Thesis (Ph. D. in Physics)--Vanderbilt University, May 2005. / Title from title screen. Includes bibliographical references.

<p> A new dating method has been developed which uses fission product xenon in a manner similar to radiogenic argon in the 40Ar - 39Ar method. Measurable quantities of spontaneous fission xenon accumulate with time in rocks and minerals containing approximately 1 ppm or more of uranium as a result of the spontaneous fission of 238u. In the past, attempts have been made to date this type of sample by measuring the absolute amount of fission product xenon and the uranium content. In the new method a sample is irradiated in a thermal neutron flux to implant xenon from the neutron induced fission of 235u in amounts proportional to the uranium content. After a short cooling period (before radioactive 133xe has decayed) the sample is placed in a high vacuum system and the xenon is released by heating in a series of temperature steps. For each step the isotopic composition of the xenon is measured using a high sensitivity mass spectrameter and the ratio of spontaneous fission xenon to neutron fission xenon is calculated. Fission xenon ages for all steps are then calculated from the spontaneous to neutron fission xenon ratios and the measured integrated neutron flux. </p> <p> The method has several advantages over the conventional U - Xe technique. {a) The measurement of isotope ratios is inherently more precise than the measurement of absolute quantities. (b) Whereas the conventional U - Xe technique measures the sample date only once, the method described in this thesis measures the sample date several times. (c) If a geological disturbance has caused loss of fission xenon a valid date can still be obtained using the low mobility xenon released at high temperatures. (d) In some samples disturbed dates as well as initial formation dates can be measured. </p> <p> The method was developed and evaluated by a series of experiments which dated four zircon samples, one sphene sample, and a carbonatite sample. The zircon and sphene samples had been dated in other laboratories by the conventional u - Th - Pb and Rb - Sr methods. Fission xenon dating of these samples provided valuable new geochronological information. The experiments also showed that in addition to the advantages mentioned, the new method appears to be better able to see past metamorphic events than other methods. </p> / Thesis / Candidate in Philosophy

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The absolute abundances of fission-product xenon and krypton isotopes in six uranium minerals were determined mass spectrometrically using the isotope dilution technique. The fission products were resolved into a U spontaneous fission component, a U 235 neutron-induced fission component, and a U 238 neutron-induced fission component. The presence of U 238 of neutron-induced fission indicated that t he neutron energies in uranium minerals were higher than had been suspected. Internal consistency in the analysis was achieved only when the I 29 yield in u235 thermal neutron fission reported by Purkayastha and Martin was lowered by 20%. The number of U 238 spontaneous fissions was calculated from the lead age, U 238 content, and the U 238 pontaneous fission half life. A comparison of the number of fission products from U 238 spontaneous fission and the number of spontaneous fissions in each of the six minerals showed that five of the six minerals had lost large portions of their noble gases. A fractionation of krypton relative to xenon accompanied this loss. The results obtained for the sixth mineral indicated little or no loss of xenon and krypton. Absolute yields for U 238 spontaneous fission were therefore determined from an analysis of this mineral. / Thesis / Doctor of Philosophy (PhD)

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L'objectif de cette thèse est d'explorer les mécanismes d'émission des neutrons émis lors du processus de la fission. En particulier, la question ouverte de l'existence d'une anisotropie dynamique dans le centre de masse des fragments de fission et/ou de la possibilité d'une émission de neutrons de scission est explorée. La thèse débute par une introduction aux concepts théoriques permettant de décrire les processus de fission et les mécanismes d'émission de neutrons. La nécessité de concevoir une méthode d'analyse appropriée pour caractériser des mécanismes très subtiles est discutée, en insistant sur l'importance de maîtriser les biais expérimentaux. Le travail effectué a exigé un effort important de simulation, à travers le développement d'une procédure Monte Carlo pour décrire la fission spontanée du 252Cf, ainsi qu'une modélisation du dispositif expérimental de l'expérience CORA3.Nous proposons pour la première fois une approche simultanée et indépendante des deux mécanismes, émission de scission et anisotropie dynamique, ainsi que des valeurs quantitatives d'anisotropie et d'émission de scission mesurées expérimentalement. / The subject of this thesis is to explore the neutron emission mechanisms in the fission process. In particular a long standing open question, the existence of a dynamical anisotropy in the centre of mass of the fission fragments and/or a possible scission neutron emission is explored. The thesis starts with an overview of the theoretical concepts on the fission process and on the neutron emission mechanisms. Also the necessity to conceive an appropriate analysis method is stressed when a very subtle mechanism is studied as various approximations and/or experimental biases not completely handled can hide the physical phenomena. In the presented work a huge effort was required to write a Monte Carlo procedure based on a coherent model for the spontaneous fission of 252Cf and to couple it with the devices exploited in the CORA3 experiment.