29,647 research outputs found
Excitation of g modes in Wolf-Rayet stars by a deep opacity bump
We examine the stability of l=1 and l=2 g modes in a pair of nitrogen-rich
Wolf-Rayet stellar models characterized by differing hydrogen abundances. We
find that modes with intermediate radial orders are destabilized by a kappa
mechanism operating on an opacity bump at an envelope temperature log T ~ 6.25.
This `deep opacity bump' is due primarily to L-shell bound-free transitions of
iron. Periods of the unstable modes span ~ 11-21 hr in the model containing
some hydrogen, and ~ 3-12 hr in the hydrogen-depleted model. Based on the
latter finding, we suggest that self-excited g modes may be the source of the
9.8 hr-periodic variation of WR 123 recently reported by Lefevre et al. (2005).Comment: 5 pages, 3 figures, accepted by MNRAS letter
Imaging interstitial iron concentrations in boron-doped crystalline silicon using photoluminescence
Imaging the band-to-band photoluminescence of silicon wafers is known to provide rapid and high-resolution images of the carrier lifetime. Here, we show that such photoluminescence images, taken before and after dissociation of iron-boron pairs, allow an accurate image of the interstitial iron concentration across a boron-doped p-type silicon wafer to be generated. Such iron images can be obtained more rapidly than with existing point-by-point iron mapping techniques. However, because the technique is best used at moderate illumination intensities, it is important to adopt a generalized analysis that takes account of different injection levels across a wafer. The technique has been verified via measurement of a deliberately contaminated single-crystal silicon wafer with a range of known iron concentrations. It has also been applied to directionally solidified ingot-grown multicrystalline silicon wafers made for solar cell production, which contain a detectible amount of unwanted iron. The iron images on these wafers reveal internal gettering of iron to grain boundaries and dislocated regions during ingot growth.D.M. is supported by an Australian Research Council
QEII Fellowship. The Centre of Excellence for Advanced
Silicon Photovoltaics and Photonics at UNSW is funded by
the Australian Research Council
Measuring dopant concentrations in compensated p-type crystalline silicon via iron-acceptor pairing
We present a method for measuring the concentrations of ionized acceptors and donors in compensated p-type silicon at room temperature.Carrier lifetimemeasurements on silicon wafers that contain minute traces of iron allow the iron-acceptor pair formation rate to be determined, which in turn allows the acceptor concentration to be calculated. Coupled with an independent measurement of the resistivity and a mobility model that accounts for majority and minority impurity scatterings of charge carriers, it is then possible to also estimate the total concentration of ionized donors. The method is valid for combinations of different acceptor and donor species.D.M. is supported by an Australian Research Council
fellowship. L.J.G. would like to acknowledge SenterNovem
for support
Thematic mapper data quality and performance assessment in renewable resource/agricultural remote sensing
A "quick look" investigation of the initial LANDSAT-4, thematic mapper (TM) scene received from Goddard Space Flight Center was performed to gain early insight into the characteristics of TM data. The initial scene, containing only the first four bands of the seven bands recorded by the TM, was acquired over the Detroit, Michigan, area on July 20, 1982. It yielded abundant information for scientific investigation. A wide variety of studies were conducted to assess all aspects of TM data. They ranged from manual analyses of image products to detect obvious optical, electronic, or mechanical defects to detailed machine analyses of the digital data content for evaluation of spectral separability of vegetative/nonvegetative classes. These studies were applied to several segments extracted from the full scene. No attempt was made to perform end-to-end statistical evaluations. However, the output of these studies do identify a degree of positive performance from the TM and its potential for advancing state-of-the-art crop inventory and condition assessment technology
Spatially partitioned embedded Runge-Kutta Methods
We study spatially partitioned embedded Runge–Kutta (SPERK) schemes for partial differential equations (PDEs), in which each of the component schemes is applied over a different part of the spatial domain. Such methods may be convenient for problems in which the smoothness of the solution or the magnitudes of the PDE coefficients vary strongly in space. We focus on embedded partitioned methods as they offer greater efficiency and avoid the order reduction that may occur in non-embedded schemes. We demonstrate that the lack of conservation in partitioned schemes can lead to non-physical effects and propose conservative additive schemes based on partitioning the fluxes rather than the ordinary differential equations. A variety of SPERK schemes are presented, including an embedded pair suitable for the time evolution of fifth-order weighted non-oscillatory (WENO) spatial discretizations. Numerical experiments are provided to support the theory
AC impedance study of degradation of porous nickel battery electrodes
AC impedance spectra of porous nickel battery electrodes were recorded periodically during charge/discharge cycling in concentrated KOH solution at various temperatures. A transmission line model (TLM) was adopted to represent the impedance of the porous electrodes, and various model parameters were adjusted in a curve fitting routine to reproduce the experimental impedances. Degradation processes were deduced from changes in model parameters with electrode cycling time. In developing the TLM, impedance spectra of planar (nonporous) electrodes were used to represent the pore wall and backing plate interfacial impedances. These data were measured over a range of potentials and temperatures, and an equivalent circuit model was adopted to represent the planar electrode data. Cyclic voltammetry was used to study the characteristics of the oxygen evolution reaction on planar nickel electrodes during charging, since oxygen evolution can affect battery electrode charging efficiency and ultimately electrode cycle life if the overpotential for oxygen evolution is sufficiently low
UNICEF IWASH Project, Northern Region, Ghana : an adapted training manual for groundwater development
This report is an adapted training manual, with specific best practice recommendations for
groundwater development practitioners working in the Northern Region, Ghana. It is designed to
be used in conjunction with the existing comprehensive training manual ‘Developing
Groundwater: a guide to rural water supply’ by MacDonald, Davies, Calow and Chilton (2005).
The additional guidelines provided in this supplementary report are specific to the Northern
Region of Ghana, and have been informed by a review of groundwater development in the
region which BGS carried out on behalf of UNICEF in 2010-2011.
The Northern Region is a difficult area in which to find and develop groundwater resources. For
this reason, more resources – time and money – need to be focussed on careful borehole siting
and development in order to maximise success. This includes detailed desk and field
reconnaissance surveys; the effective use and interpretation of geophysical siting methods;
collection of good quality data during drilling and test pumping; rigorous recording and
management of data; and effective interpretation, sharing and use of hydrogeological
information by all groundwater development practitioners. This report, and the associated
manual ‘Developing Groundwater’, provide practical help for carrying out these activities
effectively.
The authors gratefully acknowledge those persons who contributed to the formation of these
guidelines, who include:
UNICEF Ghana – Othniel Habila, Kabuka Banda and David Ede
Community Water and Sanitation Agency (CWSA), Ghana – John Aduakye
Canadian International Development Agency (CIDA) – Hydrogeological Assessment
Project (HAP) – James Racicot
All participants at the UNICEF/BGS workshop and training programme held in Tamale,
Northern Region, from 7 to 18 February 2011
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