The measurement of gamma-radiation by scintillation counting

This thesis is concerned with the application of the technique of scintillation counting to the detection of gamma radiation. In counters of this type the scintillations produced in a luminescent medium are detected by means of a photo-multiplier tube and the electrical pulses at the output of the latter are amplified and counted electronically. An attempt is made to interpret the behaviour of such counters and to relate the observed counting rates to the physical processes occuring in the luminophor. Following a brief historical review of the scintillation counting technique, a theoretical analysis of the gamma scintillation counter is developed. This analysis describes how the number and energy distribution of the secondary electrons produced in a luminophor of known composition and dimensions subjected to a known flux of gamma radiation can be calculated. It shows that, providing the luminescent decay of the luminophor is rapid, each secondary electron will produce a single scintillation the intensity of which will be proportional to the energy of the electron. When the luminophor has a slow decay, on the other hand, this simple behaviour is not to be expected. The amplitude distribution of the resulting electrical pulses will be modified by statistical processes in the photo-multiplier tube, and by the resolving time of the amplifying and counting equipment. Experimental studies designed to test the validity of the theoretical analysis ore described. The first part of these studies is an investigation of the statistics of the type 1P21 photo-multiplier tube, stimulated by both continuous and pulsed light. The results of this investigation are in good accord with the theoretical predictions. The crystalline lumisophors investigated experimentally included calcium tungstate, thallium-activated potassium iodide, and thallium-activated sodium iodide. Following some preliminary studies on the effect of resolving time on counting rate, the absolute counting rates obtained when specimens of ench of these luminophors were irradiated under scatter-free conditions with the gamma radiation from sodium24, cobalt60, bromine82, iodine131 and gold198 were measured. These rates were then compared with the calculated rates of production of secondary electrons in the media. In the case of calcium tunstate under appropriate conditions, good agreement between counting rate end rate of secondary electron production was obtained. In the alkali halide luminophors, on the other hand, counting rates were abnormally high because of their relatively slow phosphorescent decay. These findings were confirmed by the study of the pulse amplitude distributions obtained with the luminophors under various experimental conditions. Studies on the variation in luminescent efficiency with temperature of eaoh of these luminophors, and on the spectral distributions of the emitted light are also described. Comparative measurements on a large number of liquid luminescent systems were made, with particular reference to the variation in luminescent efficiency with concentration of solute. These variations are interpreted in terms of a general theory due to Johnson and Williams. The system p-terpheny1-benzene was selected for more detailed study and the absolute counting rates obtained in this medium when irradiated by the gamma rays from ench of the five radio-isotopes already mentioned were measured. When a correction is applied for the electrons scattered into the medium from the walls of the containing vessel, the observed rates are found to be in good agreement with the calculated rates of production of secondary electrons. It was observed in the course of the above studies that both pure liquids such as water and transparent solids such as Perspex luminesce under gamma irradiation, and this luminescence is shown to be often explicable in terms of the Cerenkov effect. Detailed studies of this effect in Perspex were carried out, and the angular distributions and relative intensities of the emission due to each of the radio-isotopes: sodium24, cobalt60, bromine82 and iodine131 were measured. Good agreement is obtained between the observed results and the classical theory due to Frank and Tamm for the effect. Finally, experimental studies made on the luminescent decay of a copper-activated zinc sulphide phosphor of long afterglow following excitation by gamma radiation from each of the five radio-isotopes mentioned are described. The decay is shown to be non-exponential in form, but provided that certain precautions are observed, the counting rate at a known time after irradiation can be related to the gamma radiation flux through the phosphor during the exposure. The thesis ends with a critical discussion of the experimental results and an assessment of their implications, together with some suggestions for future research in this field. (Abstract shortened by ProQuest.)<p

Flow and Transport in Regions with Aquatic Vegetation

This review describes mean and turbulent flow and mass transport in the presence of aquatic vegetation. Within emergent canopies, the turbulent length scales are set by the stem diameter and spacing, and the mean flow is determined by the distribution of the canopy frontal area. Near sparse submerged canopies, the bed roughness and near-bed turbulence are enhanced, but the velocity profile remains logarithmic. For dense submerged canopies, the drag discontinuity at the top of the canopy generates a shear layer, which contains canopy-scale vortices that control the exchange of mass and momentum between the canopy and the overflow. The canopy-scale vortices penetrate a finite distance into the canopy, δe, set by the canopy drag. This length scale segregates the canopy into two regions: The upper canopy experiences energetic turbulent transport, controlled by canopy-scale vortices, whereas the lower canopy experiences diminished transport, associated with the smaller stem-scale turbulence. The canopy-scale vortices induce a waving motion in flexible blades, called a monami.National Science Foundation (U.S.) (EAR 0309188)National Science Foundation (U.S.) (EAR 0125056)National Science Foundation (U.S.) (EAR0738352)National Science Foundation (U.S.) (OCE0751358

The clinical assessment study of the foot (CASF): study protocol for a prospective observational study of foot pain and foot osteoarthritis in the general population.

BACKGROUND: Symptomatic osteoarthritis (OA) affects approximately 10% of adults aged over 60 years. The foot joint complex is commonly affected by OA, yet there is relatively little research into OA of the foot, compared with other frequently affected sites such as the knee and hand. Existing epidemiological studies of foot OA have focussed predominantly on the first metatarsophalangeal joint at the expense of other joints. This three-year prospective population-based observational cohort study will describe the prevalence of symptomatic radiographic foot OA, relate its occurrence to symptoms, examination findings and life-style-factors, describe the natural history of foot OA, and examine how it presents to, and is diagnosed and managed in primary care. METHODS: All adults aged 50 years and over registered with four general practices in North Staffordshire, UK, will be invited to participate in a postal Health Survey questionnaire. Respondents to the questionnaire who indicate that they have experienced foot pain in the preceding twelve months will be invited to attend a research clinic for a detailed clinical assessment. This assessment will consist of: clinical interview; physical examination; digital photography of both feet and ankles; plain x-rays of both feet, ankles and hands; ultrasound examination of the plantar fascia; anthropometric measurement; and a further self-complete questionnaire. Follow-up will be undertaken in consenting participants by postal questionnaire at 18 months (clinic attenders only) and three years (clinic attenders and survey participants), and also by review of medical records. DISCUSSION: This three-year prospective epidemiological study will combine survey data, comprehensive clinical, x-ray and ultrasound assessment, and review of primary care records to identify radiographic phenotypes of foot OA in a population of community-dwelling older adults, and describe their impact on symptoms, function and clinical examination findings, and their presentation, diagnosis and management in primary care

Vegetation phenology as a key driver for fire occurrence in the UK and comparable humid temperate regions

Background Fire activity in the UK and comparable regions of northwest Europe is generally out of phase with peak fire weather conditions. Aims Here, we assess the potential effect of phenology on fire occurrence patterns for the UK. Methods We examined fire occurrence and vegetation phenology in the UK for 2012–2023, mapped onto the main fire-affected vegetation cover types within distinct precipitation regions, allowing the fire occurrence for fuels in different phenological phases to be explored across distinct ‘fuel’ types and regions. Key results The UK’s fire regime is characterised by burning in semi-natural grasslands and evergreen dwarf shrub ecosystems in early spring when vegetation is still dormant. During the high-greenness phase in late spring and summer, fire activity is reduced by a factor of 5–6 despite typically elevated fire weather conditions within that period. Conclusions and implications Semi-natural vegetation in the UK is very resistant to burning during the high-greenness phase. However, this ‘fire barrier’ is diminished during severe drought episodes, which are predicted to become more extreme in the coming decades. Incorporating phenology information into models therefore has great potential for improving future fire danger and behaviour predictions in the UK and comparable humid temperate regions

A case study of boundary layer ventilation by convection and coastal processes

It is often assumed that ventilation of the atmospheric boundary layer is weak in the absence of fronts, but is this always true? In this paper we investigate the processes responsible for ventilation of the atmospheric boundary layer during a nonfrontal day that occurred on 9 May 2005 using the UK Met Office Unified Model. Pollution sources are represented by the constant emission of a passive tracer everywhere over land. The ventilation processes observed include shallow convection, turbulent mixing followed by large-scale ascent, a sea breeze circulation and coastal outflow. Vertical distributions of tracer are validated qualitatively with AMPEP (Aircraft Measurement of chemical Processing Export fluxes of Pollutants over the UK) CO aircraft measurements and are shown to agree impressively well. Budget calculations of tracers are performed in order to determine the relative importance of these ventilation processes. Coastal outflow and the sea breeze circulation were found to ventilate 26% of the boundary layer tracer by sunset of which 2% was above 2 km. A combination of coastal outflow, the sea breeze circulation, turbulent mixing and large-scale ascent ventilated 46% of the boundary layer tracer, of which 10% was above 2 km. Finally, coastal outflow, the sea breeze circulation, turbulent mixing, large-scale ascent and shallow convection together ventilated 52% of the tracer into the free troposphere, of which 26% was above 2 km. Hence this study shows that significant ventilation of the boundary layer can occur in the absence of fronts (and thus during high-pressure events). Turbulent mixing and convection processes can double the amount of pollution ventilated from the boundary layer

Evaluating the structure and magnitude of the ash plume during the initial phase of the 2010 Eyjafjallajökull eruption using lidar observations and NAME simulations

The Eyjafjallajökull volcano in Iceland erupted explosively on 14 April 2010, emitting a plume of ash into the atmosphere. The ash was transported from Iceland toward Europe where mostly cloud-free skies allowed ground-based lidars at Chilbolton in England and Leipzig in Germany to estimate the mass concentration in the ash cloud as it passed overhead. The UK Met Office's Numerical Atmospheric-dispersion Modeling Environment (NAME) has been used to simulate the evolution of the ash cloud from the Eyjafjallajökull volcano during the initial phase of the ash emissions, 14–16 April 2010. NAME captures the timing and sloped structure of the ash layer observed over Leipzig, close to the central axis of the ash cloud. Relatively small errors in the ash cloud position, probably caused by the cumulative effect of errors in the driving meteorology en route, result in a timing error at distances far from the central axis of the ash cloud. Taking the timing error into account, NAME is able to capture the sloped ash layer over the UK. Comparison of the lidar observations and NAME simulations has allowed an estimation of the plume height time series to be made. It is necessary to include in the model input the large variations in plume height in order to accurately predict the ash cloud structure at long range. Quantitative comparison with the mass concentrations at Leipzig and Chilbolton suggest that around 3% of the total emitted mass is transported as far as these sites by small (<100 μm diameter) ash particles

Simonsenia aveniformis sp nov (Bacillariophyceae), molecular phylogeny and systematics of the genus, and a new type of canal raphe system

The genus Simonsenia is reviewed and S. aveniformis described as new for science by light and electron microscopy. The new species originated from estuarine environments in southern Iberia (Atlantic coast) and was isolated into culture. In LM, Simonsenia resembles Nitzschia, with bridges (fibulae) beneath the raphe, which is marginal. It is only electron microscope (EM) examination that reveals the true structure of the raphe system, which consists of a raphe canal raised on a keel (wing), supported by rib like braces (fenestral bars) and tube-like portulae; between the portulae the keel is perforated by open windows (fenestrae). Based on the presence of portulae and a fenestrated keel, Simonsenia has been proposed to be intermediate between Bacillariaceae and Surirellaceae. However, an rbcL phylogeny revealed that Simonsenia belongs firmly in the Bacillariaceae, with which it shares a similar chloroplast arrangement, rather than in the Surirellaceae. Lack of homology between the surirelloid and simonsenioid keels is reflected in subtle differences in the morphology and ontogeny of the portulae and fenestrae. The diversity of Simonsenia has probably been underestimated, particularly in the marine environment.Polish National Science Centre in Cracow within the Maestro program [N 2012/04/A/ST10/00544]; Sciences and Technologies Foundation-FCT (Portugal) [SFRH/BD/62405/2009]info:eu-repo/semantics/publishedVersio