Table of contents

The optical path transformation is applied to reduce the one-speed neutron transport equation for a class of composite subcritical slabs to single-region problems. The class idealises, within the uncertainty of the one-speed model, a variety of practical situations such as U-D₂O-C-Zr-Pb or Pu-U-Na-Fe symmetric reactor assemblies; which may possibly contain a symmetrically anisotropic neutron source. A closed form double series solution, which turns out to be quite convenient for design and optimisation purposes, has been obtained, in terms of discontinuous functions for the multi-regional angular flux by application of a double finite Legendre transform. The double transform blends the conventional P_N method with the spatial polynomial expansion method. Disadvantage factor evaluations for a U-C lattice cell resulting from a low-order P₀P₁ approximation of this method are found to be in full agreement with hybrid diffusion-transport estimates.

The steady-state neutron-source method is applied to measure the thermal neutron absorption cross section of geological samples. On standard rocks the technique gives results consistent with previous studies. Measurements of North Sea sediments with high contents of clay minerals and large absorption cross sections are also performed, and demonstrate that such materials are within the range where the steady-state neutron-source method may be safely employed.

Holographic gratings are written in copper-doped LiNbO₃ crystals at temperatures of about 180 degrees C. At room temperature the gratings are fixed. The subsequent development process under homogeneous illumination is investigated by the microphotometric method. Measurements of the light intensity pattern at the exit face of the crystal reveal not only the change of the amplitude but also a phase shift of nearly pi . The dynamics of the development process can be fully understood by the assumption of a quasi-permanent modulated charge distribution and a second modulated charge distribution of optically excitable ions.

It is well known that if a circular aperture is placed inside a laser cavity, the resonant field spatial distribution is different from the Gaussian distribution. In this paper, the authors study the radial distribution of the resonant field progressive components, and show that the latter are characterised by distinct widths. The values of these widths depend on the diaphragm opening. To compute the resonant field distribution, they use a numerical method based on a field expansion in terms of a set of Gauss-Laguerre functions.

The authors report the use of thin-film cadmium sulphide bolometers for the detection of 9.4 GHz ultrasonic waves at a temperature of 4.2 K. As this bolometer is a thin-film device, it provides both a fast response time and a relatively strain-free bond to the sample. With the addition of an aluminium layer the working temperature range of this bolometer has been extended to 2 K. These bolometers also work in the presence of a magnetic field.

The theory of the scattering of compression waves in viscous fluids is examined. The effects of fluid viscosity, of differences in density and in elastic modulus between the particles and the fluid, of heat transfer and of concentration are considered. Ultrasonic phase velocity and attenuation are derived. Results for the phase velocity are compared with several other formulations. The feasibility of using ultrasound to characterise suspensions is discussed.

The authors have measured the conductivity of models of porous media made of binary mixtures of insulating spheres, the pore spaces of which are filled with conducting fluid. Although the sphere diameter ratios used in the experiments, 4 and 11, lead to packings with very different geometrical structures, the experimental points can all be fitted using Archie's law with an exponent m=1.46. They find just a small difference between the conductivities of the two types of packings.

The possibility of the occurrence of steady state negative electron mobility in externally ionised gas mixtures is considered. The requirements for the existence of the effect are discussed, using the example of externally ionised argon-fluorine gas mixtures. Results of numerical calculations of the drift velocity in electron-beam irradiated Ar/F₂ gas mixtures are presented, which predict the occurrence of the effect in actual cases.

It is demonstrated that the effect of the electromagnetic field of high-frequency surface waves on self-excited ionisation waves results in the suppression and frequency shift of the low-frequency waves. The experiments are performed in Ne and He-Ne gas discharge plasma. The investigations are directed towards application to gaseous lasers.

For pt.I see ibid., vol.21, p.1597 (1988). The authors have investigated theoretically the interaction of self-excited ionisation waves with high-frequency surface waves with a view to obtaining the dispersion of the ionisation waves at the present surface wave field. Mainly the action of ponderomotive force and additional electron heating due to excitation of the high-frequency wave are considered as non-linear mechanisms responsible for the changes of the dispersion characteristics of the ionisation waves. The theoretical results are in good agreement with the experimental data presented in part I for the suppression of ionisation waves by the field of the high-frequency surface waves and their frequency shift.

In Part one, the formation and development of leader filaments in large air-gap discharges are studied by Schlieren techniques. The time evolution of the thermal diameter of the leader at a given point in space is studied for several gap spacings ranging from 1 to 16.7 m and for different wave forms and voltage levels. In Part two, the authors review the main phases of the positive leader development. Experimental results obtained by Schlieren methods are used to test previous models of leader behaviour. Information is obtained about reduced field values and neutral temperature values in the leader channel.

The range profile of Hg⁺ implanted in Li_0.16Na_0.84NbO₃ at energies from 50 to 350 keV is measured by MeV ⁴He⁺ Rutherford back-scattering. A computer program based on the Biersack model has been written to facilitate comparison with the experimental results. It is found that the measured projected range is in good agreement with the calculated value and a marked improvement in the range straggling is obtained when the second-order energy loss is taken into consideration.

Brightness and electrical characteristics of powder DCEL displays have been systematically measured during both the forming operation and simulated life testing. Of particular interest and novelty are the effects of different gaseous environments on display behaviour. Forming in inert or reducing gases is slow and a new phenomenon of 'sticking' is reported where the forming rate reduces to extremely low values (<0.02 V h^-1) with an associated decrease in quantum efficiency. A new mechanism for forming is proposed centred around a changing effective barrier height for electron injection, and it is concluded that the width of the formed layer does not change during most of the forming process, contrary to previous theories. This new mechanism also operates during pulsed DC life testing of the panels which displayed the best maintenance characteristics.

For part I see ibid., vol.21, p.1627 (1988). A new way to describe the mechanism of forming of powder DC electroluminescent panels is proposed that is based on the hypothesis that the surface regions of the phosphor particles adjacent to the anode are oxidised. This irreversible chemical transformation produces the narrow resistive formed layer in which electroluminescence can occur. Electron conduction and photon production is controlled not only by the applied electric field but also by the internal fields within the particles derived from surface-adsorbed oxygen ions. The new observations reported in part I are explained and additional, confirmatory experiments are described. It is argued that identical physical and chemical processes control the brightness of pulsed DC-operated displays and some implications for the solution of the maintenance problem are discussed.

Study of a single-loop RF surface coil operating in close proximity to a conducting medium yields a simple theoretical description of coil behaviour. The results are used to design a new class of surface coil resonator with improved farfield signal response, useful for examination of deep tissues.

The breakdown voltage of a high-pressure argon arc is increased by about 80% if water is present in the vessel, and is further increased if the discharge is forced close to the water surface by a metal conductor outside the arc chamber.

The sensitivity of CoPc and CuPc thin-film gas sensors to water vapour has been investigated as a function of relative humidity in air, nitrogen and oxygen. Qualitatively similar behaviour was observed for these two phthalocyanines. Water behaves as a donor impurity and there is evidence from both desorption data and Arrhenius plots of the dark conductivity that two types of adsorption or bonding are involved. The significant response of these two materials to humidity indicates that the role of water vapour must be given consideration in studies of the viability of phthalocyanines for detection of toxic gases such as NO₂ and NH₃.

The authors report a method for the preparation of contacts of very low surface resistivity (contact resistance multiplied by contact area) to polycrystalline YBaCuO thin films. The method provides contacts with resistivities smaller than 3*10^-7 Omega cm², which are obtained by the application of silver paint dots onto the film surface prior to annealing. Only one annealing procedure is required for the preparation of samples and contacts. Above the critical current both the four-terminal and two-terminal resistances obeyed Ohm's law for several current decades.

A simple theoretical model attributes the fast current fluctuations in liquid-metal-ion sources (LMIS) to vibrations of the liquid emitter under the competing influences of electric and surface tension forces. The model agrees well with experiment.

The microhardness of three sets of Ti-N films containing the epsilon -Ti₂N phase is investigated. Experiments show that Ti-N films with highest microhardness do not have to contain the epsilon -Ti₂N phase, as has usually been reported previously. The hardest films always contain the delta -TiN_x phase only.