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No description available.

A brief description is given of gas proportional detectors, with emphasis on the development and behaviour of multiwire proportional counters (MWPC's). In particular, the behaviour of MWPC's as position sensitive detectors is discussed and a description given of the various techniques employed in obtaining and processing positional information. A theoretical analysis of a recently discovered phenomenon, known as angular localisation of the electron avalanche, is presented and a study is made of induced charges, due to this avalanche, on nearby anodes and cathodes. Formulae are developed which allow the calculation of the induced charges, and theoretical predictions are given of how the effect may be used to provide position signals. An experimental system is described from which evidence has been obtained that a high degree of angular localisation does indeed exist. Comparisons are made between these experimental measurements and the predictions of the theoretical model. Various fundamental limitations are discussed which indicate the feasibility of using the localisation effect for positional interpolation between anode wires. The experimental work is extended to observe the effects of using several common counter gas mixtures and also of varying the chamber geometry. Finally some applications are discussed and suggestions are made for future research.

This thesis summarises the authors contributions to the field of ionising radiation effects by reviewing a number of studies in physics and biology made over the past 20 years. These studies relate primarily to aspects of radiological protection and chapters I and II provide some background and an historical introduction to this important branch of radiation research. Chapter III deals with work on two-dimensional multilayer structures and in particular their utilisation as long-spacing pseudo crystals for the dispersion of low energy X-radiation. Chapter IV deals with investigations of the gas gain and energy resolution of gas proportional counters and their utilisation for the study of energy deposition processes. Chapter V deals with the development of defensible radiological protection criteria for, and the measurement of doses to, superficial tissues such as the skin, eye and gonads. Enphasis has been placed upon the need for parallel developments in the understanding of both the necessary radiobiology and physics. Studies in radiobiology and morphology can identify the origin and depth of target cells while fundamental physics developments may be necessary for the production of appropriate radiation detectors. The superficial tissues may be subjected to high localised radiation exposures from poorly penetrating radiations such as alpha, beta and low energy X-radiation. Chapter VI reviews the use of animal models for the investigation of the effects of non-uniform skin exposure such as those which arise when the skin is subjected to radioactive particle exposures. The discussion is framed in terms of the limitations of current recommendations of the International Commission on Radiological Protection (ICRP) for skin dose limitation. The rationale of much of the authors work has been to carry out fundamental research in order to identify weaknesses in ICRP recommendations and, where they exist, to provide information to strengthen or underwrite revised radiological protection criteria. Such new data have been produced, for both stochastic and nonstochastic biological effects, through extensive multi-disciplinary collaborative research programmes, which are described. Chapter VII deals with the development of biological dosimetry techniques, particularly those capable of providing information on the uniformity of radiation exposures. The development of skin and hair as monitors of skin dose, and as indicators of the uniformity of whole body exposures, is particularly appropriate to the need for dose assessments in unscheduled exposures which have been underlined by the Chernobyl reactor accident. Chapter VIII deals with the evaluation of radiation risks from human radiation exposure experience. Particular emphasis is laid on the importance of the Japanese bomb survivor data and the controversy regarding radiation risks which has arisen in recent years following the need to revise the survivors radiation doses. The authors original estimates of revised risks and more recent analyses of the latest Japanese epidemiology data are presented. More than 80 publications are referred to, the majority of which are included as copies at the end of each chapter, together with suggestions for further reading. A list of the authors major publications and curriculum vitae are also included.

This work forms an extension to krypton and xenon of previous studies of argon in this laboratory, and aims at a critical comparison between the charge transport properties of argon, krypton and xenon. A pulse of 40KeV electrons, of between 5 and 300 nanoseconds duration penetrates one of the electrodes between which the specimen is contained. Electron-hole pairs are generated in a region near the surface of the specimen, and carriers of one sign are extracted by an applied electric field. The drift of carriers through the specimen is detected by amplifying and displaying on an oscilloscope the voltage pulse which appears across a load resistor in series with the electrodes and the battery. The drift velocity v of electrons has been measured in these substances at electric fields E between 20V.cm-1 and 40kV.cm-1. In all cases, v is determined principally by acoustic mode scattering. The low field mobilities at temperatures near the triple point are: Units: cm2V-1sec-1. In both solid and liquid, there is a transition at intermediate fields to a region where v ? E1/2. This behaviour has been described in terms of the Shockley 'hot electron' theory. At high fields v becomes independent of E in both liquids and solids. Deviations from Shockley's theory have been explained qualitatively in terms of the recent theory of Cohen and Lekner. Holes have been found to be mobile only in solid xenon, and a tentative interpretation of the results in terms of the small polar on theory is given. The drift velocity of holes was found to depend linearly on electric field up to at least 30kV.cm-1. The hole mobility at 158°K is approximately 2 x 10-2cm2V-1sec-1. It is suggested that deviations from the small polar on theory below about 130°K are caused by hole trapping. It is important to obtain very pure samples for drift velocity measurements, and an evaluation of two purification techniques employed is given.

The first section of this work consists of an analysis of three wave-functions for the ground state of the two-electron series which begins with H-. Two of the wave-functions include electron correlation. One does so by means of superposition of configurations and the other by the inclusion of explicit correlation factors. For comparison, the third function is of the Hartree-Fock type. The correlation within the wave-functions is demonstrated by presenting various density functions and density difference maps relative to the uncorrelated approach. Expectation values and physical properties such as X-ray scattering factors, diamagnetic susceptibilities and nuclear diamagnetic shielding factors are also presented. By this means it is possible to assess the correlation predicted by the wave-functions, with comparison being made where possible with results from a more accurate wave-function. In addition, the production of the natural expansion for one of the correlated functions gives extra information about the effects of radial and angular correlation. The second part consists of a study of a good one-centre treatment of the diatomic molecular ion HeH+. Both the molecular formation and the electron correlation in the system are studied by means of density differences and expectation values. Natural orbitals are used to enable comparison to be made with another HeH+ wave-function. This comparison shows that the two calculations are similar, even though conceived quite differently. A development of the general theory of natural spin orbitals is given as an appendix. Special emphasis is given to this technique as regards two-electron systems.

Drift mobility techniques have been used to measure the transport of excess electrons and holes in orthorhombic S and vitreous Se under hydrostatic pressures p of up to 6 kbar in the temperature range from -50°C to 80°C. In the S crystals, where electrons propagate by an intermolecular hopping mechanism, a strong pressure dependence was found for the electron mobility, [special characters omitted] exp (p/3.15 kbar) the activation energy, however, remained unchanged. It is concluded, on the basis of small polaron theory, that the observed effect is caused by an increase of the intermolecular resonance integral J. In fact, a LCAO calculation of the intermolecular overlap integral as a function of pressure predicts changes in J of the right order of magnitude. At low temperatures the hole mobility depends on the pressure as uh exp (p/3.15 kbar). If the temperature is increased, however, a progressively weaker dependence is found. This can be described phenomenologically by taking the trap density to be dependent on the pressure as [special characters omitted] exp (-P/5 kbar). In vitreous Se no pressure dependence of the mobilities could be detected and the implications of this result on the conduction mechanism are discussed. It is also suggested that the pronounced pressure dependence of the steady state dark- and photo-conductivity in this material is primarily due to the effect of pressure on the injecting properties of the contacts.

Wavefunctions possessing effective nuclear charges which are explicitly angularly dependent have been used as a simple minimal basis set for a one-centre study of HeH+. Correlation effects were introduced by using spatially different orbitals for different electrons of opposite spins. Allowance for the effects of correlation and also molecular formation within the confines of the model are discussed in terms of changes which occur in the electron density distribution. A comparison is made with results obtained from a corresponding analysis of an extensive, single-centre, configuration-interaction wavefunction. Although several points of similarity emerge, the analysis primarily indicates features and shortcomings of the basis orbitals which should aid any future applications. Also a study has been made of the LiH molecule which utilizes the one-particle density for an analysis of the effects of electron correlation. Two wavefunctions are examined in detail for their effectivness in representing correlation. The first of these is the GI method which uses spin projection to give a wavefunction which may be interpreted by means of the Independent Particle Model, the second is a very accurate configuration-interaction calculation. Correlation effects are observed within the model by constructing the uncorrelated wavefunction from the appropriate doubly occupied first natural configuration calculated for each of the above cases. The analysis is presented in terms of density difference maps and profiles which render the results in a particularly clear and revealing form.