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- Szymanski, J. J., et al.
(författare)
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A study of the decay mu –> e gamma by the MEGA experiment
- 1996
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Ingår i: Proceedings, 9th Meeting, DPF'96, Minneapolis, USA, August 11-15, 1996. Vol. 1, 2.. ; , s. 1450-1452
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Konferensbidrag (refereegranskat)
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- Cooper, M. D., et al.
(författare)
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Construction and performance of MEGA's low-mass, high-rate cylindrical MWPCs
- 1998
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Ingår i: Nuclear Instruments and Methods in Physics Research Section A. - : Elsevier. - 0168-9002 .- 1872-9576. ; 417:1, s. 24-49
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Tidskriftsartikel (refereegranskat)abstract
- A design for extremely low mass, high-resolution multiwire proportional chambers (MWPC) was achieved by the MEGA collaboration in its experiment to search for the lepton family number violating decay μ → eγ. To extend the present branching ratio limit by over an order of magnitude, these MWPCs were operated in high particle fluxes. They showed minimal effects of aging, and evidenced spatial and energy resolutions for the orbiting positrons from muon decay which were consistent with our design parameters. The unique features of these chambers, their assembly into the MEGA positron spectrometer, and their performance during the experiment are described in this paper.
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- Antonarakis, S. E., et al.
(författare)
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Factor VIII gene inversions in severe hemophilia A : Results of an international consortium study
- 1995
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Ingår i: Blood. - : American Society of Hematology. - 0006-4971 .- 1528-0020. ; 86:6, s. 2206-2212
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Tidskriftsartikel (refereegranskat)abstract
- Twenty-two molecular diagnostic laboratories from 14 countries participated in a consortium study to estimate the impact of Factor VIII gene inversions in severe hemophilia A. A total of 2,093 patients with severe hemophilia A were studied; of those, 740 (35%) had a type 1 (distal) factor VIII inversion, and 140 (7%) showed a type 2 (proximal) inversion. In 25 cases, the molecular analysis showed additional abnormal or polymorphic patterns. Ninety-eight percent of 532 mothers of patients with inversions were carriers of the abnormal factor VIII gene; when only mothers of nonfamilial cases were studied, 9 de novo inversions in maternal germ cells ware observed among 225 cases (≃ 1 de novo maternal origin of the inversion in 25 mothers of sporadic cases). When the maternal grandparental origin was examined, the inversions occurred de novo in male germ cells in 69 cases and female germ cells in 1 case. The presence of factor VIII inversions is not a major predisposing factor for the development of factor VIII inhibitors; however, slightly more patients with severe hemophilia A and factor VIII inversions develop inhibitors (130 of 642 [20%]) than patients with severe hemophilia A without inversions (131 of 821 [16%]).
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- Nisbet, R. M., et al.
(författare)
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Primary-productivity gradients and short-term population dynamics in open systems
- 1997
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Ingår i: Ecological Monographs. ; 67:4, s. 535-553
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Tidskriftsartikel (refereegranskat)abstract
- We present three models representing the trophic and behavioral dynamics of a simple food chain (primary producers, grazers, and predators) at temporal scales shorter than the scale of consumer reproduction, and at the spatial scales typically employed in field experiments. These models incorporate flexible behavioral responses of organisms to their predators and resources in spatially heterogeneous environments that are open to immigration and emigration. The basic models include passive immigration at all trophic levels, producer growth rates and losses to grazer consumption, grazer emigration rate as a behavioral response to producer and predator densities, grazer losses to predator consumption, and predator emigration as a function of grazer density. We model this system as: (1) a set of ordinary differential equations (''well-mixed model''); (2) a set of partial differential equations describing a population of discrete grazers foraging on discrete patches of primary producers (''discrete-grazer model''); and (3) a set of simulation rules describing the movement and foraging of individual grazers and the growth of primary producers on discrete patches in explicit space (''individual-based model''). The ordinary differential-equation models produced similar results to individual-based models with well-mixed producers, and the discrete-grazer and individual-based models produced similar results when grazers possessed a long-term memory of patch reward rates. The well-mixed and discrete-grazer models thus represent specific, limiting cases of the general individual-based model. Multiple equilibria and sustained oscillations are possible but are less likely in the discrete-grazer and individual-based models than in the well-mixed model, because localized foraging of discrete grazers leads to the rapid development of spatial heterogeneity in producer biomass and, hence, to a decrease in overall primary production. All models predict that stable equilibrium densities of all trophic levels increase with enrichment, provided grazers increase their emigration rates as predator density increases. If increasing predator density leads to decreasing grazer-emigration rates, predator and grazer densities increase, but producer biomass may increase or decrease with enrichment. These results contrast with predictions from models that assume ideal free distributions of grazers and/or predators with respect to their resources. Our models also predict that densities at all trophic levels will increase with increasing producer immigration, and that producer density will decline with increasing grazer immigration and increase with increasing predator immigration. Our qualitative findings on enrichment are used to interpret an experiment dealing with the short-term dynamics of a stream community open to grazers and predators.
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- Cooper, S. D., et al.
(författare)
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Quantifying spatial heterogeneity in streams
- 1997
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Ingår i: Journal of the North American Benthological Society. ; 16:1, s. 174-188
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Tidskriftsartikel (refereegranskat)abstract
- Although theoretical and empirical studies show that spatial heterogeneity has important effects on the dynamics of populations and the structure of communities, there has been little rigorous quantification of terms like ''patchiness'' or ''spatial heterogeneity'' in studies of lotic systems. In order to compare the spatial heterogeneity of different systems and understand the causes and consequences of that heterogeneity, we must first be able to quantitatively measure it. Spatial heterogeneity has many aspects that change with the scale of our observations, so we need a battery of descriptive measures that explicitly consider the scale-dependence of ecological pattern Response variables exhibiting similar frequency distributions (i.e., similar overall variability) can have very different spatial distributions; consequently, descriptions of spatial heterogeneity require spatial data, i.e., data related to geographic locations (maps). We review statistical techniques for quantitatively describing aspects of heterogeneity in spatial data, emphasizing the decomposition of heterogeneity into different scales of variation (trends, overall variability and spatial dependence or autocorrelation). Gradients in spatial data can be evaluated using trend analyses (e.g., regressions), whereas the spatial structure of variation around trends can be evaluated using geostatistical methods. The central concept of geostatistics is spatial dependence, which is the degree to which values of a response variable differ as a function of the distance (lag) between sampling locations. Semivariograms plot variation among samples separated by a common lag Versus lag, and can be objectively decomposed by piece-wise regression techniques to estimate the strength and scales of spatial dependence. A variety of other methods can be used to quantify spatial heterogeneity from categorical and numerical maps depending on the question of interest and the underlying structure of the spatial data (e.g., methods derived from fractal geometry and information theory, nearest neighbor analysis, spectral analysis, Mantel's test). Spatial heterogeneity in stream organisms is driven by local variation in environmental conditions, by interactions between individuals of the same or different species, and by the effects of organisms on their abiotic environment. By applying geostatistical methods to spatial data collected from field experiments, stream ecologists can evaluate the effects of biotic and abiotic factors on the spatial arrangement of organisms in streams. We present examples of data obtained from experiments examining how consumers affect, and respond to, spatial heterogeneity in their resources. The results indicate that consumer-resource feedbacks should be considered when modeling the causes and consequences of spatial heterogeneity in streams.
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- Jenkins, T. M., et al.
(författare)
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Effects of population density on individual growth of brown trout in streams
- 1999
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Ingår i: Ecology. ; 80:3, s. 941-956
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Tidskriftsartikel (refereegranskat)abstract
- Some studies suggest that lotic populations of brown trout (Salmo trutta) are regulated through density-dependent mortality and emigration to the extent that mean growth rates of resident survivors are unrelated to trout densities. To test this, we studied the relationship between density and growth, mortality, and emigration of brown trout in two alpine streams and a set of stream channels in eastern California. We sampled trout at the scale of "segments" (5-31 m long riffles, runs, and pools) and "sections" (340-500 m in length) of Convict Creek over a 3-yr period. Trout were also sampled during 6 yr in seven 90-m sections of Mammoth Creek. For 2 yr, we manipulated trout densities in Convict Creek by removing trout from two sections and adding trout to two other sections. We also manipulated densities in seven 50-m stream channels, using a natural size distribution of trout in one year and underyearlings only in a second year. In both streams, average size (body length or mass) of underyearlings in fall was negatively related to trout density and was furthermore affected by sampling location and year. The strong, negative relationship between individual mass and density of trout could be detected at the spatial scale of whole sections, but not at the scale of individual segments. The Convict Creek and stream channel experiments also revealed strong negative effects of density on average mass of underyearlings in fall, and on proportional mass increase of yearling and older trout from spring to fall. In contrast, mortality and emigration were unrelated to initial stocking densities in the channels. In all our data, the negative effects on growth were most pronounced at densities <1 trout/m(2) and the growth-density relationships were well described by negative power curves. Large individuals were always less affected by increasing trout density than were small individuals, suggesting a competitive advantage of large over small trout that increased with density. We conclude that individual growth of brown trout in streams can be affected by trout density to an extent that suggests a substantial influence on population regulation. Results from our multiyear, multiscale, and experimental study indicate that density dependence in the growth of stream salmonids will be difficult to detect in purely observational data, especially in systems with relatively high fish densities (where the growth-density relationship has a flat slope), when data are collected and analyzed at small spatial scales, and when insufficient information is collected to assess the contribution of interannual variation in growth.
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