This work is concerned with physics-aspects of safety, quality control (QC) and dosimetry audit in high dose rate (HDR) gynaecological brachytherapy. A survey of brachytherapy QC practice across the UK was conducted. Areas of least consistency were addressed, including test method development and establishment of clinical performance requirements. ‘End to end’ dosimetry auditing was not being utilised and its implementation was the main focus of this work. Three candidate dosimeters were evaluated for use in audit: Fibre optic thermoluminescence detector, Gafchromic EBT3® radiochromic film, and Presage® radiochromic plastic. Film dosimetry was selected, fully characterised, triple-channel dosimetry evaluated, and uncertainty reduction methods implemented. A novel ‘end to end’ audit methodology was developed, the BRachytherapy Applicator Dosimetry (BRAD) system, to measure dose distributions around clinical brachytherapy applicators and compare to treatment planning system calculations. MCNP5 Monte Carlo code was used to support the design of the BRAD system and validate the use of film dosimetry. 46 radiotherapy centres in the UK were audited. Delivery of the intended prescription dose was confirmed to be within clinically acceptable levels at all centres, mean difference 0.6% for plastic and 3.0% for metal applicators (±3.0% k=1). The intended dose distribution was faithfully delivered to the film-measured dose planes with a mean gamma passing rate of 97.8% at 3% (local) 2 mm criteria. Two audits had results that required follow-up and both were resolved. Each audit included a review of local brachytherapy physics practice and opportunities for improvement were reported, including imaging, applicator reconstruction, planning procedures, QC tests, and staff training. The brachytherapy audit provided the first comprehensive validation of ‘end to end’ clinical brachytherapy dosimetry, from applicator imaging to treatment delivery, combined with a review of clinical physics practice. The BRAD system is retained in the Institute of Physics and Engineering in Medicine (IPEM) phantom library.

This thesis describes an experimental study of exciton recombination in isolated semiconducting single-wall carbon nanotubes synthesised by different methods and wrapped in different polymers PFO (9,9-dioctylfluorenyl-2,7-diyl), PFO-BPy (9,9-Dioctyfluorenyl-2,7-diyl-Bipyridine) and P3HT (poly 3-hexylthiophene). We present a comprehensive study using femtosecond transient absorption measurements of the kinetics of exciton recombination, where diffusion of excitons in the confined one-dimensional system significantly affects their optical and electronic properties. In all studied samples of isolated nanotubes, an exciton-exciton process dominated the recombination under high excitation, and exhibited a distinct crossover to a diffusion-limited regime with anomalous kinetics at late times. We attribute the reaction-diffusion crossover to a finite reaction probability per exciton-exciton encounter. We have demonstrated a methodology to determine the microscopic parameters controlling reaction and diffusion processes, based on measurements at high initial density where the optical absorption is fully saturated. In studies of the same nanotube species synthesized by both HiPCO (high pressure catalytic decomposition of carbon monoxide) and CoMoCAT (cobalt and molybdenum catalysts based chemical vapour deposition technique) methods and wrapped by different polymers, the exciton reaction probability was approximately constant, corresponding to on average one in five exciton-exciton interactions resulting in an exciton recombination. On the other hand, there was significant variation in the diffusive hopping time for samples synthesized by different processes or subject to different processing. This is consistent with the wide range of values for the diffusion coefficient reported in the literature. We have found that excitons in nanotubes synthesized by the HiPCO are more mobile than CoMoCAT nanotubes, by a factor of ~2 for nanotubes wrapped in P3HT. This may be associated with a lower defect concentration in HiPCO nanotubes compared to CoMoCAT. The nanomaterials studied in this thesis are promising for nanotube-organic hybrids based light emitting and harvesting devices. Our findings will not only facilitate the selection of materials in these applications but also represent experimental data that challenges existing theoretical models for kinetics of non-equilibrium stochastic systems.

Single photon emission computed tomography (SPECT) of the myocardium accounts for the vast majority of nuclear cardiology procedures performed worldwide. This is mainly due to its wide availability, and its established and clinically validated diagnostic and prognostic accuracy. It is due to the availability of quantitative software allowing for reliable and reproducible assessments of myocardial perfusion and function. For many SPECT images were obtained using the Filtered Back Projection (FBP) reconstruction algorithm. Recently, due major advances in hardware and software developments 3-D Iterative Reconstruction algorithms became widely available in nuclear cardiology departments. These SPECT reconstruction algorithms offer improved spatial resolution and image contrast as well as accurate implementation of attenuation and scatter corrections. More recently, the hybrid imaging modality SPECT/CT became more widely available radiotracer distribution as well as for computed tomography. In recent years, research into the effect of attenuation and scatter corrections has concluded that combining CT attenuation correction with scatter correcting provides a precise quantification of radiopharmaceutical distribution obtained in SPECT research. Although the method has been proven to attain “clean” images for a variety of diagnostic means, its effects under respiratory cardiac motions using SPECT and high resolution algorithms in 3-D iterative reconstruction imaging is yet to be tested. This study aims to determine the efficacy and quantitative accuracy for 3-D iterative reconstruction algorithms when compared to FBP techniques for cardiac SPECT images. The study will attempt to investigate the issue of respiratory induced cardiac motion and to demonstrate its effect on spatial resolution, image contrast and quantification in nuclear cardiology on SPECT images reconstructed using FBP and Flash (OSEM) 3-D algorithms. We carried out two types of investigations involving experimental measurements carried out on three different phantoms, and Monte Carlo simulations using the GATE package. For the experimental part, we used a Siemens Symbia T16 SPECT/CT scanner to acquire SPECT/CT images using a Tc99m point source, a cardiac insert and a cardiac anthropomorphic phantom; data was acquired, reconstructed and analysed using standard clinical protocols available on Symbia T16 SPECT/CT scanner. We also looked at CT based attenuation and scatter corrections to study their effect on image quality and quantification.

No description available.

The discretisation of geometry into a mesh is a critical part of numerical simulation. Fully automatic' push-button' algorithms are available for generating tri and tet meshes but they are often eschewed in favour of multiblock structured meshes because they offer significant advantages. However, their generation technology is considerably less mature and it is standard practice in industry for skilled personnel to interactively construct multiblock decompositions in specialised software graphical interfaces. There is generally a poor understanding of the essential underlying properties of quad and hex meshes and heuristic experience-based techniques are used to build the block structures in stages. This is a very time consuming and expensive process for complex geometries and it stands as the biggest bottleneck in the simulation process. In this thesis, new reasoned approaches to automatically generating multiblock decompositions in 2-D and 3-D are explored. A recent precise theoretical study of the properties of quad meshes has shown that mesh singularities, where the regular grid structure is disrupted, are vitally important features and the overall shape and topology of a multiblock structured mesh hinges on their properties. Inspired by this insight, an original method is developed for using the medial axis to find a valid and effective arrangement of mesh singularities. A novel cross-field generation algorithm on a tet mesh discretisation of a surface is also developed with a similar line of reasoning. This method can create well-shaped multiblock decompositions that are adjusted to suit target element sizes and alignments. Finally, preliminary work on extending the approach to 3-D for generating a cross-frame field on a tet mesh is presented. New ideas and perspectives are offered for understanding the 3-D problem's fundamental difficulties and a novel initialisation method is used with a following optimisation method to produce cross-frame fields with conspicuous line singularity networks.

This thesis presents the study of magnetohydrodynamic (MHO) waves and oscillations in quiet Sun chromospheric fine structures near the solar disk center using multiwavelength, high temporal and spatial resolution observations. Transverse oscillations in chromospheric on-disk mottles that appear to be rooted in photospheric magnetic bright points are detected with great accuracy and interpreted in terms of MHO kink waves. Measured kink wave characteristiCs, such as periods (ranging from 70 to 280 s and peak at -165-s), phase speeds 40-120-km S·l, and transverse velocity amplitudes (2-18 km S-l), are similar to results found for limb spicules. An analysis of oscillation signals at different positions along the mottles provides evidence for upward and downward propagating and standing transverse waves. Wave characteristics including the maximum transverse velocity amplitude and the phase speed are measured as a function of distance along the structure's length. Solar magnetoseismology (SMS) is applied to these measured parameters to obtain diagnostic information on key plasma parameters (e.g., magnetic field, density, temperature, flow speed) of these localized waveguides. Using the SMS tool, the magnetic field strength of the mottle along the -2 Mm length is found to decrease by a factor of 12, while the local plasma density scale height is -280±80-km.

No description available.

Polarizable force fields are a new and promising way of modelling water solutions using molecular dynamic computer simulations. In this work, we discuss the general workflow of characterising new water and ion models and apply this scheme on the SW10e water model with polarizable ions. We thereby investigate several physico-chemical properties in terms of physical meaningfulness, experimental characteristics, simulation methods and performance of the polarizable model under test, namely the density, hydration enthalpy and Gibbs free energy, structural properties, residence time, the potential of mean force, surface tension and the osmotic pressure. Recently, the osmotic pressure was discovered as a new and reliable method to characterise the quality of a model for aqueous solutions. Therefore, we thoroughly investigated one computational way of calculating the osmotic pressure: the membrane method. In a novel approach, we use the polarizability of ions to adjust the model to fit to experimental values at high salt concentrations. We are able to create a consistent ion and water model reproducing bulk properties and interfacial properties at low and high salt concentrations.

The energy and angular distributions of low energy electrons « 400 eV) inelastically scattered from a clean copper surface have been determined in the neighbourhood of the specularly reflected beam. The intensity of these electrons is strongly influenced by elastic diffraction from the lattice. In certain circumstances the relationship between the elastic and inelastic cross-sections can be explained in terms of a simple, kinematical theory developed by C. B. Duke and others. This theory, however, is inadequate when multiple diffraction is pronounced. The angular distribution in scattered intensity is insensitive to the diffracting conditions and can not be explained in terms of simple models. The complexity of the electronic structure of copper surfaces is emphasized

The manifolds Hp,q [Symbol appears here. To view, please open pdf attachment] are a family of non-compact hyperboloids carrying inhomogeneous Euclidean metrics. In supergravity they appear as an interesting class of dimensional reductions, related to the well known sphere reductions by a simple analytic continuation. The spectrum of lower dimensional modes in these backgrounds is still poorly understood. In this thesis, we construct the complete Pauli reduction of type IIA supergravity on H2,2 [Symbol appears here. To view, please open pdf attachment] . We carefully analyse the spectrum of gravitational waves in the resulting Salam-Sezgin background, and identify the boundary conditions needed to render these modes normalisable. We give these boundary conditions a codimension-2 braneworld interpretation. We then exhibit a supersymmetric braneworld geometry based on the NS5-brane. In the remainder of this thesis we apply holographic methods to the problem of the fractionalisation transition in condensed matter theory. We exhibit a phase transition between a superconducting and a fractionalised phase in a bottom-up Einstein-Maxwell-Dilaton theory, and discuss the importance of entropy scaling in achieving this.