Cosmogenic nuclides indicate that boulder fields are dynamic, ancient, multigenerational features

Boulder fields are found throughout the world; yet, the history of these features, as well as the processes that form them, remain poorly understood. In high and mid-latitudes, boulder fields are thought to form and be active during glacial periods; however, few quantitative data support this assertion. Here, we use in situ cosmogenic 10Be and 26Al to quantify the near-surface history of 52 samples in and around the largest boulder field in North America, Hickory Run, in central Pennsylvania, USA. Boulder surface 10Be concentrations (n = 43) increase downslope, indicate minimum near-surface histories of 70-600 k.y., and are not correlated with lithology or boulder size. Measurements of samples from the top and bottom of one boulder and three underlying clasts as well as 26Al/10Be ratios (n = 25) suggest that at least some boulders have complex exposure histories caused by flipping and/or cover by other rocks, soil, or ice. Cosmogenic nuclide data demonstrate that Hickory Run, and likely other boulder fields, are dynamic features that persist through multiple glacial-interglacial cycles because of boulder resistance to weathering and erosion. Long and complex boulder histories suggest that climatic interpretations based on the presence of these rocky landforms are likely over simplifications

Thinking about growth : a cognitive mapping approach to understanding small business development

School of Managemen

Scaling of Energy Gain with Plasma Parameters in a Plasma Wakefield Accelerator

We have recently demonstrating the doubling of the energy of particles of the ultra-short, ultra-relativistic electron bunches of the Stanford Linear Accelerator Center [1]. This energy doubling occurred in a plasma only 85 cm-long with a density of {approx} 2.6 x 10{sup 17} e{sup -}/cm{sup -3}. This milestone is the result of systematic measurements that show the scaling of the energy gain with plasma length and density, and show the reproducibility and the stability of the acceleration process. We show that the energy gain increases linearly with plasma length from 13 to 31 cm. These are key steps toward the application of beam-driven plasma accelerators or plasma wakefield accelerators (PWFA) to doubling the energy of a future linear collider without doubling its length

Electron Bunch Length Measurements in the E-167 Plasma Wakefield Experiment

Bunch length is of prime importance to beam driven plasma wakefield acceleration experiments due to its inverse relationship to the amplitude of the accelerating wake. We present here a summary of work done by the E167 collaboration measuring the SLAC ultra-short bunches via autocorrelation of coherent transition radiation. We have studied material transmission properties and improved our autocorrelation traces using materials with better spectral characteristics

Native dominants in British woodland – a potential cause of reduced species-richness?

The invasion of native habitats by alien species has received considerable attention. However, in Britain high levels of dominance by a small number of aggressive native plant species may have an equal, or greater, impact on the richness of native woodlands. Here, we examine this hypothesis by modelling the realized niche of native-dominant species along the principal coenocline of British woodlands, and examined niche overlaps with 78 woodland specialist species and two alien species. Four native species had a much greater cover than all other field-layer species, and between them they entirely covered the response range of all other field-layer species, replacing one another along the coenocline. These findings, combined with autecological information suggest that Hedera helix, Mercurialis perennis, Pteridium aquilinum and Rubus fruticosus have the potential to become ‘over-dominant’ and perhaps may impinge on other field-layer species. Our results also identified which field-layer species are likely to be impacted by a change in abundance of each of these dominant-species, and as such, provide a novel quantitative method of risk assessment to aid conservation policy. Understanding how woodland communities remain diverse, even in the presence of aggressive native species, may provide insights into how the impact of exotic invasive species can be managed