Comparison of quantum mechanical and classical trajectory calculations of cross sections for ion-atom impact ionization of negative - and positive -ions for heavy ion fusion applications

Stripping cross sections in nitrogen have been calculated using the classical trajectory approximation and the Born approximation of quantum mechanics for the outer shell electrons of 3.2GeV I$^{-}$ and Cs$^{+}$ ions. A large difference in cross section, up to a factor of six, calculated in quantum mechanics and classical mechanics, has been obtained. Because at such high velocities the Born approximation is well validated, the classical trajectory approach fails to correctly predict the stripping cross sections at high energies for electron orbitals with low ionization potential.Comment: submitted to Phys. Rev.

Nucleation and Growth of GaN/AlN Quantum Dots

We study the nucleation of GaN islands grown by plasma-assisted molecular-beam epitaxy on AlN(0001) in a Stranski-Krastanov mode. In particular, we assess the variation of their height and density as a function of GaN coverage. We show that the GaN growth passes four stages: initially, the growth is layer-by-layer; subsequently, two-dimensional precursor islands form, which transform into genuine three-dimensional islands. During the latter stage, island height and density increase with GaN coverage until the density saturates. During further GaN growth, the density remains constant and a bimodal height distribution appears. The variation of island height and density as a function of substrate temperature is discussed in the framework of an equilibrium model for Stranski-Krastanov growth.Comment: Submitted to PRB, 10 pages, 15 figure

A survey exploring private farm advisor perspectives of agri-environment schemes : The case of England's Environmental Stewardship programme

Most stakeholder-based research concerning agri-environmental schemes (AES) derives from work engaging with farmers and land managers. Consequently, the voices and opinions of other actors involved in AES tends to be unrepresented in the wider literature. One group of actors that seem particularly overlooked in this respect are private (independent) farm advisors (i.e., the consultants contracted by farmers and land managers to advise-on AES and agronomic matters). To begin to rectify this knowledge gap we developed an exploratory online survey to explore private farm advisor perspectives in the UK; specifically, the situation in England and advisors' experience of Natural England's Environmental Stewardship programme. A total of 251 Natural England registered farm advisors (29.9%) completed our survey. The majority of these had knowledge and expertise in relation to two (31.5%) or three (42.2%) Environmental Stewardship schemes, with proficiency in ELS (93.4%) and HLS (82.8%) being the most common. On average, advisors had 9.6 ± 5.6 yrs of experience and operated (75.3%) in a single region of England. Although our results concentrated upon a relatively simple set of initial topics of inquiry, the survey revealed a number of interesting findings. Firstly; for example, that in the opinion of the advisors working with farmers applying for Environmental Stewardship schemes, the 'knowledge-exchange encounter' occurring between themselves, their clients and Natural England is fundamental to the environmental effectiveness of these schemes as well as their farm business compatibility. Secondly, respondents suggested that beneath this 'encounter' lie tensions arising from the competing agendas and objectives of the different actors involved which can affect the content of agreements; for instance, farmer selection of management options versus Natural England's target environmental objectives. Farm advisors suggested that they had to negotiate this balance whilst also serving the needs of their clients. Thirdly, respondents raised issues concerning the complicated nature of scheme arrangements, both from their own and farmers' perspectives, as well as the adequacy of payments to cover input costs and matters regarding contractual compliance, all of which theyproposed affected farmer participation. Looking ahead, we believe that future AES should account for all of these issues in their design to aid long-term farmer participation, effective agreement implementation and beneficial environmental management

Bioresorbable composite bone fracture repair plates: manufacture and characterisation

This study reports on Bioresorbable composites manufactured using PLA as matrix and phosphate-based glass fibres as reinforcement. Composites were manufactured with varying volume fraction (from 25% - 45%) and mechanically tested

Processes controlling atmospheric dispersion through city centres

We develop a process-based model for the dispersion of a passive scalar in the turbulent flow around the buildings of a city centre. The street network model is based on dividing the airspace of the streets and intersections into boxes, within which the turbulence renders the air well mixed. Mean flow advection through the network of street and intersection boxes then mediates further lateral dispersion. At the same time turbulent mixing in the vertical detrains scalar from the streets and intersections into the turbulent boundary layer above the buildings. When the geometry is regular, the street network model has an analytical solution that describes the variation in concentration in a near-field downwind of a single source, where the majority of scalar lies below roof level. The power of the analytical solution is that it demonstrates how the concentration is determined by only three parameters. The plume direction parameter describes the branching of scalar at the street intersections and hence determines the direction of the plume centreline, which may be very different from the above-roof wind direction. The transmission parameter determines the distance travelled before the majority of scalar is detrained into the atmospheric boundary layer above roof level and conventional atmospheric turbulence takes over as the dominant mixing process. Finally, a normalised source strength multiplies this pattern of concentration. This analytical solution converges to a Gaussian plume after a large number of intersections have been traversed, providing theoretical justification for previous studies that have developed empirical fits to Gaussian plume models. The analytical solution is shown to compare well with very high-resolution simulations and with wind tunnel experiments, although re-entrainment of scalar previously detrained into the boundary layer above roofs, which is not accounted for in the analytical solution, is shown to become an important process further downwind from the source

Viking navigation

A comprehensive description of the navigation of the Viking spacecraft throughout their flight from Earth launch to Mars landing is given. The flight path design, actual inflight control, and postflight reconstruction are discussed in detail. The preflight analyses upon which the operational strategies and performance predictions were based are discussed. The inflight results are then discussed and compared with the preflight predictions and, finally, the results of any postflight analyses are presented

Rethinking place and the social work office in the delivery of children's social work services

Limited attention has been given to the concept of place in social work research and practice. This paper draws on the national evaluation of social work practices (SWPs) in England undertaken between 2009 and 2012. SWPs were pilot organisations providing independent social work services for children in out-of-home care in five sites. One factor distinguishing some of these pilots was their attention to place. The evaluation employed a mixed methods approach and we use data from interviews with 121 children and young people in out-of-home care, 19 birth parents and 31 interviews with SWP staff which explored their views and experiences of the SWP offices. Children and young people were alert to the stigma which could attach to social work premises and appreciated offices which were planned and furnished to appear less institutional and more ‘normal’. Daily interactions with staff which conveyed a sense of recognition and value to service users also contributed to a view of some SWP offices as accessible and welcoming places. Both children and parents appreciated offices that provided fun activities that positioned them as active rather than passive. Staff valued opportunities for influencing planning decisions about offices and place was seen to confer a value on them as well as on service users. However, not all the SWPs were able to achieve these aspects of place, and engaging children and families in place was less likely when the service user population was widely dispersed. Recognising the importance of place and how place is constructed through relationships between people as well as through the physical environment appeared to be key to creating offices that combated the stigma attached to out-of-home care. Those leading and managing children’s services should explore ways of involving local communities in planning social work offices and turn attention to making these offices accessible, welcoming, places

Lessons for major system change: centralization of stroke services in two metropolitan areas of England

OBJECTIVES: Our aim was to identify the factors influencing the selection of a model of acute stroke service centralization to create fewer high-volume specialist units in two metropolitan areas of England (London and Greater Manchester). It considers the reasons why services were more fully centralized in London than in Greater Manchester. METHODS: In both areas, we analysed 316 documents and conducted 45 interviews with people leading transformation, service user organizations, providers and commissioners. Inductive and deductive analyses were used to compare the processes underpinning change in each area, with reference to propositions for achieving major system change taken from a realist review of the existing literature (the Best framework), which we critique and develop further. RESULTS: In London, system leadership was used to overcome resistance to centralization and align stakeholders to implement a centralized service model. In Greater Manchester, programme leaders relied on achieving change by consensus and, lacking decision-making authority over providers, accommodated rather than challenged resistance by implementing a less radical transformation of services. CONCLUSIONS: A combination of system (top-down) and distributed (bottom-up) leadership is important in enabling change. System leadership provides the political authority required to coordinate stakeholders and to capitalize on clinical leadership by aligning it with transformation goals. Policy makers should examine how the structures of system authority, with performance management and financial levers, can be employed to coordinate transformation by aligning the disparate interests of providers and commissioners