Management and protection of quality of groundwater resources in the english midlands

The aquifers of the Severn-Trent region of the U.K. are important sources of public water supply. They are threatened by a variety of local and diffuse quality problems and significant contamination of these aquifers has occurred in some locations. This paper outlines some of these problems and describes the groundwater quality management and resource protection strategy adopted for the Severn-Trent area.     

Area detector corrections for high quality synchrotron X-ray structure factor measurements

Correction procedures for obtaining accurate X-ray structure factors from large area detectors are considered, including subpanel effects, over excited pixels and careful intensity corrections. Problems associated with data normalization, the use of a pixel response correction from a glass standard and minimization of systematic errors are also discussed. Data from glassy GeSe₂ and liquid water measured with a Perkin Elmer amorphous-Silicon detector are used to demonstrate the effectiveness of these correction procedures. This requires reduction of systematic errors in the measured intensity to around the 0.1% level.     

A comparison of three estimators of the Weibull parameters

Using mean square error as the criterion, we compare two least squares estimates of the Weibull parameters based on non‐parametric estimates of the unreliability with the maximum likelihood estimates (MLEs). The two non‐parametric estimators are that of Herd–Johnson and one recently proposed by Zimmer. Data was generated using computer simulation with three small sample sizes (5, 10 and 15) with three multiply‐censored patterns for each sample size. Our results indicate that the MLE is a better estimator of the Weibull characteristic value, θ, than the least squares estimators considered. No firm conclusions may be made regarding the best estimate of the Weibull shape parameter, although the use of maximum likelihood is not recommended for small sample sizes. Whenever least squares estimation of both Weibull parameters is appropriate, we recommend the use of the Zimmer estimator of reliability. Copyright © 2001 John Wiley & Sons, Ltd.     

Charge-coupled-device detection of soft x rays for grazing-incidence spectrometers

We describe calculations of the sensitivity of CCD's to soft x rays incident at grazing angles. Soft-x-ray spectrometers based on a Rowland-circle geometry have a focal plane and hence detector surface at grazing incidence to the soft × rays. We model the penetration of the grazing-incidence soft × rays to the sensitive region of the CCD and predict the charge-collection efficiency as a function of wavelength. The results show significant advantages over microchannel plate intensified detection.     

The passage of homopolymeric RNA through small solid-state nanopores

Solid-state nanopores are widely acknowledged as tools with which to study local structure in biological molecules. Individual molecules are forced through a nanopore, causing a characteristic change in an ionic current that depends on the molecules' local diameter and charge distribution. Here, the translocation measurements of long (~5-30 kilobases) single-stranded poly(U) and poly(A) molecules through nanopores ranging from 1.5 to 8 nm in diameter are presented. Individual molecules are found to be able to cause multiple levels of conductance blockade upon traversing the pore. By analyzing these conductance blockades and their relative incidence as a function of nanopore diameter, it is concluded that the smallest conductance blockades likely correspond to molecules that translocate through the pore in predominantly head-to-tail fashion. The larger conductance blockades are likely caused by molecules that arrive at the nanopore entrance with many strands simultaneously. These measurements constitute the first demonstration that single-stranded RNA can be captured in solid-state nanopores that are smaller than the diameter of double-stranded RNA. These results further the understanding of the conductance blockades caused by nucleic acids in solid-state nanopores, relevant for future applications, such as the direct determination of RNA secondary structure.     

Upgrading of low-grade gold ore samples for improved particle characterisation using Micro-CT and SEM/EDX

There is a considerable challenge in accurate characterisation of gold (Au) particles in low-grade plant ore mineral samples. This is particularly true for automated mineralogical tools such as X-ray micro-computed tomography (Micro-CT) and scanning electron microscopy (SEM), where the need for statistically meaningful numbers of particles requires many sections to be analysed. Whiles the Vertical Gas Stream (VGS) elutriator is suitable for coarse particle upgrading (i.e. >38 μm), the performance is poor for finer particles (i.e. <38 μm). Consequently, the system has been modified to Vertical Water Stream (VWS) elutriator using higher density fluid (i.e. water) to enable analysis of Au particles below 38 μm. In this work, the VGS system was used to upgrade Au particles in the ?53 + 38 μm size fraction (in rougher concentrate, rougher tailings, regrind mill discharge and regrind cyclone underflow) and the VWS system was used to upgrade Au particles in the ?38 μm size fraction of the regrind mill discharge sample. The VWS elutriator was calibrated using galena (specific gravity, S.G. of 7.58) and quartz (S.G. of 2.65) particles of <38 μm size as model minerals. From the calibration tests, partition curves as a function of particle size were generated. Using these measurements, theoretical partition curves for Au (S.G. of 19.3) have been calculated. The VWS concentrate was characterised using Micro-CT and compared with SEM coupled with energy dispersive X-ray (EDX) analysis of ?53 + 38 μm ore size fraction of the VGS concentrate of the four sample streams. The Micro-CT analysis of VWS Au concentrate showed that sufficient particles (Au) can be upgraded. SEM/EDX results indicate that regrind does not affect changes in free Au particle morphology, aspect ratio and frequency of shearing damage in the ?53 + 38 μm size fraction. Cyclone classification of the regrind mill discharge in the ?53 + 38 μm size fraction appears to perform surface cleaning by exposing obscuring silver (Ag) surfaces on Au particles in the mill discharge sample.     

Waveguiding Fresnel Lenses: Modelling and Fabrication

Abstract

This paper examines the methods, problems and results of modelling Fresnel lenses, and the progress towards fabricating experimental devices that has resulted from these evaluations. The computer model used ray-tracing through phase-weighted two-dimensional Fresnel lenses overlaid onto optical waveguide structures. Several types of lenses were considered, digital phase shift, analogue and photolithographically optimized forms being selected for particular study. The evaluation was directed to determining the effect on performance of changes in dimensions, selected focal length and materials. Negative form lens structures were included, as were a range of fabrication methodologies, so that substrate anisotropy and overlayer refractive index were necessarily surveyed. The lens fabrication will ultimately employ three main technologies: dilute-melt proton-exchange in LiNbO₃, GaAlAs and Nb₂O₅ layers as the guiding films, with lens overlays in a range of deposited films with refractive indices up to 3·85. We report preliminary experimental results leading towards such lenses. For a typical lens, aberration and optical power transfer efficiency were measured and examined in relation to the expected coupling efficiency of the lens overlay to the optical waveguide (the ‘overlap’ integral).     

Design and imaging performance of achromatic diffractive-refractive x-ray and gamma-ray Fresnel lenses

Achromatic combinations of a diffractive phase Fresnel lens and a refractive correcting element have been proposed for x-ray and gamma-ray astronomy and for microlithography, but considerations of absorption often dictate that the refractive component be given a stepped profile, resulting in a double Fresnel lens. The imaging performance of corrected Fresnel lenses, with and without stepping, is investigated, and the trade-off between resolution and useful bandwidth in different circumstances is discussed. Provided that the focal ratio is large, correction lenses made from low atomic number materials can be used with x rays in the range of approximately 10-100 keV without stepping. The use of stepping extends the possibility of correction to higher-aperture systems, to energies as low as a few kilo electron volts, and to gamma rays of mega electron volt energy.     

Isoniazid poisoning: Pharmacokinetics and effect of hemodialysis in a massive ingestion

Isoniazid is a rare overdose that causes seizures and there is limited evidence to guide treatment. We report a 20‐year‐old female migrant who presented with recurrent seizures after ingesting 25 g of isoniazid. She was treated with activated charcoal, repeated doses of midazolam for the seizures, and given multiple doses of pyridoxine (14 mg), limited by availability. She was admitted to intensive care, and 5.5 hours post‐ingestion, she was commenced on continuous veno‐venous hemodiafiltration (CVVHDF). She was extubated after 24 hours and CVVHDF was ceased 6 hours later (30 hours post‐overdose). Her renal function remained normal and her initial lactate was the highest at 2.3. She made a full recovery. Five plasma samples were collected before, during, and after CVVHDF, and isoniazid was quantified with liquid chromatography‐tandem mass spectrometry. A pharmacokinetic analysis of time‐isoniazid concentration data was fitted to a two‐compartment model with first‐order input (with fixed k_a) with the effect of CVVHDF modeled as a time‐dependent covariate. This suggested that there was initially good clearance with CVVHDF (4 times endogenous clearance), which rapidly declined within hours.