A new model of cosmogenic production of radiocarbon 14C in the atmosphere

We present the results of full new calculation of radiocarbon 14C production in the Earth atmosphere, using a numerical Monte-Carlo model. We provide, for the first time, a tabulated 14C yield function for the energy of primary cosmic ray particles ranging from 0.1 to 1000 GeV/nucleon. We have calculated the global production rate of 14C, which is 1.64 and 1.88 atoms/cm2/s for the modern time and for the pre-industrial epoch, respectively. This is close to the values obtained from the carbon cycle reservoir inventory. We argue that earlier models overestimated the global 14C production rate because of outdated spectra of cosmic ray heavier nuclei. The mean contribution of solar energetic particles to the global 14C is calculated as about 0.25% for the modern epoch. Our model provides a new tool to calculate the 14C production in the Earth's atmosphere, which can be applied, e.g., to reconstructions of solar activity in the past.Comment: Published in EPSL, 337, 114, 201

High magnetic field effects in low-dimensional carbon nanostructures

This thesis describes studies of graphene, single walled carbon nanotubes (SWNTs) and InSb. Optical and electronic measurements probe the effects of high magnetic fields on these low-dimensional systems. Chapter 1 introduces a theoretical description and background behind the materials and physical phenomena studied in this work. The structure and unique properties of carbon nano-materials are described. The experimental methods used in this thesis are described in Chapter 2. Chapter 3 describes magnetotransport measurements on InSb/AlInSb heterostructures revealing that the large energy gaps, and extremely high mobility, associated with this system leads to exceptionally well defined quantum Hall plateaux for both even (Landau level) and odd (spin-split) filling factors. Even higher cyclotron energy gaps are expected in graphene. Chapter 4 reveals that due to a combination of large cyclotron energy gaps and fast electron-phonon energy loss rates, the quantum Hall effect (QHE) in graphene can be observed to unprecedented current densities (43 A/m) and temperatures (> 45 K). The behaviour of epitaxial graphene grown on silicon carbide in the quantum Hall regime is shown to be characterised by a strongly magnetic field dependent carrier density due to charge transfer from surface donor states in the substrate. Chapter 5 shows that polymer wrapping of SWNTs can achieve high quality purified samples. Individual SWNTs were probed using micro-photoluminescence measurements in magnetic fields up to 30 T. The combination of high magnetic fields and high spectral and spatial resolution allowed a detailed study of exciton fine structure. High intensity laser irradiation is shown to induce bound excitons in pristine tubes. The optical properties of a number of tubes are dominated by defect sites which may be imaged along the tube using the magnetic brightening of dark excitons associated with such defects

Disorder induced Dirac-point physics in epitaxial graphene from temperature-dependent magneto-transport measurements

We report a study of disorder effects on epitaxial graphene in the vicinity of the Dirac point by magneto-transport. Hall effect measurements show that the carrier density increases quadratically with temperature, in good agreement with theoretical predictions which take into account intrinsic thermal excitation combined with electron-hole puddles induced by charged impurities. We deduce disorder strengths in the range 10.2 $\sim$ 31.2 meV, depending on the sample treatment. We investigate the scattering mechanisms and estimate the impurity density to be $3.0 \sim 9.1 \times 10^{10}$ cm$^{-2}$ for our samples. An asymmetry in the electron/hole scattering is observed and is consistent with theoretical calculations for graphene on SiC substrates. We also show that the minimum conductivity increases with increasing disorder potential, in good agreement with quantum-mechanical numerical calculations.Comment: 6 pages, 3 figure

Determination of the QCD color factor ratio CA/CF from the scale dependence of multiplicity in three jet events

I examine the determination of the QCD color factor ratio CA/CF from the scale evolution of particle multiplicity in e+e- three jet events. I fit an analytic expression for the multiplicity in three jet events to event samples generated with QCD multihadronic event generators. I demonstrate that a one parameter fit of CA/CF yields the expected result CA/CF=2.25 in the limit of asymptotically large energies if energy conservation is included in the calculation. In contrast, a two parameter fit of CA/CF and a constant offset to the gluon jet multiplicity, proposed in a recent study, does not yield CA/CF=2.25 in this limit. I apply the one parameter fit method to recently published data of the DELPHI experiment at LEP and determine the effective value of CA/CF from this technique, at the finite energy of the Z0 boson, to be 1.74+-0.03+-0.10, where the first uncertainty is statistical and the second is systematic.Comment: 20 pages including 6 figures Version 2 corrects typographical error in equation (2

Renormalon resummation and exponentiation of soft and collinear gluon radiation in the thrust distribution

The thrust distribution in e+e- annihilation is calculated exploiting its exponentiation property in the two-jet region t = 1-T << 1. We present a general method (DGE) to calculate a large class of logarithmically enhanced terms, using the dispersive approach in renormalon calculus. Dressed Gluon Exponentiation is based on the fact that the exponentiation kernel is associated primarily with a single gluon emission, and therefore the exponent is naturally represented as an integral over the running coupling. Fixing the definition of Lambda is enough to guarantee consistency with the exact exponent to next-to-leading logarithmic accuracy. Renormalization scale dependence is avoided by keeping all the logs. Sub-leading logs, that are usually neglected, are factorially enhanced and are therefore important. Renormalization-group invariance as well as infrared renormalon divergence are recovered in the sum of all the logs. The logarithmically enhanced cross-section is evaluated by Borel summation. Renormalon ambiguity is then used to study power corrections in the peak region Qt \gsim Lambda, where the hierarchy between the renormalon closest to the origin (~1/Qt) and others (~1/(Qt)^n) starts to break down. The exponentiated power-corrections can be described by a shape-function, as advocated by Korchemsky and Sterman. Our calculation suggests that the even central moments of the shape-function are suppressed. Good fits are obtained yielding alpha_s^{MSbar} (M_Z) = 0.110 \pm 0.001, with a theoretical uncertainty of ~5%.Comment: Correction in section 3.2, eqs. (46) to (50); journal versio

Measuring Effects of Housing Densities on Property Values Using Locally Weighted Regression

The objective of this study is to determine spatial differences in the marginal values of housing density and their implications for housing development projects. In order to achieve this objective, the values households place on neighborhood density were measured using locally weighted regression in a hedonic housing-price framework. The neighborhood housing density coefficient in the global model confirms the positive and significant value of lower neighborhood density. The spatial distribution of the housing density marginal effects from the local model shows the variation of site-specific values of neighborhood housing density. The marginal effects of lower neighborhood density gradually increase closer to the Town of Farragut, on the west end of the county. The Town of Farragut and adjacent areas show the highest marginal effects of lower neighborhood density on housing price. It was found that the marginal implicit price for one fewer house per acre in the Farragut area was $1 1 ,964 whereas the whole Knox County area was$3,53 1 . Based on these marginal implicit prices, open space in Farragut is valued 3 .4 times greater than open space in the whole Knox County study area. Understanding the spatial variation in values of neighborhood density across specific locations can lead to smart growth policies that are more appropriate for site-specific conditions. For example, a site-specific conservation subdivision ordinance could be developed based on this implicit price ratio. Each conservation subdivision is required to set aside a minimum percentage of its adjusted tract acreage as open space. Typically, the minimum amount of adjusted tract acreage is defined rather arbitrarily without systematic consideration of households\u27 preferences about open space. For instance, based on the ratio found in this study, 3.4 times more open space within a conservation subdivision could be set aside for open space in the Farragut area compared with the overall area. Since the Farragut area has a smaller neighborhood density than the overall area, more abundant open space outside a subdivision in the Farragut area should substitute for the minimum open space requirement within the subdivision. Thus, the open space that is set aside for a subdivision in the Farragut area should be less than 3.4 times the amount set aside for a subdivision in the overall area. In conclusion, smart growth policies promoting only high-density development are incapable of providing a cure for urban sprawl without also considering spatial variation in the values of neighborhood density

Talc-dominated seafloor deposits reveal a new class of hydrothermal system

The Von Damm Vent Field (VDVF) is located on the flanks of the Mid-Cayman Spreading Centre, 13?km west of the axial rift, within a gabbro and peridotite basement. Unlike any other active vent field, hydrothermal precipitates at the VDVF comprise 85–90% by volume of the magnesium silicate mineral, talc. Hydrothermal fluids vent from a 3-m high, 1-m diameter chimney and other orifices at up to 215?°C with low metal concentrations, intermediate pH (5.8) and high concentrations (667?mmol?kg?1) of chloride relative to seawater. Here we show that the VDVF vent fluid is generated by interaction of seawater with a mafic and ultramafic basement which precipitates talc on mixing with seawater. The heat flux at the VDVF is measured at 487±101?MW, comparable to the most powerful magma-driven hydrothermal systems known, and may represent a significant mode of off-axis oceanic crustal cooling not previously recognized or accounted for in global models