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A fine-grained, up to 3-m-thick tephra bed in southwestern Saskatchewan, herein named Duncairn tephra (Dt), is derived from an early Pleistocene eruption in the Jemez Mountains volcanic field of New Mexico, requiring a trajectory of northward tephra dispersal of ~1500 km. An unusually low CaO content in its glass shards denies a source in the closer Yellowstone and Heise volcanic fields, whereas a Pleistocene tephra bed (LSMt) in the La Sal Mountains of Utah has a very similar glass chemistry to that of the Dt, supporting a more southerly source. Comprehensive characterization of these two distal tephra beds along with samples collected near the Valles caldera in New Mexico, including grain size, mineral assemblage, major- and trace-element composition of glass and minerals, paleomagnetism, and fission-track dating, justify this correlation. Two glass populations each exist in the Dt and LSMt. The proximal correlative of Dt1 is the plinian Tsankawi Pumice and co-ignimbritic ash of the first ignimbrite (Qbt1g) of the 1.24 Ma Tshirege Member of the Bandelier Tuff. The correlative of Dt2 and LSMt is the co-ignimbritic ash of Qbt2. Mixing of Dt1 and Dt2 probably occurred during northward transport in a jet stream.
Field studies of grazing management have frequently concluded that the magnitude and direction of vegetation response is dependent on initial vegetation condition. On upland heath, this dependence reflects the importance of small-scale ecological processes (e.g. plant competition), and local neighbourhood effects (e.g. spatial distribution of plant species), in driving the vegetation dynamics. These small-scale effects, together with variation in grazing patterns, increase the difficulty of deriving general rules about the effect of grazing on vegetation change from field studies. However, we need to determine the impacts of such grazing-related vegetation change upon biodiversity, (e.g. birds). For many bird species it is impractical to use experimental approaches due to low breeding densities, and the influence of other site and management effects (e.g. predator control). To predict the effect of management changes on them requires an accurate assessment of the large-scale effects of grazing management on the ecological landscape using data from small-scale field studies. This paper sets out an approach that integrates field studies with theoretical models to investigate the large-scale effects of grazing management on plant and bird communities on upland heath.
We describe the performance of the Boolardy Engineering Test Array, the prototype for the Australian Square Kilometre Array Pathfinder telescope. Boolardy Engineering Test Array is the first aperture synthesis radio telescope to use phased array feed technology, giving it the ability to electronically form up to nine dual-polarisation beams. We report the methods developed for forming and measuring the beams, and the adaptations that have been made to the traditional calibration and imaging procedures in order to allow BETA to function as a multi-beam aperture synthesis telescope. We describe the commissioning of the instrument and present details of Boolardy Engineering Test Array’s performance: sensitivity, beam characteristics, polarimetric properties, and image quality. We summarise the astronomical science that it has produced and draw lessons from operating Boolardy Engineering Test Array that will be relevant to the commissioning and operation of the final Australian Square Kilometre Array Path telescope.
A detailed understanding of the response of mineral phases to the radiation fields experienced in a geological disposal facility (GDF) is currently poorly constrained. Prolongued ion irradiation has the potential to affect both the physical integrity and oxidation state of materials and therefore may alter a structure's ability to react with radionuclides. Radiohalos (spheres of radiation damage in minerals surrounding radioactive (α-emitting) inclusions) provide useful analogues for studying long term α-particle damage accumulation. In this study, silicate minerals adjacent to Th- and U-rich monazite and zircon were probed for redox changes and long/short range disorder using microfocus X-ray absorption spectroscopy (XAS) and high resolution X-ray diffraction (XRD) at Beamline I18, Diamond Light Source. Fe3+ → Fe2+ reduction has been demonstrated in an amphibole sample containing structural OH– groups – a trend not observed in anhydrous phases such as garnet. Coincident with the findings of Pattrick et al. (2013), the radiolytic breakdown of OH– groups is postulated to liberate Fe3+ reducing electrons. Across all samples, high point defect densities and minor lattice aberrations are apparent adjacent to the radioactive inclusion, demonstrated by micro-XRD.
Geological disposal facilities (GDF) are intended to isolate and contain radioactive waste within multiple protective barriers, deep underground, to ensure that no harmful quantities of radioactivity reach the surface environment. The last line of defense in a multi-barrier GDF is the geosphere, where iron is present in the host rock mineralogy as either Fe(II) or Fe(III), and in groundwater as Fe(II) under reducing conditions. The mobility of risk-driving radionuclides, including uranium and technetium, in the environment is affected significantly by their valence state. Due to its low redox potential, Fe(II) can mediate reduction of these radionuclides from their oxidized, highly mobile, soluble state to their reduced, insoluble state, preventing them from reaching the biosphere. Here a study of five types of potential host rocks, two granitoids, an andesite, a mudstone and a clay-rich carbonate, is reported. The bulk rocks and their minerals were analysed for iron content, Fe(II/III) ratio, and for the speciation and fine-grained nature of alteration product minerals that might have important controls on groundwater interaction. Total iron content varies between 0.9% in clays to 5.6% in the andesite. X-ray absorption spectroscopy reveals that Fe in the granitoids and andesite is predominantly Fe(II), and in mudstones, argillaceous limestone and terrestrial sandstone is predominantly Fe(III). The redox reactivity of the potential host rocks both in the presence and absence of Fe(II)-containing 'model' groundwater was investigated using an azo dye as a probe molecule. Reduction rates as determined by reactivity with the azo dye were correlated with the ability of the rocks to uptake Fe(II) from groundwater rather than with initial Fe(II) content. Potential GDF host rocks must be characterized in terms of mineralogy, texture, grain size and bulk geochemistry to assess how they might interact with groundwater. This study highlights the importance of redox reactivity, not just total iron and Fe(II)/(III) ratio, when considering the host rock performance as a barrier material to limit transport of radionuclides from the GDF.
Emission features from ionized carbon dioxide and carbon monoxide were measured in the 1900- to 4300-Å spectral region. The Lyman-α 1216-Å line of atomic hydrogen and the 1304-, 1356-, and 2972-Å lines of atomic oxygen were observed.
The ESRO satellite COS-B carries one single experiment aiming at the measurement of arrival direction and energy of celestial gamma rays with energies between 25 MeV and 10 GeV. The experiment is conventional in design and consists of a veto counter, a wire spark chamber, a telescope and an energy calorimeter.
The energy measurement is obtained by a CsI scintillation crystal of 4.7 radiation length thickness. The expected energy resolution at 100 MeV is 50% FWHM. The other detector elements are designed as to cause the least possible degradation of the energy measurement.
The possibilities for the detection of a small contribution of π -origin gamma rays in the presence of a power-law type background spectrum will be discussed.
This paper describes the system architecture of a newly constructed radio telescope – the Boolardy engineering test array, which is a prototype of the Australian square kilometre array pathfinder telescope. Phased array feed technology is used to form multiple simultaneous beams per antenna, providing astronomers with unprecedented survey speed. The test array described here is a six-antenna interferometer, fitted with prototype signal processing hardware capable of forming at least nine dual-polarisation beams simultaneously, allowing several square degrees to be imaged in a single pointed observation. The main purpose of the test array is to develop beamforming and wide-field calibration methods for use with the full telescope, but it will also be capable of limited early science demonstrations.
Combined microfocus XAS and XRD analysis of α-particle radiation damage haloes around thorium-containing monazite in Fe-rich biotite reveals changes in both short- and long-range order. The total α-particles flux derived from the Th and U in the monazite over 1.8 Ga was 0.022 α particles per atomic component of the monazite and this caused increasing amounts of structural damage as the monazite emitter is approached. Short-range order disruption revealed by Fe K-edge EXAFS is manifest by a high variability in Fe–Fe bond lengths and a marked decrease in coordination number. XANES examination of the Fe K-edge shows a decrease in energy of the main absorption by up to 1 eV, revealing reduction of the Fe3+ components of the biotite by interaction with the 24He2+, the result of low and thermal energy electrons produced by the cascade of electron collisions. Changes in d spacings in the XRD patterns reveal the development of polycrystallinity and new domains of damaged biotite structure with evidence of displaced atoms due to ionization interactions and nuclear collisions. The damage in biotite is considered to have been facilitated by destruction of OH groups by radiolysis and the development of Frenkel pairs causing an increase in the trioctahedral layer distances and contraction within the trioctahedral layers. The large amount of radiation damage close to the monazite can be explained by examining the electronic stopping flux.
Murray C. Grant, Royal Society for the Protection of Birds Scotland and RPS Planning & Development,
James W. Pearce-Higgins, Royal Society for the Protection of Birds Scotland and British Trust for Ornithology
Moorlands represent the largest extent of semi-natural habitat remaining in Britain. They provide habitat for some of Britain’s most spectacular and charismatic bird species, and yet may often seem vast tracts of barren, inhospitable land, devoid of life. Moorlands occur in other temperate-zone uplands (e.g. in Scandinavia and Russia), as well as in some high-altitude tropical zones (e.g. the Andean páramos) (Holden et al., 2007). Britain’s moorland bird assemblages have been extensively studied and this chapter summarises current knowledge of their regional and habitat variation.
Definitions of moorland vary, but here we use the term to mean the open habitats lying between the upper edge of enclosed farmland and the original climax upper tree-line. We exclude the alpine zone (see Chapter 9). Throughout much of Britain, moorlands are found at altitudes from c. 300 m up to 700 m, but descend to sea level in northern Scotland. They occur in areas of high precipitation, generally on infertile acidic soils, but sometimes overlie base-rich bedrock (Thompson et al., 1995). The main habitat types comprise dwarf-shrub heath, blanket bog and acid grassland (see Fig. 8.1 for examples). Dwarf-shrub heath has >25% dwarf-shrub cover, with heather Calluna vulgaris usually dominant. Indeed, British moorland includes the world’s largest expanse of heather dominated habitats. Wet, cool climate and flat topography facilitates the development of deep peat soils on which blanket bog occurs, and where typical plants include cottongrass Eriophorum spp. and bog mosses Sphagnum spp. Acid grasslands are often dominated by mat grass Nardus stricta and heath rush Juncus squarrosus. These habitat types together cover in excess of 5 million ha of Britain, over 20% of the total land area (Haines-Young et al., 2000; Fig. 8.2).
A study has been carried out on the evolution of light induced defects in protocrystalline (diluted) a-Si:H films under 1 sun illumination. A room temperature reversal is observed in the photocurrents at 25°C, which is consistent with the relaxation in the recombination currents on corresponding p-i-n solar cells. It is also consistent with the pressure of “fast” states such as have been observed after high intensity illumination. Even with the limitations imposed by the relaxation in the light induced changes on the subgap absorption measurements, the evolution of distinctly different gap states centered around 0.9 and 1.15eV from the conduction band was identified. The kinetics of the electron occupied states, kN(E), at these two energies is compared with that of the neutral dangling bond (D0) densities as measured with electron spin resonance. Because of the similarity between the preliminary results of these kinetics it has not been possible to identify which states correspond to the D0 nor to draw any reliable conclusions about the nature of the different states.
The effect of heavily doping silicon3with boron (up to 9.0×19/cm3) or antimony and arsenic (up to 1.0×19/cm3) on the nucleation and growth of bulk stacking faults (BSF) was investigated by the use of three-step heat treatment; 700°C-24 hrs., 900 °C 24 hrs., and 1050 °C up to 48 hrs.
The length and density of BSF were determined by optical microscopy after a Secco or Schimmel etch of cleaved specimens. From the effect of heavy doping on the density of BSF we imply the nucleation rate is doping level dependant. The effect of the low temperature (700°C) heat treatment time on the relationship between the density and the length of BSF indicates that the growth of BSF may be a supply-limited process. The effect of a high temperature (1300°C) oxygen dispersion treatment was also investigated. The results are analyzed in terms of a vacancy model.
The effect of pressure on self diffusion in crystalline silicon is being studied using 30Si as a tracer. Diffusion experiments have been carried out in the pressure range of 1 to 35000 atmospheres at 1000°C. The 30Si is observed to diffuse faster at high pressures, indicating a negative activation volume.
Boron diffusion profiles in single crystal silicon from highly doped polysilicon sources have been measured using SIMS after diffusion at 950°C for various times. These data have been analyzed to determine D(c) of boron in the single crystal. It is ncrmally assumed that at high boron concentrations D increases linearly with concentration. However, the shape of these profiles indicates that for a polysilicon source, this behavior does not appear to hold. Using Bolzmann-Matano analysis, D(c) was found to be insensitive to boron concentrations above 3.0E19 atoms/cm3. The results of this analysis were confirmed by using them as input to a diffusion simulation computer program and excellent agreement with the experimental profiles was obtained. The value of D was found to be unusually high at all concentrations in the single crystal and increased almost linearly with the doping level of the polysilicon. The effect of the furnace ramp-down cycle on the profiles near the crystal surface have also been investigated.
In this paper we report preliminary observations of oxygen precipitation in degenerately-doped silicon using etching, optical microscopy and transmission electron microscopy. It was found that n+ material was resistant to precipitation, but p+ material precipitated readily. A multistep heat treatment starting with a low temperature step to achieve a high supersaturation ratio was sucessfully used to induce precipitation in n+ material.
This paper presents the results of a systematic TEM investigation to understand the chemical, morphological and microstructural makeup of photoresist residues following oxygen plasma ashing. The investigation was carried out on generic non-product control wafers and product wafers with a view to help aid in the identification of a suitable post-ashing cleaning process. Specimens in planview were prepared by mechanical grinding and Argon ion milling from the substrate end of the sample. Energy Dispersive X-ray Spectrometry (EDS) was used to obtain the composition and selected area electron diffraction (SAD) was used to obtain information on crystallinity. The residues were essentially amorphous and were found to be arsenic-rich in composition and exhibited a dropletlike morphology decorating areas where the photoresist was used as an implantation mask. In addition, hexagonally shaped crystals and spherical particulates of varying dimensions were seen in several regions and were found to be carbon rich.
The use of poly-buffered LOCOS processing is a common feature of many
sub-micron integrated circuit fabrication processes. However, the silicon
layer interposed between the nitride oxidation mask and the pad oxide is
often difficult to remove. Different strategies involve dry and/or wet
etching of the film. We have demonstrated the utility of adding metals such
as Fe or Cu to a conventional phosphoric bath used to etch silicon nitride.
Briefly, the presence of the metals is thought to result in a classic
oxidation-reduction reaction between the metal and the silicon. Additions of
60ppm of Cu+2 resulted in etch rates of 20Å /min. on undoped
polysilicon at a process temperature of 165°C, whereas, the etch rate of
SiO2 was less than 1Å/min. Similar results were obtained for
additions of Fe+3 and other metals.
We have revisited the issue of p-layer optimization for amorphous silicon (a-Si:H) solar cells, correlating spectroscopic ellipsometry (SE) measurements of the p-layer in the device configuration with light current-voltage (J-V) characteristics of the completed solar cell. Working with p-layer gas mixtures of H2/SiH4/BF3 in rf plasma-enhanced chemical vapor deposition (PECVD), we have found that the maximum open circuit voltage (Voc) for n-i-p solar cells is obtained using p-layers prepared with the maximum possible hydrogen-dilution gas-flow ratio R=[H2]/[SiH4], but without crossing the thickness-dependent transition from the a-Si:H growth regime into the mixed-phase amorphous + microcrystalline [(a+μc)-Si:H] regime for the ∼200 Å p-layers. As a result, optimum single-step p-layers are obtained under conditions similar to those applied for optimum i-layers, i.e., by operating in the so-called “protocrystalline” Si:H film growth regime. The remarkable dependence of the p-layer phase (amorphous vs. microcrystalline) and n-i-p solar cell Voc on the nature of the underlying i-layer surface also supports this conclusion.