Calibration of the TWIST high-precision drift chambers

A method for the precise measurement of drift times for the high-precision drift chambers used in the TWIST detector is described. It is based on the iterative correction of the space-time relationships by the time residuals of the track fit, resulting in a measurement of the effective drift times. The corrected drift time maps are parametrised individually for each chamber using spline functions. Biases introduced by the reconstruction itself are taken into account as well, making it necessary to apply the procedure to both data and simulation. The described calibration is shown to improve the reconstruction performance and to extend significantly the physics reach of the experiment.Comment: 8 pages, 5 figure

Factors mediating structure and trophic interactions of estuarine nekton communities

Understanding how communities and species assemblages persist is among the most fundamental objectives in ecology, particularly as human modifications to the landscape increase. Through application of traditional community metrics with emerging biochemical tracers in combination with community/food web ecology theory, I provide an evaluation of the effects of anthropogenically-altered freshwater flow disturbance on estuarine nekton community structure and trophic interactions. These two parameters are central toward understanding the functioning of aquatic communities and ensuring their persistence. This dissertation provides data regarding the effects of human-altered freshwater flow on estuarine nekton communities in tidal rivers and, in doing so, has fostered valuable findings regarding the application of stable isotopes to estuarine fishes and large vertebrates. Specifically, this research demonstrates that losses of estuarine nekton community biodiversity (Chapter 2), the shift in resource availability to lower trophic level species (Chapter 5), and changes to energy flow pathways leading to higher trophic level consumers (Chapter 6), are all associated with high flow events. This dissertation further demonstrates that the application of stable isotopes requires consideration of a species life history characteristics, as interpretation of a species diet and trophic roles can be complex (Chapters 3 and 4). Collectively, these findings suggest that high flow events affect the structure and trophic interactions of estuarine nekton communities and provide a greater understanding of the impacts of such anthropogenic-mediated stressors on these complex ecosystems. Whether altered high-flow disturbance events result in adverse or beneficial effects on the persistence of estuaries remains to be established. However, in order to maintain and/or restore the integrity of an ecosystem requires that conservation and management actions be firmly grounded in scientific understanding. This becomes especially relevant as worldwide changes to hydrologic connectivity continue with increasing anthropogenic pressures. This research demonstrates the potential for the simplification of food webs and changes to dominant trophic assemblages that are associated with flow alteration. For the commercially, recreationally and ecologically valuable species that define estuarine nekton communities, these observations emphasize the necessity of research and management programs aimed at maintaining the integrity of these highly-valued ecosystems

Nonharmonic oscillations of nanosized cantilevers due to quantum-size effects

Using a one-dimensional jellium model and standard beam theory we calculate the spring constant of a vibrating nanowire cantilever. By using the asymptotic energy eigenvalues of the standing electron waves over the nanometer-sized cross-section area, the change in the grand canonical potential is calculated and hence the force and the spring constant. As the wire is bent more electron states fits in its cross section. This has an impact on the spring"constant" which oscillates slightly with the bending of the wire. In this way we obtain an amplitude-dependent resonance frequency of the oscillations that should be detectable.Comment: 6 pages, 5 figure