Dissertation
ATOMIC DEFECTS OF CZOCHRALSKI GROWN CERIUM AND CALCIUM CO-DOPED YTTRIUM ALUMINUM GARNET, CERIUM AND LITHIUM CO-DOPED YTTRIUM ALUMINUM GARNET, AND DOPED LITHIUM ALUMINATE
Doctor of Philosophy (PhD), Washington State University
01/2016
Handle:
https://hdl.handle.net/2376/118332
Abstract
The development and improvement of scintillators for radiation detection is an important field of research that can affect homeland security, medical imaging, and high energy physics research. One method to improve scintillators is by co-doping to influence the defect and luminescent properties. In this dissertation, cerium doped yttrium aluminum garnet single crystals were produced by the Czochralski method and co-doped with calcium and lithium. Similar to other calcium co-doped scintillators, calcium co-doping in yttrium aluminum garnet was revealed to significantly improve the short and long decay times and reduce thermoluminescence signal between 30 K and 698 K. Additionally, positron annihilation spectroscopy revealed an increase in vacancy defects, and the mechanism for improved decay times was found with photoluminescence which revealed the presence of a non-radiative decay pathway, which has not been observed before.
Lithium co-doping increased the scintillation light output and significantly reduced longer decay times while not affecting the short decay time. No Ce(IV) was detected despite co-doping with a mono-valent ion, and thermoluminescent signal was discovered to have decreased significantly. Photoluminescence demonstrated no significant change in the emission intensity for the lithium co-doped on stoichiometric sample while the 10% excess alumina samples, which were shown to contain secondary phase, exhibited a significant increase in photoluminescent light output. Overall, lithium co-doping improves scintillation properties by interfering with charge carrier trapping.
Finally, rare-earth activation using cerium, europium, and praseodymium of lithium aluminate grown by the Czochralski method for neutron detection was investigated. No clear luminescent activation by the rare-earth ions was identified and it was concluded that lithium aluminate is not a suitable host for Ce, Eu, and Pr rare-earth ions. Despite this, several luminescent defect and impurity centers were identified using photoluminescence, and the depth of a shallow charge carrier trap was calculated using the initial rise method. Additionally, the intrinsic neutron detection efficiency, while using natural lithium, for a 10 mm Ø x 10 mm tall right cylinder was found to be 70% using Monte Carlo N-Particle Transport Code, and distinct neutron detection was demonstrated with the Czochralski grown samples despite poor luminescence.
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Details
- Title
- ATOMIC DEFECTS OF CZOCHRALSKI GROWN CERIUM AND CALCIUM CO-DOPED YTTRIUM ALUMINUM GARNET, CERIUM AND LITHIUM CO-DOPED YTTRIUM ALUMINUM GARNET, AND DOPED LITHIUM ALUMINATE
- Creators
- Peter Thomas Dickens
- Contributors
- Kelvin G Lynn (Advisor)Gary Collins (Committee Member)John McCloy (Committee Member)Matthew D McCluskey (Committee Member)
- Awarding Institution
- Washington State University
- Academic Unit
- Materials Science and Engineering Program
- Theses and Dissertations
- Doctor of Philosophy (PhD), Washington State University
- Number of pages
- 170
- Identifiers
- 99900581722001842
- Language
- English
- Resource Type
- Dissertation