christina dugan - Academia.edu (original) (raw)

Papers by christina dugan

UU 18. NUMBER OF PAGES 101 19a. NAME OF RESPONSIBLE PERSON

Hydrothermally grown structures of uranium dioxide (UO2), thorium dioxide (ThO2), and UxTh1-xO2 a... more Hydrothermally grown structures of uranium dioxide (UO2), thorium dioxide (ThO2), and UxTh1-xO2 alloys were characterized for suitability in uranium-based neutron detectors and nuclear reactor fuels. ThO2 was studied for its potential in an envisioned UO2/ThO2 heterojunction. A U0.71Th0.29O2 alloy was studied because of its resistance to oxidation, and potential to stabilize the surface of UO2. The majority carrier mobility of the UO2 samples, as measured by Hall effect, was consistent with established literature values on samples requiring substantial surface preparation. Direct current I(V) measurements provided evidence that the Hall currents may penetrate below any surface oxidized region caused by ambient air exposure. The depth of this surface structure was further investigated using angular resolved x-ray photoemission spectroscopy (ARPES). The U0.71Th0.29O2 effective Debye temperature of 217± 24 K was measured using temperature-dependent x-ray photoemission spectroscopy (XPS). The specific heat capacity for the U0.71Th0.29O2 alloy was calculated from the Debye temperature in order to compare to the heat capacity obtained from the more traditional modulatedtemperature differential scanning calorimetry (MDSC). The XPS derived Debye temperature specific heat capacity was lower than with MDSC, showing how surface oxidation and hydrolysis effect surface stability. The U 4f7/2 peak energy was observed using ARPES, providing evidence of U 4+ and U 6+ oxidation states at the surface with an increased concentration of the U 4+ oxidation states further from the surface. The U 5f peaks observed in ARPES provided v evidence of U 4+ at the surface but does not preclude the presence of U 6+. Throughout the surface measurement, the electronic band-gap energy remained at 2.2 eV, confirming the dominance of UO2 in establishing the electronic structure. These surface characteristics also revealed a vacuum reduced UO2 surface and excess oxygen incorporated into the lattice to form a UO2+x selvedge region. vi Acknowledgments First and foremost I would like to thank God, for with God nothing shall be impossible. God truly blessed me with a great advisor and dissertation committee, wonderful lab technicians, and loving parents. Without the assistance of the following personnel, none of this would have been possible. I would like to show my sincere gratitude to my faculty advisor, Dr. James Petrosky, for his guidance and support throughout the course of this dissertation effort. I would like to thank the other two members of my committee, Dr. John McClory and Lt Col Sheena Winder for their time, advisement, and knowledge of nuclear physics and material science. Their insight and experience was certainly appreciated. I want to acknowledge Air Force Research Laboratory for providing hydrothermally-grown, single-crystal actinide samples and assistance with modulated-temperature differential scanning calorimetry. I would like to thank the lab technician, Greg Smith for his constant support, patience, and time. The numerous hours he dedicated to repairing equipment, building new chambers, and troubleshooting are much appreciated. Last but not least, I would like to thank my parents, who support me throughout my life in all my endeavors.

Physica Status Solidi B-basic Solid State Physics, Aug 26, 2022

The interband transitions of UO2 are validated independently through cathode luminescence. A pict... more The interband transitions of UO2 are validated independently through cathode luminescence. A picture emerges consistent with density functional theory. While theory is generally consistent with experiment, it is evident from the comparison of UO2 and ThO2 that the choice of functional can significantly alter the bandgap and some details of the band structure, in particular at the conduction band minimum. Strictly ab initio predictions of the optical properties of the actinide compounds, based on density functional theory alone, continue to be somewhat elusive.

Applied Optics, Feb 15, 2023

Lithium compounds such as lithium hydride ( L i H ) and anhydrous lithium hydroxide ( L i O H ) h... more Lithium compounds such as lithium hydride ( L i H ) and anhydrous lithium hydroxide ( L i O H ) have various applications in industry but are highly reactive when exposed to moisture and C O 2 . These reactions create new molecular compounds that degrade applications. Environmental conditions such as temperature and moisture are examples of environmental conditions that are of interest for these reactions. To interrogate the effects of such weatherization, experiments were conducted in an environmental chamber (Plas-Labs 890-THC glove box) employing a pulsed laser and an echelle spectrograph in a novel single setup to conduct both Raman spectroscopy and laser-induced breakdown spectroscopy (LIBS) in tandem. These measurements in conjunction with data fusion and machine learning techniques are used to develop training and testing of environmental conditioning of Li compounds. Modeling of environmental characterizations involving lithium-based compounds enabled by the presented measurements and analytical techniques has significant implications on industrial technologies, such as batteries, and other nuclear applications.

Applied Optics

Lithium compounds such as lithium hydride ( L i H ) and anhydrous lithium hydroxide ( L i O H ) h... more Lithium compounds such as lithium hydride ( L i H ) and anhydrous lithium hydroxide ( L i O H ) have various applications in industry but are highly reactive when exposed to moisture and C O 2 . These reactions create new molecular compounds that degrade applications. Environmental conditions such as temperature and moisture are examples of environmental conditions that are of interest for these reactions. To interrogate the effects of such weatherization, experiments were conducted in an environmental chamber (Plas-Labs 890-THC glove box) employing a pulsed laser and an echelle spectrograph in a novel single setup to conduct both Raman spectroscopy and laser-induced breakdown spectroscopy (LIBS) in tandem. These measurements in conjunction with data fusion and machine learning techniques are used to develop training and testing of environmental conditioning of Li compounds. Modeling of environmental characterizations involving lithium-based compounds enabled by the presented measur...

Uranium dioxide (UO2), thorium dioxide (ThO2), and UxTh1-xO2 alloys are characterized for suitabi... more Uranium dioxide (UO2), thorium dioxide (ThO2), and UxTh1-xO2 alloys are characterized for suitability in uranium-based neutron detectors. ThO2 was studied for an envisioned UO2/ThO2 heterojunction. A U0.71Th0.29O2 alloy was studied because of its resistance to oxidation and potential use in surface passivation. The U0.71Th0.29O2 effective Debye temperature of 217± 24 K was measured using temperature-dependent x-ray photoemission spectroscopy (XPS). The specific heat capacity for the U0.71Th0.29O2 alloy was calculated from the Debye temperature and compared to the heat capacity obtained from modulated-temperature differential scanning calorimetry (MDSC). The XPS derived Debye temperature specific heat capacity was lower than with MDSC due to effects of a vacuum reduced crystal surface. Angle resolved XPS provides depth profiling of a UO2 surface. The U 4f7/2 peaks are evident of U4+ and U6+ oxidation states with an increase in the U4+ oxidation state further from the surface. The pre...

UU 18. NUMBER OF PAGES 101 19a. NAME OF RESPONSIBLE PERSON

Journal of Applied Physics, 2020

The infrared-active phonon modes, in single-crystal samples of thorium dioxide (ThO2) and uranium... more The infrared-active phonon modes, in single-crystal samples of thorium dioxide (ThO2) and uranium dioxide (UO2), were investigated using spectroscopic ellipsometry and compared with density functional theory. Both ThO2 and UO2 are found to have one infrared-active phonon mode pair [consisting of one transverse optic (TO) and one associated longitudinal optic (LO) mode], which is responsible for the dominant features in the ellipsometric data. At room temperature, our results for the mode pair’s resonant frequencies and broadening parameters are comparable with previous reflectance spectroscopy characterizations and density functional theory predictions. For ThO2, our ellipsometry and density function theory results both show that the LO mode broadening parameter is larger than the TO mode broadening. This signifies mode anharmonicity, which can be attributed to the intrinsic phonon–phonon interaction. In addition to the main mode pair, a broad low-amplitude impurity-like vibrational...

Applied Physics Letters, 2019

Band-to-band transition energy parameters for single-crystal actinide samples of uranium oxide an... more Band-to-band transition energy parameters for single-crystal actinide samples of uranium oxide and thorium oxide were determined and compared using spectroscopic ellipsometry and critical-point dielectric function analyses. Spectroscopic ellipsometry measurements from the near-infrared to the vacuum ultraviolet spectral region were used to determine the dielectric functions of uranium oxide and thorium oxide. The critical-point structure is similar between UO2 and ThO2 but strongly blue shifted for ThO2. We find bandgap energies of 2.1 eV and 5.4 eV for UO2 and ThO2, respectively.

The European Physical Journal B, 2018

The semiconductor and optical properties of UO2 are investigated. The very long drift carrier lif... more The semiconductor and optical properties of UO2 are investigated. The very long drift carrier lifetimes, obtained from current-voltage I(V) and capacitance-voltage C(V) measurements, along with the well-defined optical properties provide little evidence of an abundance of material defects away from the surface region. Schottky barrier formation may be possible, but very much dependent on the choice of contact and surface stoichiometry and we find that Ohmic contacts are in fact favored. Depth resolved photoemission provided evidence of a chemical shift at the surface. Density functional theory, with the Heyd-Scuseria-Ernzerhof (HSE) functional, indicates a band gap of a 2.19 eV and an anti-ferromagnetic ground state. Ellipsometry measurements indicates at UO2 is relatively isotropic with a band gap of approximately 2.0 eV band gap, consistent with theoretical expectations.

physica status solidi (RRL) - Rapid Research Letters, 2018

The effective Debye temperature (Θ DE) of the surface of a single crystal of U 0.71 Th 0.79 O 2 a... more The effective Debye temperature (Θ DE) of the surface of a single crystal of U 0.71 Th 0.79 O 2 alloy prepared by hydrothermal synthesis is obtained from temperature-dependent X-ray photoemission in the temperature range of 300-623 K. The effective average Debye temperature is determined to be 217 AE 24 K. X-ray fluorescence (XRF) spectroscopy confirms the crystal's composition as U 0.71 Th 0.79 O 2. Photoemission spectroscopy reveals both thorium and uranium 4f states, which includes uranium and thorium in 4þ oxidation states. The Debye temperature of the single crystal U 0.71 Th 0.79 O 2 alloy, as measured via photoemission, is lower than urania or thoria literature values, indicating either higher defect scattering or the presence of added vibrational modes within the alloyed sample. The World Nuclear Association 2013 Global Nuclear Fuel Market Report indicates a 31% increase in uranium demand over 2013-2023. In addition, the world wide electricity demand is expected to double from 2004 to 2030. [1] Recent research is focused on alternative radioactive isotope fuels, such as thorium, to allow for new breeding or conversion fuels. Thorium is 3-4 times more abundant than uranium, and natural thorium is nearly 100% of the desired isotope 232 Th. Therefore, for all thorium derived fuel, no enrichment is needed, which would add to its cost. In comparison, natural uranium is 0.71% 235

Lithium tetraborate or LTB crystals are being developed for possible use in solid state neutron d... more Lithium tetraborate or LTB crystals are being developed for possible use in solid state neutron detectors. Already used in thermo luminescence dosimeters, LTB is of interest due to its large cross section for neutron capture by lithium and boron. The reaction between lithium and a neutron produces an alpha particle and tritium. When boron interacts with a neutron an alpha particle and lithium are produced. These reactions are the basis for neutron detection, and a LTB crystal enriched with Mn should show improved efficiency for such detection. There is, however, a lack of fundamental characterization information regarding this useful material, particularly with regard to its electronic configuration. In this study, cathodoluminescent spectroscopy has been used to determine the energy level structure of undoped and manganese doped lithium tetraborate crystals. Measurements were made at various beam energies from 5 to 10 KeV on samples maintained at room temperature. Self trapped exci...

ABSTRACT Lithium tetraborate or LTB crystals are being developed for possible use in solid state ... more ABSTRACT Lithium tetraborate or LTB crystals are being developed for possible use in solid state neutron detectors. Already used in thermo luminescence dosimeters, LTB is of interest due to its large cross section for neutron capture by lithium and boron. The reaction between lithium and a neutron produces an alpha particle and tritium. When boron interacts with a neutron an alpha particle and lithium are produced. These reactions are the basis for neutron detection, and an LTB crystal enriched with Mn should show improved efficiency for neutron detection. There is, however, a lack of fundamental characterization information regarding this useful material, particularly with regard to its electronic configuration. In this study, photoemission spectroscopy has been used to determine the energy level structure of manganese doped Lithium Tetraborate crystals. Measurements were made using UV photons from the storage ring of the synchrotron at Louisiana State University. Comparison will be made between Mn doped LTB crystals and undoped crystals.

MRS Proc., 2011

Photoemission spectroscopy using synchrotron radiation was used to determine the energy level str... more Photoemission spectroscopy using synchrotron radiation was used to determine the energy level structure of Mn doped 2 4 7 Li B O crystals. Photoemission studies provided evidence of Mn in the bulk crystal at 47.2 eV. Valence band analysis provided the presence of surface states but no acceptor sites. Cathodoluminescence studies were also made on undoped and Mn doped 2 4 7 Li B O using various beam energies from 5 to 10 KeV at room temperature. Self trapped exciton emission states are evident in the undoped and Mn doped Li 2 B 4 O 7 sample ranging in energies from 3.1 to 4.1 eV.

Applied Physics Letters, 2013

We find Mn surface segregation for single crystals of Mn doped Li2B4O7, nominally Li1.95Mn0.05B4O... more We find Mn surface segregation for single crystals of Mn doped Li2B4O7, nominally Li1.95Mn0.05B4O7(001), but as the temperature increases, evidence of this Mn surface segregation diminishes significantly. At room temperature, the surface photovoltaic charging is significant for this pyroelectric material but is quenched at a temperature well below that seen for the undoped Li2B4O7 samples. The suppression of surface charging in the region of 120 °C that accompanies the temperature of Mn dissolution in the bulk of Li2B4O7, i.e., the reversal of Mn surface segregation (215 °C), suggests that along the (001) direction, ionic transport must be considered as significant.

The interband transitions of urania (UO2) are validated independently through cathode luminescenc... more The interband transitions of urania (UO2) are validated independently through cathode luminescence of UO2. A picture emerges consistent with density functional theory. While theory is generally consistent with experiment, it is evident that the choice of functional can significantly alter the band gap and some details of the band structure, in particular at the conduction band minimum. Strictly ab initio predictions of the optical properties of the actinide compounds, based on density functional theory alone continues to be somewhat elusive.

UU 18. NUMBER OF PAGES 101 19a. NAME OF RESPONSIBLE PERSON

Hydrothermally grown structures of uranium dioxide (UO2), thorium dioxide (ThO2), and UxTh1-xO2 a... more Hydrothermally grown structures of uranium dioxide (UO2), thorium dioxide (ThO2), and UxTh1-xO2 alloys were characterized for suitability in uranium-based neutron detectors and nuclear reactor fuels. ThO2 was studied for its potential in an envisioned UO2/ThO2 heterojunction. A U0.71Th0.29O2 alloy was studied because of its resistance to oxidation, and potential to stabilize the surface of UO2. The majority carrier mobility of the UO2 samples, as measured by Hall effect, was consistent with established literature values on samples requiring substantial surface preparation. Direct current I(V) measurements provided evidence that the Hall currents may penetrate below any surface oxidized region caused by ambient air exposure. The depth of this surface structure was further investigated using angular resolved x-ray photoemission spectroscopy (ARPES). The U0.71Th0.29O2 effective Debye temperature of 217± 24 K was measured using temperature-dependent x-ray photoemission spectroscopy (XPS). The specific heat capacity for the U0.71Th0.29O2 alloy was calculated from the Debye temperature in order to compare to the heat capacity obtained from the more traditional modulatedtemperature differential scanning calorimetry (MDSC). The XPS derived Debye temperature specific heat capacity was lower than with MDSC, showing how surface oxidation and hydrolysis effect surface stability. The U 4f7/2 peak energy was observed using ARPES, providing evidence of U 4+ and U 6+ oxidation states at the surface with an increased concentration of the U 4+ oxidation states further from the surface. The U 5f peaks observed in ARPES provided v evidence of U 4+ at the surface but does not preclude the presence of U 6+. Throughout the surface measurement, the electronic band-gap energy remained at 2.2 eV, confirming the dominance of UO2 in establishing the electronic structure. These surface characteristics also revealed a vacuum reduced UO2 surface and excess oxygen incorporated into the lattice to form a UO2+x selvedge region. vi Acknowledgments First and foremost I would like to thank God, for with God nothing shall be impossible. God truly blessed me with a great advisor and dissertation committee, wonderful lab technicians, and loving parents. Without the assistance of the following personnel, none of this would have been possible. I would like to show my sincere gratitude to my faculty advisor, Dr. James Petrosky, for his guidance and support throughout the course of this dissertation effort. I would like to thank the other two members of my committee, Dr. John McClory and Lt Col Sheena Winder for their time, advisement, and knowledge of nuclear physics and material science. Their insight and experience was certainly appreciated. I want to acknowledge Air Force Research Laboratory for providing hydrothermally-grown, single-crystal actinide samples and assistance with modulated-temperature differential scanning calorimetry. I would like to thank the lab technician, Greg Smith for his constant support, patience, and time. The numerous hours he dedicated to repairing equipment, building new chambers, and troubleshooting are much appreciated. Last but not least, I would like to thank my parents, who support me throughout my life in all my endeavors.

Physica Status Solidi B-basic Solid State Physics, Aug 26, 2022

The interband transitions of UO2 are validated independently through cathode luminescence. A pict... more The interband transitions of UO2 are validated independently through cathode luminescence. A picture emerges consistent with density functional theory. While theory is generally consistent with experiment, it is evident from the comparison of UO2 and ThO2 that the choice of functional can significantly alter the bandgap and some details of the band structure, in particular at the conduction band minimum. Strictly ab initio predictions of the optical properties of the actinide compounds, based on density functional theory alone, continue to be somewhat elusive.

Applied Optics, Feb 15, 2023

Lithium compounds such as lithium hydride ( L i H ) and anhydrous lithium hydroxide ( L i O H ) h... more Lithium compounds such as lithium hydride ( L i H ) and anhydrous lithium hydroxide ( L i O H ) have various applications in industry but are highly reactive when exposed to moisture and C O 2 . These reactions create new molecular compounds that degrade applications. Environmental conditions such as temperature and moisture are examples of environmental conditions that are of interest for these reactions. To interrogate the effects of such weatherization, experiments were conducted in an environmental chamber (Plas-Labs 890-THC glove box) employing a pulsed laser and an echelle spectrograph in a novel single setup to conduct both Raman spectroscopy and laser-induced breakdown spectroscopy (LIBS) in tandem. These measurements in conjunction with data fusion and machine learning techniques are used to develop training and testing of environmental conditioning of Li compounds. Modeling of environmental characterizations involving lithium-based compounds enabled by the presented measurements and analytical techniques has significant implications on industrial technologies, such as batteries, and other nuclear applications.

Applied Optics

Lithium compounds such as lithium hydride ( L i H ) and anhydrous lithium hydroxide ( L i O H ) h... more Lithium compounds such as lithium hydride ( L i H ) and anhydrous lithium hydroxide ( L i O H ) have various applications in industry but are highly reactive when exposed to moisture and C O 2 . These reactions create new molecular compounds that degrade applications. Environmental conditions such as temperature and moisture are examples of environmental conditions that are of interest for these reactions. To interrogate the effects of such weatherization, experiments were conducted in an environmental chamber (Plas-Labs 890-THC glove box) employing a pulsed laser and an echelle spectrograph in a novel single setup to conduct both Raman spectroscopy and laser-induced breakdown spectroscopy (LIBS) in tandem. These measurements in conjunction with data fusion and machine learning techniques are used to develop training and testing of environmental conditioning of Li compounds. Modeling of environmental characterizations involving lithium-based compounds enabled by the presented measur...

Uranium dioxide (UO2), thorium dioxide (ThO2), and UxTh1-xO2 alloys are characterized for suitabi... more Uranium dioxide (UO2), thorium dioxide (ThO2), and UxTh1-xO2 alloys are characterized for suitability in uranium-based neutron detectors. ThO2 was studied for an envisioned UO2/ThO2 heterojunction. A U0.71Th0.29O2 alloy was studied because of its resistance to oxidation and potential use in surface passivation. The U0.71Th0.29O2 effective Debye temperature of 217± 24 K was measured using temperature-dependent x-ray photoemission spectroscopy (XPS). The specific heat capacity for the U0.71Th0.29O2 alloy was calculated from the Debye temperature and compared to the heat capacity obtained from modulated-temperature differential scanning calorimetry (MDSC). The XPS derived Debye temperature specific heat capacity was lower than with MDSC due to effects of a vacuum reduced crystal surface. Angle resolved XPS provides depth profiling of a UO2 surface. The U 4f7/2 peaks are evident of U4+ and U6+ oxidation states with an increase in the U4+ oxidation state further from the surface. The pre...

UU 18. NUMBER OF PAGES 101 19a. NAME OF RESPONSIBLE PERSON

Journal of Applied Physics, 2020

The infrared-active phonon modes, in single-crystal samples of thorium dioxide (ThO2) and uranium... more The infrared-active phonon modes, in single-crystal samples of thorium dioxide (ThO2) and uranium dioxide (UO2), were investigated using spectroscopic ellipsometry and compared with density functional theory. Both ThO2 and UO2 are found to have one infrared-active phonon mode pair [consisting of one transverse optic (TO) and one associated longitudinal optic (LO) mode], which is responsible for the dominant features in the ellipsometric data. At room temperature, our results for the mode pair’s resonant frequencies and broadening parameters are comparable with previous reflectance spectroscopy characterizations and density functional theory predictions. For ThO2, our ellipsometry and density function theory results both show that the LO mode broadening parameter is larger than the TO mode broadening. This signifies mode anharmonicity, which can be attributed to the intrinsic phonon–phonon interaction. In addition to the main mode pair, a broad low-amplitude impurity-like vibrational...

Applied Physics Letters, 2019

Band-to-band transition energy parameters for single-crystal actinide samples of uranium oxide an... more Band-to-band transition energy parameters for single-crystal actinide samples of uranium oxide and thorium oxide were determined and compared using spectroscopic ellipsometry and critical-point dielectric function analyses. Spectroscopic ellipsometry measurements from the near-infrared to the vacuum ultraviolet spectral region were used to determine the dielectric functions of uranium oxide and thorium oxide. The critical-point structure is similar between UO2 and ThO2 but strongly blue shifted for ThO2. We find bandgap energies of 2.1 eV and 5.4 eV for UO2 and ThO2, respectively.

The European Physical Journal B, 2018

The semiconductor and optical properties of UO2 are investigated. The very long drift carrier lif... more The semiconductor and optical properties of UO2 are investigated. The very long drift carrier lifetimes, obtained from current-voltage I(V) and capacitance-voltage C(V) measurements, along with the well-defined optical properties provide little evidence of an abundance of material defects away from the surface region. Schottky barrier formation may be possible, but very much dependent on the choice of contact and surface stoichiometry and we find that Ohmic contacts are in fact favored. Depth resolved photoemission provided evidence of a chemical shift at the surface. Density functional theory, with the Heyd-Scuseria-Ernzerhof (HSE) functional, indicates a band gap of a 2.19 eV and an anti-ferromagnetic ground state. Ellipsometry measurements indicates at UO2 is relatively isotropic with a band gap of approximately 2.0 eV band gap, consistent with theoretical expectations.

physica status solidi (RRL) - Rapid Research Letters, 2018

The effective Debye temperature (Θ DE) of the surface of a single crystal of U 0.71 Th 0.79 O 2 a... more The effective Debye temperature (Θ DE) of the surface of a single crystal of U 0.71 Th 0.79 O 2 alloy prepared by hydrothermal synthesis is obtained from temperature-dependent X-ray photoemission in the temperature range of 300-623 K. The effective average Debye temperature is determined to be 217 AE 24 K. X-ray fluorescence (XRF) spectroscopy confirms the crystal's composition as U 0.71 Th 0.79 O 2. Photoemission spectroscopy reveals both thorium and uranium 4f states, which includes uranium and thorium in 4þ oxidation states. The Debye temperature of the single crystal U 0.71 Th 0.79 O 2 alloy, as measured via photoemission, is lower than urania or thoria literature values, indicating either higher defect scattering or the presence of added vibrational modes within the alloyed sample. The World Nuclear Association 2013 Global Nuclear Fuel Market Report indicates a 31% increase in uranium demand over 2013-2023. In addition, the world wide electricity demand is expected to double from 2004 to 2030. [1] Recent research is focused on alternative radioactive isotope fuels, such as thorium, to allow for new breeding or conversion fuels. Thorium is 3-4 times more abundant than uranium, and natural thorium is nearly 100% of the desired isotope 232 Th. Therefore, for all thorium derived fuel, no enrichment is needed, which would add to its cost. In comparison, natural uranium is 0.71% 235

Lithium tetraborate or LTB crystals are being developed for possible use in solid state neutron d... more Lithium tetraborate or LTB crystals are being developed for possible use in solid state neutron detectors. Already used in thermo luminescence dosimeters, LTB is of interest due to its large cross section for neutron capture by lithium and boron. The reaction between lithium and a neutron produces an alpha particle and tritium. When boron interacts with a neutron an alpha particle and lithium are produced. These reactions are the basis for neutron detection, and a LTB crystal enriched with Mn should show improved efficiency for such detection. There is, however, a lack of fundamental characterization information regarding this useful material, particularly with regard to its electronic configuration. In this study, cathodoluminescent spectroscopy has been used to determine the energy level structure of undoped and manganese doped lithium tetraborate crystals. Measurements were made at various beam energies from 5 to 10 KeV on samples maintained at room temperature. Self trapped exci...

ABSTRACT Lithium tetraborate or LTB crystals are being developed for possible use in solid state ... more ABSTRACT Lithium tetraborate or LTB crystals are being developed for possible use in solid state neutron detectors. Already used in thermo luminescence dosimeters, LTB is of interest due to its large cross section for neutron capture by lithium and boron. The reaction between lithium and a neutron produces an alpha particle and tritium. When boron interacts with a neutron an alpha particle and lithium are produced. These reactions are the basis for neutron detection, and an LTB crystal enriched with Mn should show improved efficiency for neutron detection. There is, however, a lack of fundamental characterization information regarding this useful material, particularly with regard to its electronic configuration. In this study, photoemission spectroscopy has been used to determine the energy level structure of manganese doped Lithium Tetraborate crystals. Measurements were made using UV photons from the storage ring of the synchrotron at Louisiana State University. Comparison will be made between Mn doped LTB crystals and undoped crystals.

MRS Proc., 2011

Photoemission spectroscopy using synchrotron radiation was used to determine the energy level str... more Photoemission spectroscopy using synchrotron radiation was used to determine the energy level structure of Mn doped 2 4 7 Li B O crystals. Photoemission studies provided evidence of Mn in the bulk crystal at 47.2 eV. Valence band analysis provided the presence of surface states but no acceptor sites. Cathodoluminescence studies were also made on undoped and Mn doped 2 4 7 Li B O using various beam energies from 5 to 10 KeV at room temperature. Self trapped exciton emission states are evident in the undoped and Mn doped Li 2 B 4 O 7 sample ranging in energies from 3.1 to 4.1 eV.

Applied Physics Letters, 2013

We find Mn surface segregation for single crystals of Mn doped Li2B4O7, nominally Li1.95Mn0.05B4O... more We find Mn surface segregation for single crystals of Mn doped Li2B4O7, nominally Li1.95Mn0.05B4O7(001), but as the temperature increases, evidence of this Mn surface segregation diminishes significantly. At room temperature, the surface photovoltaic charging is significant for this pyroelectric material but is quenched at a temperature well below that seen for the undoped Li2B4O7 samples. The suppression of surface charging in the region of 120 °C that accompanies the temperature of Mn dissolution in the bulk of Li2B4O7, i.e., the reversal of Mn surface segregation (215 °C), suggests that along the (001) direction, ionic transport must be considered as significant.

The interband transitions of urania (UO2) are validated independently through cathode luminescenc... more The interband transitions of urania (UO2) are validated independently through cathode luminescence of UO2. A picture emerges consistent with density functional theory. While theory is generally consistent with experiment, it is evident that the choice of functional can significantly alter the band gap and some details of the band structure, in particular at the conduction band minimum. Strictly ab initio predictions of the optical properties of the actinide compounds, based on density functional theory alone continues to be somewhat elusive.