Uranium Oxides for Solid-State Direct-Conversion Neutron Detection
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This study is focused on studying the potential of uranium oxides as solid-state direct-conversion neutron detectors. By modeling uranium oxides as neutron detectors, we found that thick detectors with suitable uranium isotopes would be required to achieve absorption probabilities of a few percentages: 1.5–4% for 10 MeV neutrons for natural uranium oxides, ∼2% for 1 MeV neutrons for ²³⁵U-based uranium oxides, and 7–9% for thermal neutrons for natural uranium oxides, assuming each detector is 1 cm thick. The leakage current associated with α-decay of uranium acts as the major source of noise in these materials, which imposes a minimum limit on the charge transport properties required to consider them for neutron detection. A charge carrier mobility of 1–10 cm²⋅V⁻¹⋅s⁻¹ and carrier lifetime of 10⁻⁵–10⁻⁴ s would be required to achieve a signal above the leakage current. α-U₃O₈ pellets were fabricated with ∼95% theoretical density and resistivity values on the order of 10²–10⁴ Ω-cm. The resistivity decreased slightly when the pellet composition was off-stoichiometric. The charge carrier mobility of these pellets could not be determined due to the absence of a measurable signal and was estimated as less than 1 cm²⋅V⁻¹⋅s⁻¹ based on Hall effect measurements. UO₃ pellets were fabricated with 85–90% of theoretical density. Most of these pellets, upon investigation with x-ray diffraction, were found to consist of ∼90% γ-UO₃ and ∼10% of α-UO₂(OH)₂. The direct and indirect band gap values of these pellets were ∼2 and ∼2.9 eV respectively and the resistivity values were on the order of 10⁹–10¹⁰ Ω-cm. The charge carrier mobility could not be determined due to the absence of a measurable signal but the maximum value was estimated on the order of 10⁻²–10⁻³ cm²⋅V⁻¹⋅s⁻¹ based on time-of-flight photoconductivity and steady-state space-charge-limited current methods. Our calculations showed that polaronic effects may be contributing to low charge carrier mobility in these materials. The charge carrier mobility of uranium oxides must be increased to be able to measure and use these materials for neutron detection. Based on our understanding of these materials, we have proposed various methods that may be used to increase the charge carrier mobility, however, until the charge carrier mobility measurements are carried out, the potential of these materials for neutron detection will remain unclear.
Table of Contents
Introduction -- Theory and modeling -- Methods: Theory and experimental procedure -- Results and discussions: U₃O₈ -- Results and discussions: UO₃ -- Conclusion and future work -- Appendix A. Example VAST input files -- Appendix B. I-V measurements on U₃O₈ -- Appendix C. Hall Effect measurements on U₃O₈ -- Appendix D. Band Gap analysis of UO₃ -- Appendix E. I-V measurements on UO₃ -- Appendix F. High voltage measurements on UO₃ -- Appendix G. Electronic structure of mixed crystals of UO₃
Ph.D. (Doctor of Philosophy)