Thesis (Ph.D.)--University of California, Berkeley, 1964. / "Physics, UC-34" -t.p. "TID-4500 (30th Ed.)" -t.p. Includes bibliographical references (p. 54-56).

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An accurately colliraated monenergetic fast neutron beam of small angular width was produced by bombarding a deuterium target with 2 MeV deuterons. Background reduction was achieved by using the associated particle technique and extensive shielding. Dual parameter pulse height analysis was used to reduce the coincidence time resolution between the detected neutrons and the associated He³ recoil particle to 4.6 nanoseconds by taking into account the variations in flight times of the He³ particles. A rotating thin film deuterated polyethylene target was developed which enabled a substantial increase in the neutron flux to be made. The beam of 70 neutrons/second/millisteradian produced, was of sufficient intensity to perform a small angle scattering experiment. The technique was tested by a measurement of the angular distribution from 40 degrees to 10 degrees of neutrons elastically scattered from lead. / Science, Faculty of / Physics and Astronomy, Department of / Graduate

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The high temperature behaviour of the fluorite oxides UO₂, THO₂ and Y₂O₃-stabilised ZrO ₂ have been investigated using a variety of neutron scattering techniques. Interest has centred on the cause of the anomalously large enthalpy of UO₂ at temperatures above 15OOK, an understanding of which is important in view of its use as a fission reactor fuel. High temperature techniques have been developed which enable the performance of neutron scattering at temperatures up to 3OOOK. Bragg diffraction measurements have shown that a growing fraction of anions vacate their regular sites above 21OOK in UO₂ and above 23OOK in ThO₂ attaining vacancy concentrations of ~2O% at 29OOK in both materials. Quasielastic scattering investigations have confirmed the occurrence of anion Frenkel disorder in UO2 at high temperatures and have shown that the disorder is of a dynamic nature. Both sets of results may be interpreted in terms of fluctuating, dynamic clusters of vacancies and inter stitials, having lifetimes of a few phonon periods. The elastic constants of UO₂, which have been determined up to 293OK from measurements of the long wavelength acoustic phonons, show an increased rate of softening above 24OOK. The zone-centre optic phonons broaden rapidly above 2OOOK in UO₂- The full phonon dispersion relation of ThO₂, measured at 293K, resembles closely that of UO₂. The results provide the first direct, unambiguous evidence of thermally-induced Frenkel disorder in UO₂, which is analogous to the disorder observed in the fast-ion phase of the fluorite halides, such as CaF₂. Quasielastic scattering techniques have also been used to study the defect structure of Y ₂O ₃ -stabilised ZrO ₂ between 293K and 2OOOK. Many general features of the scattering observed at room temperature may be explained in terms of nearest neighbour relaxations around oxygen vacancies. The diffuse scattering broadens in energy above 1OOOK, signifying that these clusters of relaxed ions have a finite lifetime of ~5ps at 15OOK. The results have been interpreted in terms of a vacancy hopping model which is consistent with the enhanced conductivity observed in this temperature range.