Recombination in cadmium mercury telluride photodetectors

Cadmium mercury telluride is of considerable importance as a material for the detection of IR radiation. Carrier lifetime has been studied intensively as it is the principal factor controlling detector performance. Bulk lifetime is dominated by Auger processes in the narrow bandgap material sensitive between 8 and 14 μm, while it is dominated by radiative recombination in the wider bandgap material sensitive below 5 μm. Auger processes have been studied by observing the photoconductive decay as a function of temperature. This has led to an experimental determination of the overlap integral as 0.3. A fresh calculation of radiative lifetime by the van Roosbroek-Shockley method has led to an analytic expression that agrees well with observed lifetime. Recombination at discontinuities (contacts, surfaces and flaws introduced in processing) are of importance in the photoconductive detectors. Surface recombination velocity can be reduced to low values (less than 200 cm s⁻¹) in n-type material by obtaining an accumulated surface. The rate limiting processes are then transitions between filled surface states and holes. No such accumulation appears to occur at the contacts or lines of damage introduced in processing. As a result there is considerable recombination at these features. When lifetime is controlled by transit time effects it is called sweepout. In sweepout the dependence of ambipolar mobility on majority carrier concentration leads to novel effects. Auger lifetime is reduced in low carrier concentration samples by optical injection of carriers by the background. This effect has often been ascribed to Shockley-Read recombination. These results are being used in modelling of detector performance that reproduces most of the features seen in practical detectors.     

High coverage fluid-phase floating lipid bilayers supported by ω-thiolipid self-assembled monolayers

Large area lipid bilayers, on solid surfaces, are useful in physical studies of biological membranes. It is advantageous to minimize the interactions of these bilayers with the substrate and this can be achieved via the formation of a floating supported bilayer (FSB) upon either a surface bound phospholipid bilayer or monolayer. The FSB''s independence is enabled by the continuous water layer (greater than 15 Å) that remains between the two. However, previous FSBs have had limited stability and low density. Here, we demonstrate by surface plasmon resonance and neutron reflectivity, the formation of a complete self-assembled monolayer (SAM) on gold surfaces by a synthetic phosphatidylcholine bearing a thiol group at the end of one fatty acyl chain. Furthermore, a very dense FSB (more than 96%) of saturated phosphatidylcholine can be formed on this SAM by sequential Langmuir–Blodgett and Langmuir–Schaefer procedures. Neutron reflectivity used both isotopic and magnetic contrast to enhance the accuracy of the data fits. This system offers the means to study transmembrane proteins, membrane potential effects (using the gold as an electrode) and even model bacterial outer membranes. Using unsaturated phosphatidylcholines, which have previously failed to form stable FSBs, we achieved a coverage of 73%.     

Using endemic road features to create self-explaining roads and reduce vehicle speeds

This paper describes a project undertaken to establish a self-explaining roads (SER) design programme on existing streets in an urban area. The methodology focussed on developing a process to identify functional road categories and designs based on endemic road characteristics taken from functional exemplars in the study area. The study area was divided into two sections, one to receive SER treatments designed to maximise visual differences between road categories, and a matched control area to remain untreated for purposes of comparison. The SER design for local roads included increased landscaping and community islands to limit forward visibility, and removal of road markings to create a visually distinct road environment. In comparison, roads categorised as collectors received increased delineation, addition of cycle lanes, and improved amenity for pedestrians. Speed data collected 3 months after implementation showed a significant reduction in vehicle speeds on local roads and increased homogeneity of speeds on both local and collector roads. The objective speed data, combined with residents’ speed choice ratings, indicated that the project was successful in creating two discriminably different road categories.     

The role of exportin-t in selective nuclear export of mature tRNAs

Exportin-t (Xpo-t) is a vertebrate nuclear export receptor for tRNAs that binds tRNA cooperatively with GTP-loaded Ran. Xpo-t antibodies are shown to efficiently block tRNA export from Xenopus oocyte nuclei suggesting that it is responsible for at least the majority of tRNA export in these cells. We examine the mechanism by which Xpo-t-RanGTP specifically exports mature tRNAs rather than other forms of nuclear RNA, including tRNA precursors. Chemical and enzymatic footprinting together with phosphate modification interference reveals an extensive interaction between the backbone of the TPsiC and acceptor arms of tRNAPhe and Xpo-t-RanGTP. Analysis of mutant or precursor tRNA forms demonstrates that, aside from these recognition elements, accurate 5' and 3' end-processing of tRNA affects Xpo-t-RanGTP interaction and nuclear export, while aminoacylation is not essential. Intron-containing, end-processed, pre-tRNAs can be bound by Xpo-t-RanGTP and are rapidly exported from the nucleus if Xpo-t is present in excess. These results suggest that at least two mechanisms are involved in discrimination of pre-tRNAs and mature tRNAs prior to nuclear export.     

Memory and learning abilities in everyday life of the elderly

Self-evaluations by adults (varying in age from 45-92 years) of their memory and learning abilities were investigated and related to performance on laboratory and ecological memory tasks. Hardly any association was found between subjective and objective measures. Self-evaluations were strongly influenced by (systematically varied) frames of reference: optimistic in comparisons with other people, pessimistic in comparisons with their own previous level of functioning. The most frequent problems were 'learning something new' and 'remembering names'. In contrast to external memory aids, cognitive strategies were rarely used spontaneously. Strategy training led to significant improvement of performance, that remained stable at follow-up. A further opportunity for improving performance was realized by ergonomic adaptations of computerized systems (teleshopping). Problems in learning to use such systems were strongly reduced by decreasing the load on working memory and by adapting the system to existing knowledge and skills of the users. A general observation in the different projects was that age-differences could explain only a small percentage of the variance in subjective and objective memory measures.     

The nuclear cap-binding complex is a novel target of growth factor receptor-coupled signal transduction

In an attempt to further understand how nuclear events (such as gene expression, nuclear import/export, and cell cycle checkpoint control) might be subject to regulation by extracellular stimuli, we sought to identify nuclear activities under growth factor control. Using a sensitive photoaffinity labeling assay that measured [alpha-32P]GTP incorporation into nuclear proteins, we identified the 20-kDa subunit of the nuclear cap-binding complex (CBC) as a protein whose binding activity is greatly enhanced by the extracellular stimulation of serum-arrested cells. The CBC represents a 20- and 80-kDa heterodimer (the subunits independently referred to as CBP20 and CBP80, respectively) that binds the 7-methylguanosine cap on RNAs transcribed by RNA polymerase II. This binding facilitates precursor messenger RNA splicing and export. We have demonstrated that the [alpha-32P]GTP incorporation into CBP20 was correlated with an increased ability of the CBC to bind capped RNA and have used the [alpha-32P]GTP photoaffinity assay to characterize the activation of the CBC in response to growth factors. We show that the CBC is activated by heregulin in HeLa cells and by nerve growth factor in PC12 cells as well as during the G1/S phase of the cell cycle and when cells are stressed with UV irradiation. Additionally, we show that cap-dependent splicing of precursor mRNA, a functional outcome of CBC activation, can be catalyzed by growth factor addition to serum-arrested cells. Taken together, these data identify the CBC as a nuclear target for growth factor-coupled signal transduction and suggest novel mechanisms by which growth factors can influence gene expression and cell growth.     

Dietary management of acute diarrhoea in childhood

Gastroenteritis in children is usually treated with the graded introduction of milk feeds after rehydration. Although having never been rigorously tested, the practice of gradually increasing milk strength over several days has been considered an appropriate means of warding against lactose intolerance and preventing sensitization to cow's milk antigens. These guidelines were formulated in Europe and North America and invariably lead to a reduction in nutrient intake. Malnourished children in developing countries, however, may experience an average 5-6 episodes of acute diarrhea per year and the nutrient effects are cumulative. A recent study from Latin America explored whether continued feeding is safe for infants under age 6 months and whether malnourished children respond adversely. Infants randomly assigned to receive full strength cow's milk immediately after rehydration did not have more treatment failures, higher stool outputs, or longer lasting diarrhea than those whose feeds were regarded to full strength over 48 hours. It is unclear, however, whether the youngest or more malnourished subjects were overrepresented in the treatment failures. Results also indicate that deciding to change treatment should not be dictated by the presence of reducing substances in the faeces; the majority of infants with reducing substances in their stools did well. This study offers the first scientific support for rapidly reintroducing full-strength milk formula after gastroenteritis is malnourished patients under 6 months of age. The 10% of infants in which dehydration recurs after reintroducing milk feeds are still difficult to manage. In the absence of yogurt or lactose-free formula, a locally-produced modular feed of chicken, starch, and vegetable oil may be suitable.     

Recent developments in the design and fabrication of visible photonic band gap waveguide devices

In this paper, we present the design, fabrication and initial optical testing of dielectric waveguide devices which incorporate photonic crystals with photonic band gaps (PBG) in the visible region of the spectrum. In the design of our devices we use a full three-dimensional plane wave analysis to solve the photonic band structure simultaneously with the dielectric waveguide boundary conditions for a fixed lattice and waveguide geometry. This takes into account the finite thickness of the waveguide core, and the evanescent wave in the dielectric cladding layers. Furthermore, we explain how the effective Bloch mode index can be extracted from the results. This enables us to tackle important problems associated with mode coupling between the input waveguide and guided Bloch modes within the porous PBG region, such as Fresnel reflections at the interface and up-scattering from the holes. Finally, we present the recent fabrication of quasi-periodic photonic crystals and PBG waveguide bends.