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This report details the research and development of a novel class of large area thermal neutron detector, developed specifically for the application of cargo screening. Based broadly on a laminar scintillation device, developed by Barton et al, using ZnS and Li6F, the new detector achieves a comparable level of performance to standard 3He tubes, but only employs readily available components and none isotopically enriched chemicals, thereby offering a low cost solution to the problem caused by restrictions in the supply of 3He.

In this thesis we study techniques based on the method of characteristics applied to neutron transport problems. These methods have been widely used in the solution of such problems and have been implemented in a number of commercial and re- search codes, such as the CACTUS module of Serco Assurance's WIMS software. Since characteristics-based methods are widely used in the field of nuclear energy, where safety, reliability and predictability are of paramount importance, a rigorous analysis of the convergence properties of these methods is required; this topic represents the main focus of this thesis. We begin by using results from functional analysis to obtain an a priori bound on the error in the L [infinity] norm when employing the method of long characteristics (LC) in space in conjunction with a discrete ordinates (SN) method in angle. Our analysis applies to a source problem in 20 space with vacuum boundary conditions. We show that, with refinement in element diameter h, convergence of the LC method is at least O(h), and under certain assumptions, the SN scheme is also at least first order. These results are confirmed by numerical tests. Next we obtain a similar bound on the L[infinity]-error in the case when a variety of the short characteristic (SC) method, which approximates the neutron flux with an arbitrarily high-order piecewise polynomial approximation, is exploited. We prove that for a qth order polynomial approximation, we can expect at least O(hq) convergence in the SC solution. This result is again validated by numerical results. Finally, the SC method described above is implemented in a code and applied to a variety of standard theoretical benchmark problems as well as a number of realistic models, both from the literature and provided by Serco Assurance. Results from the code show close agreement with those from a variety of independent, external sources.

In this thesis aerodynamic levitation and laser heating has been combined with neutron diffraction, X-ray absorption spectroscopy (XAS), and computer simulations to investigate the structure of liquids and glasses. In order -to determine the structure of BaTi205 glass and RE0.3Ba0.7Ti2O5.15 glass the complementary techniques of neutron diffraction and XAS were combined. Molecular dynamics models based on simple empirical potentials were generated and then refined using Reverse Monte Carlo (RMC) simulations fitting to the neutron diffraction data. From these structural models the glasses were determined to consist of a network of 4, 5, and 6 coordinated Ti-O polyhedra, which was found to be consistent with the X-ray absorption near edge spectra. A decrease in the rare earth oxygen bond length with increasing rare earth atomic mass was observed in both the RMC models and the rare earth edge XAS. The contentious issue of the apparent iso-compositional liquid-liquid phase transition in the (Y203)x(AI203)1-x system was investigated using in situ aerodynamic levitation and small angle neutron scattering (SANS). Samples of x = 0.2, 0.25, 0.3, and 0.375 were studied across a temperature range of 1300-1900K, with detailed measurements at the reported liquid-liquid transition temperature of 1788K (Greaves et al., 2008). There was no observed increase in SANS intensity consistent with the nucleation of a second liquid phase at any temperature. In situ aerodynamic levitation and neutron diffraction with isotopic substitution was used to determine the atomic structure of liquid Invar (Fe65Nh5) at -1800K. Although the introduction of Ni is substitutional, a small degree of chemical was apparent in the Bhatia-Thornton (1970) partial structure factors. Significant magnetic correlations were detected -l300K above the Invar Curie temperature. Anomalous neutron scattering was undertaken on the unusual semiconducting liquid InSe. A first peak coordination number of 3 suggested that the underlying cause of the unusual behaviour is not related to In-In homopolar bonding.

Following a literature survey into neutron detection methods, a conceptual design for a novel dose meter was proposed consisting of a pair of CdlnTe (ClT) detectors with a thin sandwiched layer of 6LiF between, achieved by coating one of the detectors with 6LiF. A detector was constructed using two 10 x 10 x 2 mm ClT detectors, of which one was coated with a 4 I-Im 6LiF layer. Coincidence counting of the alpha particle and triton arising from the 6U (n, a) 3H reaction was used to reduce the gamma background. Development of this design was carried out by undertaking experimental work on a simple prototype and by conducting Geant4 simulations. The Geant4 simulation work included comparison of simulations with experimental data to validate the modelling work carried out. Results from this work show that neutron detection is possible using this technique, using a 6UF layer of a few microns thickness, and that inference of neutron energy from the detector response is possible in principle. Also identified as a result of the modelling work were drawbacks and limitations with the use of ClT, notably the opacity of the 2 mm thick detectors used in the experimental work to thermal neutrons. This affected detector performance significantly, leading to detector separation playing a key role in detector efficiency, with efficiency also varying strongly with neutron field direction. The presence of neutrons was detected in a mixed neutron and gamma field, although 5 cm of lead shielding was required in order to detect a response above background. A review of the different methods of neutron detection is also presented with a discussion on their applicability to monitoring worker dose in the workplace. Having reviewed the performance of available dose meters, it has been concluded that there is scope for improved novel instruments as the response of existing instruments available has a strong energy dependence.

Coherent Inelastic Neutron Scattering (CINS) cross-sections are usually measured using single crystal samples and triple axis spectrometers; the resulting dispersion curves can then be analysed in terms of Born-von Karman force constant models or IS often very difficult (if not impossible) to obtain single crystal samples of sufficient size. Also, the development of pulsed neutron sources means that data may be collected over a wide range of energies and momentum transfer (Q). These two factors mean that we are now dealing with a vastly greater data set, which contains a demonstrably greater information density.

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