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  • 1
    Electronic Resource
    Electronic Resource
    Palo Alto, Calif. : Annual Reviews
    Annual Review of Phytopathology 32 (1994), S. 503-521 
    ISSN: 0066-4286
    Source: Annual Reviews Electronic Back Volume Collection 1932-2001ff
    Topics: Agriculture, Forestry, Horticulture, Fishery, Domestic Science, Nutrition , Biology
    Type of Medium: Electronic Resource
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  • 2
    Electronic Resource
    Electronic Resource
    Springer
    Intensive care medicine 17 (1991), S. 187-187 
    ISSN: 1432-1238
    Source: Springer Online Journal Archives 1860-2000
    Topics: Medicine
    Type of Medium: Electronic Resource
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  • 3
    Electronic Resource
    Electronic Resource
    Springer
    Journal of mathematical biology 28 (1990), S. 529-565 
    ISSN: 1432-1416
    Keywords: Space time ; Integral equation ; Dispersal ; Asymptotic velocity of propagation ; Approximation formulae ; Cumulant generating function ; Fisher/Skellam diffusion model
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology , Mathematics
    Notes: Abstract We consider the velocity with which an invading population spreads over space. For a general linear model, originally due to Diekmann and Thieme, it is shown that the asymptotic velocity of population expansion can be calculated if information is available on: (i) the net-reproduction, R o; i.e. the expected number of offspring produced by one individual throughout its life, and (ii) the (normalized) reproduction-and-dispersal kernel, β(a, χ − ξ); i.e. the density of newborns produced per unit of time at position χ by an individual of age a born at ξ By means of numerical examples we study the effect of the net-reproduction and the shape of the reproduction-and-dispersal kernel on the velocity of population expansion. The reproduction-and-dispersal kernel is difficult to measure in full. This leads us to derive approximation formulas in terms of easily measurable parameters. The relation between the velocity of population expansion calculated from the general model and that from the Fisher/Skellam diffusion model is discussed. As a final step we use the model to analyse some real-life examples, thus showing how it can be put to work.
    Type of Medium: Electronic Resource
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  • 4
    Electronic Resource
    Electronic Resource
    Springer
    Oecologia 91 (1992), S. 371-376 
    ISSN: 1432-1939
    Keywords: Earthworms ; Active dispersal ; Passive dispersal ; Model
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology
    Notes: Summary In this paper a simple model is used to study the dispersal of earthworm populations into new habitats. Simple models do not describe processes accurately, but can help gain insight into the functioning of ecosystems or processes in ecosystems. Using information on reproduction, survival and dispersal at the level of the individual, the velocity of earthworm population expansion was calculated. Dispersal of earthworms can be active or passive. The parameters of active and passive dispersal were calculated from field experiments in one of the Dutch polders. Parameters of reproduction and survival were estimated from published data. The effects of processes at the individual level on the velocity of population expansion were studied for two species (Aporrectodea caliginosa and Lumbricus rubellus). The model shows that passive transport has a major influence on the velocity of population expansion, which is strongly increased even if this transport involves only a very small part of the population. At a high level of passive transport, however, death induced by this mode of dispersal can have a negative influence on population expansion. In the discussion it is indicated that optimising growth conditions of the earthworms might be the easiest way to promote population expansion. However, promoting dispersal by passive mechanisms can also be very important.
    Type of Medium: Electronic Resource
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  • 5
    ISSN: 1434-601X
    Keywords: 14.80.Pb ; 13.10. + q ; 12.20.Fv
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract Motivated by the observation of energy- and momentum-correlatede + e − pairs in heavy ion collisions, a search for resonances ine + e − scattering at the corresponding energies has been made. Thee + e − decay channel is analyzed in a time window from ∼ 10−13 s to ∼ 10−11 s with a set-up optimized for low-background detection of delayede + e − decays. The two-photon decay channel of a hypothetical resonance is investigated by measuring the two-photon annihilation-in-flight excitation function. New upper limits for the partiale + e − decay width Γee of a few meV are derived for total centre-of-mass energies √s between 1.78 and 1.92 MeV, taking into account the dilepton as well as the two-photon decay of a neutral resonance.
    Type of Medium: Electronic Resource
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  • 6
    ISSN: 1434-601X
    Keywords: 14.60.Cd ; 25.70.Cd ; 25.70.Ef
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract We present the results obtained from systematic studies of positron creation for a series of heavy-collision systems, with united chargeZ u =Z 1 +Z 2 ranging fromZ u =164 (Pb + Pb) toZ u =184 (U+U) at bombarding energies close to the Coulomb barrier, using the Orangeβ-spectrometer at GSI. For each collision system studied, the dominating continuous distributions due to quasiatomic and nuclear positron emission are determined accurately. This is essential in obtaining the characteristics of the still unexplained monoenergetic positron lines which appear in the energy range between 200 keV and 400 keV. Our results are compared with coupled-channels calculations for quasi-atomic positron creation. The latter describe quite well the global features of the measured spectra, but overestimate systematically their absolute values. From the comparison, a common normalization factor of about 0.75 can be established for the calculated spectra. In particular, the dependence onZ u of the measured emission probabilities was found to follow a power law (∝Z u 195±1), in fair agreement with the theoretical prediction.
    Type of Medium: Electronic Resource
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  • 7
    ISSN: 1434-601X
    Keywords: 14.60.Cd ; 25.70.Cd ; 25.70.Ef
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract We present the results obtained from a series of ρ+ρ−-coincidence measurements in heavy-ion collisions using the double-Orangeβ-spectrometer at GSI. The collision systems U+U, U+Pb, and U+Ta were investigated at bombarding energies close to and slightly above (U+Ta) the Coulomb barrier. For all systems studied, very narrow (FWHM−20 keV) ρ+ρ− lines were observed in the sum-energy spectra, with kinetic energies ranging from ∼555 keV to ∼810 keV, superimposed on a continuous distribution mainly due to uncorrelated ρ+ρ− emission. Particularly in the U+Ta system, a pronounced sum-energy line appears at ∼634 keV, predominantly in deep-inelastic collisions. In some cases (e.g. U+Pb) the line characteristics is consistent with a two-body decay mode of an emitter which moves with the c.m. velocity of the colliding ions. However, other lines, and in particular the 634 keV line (U+Ta), exhibit a rather isotropical opening-angle distribution whereas their energy is unequally shared between positrons and electrons, thus being in clear disagreement with this scenario. In general, the data preclude an emission from the separated (moving) nuclei, and, in the latter cases, provide evidence that the e+e−-pair decay occurs in the vicinity of the Coulomb field of a third heavy (positively charged) partner having only a small transverse velocity (|v|⊥〈0.02c)
    Type of Medium: Electronic Resource
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  • 8
    ISSN: 1434-6079
    Keywords: 34.80.Kw ; 32.30.Rj
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract The x-ray spectra induced by spontaneous capture of free cooling electrons into bare and hydrogenlike heavy ions have been observed. Au78+, Au79+ and U91+ ions were stored in the storage ring ESR at velocities between 60% and 65% of the velocity of light and the x rays were measured in coincidence with stored particles having captured one electron in the electron cooler. The lines observed can be attributed to the direct transition of cooling electrons into theK shell of the projectile and to the characteristicL →K transitions. The latter are mainly induced by capture into higher shells and subsequent cascade transitions. The measurements are compared to detailed calculations within the framework of presently available theories. Although the relative structure of the measured spectra is in fair agreement with the theoretical prediction, the total charge-changing rate in the electron cooler is about a factor of three smaller than expected.
    Type of Medium: Electronic Resource
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