopathic interactions from resource competition (Weidenhamer et al. 1989 Thijs et al. 1994 Weidenhamer 1996) as long as autotoxicity and/or hormesis are absent (An et al. 1993 Belz et al. 2005 Sinkkonen 2001, 2003, 2007). Density bioassays are clearly consistent with a holistic approach. 

Density-dependent effects in laboratory, greenhouse, and field bioassays. These, proposed almost a decade ago by Weidenhamer et al. (1987, 1989), are almost never considered. Simply put, the hypothesis states that growth reductions (phytotoxicity) observed at low plant densities will diminish as the density of the test species is increased. 

Second, density-dependent effects must be considered in bioassays. Plants compete for toxins just as they do for nutrients (Hoffman Lavy 1978 Weidenhamer et al. 1987, 1989 Thijs et al. 1994). In bioassays, the practical consequence of this is that the greatest inhibitory effects will be observed with the lowest density of assay species. In assays of a toxic soil or litter, the greatest effects will be observed with the largest pot volumes. While the ability to use low plant densities and large pot volumes may be limited depending on the availability of experimental material, the influence of these factors needs to be remembered. 

Figure A3.6.13. Density dependence of die photolytic cage effect of iodine in compressed liquid n-pentane (circles), n-hexane (triangles), and n-heptane (squares) [38], The solid curves represent calculations using the diffusion model [37], the dotted and dashed curves are from static caging models using Camahan-Starling packing fractions and calculated radial distribution fiinctions, respectively [38],...

In planning cathodic protection, the specific resistivity of the water, the size of the surfaces to be protected and the required protection current densities have to be determined. The protection current density depends on the type and quality of the coating. Thermosetting resins (e.g., tar-epoxy resin coatings) are particularly effective and are mostly used today on coastal structures. They are chemically... 

V Z V" and the measurement of the actual component volumes V becomes a difficult undertaking. In these systems, a volume change—either shrinkage or expansion—is experienced upon mixing of the components. In addition, thermal effects may accompany the formation of the mixture or solution. The volumetric composition of liquid mixtures is expected to vary with temperature owing to the density dependence on temperature. 

Airborne particulate matter may comprise liquid (aerosols, mists or fogs) or solids (dust, fumes). Refer to Figure 5.2. Some causes of dust and aerosol formation are listed in Table 4.3. In either case dispersion, by spraying or fragmentation, will result in a considerable increase in the surface area of the chemical. This increases the reactivity, e.g. to render some chemicals pyrophoric, explosive or prone to spontaneous combustion it also increases the ease of entry into the body. The behaviour of an airborne particle depends upon its size (e.g. equivalent diameter), shape and density. The effect of particle diameter on terminal settling velocity is shown in Table 4.4. As a result ... 

In high-mobility liquids, the quasi-free electron is often visualized as having an effective mass m different fron the usual electron mass m. It arises due to multiple scattering of the electron while the mean free path remains long. The ratio of mean acceleration to an external force can be defined as the inverse effective mass. Often, the effective mass is equated to the electron mass m when its value is unknown and difficult to determine. In LRGs values of mVm 0.3 to 0.5 have been estimated (Asaf and Steinberger,1974). Ascarelli (1986) uses mVm = 0.27 in LXe and a density-dependent value in LAr. 

Radiation causes dominant lethal mutations in the medaka (Oryzias latipes) (Shima and Shimada 1991). Mosquitofish (Gambusia spp.) from radionuclide-contaminated ponds in South Carolina differed from conspecifics in reference ponds, as judged by the frequency of DNA markers, and this is consistent with the hypothesis that these DNA markers may originate from genetic elements that provide a selective advantage in contaminated habitats (Theodorakis et al. 1998). Ionizing radiation at low-level chronic exposure reportedly has no deleterious genetic effects on aquatic populations because exposure is compensated by density-dependent responses in fecundity (IAEA 1976). However, this needs verification. 

The effective potential has a form similar to that for the time-independent case with the density argument replaced by the TD density variable. Also some of the functionals may be current density dependent. 

M ) we find the binding energy uasi, which is density depended due to the effective masses. The corresponding wave function is used to evaluate the Pauli blocking term... 

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