1 Fabrication and Characterization of a Silicon Carbide Alpha Detector For

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Fabrication and Characterization of a Silicon Carbide Alpha Detector for Molten Salt

Application

Dissertation

Presented in Partial Fulfillment of the Requirements for the Degree Doctor of Philosophy

in the Graduate School of The Ohio State University

Joshua T. Jarrell, B.S., M.S.

Graduate Program in Nuclear Engineering

The Ohio State University

2018

Dissertation Committee

Dr. Lei R. Cao, Advisor

Dr. Thomas Blue

Dr. Marat Khafizov

Copyrighted by

Joshua T. Jarrell

2018

Abstract

There exists a need for monitoring the actinide concentrations in elevated temperature molten salt environments. Reprocessing of used nuclear fuel through pyroprocessing is being investigated as a viable method to manage the growing stockpile of used nuclear fuel. Idaho National Laboratory has demonstrated the ability to reprocess both breeder and blanket fuel from the Experimental Breeder Reactor II using an electrorefining system.

This system uses a molten eutectic salt mixture of lithium chloride and potassium chloride.

This electrorefining system can produce high purity uranium ingots and mixed uranium- plutonium ingots. The fundamental electrochemistry used for this process precludes the separation of high purity plutonium when operated within the suggested process limits.

However, special nuclear material may be diverted by operating outside of the normal process. Complete draw down of the uranium dissolved into the molten salt would allow for the subsequent removal of high purity plutonium. Monitoring of the operational history of the electrorefiner is therefore essential to address these non-proliferation and safeguard concerns. There is thus a need to monitor the concentrations of individual elements and isotopes present in the electrorefiner salt. Currently, such assays require time on the order of weeks to provide an accurate description of isotopic concentrations within the salt. Thus, a near real-time measurement system for the actinide isotopic concentrations within the salt is needed. All actinide isotopes of interest to non-proliferation and safeguards interests

ii emit characteristic alpha particles. Semiconductor radiation detectors have been shown to provide a compact, high energy resolution solution to spectroscopic measurement needs.

Silicon carbide, a wide band-gap semiconductor, provides elevated temperature operation capability and corrosion resistance in the molten salt environment that is superior to silicon.

As a result, for this work, alpha radiation detectors comprised of 4H-SiC with Schottky barrier contacts have been fabricated and shown to operate above 500oC. Detector contact compositions of nickel-platinum was explored as possible Schottky contact structures. The electrical and diode characteristics of the detectors were measured. Alpha spectra from multiple source isotopes and source geometries were obtained in vacuum with the detector heated from 20oC to 500oC. The resulting detector behavior including alpha spectrum centroid position and detector energy resolution were measured. To avoid energy attenuation in the molten salt, a repeatable method for depositing actinides to the surface of the detector was devised that allows for repeated spectroscopic measurements by a single detector. The resilience of detector performance to submersion in a molten salt was investigated as well as energy resolution during elevated temperature operation. Detectors were characterized prior to being submerged in a 500oC molten LiCl-KCl eutectic salt for increasing time intervals. After submersion, the detectors were again characterized to identify any degradation. Detector packaging capable of withstanding the corrosive 500oC molten salt environment was developed which allows for electrical connections between the detector and spectrometry equipment. The packaging was designed to allow for actinide deposition on the active area of the detector, allowing for accurate calculations of the actinide mass deposited by a known current. Additionally, nuclear forensic applications of

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4H-SiC alpha detectors in conjunction with electrodeposited source fabrication were explored. A method was determined to calculate the 235U enrichment in the product stream of an enrichment facility through measurement of the 234U and 235U enrichments in an electrodeposited source fabricated from depleted uranium.

Dedication

This is dedicated to my loving wife Jennifer.

Acknowledgments

This work would not be possible without the assistance and support of many individuals.

Foremost among these are Dr. Raymond Cao and Dr. Thomas Blue. Their guidance, suggestions, and feedback proved indispensable in my development as a graduate student.

I would also like to acknowledge Dr. Lei Wang and Sha Xue for their work which contributed to the outcome of this research. Additionally, I would like to acknowledge Dr.

Marat Khafizov for serving on my thesis defense committee. I would like to acknowledge

Dr. Timothy Garcia, Dr. Benjamin Reinke and the staff of Nanotech West for their assistance with the development of the equipment and processes which were invaluable in the research presented in this work. I would also like to acknowledge the National

Academy for Nuclear Training Fellowship and the Nuclear Regulatory Commission

Fellowship programs. Their financial support of my stipend made this work possible.

Additionally, I would like to thank the Nuclear Energy University Program for their support of the project on which I worked.

Vita

November 7, 1990 ……………… Born – Orange, VA, USA

2013 …………………….…...….. B.S Physics, James Madison University

2015 …………………………..… M.S. Nuclear Engineering, The Ohio State University

2016-Present ……………………. NRC Fellow, The Ohio State University

Publications

1. Josh Jarrell, Milan Stika, Michael Simpson, Thomas E. Blue, and Lei R. Cao, “4H-SiC Alpha Spectrometry for Nuclear Forensics with Electrodeposited Sources”, J. of Radioanalytical and Nuclear Chemistry (accepted)

2. Josh Jarrell, Thomas E. Blue, and Lei R. Cao “Development of A Ni-Pt Schottky Diode for High Temperature Alpha Spectroscopy”. IEEE 2017 NSS/MIC/RTSD, 00(2):277– 287, Oct 2017.

3. Josh Jarrell, Milan Stika, Michael Simpson, Thomas E. Blue, Lei R. Cao. “Depleted Uranium and Th-232 Decay Chain Daughter Isotope Identification with 4H-SiC Alpha Spectroscopy”. Transactions of American Nuclear Society, 00(2):277–287, Oct 2017.

4. Lei Wang, Josh Jarrell, Sha Xue, Thomas Blue, Lei R. Cao. “The Fast Neutron Sensitivity of a SiC Detector”. Transactions of American Nuclear Society, 00(2):277– 287, Oct 2017.

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5. Lei Cao, Josh Jarrell, Andrew Kauffman, Susan White, Kevin Herminghuysen, Douglas E. Hardtmayer, Jeff Sanders, Shelly Li “A Radioactive Tracer Dilution Method to Determine the Mass of Molten Salt”. Journal of Radioanaltyical and Nuclear Chemistry, https://doi.org/10.1007/s10967-017-5417-5, Aug 2017.

6. Lei Cao, Eric Moore, Josh Jarrell “Heteroepitaxial Diamond Growth on 4H-SiC using Microwave Plasma Chemical Vapor Deposition”. Heliyon, Sep 2017.

7. Joshua Jarrell, Eric Moore, Thomas Blue, Lei Cao “Elevated Temperature Alpha Spectroscopy with Nickel-Platinum 4H-SiC Schottky Diodes”. Transactions of American Nuclear Society, 116 (1). 123-125, Jun 2017.

8. M. Stika, S. Padilla, J. Jarrell, T. Blue, L. R. Cao, M. Simpson “Thin-Layer Elec- trodeposition of Thorium Metal from Molten LiCl-KCl”. Journal of Electrochemical Society, vol. 164, no. 8, 2017.

9. Josh Jarrell, Milan Stika, Max Chaiken, Michael Simpson, Thomas E. Blue, Lei R. Cao “Determination of the thickness of an electrodeposited thorium film with SiC alpha detectors”. J. Radioanalytical and Nuclear Chemistry, 1127-1133 Vol. 311, no. 2., Nov 2016.

10. Milan Stika, Max Chaiken, Josh Jarrell, Thomas Blue, Lei Raymond Cao, Michael Simpson “Thin-Layer Electrodeposition of Thorium and Uranium from Molten LiCl- KCl”. ECS Transactions, 603-608 volume 75, issue 15, 2016.

11. Milan Stika, Max Chaiken, Joshua Jarrell, Thomas Blue, Lei R. Cao, Michael Simpson “Electrodeposition of Actinides on a Semiconductor Detector for Concentration Monitoring”. Transactions of American Nuclear Society, 114 (1). 340-344, Jun 2016.

12. Josh Jarell, Benjamin Reinke, Max Chaiken, Brandon Wilson, Wolfgang Windl, Brian Esser, Lei Cao, Thomas Blue “Charge Carrier Diffusion Length Determination in 4H- SiC Schottky Alpha Detectors”. Transactions of American Nuclear Society, 114 (1). 300- 304, Jun 2016.

13. Benjamin Reinke, Joshua Jarrell, Max Chaiken, Brandon A. Wilson, Thomas E. Blue, Wolfgang Windl, Bryan D. Esser, Lei Cao “Long-term 500 C testing of high- temperature 4H-SiC Schottky diode alpha article detectors for pyroprocessing”. Transactions of American Nuclear Society, 113 (1). 489-491, Nov 2015.

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Fields of Study

Major Field: Nuclear Engineering

Table of Contents

Abstract ...... ii Dedication ...... v Acknowledgments...... vi Vita ...... vii List of Tables ...... xiii List of Figures ...... xiv Chapter 1. Introduction ...... 1 1.1 The Nuclear Fuel Cycle...... 3 1.2 Pyroprocessing and Molten Salts ...... 6 1.3 Alpha Spectrometry ...... 8 1.4 Nuclear Forensics...... 9 Chapter 2. Background ...... 11 2.1 Diode Principles ...... 11 2.1.1 P-N and P-I-N Diodes ...... 14 2.1.2 Schottky Diode...... 18 2.2 Wide Band-Gap Semiconductors ...... 20 2.3 Elevated Temperature Diode Operation ...... 21 2.4 4H-Silicon Carbide ...... 23 2.5 Semiconductor Radiation Detection Principles ...... 25 2.6 Detector Characterization and Testing Equipment ...... 32 2.6.2 Electrical Characterization Equipment ...... 32 2.6.1 Electrical Characterization Methods ...... 33 2.6.2 Alpha Spectroscopy Equipment ...... 36 2.7 The Molten Salt Environment ...... 41 2.8 Electrodeposition and Electronegativity ...... 43 x

2.9 Thick Film Source Geometry...... 44 2.10 Nuclear Forensic Applications of Alpha Spectrometry ...... 48 Chapter 3. Detector Fabrication and Simulation ...... 50 3.1 Initial Detector Design ...... 50 3.2 Silicon Carbide Wafer Preparation ...... 53 3.3 Shadow masking metal deposition...... 55 3.4 Annealing for Schottky Contact...... 56 3.5 Design Revisions ...... 57 3.5.1 Double Schottky Contacts...... 57

3.5.2 Epitaxial SiO2 Passivation Layer ...... 59 3.5.3 Guard Ring ...... 61 3.5.4 Photolithography ...... 63 3.5.5 Etching ...... 66 3.5.6 Liftoff ...... 68 3.7 Electrical Connections and Wire Bonding ...... 73 3.8 Simulated Source Spectra ...... 75 Chapter 4. Detector Characterization Results ...... 91 4.1 Current-Voltage Behavior under Forward Bias ...... 91 4.1.1 Turn-On Voltage ...... 92 4.1.2 Schottky Barrier Calculation ...... 94 4.1.3 Ideality Factor ...... 95 4.2 Current-Voltage Behavior under Reverse Bias ...... 98 4.2.1 Leakage Current versus Voltage ...... 98 4.2.2 Capacitance-Voltage Behavior ...... 101 4.3 Charge Depleted Region Calculation...... 103 4.4 Room Temperature Alpha Spectrometry ...... 105 4.4.1 Triple Alpha Source ...... 110 4.4.2 Thick Film Electrodeposited Source ...... 112 4.4.3 Paper 1 Results and Discussion ...... 118 4.4.4 Paper 1 Conclusions ...... 126 4.4.5 Energy Loss in Increased Thickness Schottky Contact ...... 127 4.5 Elevated Temperature Alpha Spectrometry and Leakage Current ...... 131

4.5.1 Elevated Temperature Alpha Spectroscopy ...... 132 4.5.2 4H-SiC W-Value Determination ...... 139 4.5.3 Forward Bias Temperature Sensing ...... 140 4.6 Molten Salt Experiments...... 144 4.6.1 Thermal Shock Response from Molten Salt Submersion ...... 146 4.6.2 Long Term Detector Behavior in Molten Salt ...... 148 Chapter 5. Detector Packaging for Molten Salt Applications ...... 153 5.1 Detector Packaging Design Requirements...... 153 5.2 Molten Salt Material Compatibility ...... 153 5.3 Detector Housing ...... 154 5.4 Detector Mounting and Hermetic Sealing Requirements ...... 156 5.5 Signal Cable Considerations ...... 159 5.6 Electrical Connections ...... 161 5.7 Packaged Detector Characterization ...... 163 5.7 Alpha Spectrum from Surface Deposited Actinide Source ...... 168 Chapter 6. Nuclear Forensic Applications of 4H-SiC Alpha Spectrometry ...... 172 6.1 Paper 3 Abstract ...... 172 6.2 Paper 3 Introduction ...... 173 6.2 Electrodeposited Source Fabrication of Forensic Samples ...... 176 6.3 Paper 3 Results and Discussion ...... 176 6.4 Paper 3 Conclusions ...... 184 Chapter 7. Discussion and Conclusions ...... 185 7.1 Discussion ...... 185 7.2 Future Work ...... 186 7.2.1 General Detector Design Improvements ...... 186 7.2.3 Pyroprocessing Applications ...... 187 7.2.3 Nuclear Forensic Applications ...... 188 7.3 Conclusions ...... 189 References ...... 192

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List of Tables

Table 1 Isotopic Abundances of Pu in LWR UNF ...... 78 Table 2. Fission Fragment Energies...... 89 Table 3. Ceramic Adhesive Disc Masses ...... 158 Table 4. Product 235U enrichment from plausible waste stream 235U enrichments from gaseous diffusion with NW234=0.00286 at.% ...... 183 Table 5. Product 235U enrichment from plausible waste stream 235U enrichments from gaseous centrifugation with NW234=0.00286 at.% ...... 183

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List of Figures

Figure 1. The Mark IV Idaho National Laboratory Electrorefiner ...... 2 Figure 2. The Ramsdell classification scheme representation of the stacking order of 4H- SiC [22]...... 24 Figure 3. Keithley 2410 Sourcemeter used for electrical characterization. The Sourcemeter has both 2 and 4-wire measurement capability...... 32 Figure 4. The vacuum was established using a Pfeiffer HiCube turbopump and mechanical pump unit...... 36 Figure 5. Vacuum bell jar on top of a stainless-steel stand. A metal platform inside houses the heater and serves as the measurement stand...... 37 Figure 6. Canberra DSA-2000 Multichannel Analyzer...... 38 Figure 7. Charge sensitive Ortec 142B preamplifier ...... 38 Figure 8. Blue Wave substrate heater with a detector mounted on the surface...... 39 Figure 9. The Blue Wave heater controller, which was interfaced with a Labview program and laptop computer...... 40 Figure 10. Labjack Microcontroller used to interface between the data acquisition system, heater controller, and laptop computer...... 41 Figure 11. The electronegativity of selected elements when dissolved into a molten LiCl- KCl eutectic salt [10]...... 43 Figure 12. As the source thickness increases, the minimum incident alpha energy decreases and the spectrum broadens...... 45 Figure 13.SiC stopping power for 5485 keV alpha particles. As the energy of the alpha particle decreases, the stopping power rapidly increases...... 47 Figure 14. The 4H-SiC surface is shown in the above figure coated in SiO2. The 1 μm thick SiO2 layer is shown in green and pink from refraction effects...... 60 Figure 15. An overhead view of the guard ring and SiO2 structure of the 4H-SiC detector...... 62 Figure 16. The guard ring structure is separated from the Schottky contact by a 250 μm thick SiO2 ring. The SiC appears in the above figure in off-white, while the SiO2 appears as green and pink...... 63 Figure 17. Initially, the 4H-SiC die is cleaned and stripped of any remaining photoresist or oxides...... 70 Figure 18. A 1 μm thick layer of SiO2 is deposited on the epitaxial surface of the SiC die using PECVD...... 70 Figure 19. A 1 μm layer of photoresist is applied to the surface of the detector, spin coated to ensure uniform thickness, masked, and exposed to a mercury i-beam light to create the appropriate pattern in the photoresist...... 70 xiv

Figure 20. The photoresist is held for 2 minutes in a developer which dissolves the exposed photoresist leaving behind the pattern shown in the above figure...... 71 Figure 21. The die is etched using a plasma etcher and CF4 gas. The etching process removes the photoresist and SiO2 at a nearly 1:1 preference...... 71 Figure 22. After etching, a layer of photoresist is again applied, and the pattern developed as before the etching process. This is to ensure that the metal contacts are only deposited in contact with the 4H-SiC, not on the SiO2...... 72 Figure 23. The metal contacts are deposited on the epitaxial surface of the detector, and the photoresist is removed with a chemical solvent along with the metal deposited on top of it. This leaves behind only the metal deposited in contact with the 4H-SiC...... 72 Figure 24. Metal contacts are deposited on the bulk side of the die, and the detector is annealed at 650oC to form a Schottky contact on both the epitaxial and bulk side of the detector...... 73 Figure 25. The simulated alpha spectrum from a mixed nuclide source bearing U, Pu, and Np in ratios representative of used nuclear fuel. The simulation assumes an idealized 0.01% energy resolution...... 77 Figure 26. The resulting alpha spectrum from a mixed nuclide actinide alpha source. The source contains the expected ratios of U, Pu, and Np from PWR UNF...... 80 Figure 27. The simulated alpha spectra of each of the actinides of interest for safeguards concerns. Each of the actinide spectra have been plotted separately to illustrate the convolution of the spectra as a result of a mixed source with 0.3% detector resolution .. 81 Figure 28. The simulated combined spectra of a U, Pu, and Np bearing alpha source as measured with a 1% FWHM energy resolution detector. Many of the unique alpha peaks are visible but the 239Pu and 240Pu peaks are indistinguishable...... 82 Figure 29. The individual spectra for a detector with 1% FWHM energy resolution. At 1% resolution, several of the distinct alpha peaks are still visible...... 83 Figure 30. The resulting spectrum from a mixed U, Pu, and Np source as measured with a 2.2% energy resolution alpha detector. Little information is readily available from the spectrum concerning specific actinide concentrations...... 84 Figure 31. The stopping power of 103Ru in 4H-SiC plotted versus energy. Until the 103Ru particles reach a low energy (a few MeV), the stopping power varies slowly and linearly...... 87 Figure 32. The stopping power of 134Xe. As the heavier fission fragment, 134Xe receives a smaller fraction of the total kinetic energy from fission. As a result, the stopping power of SiC for the 134Xe and the 103Ru are similar...... 88 Figure 33. The simulated path of 134Xe fission fragments from spontaneous 240Pu fission through a 1-micron thick source layer...... 89 Figure 34. When a negative voltage is applied to the epitaxial Schottky contact to form a charge depleted region, the bulk contact is forward-biased...... 92 Figure 35. The forward-bias IV curve for an epitaxial Schottky contact detector with an ohmic bulk-side contact...... 93 Figure 36. Forward bias semilog plot of a Ni-Pt 4H-SiC alpha detector. The natural logarithm of the current is plotted against the bias voltage...... 93

Figure 37. Schottky barrier height 휙퐵 versus Schottky contact diameter. As the diameter of the contact increases, the apparent Schottky barrier height decreases...... 95 Figure 38. Diode ideality versus Schottky contact diameter. As the diameter increases, the ideality factor of the diode increases. This suggests an increase in available charge trapping sites...... 96 Figure 39. Schottky barrier height 휙퐵 versus diode ideality. As the Schottky barrier height decreases, the ideality factor of the diode decreases. It is useful to note that both parameters decrease with increasing contact diameter...... 97 Figure 40. Leakage current for a single Schottky contact detector at 20oC. As the bias voltage becomes more negative, the leakage current increases exponentially...... 99 Figure 41. Semilog plot of leakage current versus voltage for a double Schottky detector. The measured leakage current is notably lower for the double Schottky contact...... 100 Figure 42. A SiO2 passivated detector. The leakage current is reduced by an order of magnitude with the inclusion of a passivation layer...... 101 Figure 43. Capacitance versus voltage measurement in reverse bias for a Schottky contact detector...... 102 Figure 44. Calculated capacitance for a 1.5 mm diameter detector versus the calculated depleted region depth xd...... 105 Figure 45. 241Am spectra collected with a Ni-Pt Schottky contact 4H-SiC detector at 20oC at increasing bias voltage. As the bias voltage increases, the depletion depth increases resulting in a larger pulse and thus a higher centroid channel number for the spectrum...... 107 Figure 46. As the applied voltage of the detector increases, the energy resolution (FWHM) of the detector decreases linearly to approximately 160V. Beyond this the resolution varies little. Decreasing capacitance improves resolution...... 109 Figure 47. A typical 241Am alpha spectrum measured with a 4H-SiC detector. The energy resolution for this detector was 0.94%, which is not sufficient to resolve the secondary and tertiary alpha energies of the 241Am alpha spectrum...... 110 Figure 48. The characteristic alpha peaks of a 237Np, 241Am, and 244Cm alpha source from Eckert and Ziegler. The spectra were collected using an off-the-shelf Canberra silicon surface barrier detector...... 111 Figure 49. The alpha spectrum collected using a fully depleted 4H-SiC alpha detector operating at -200V bias voltage for 15 minutes...... 112 Figure 50. SEM image of electroplated 232Th sample surface. Thorium, shown in yellow, is abundant on the substrate surface, which is otherwise composed of iron, nickel and chromium – components of the stainless steel. Red dots indicate zirconium deposits from the reference electrode...... 114 Figure 51. SEM image of electroplated depleted uranium (shown in green) on a nickel substrate...... 114 Figure 52. The alpha spectrum from an electrodeposited thorium source using a Canberra silicon surface barrier alpha detector operating at 40V. The daughter isotopes are labeled for the thorium decay chain...... 115 Figure 53. The alpha spectrum from an electrodeposited thorium source measured with a 4H-SiC alpha detector at -200v bias voltage...... 116 xvi

Figure 54. The alpha spectrum from an electrodeposited depleted uranium source collected with a 4H-SiC alpha detector operating at -200V...... 117 Figure 55. The resulting alpha spectrum from an electroplated thorium source using a partially depleted 4H-SiC alpha detector. Low energy noise below channel 800 is excluded. Source was counted for 72 h to achieve a reasonable signal from the low activity natural sample...... 119 Figure 56. Visualization of energy deposition in a partially depleted detector from a relatively thick source...... 121 Figure 57. Stopping power of alpha particles in SiC from SRIM. Higher energy alpha particles have lower stopping power than lower energy particles, meaning that energy deposition is not linear for higher energy particles in a partially depleted detector...... 122 Figure 58. Change in energy during particle tracking in SiC for an alpha particle of energy 4.012 MeV through 11.55 μm of SiC. The step changes in ΔE are due to the discrete stopping power data from SRIM...... 123 Figure 59. Deposited energy in partially biased SiC calculated for incident particle energy on SiC with an 8.86 μm depletion depth...... 123 Figure 60. Expected energy spectral results in a partially biased detector...... 124 Figure 61. A 4H-SiC alpha detector with a 300 nm thick Pt layer wire bonded to a ceramic chip carrier. The connection is made using a 10 nm Au wire...... 128 Figure 62. A 4H-SiC detector with a 10 nm Pt capping layer wire bonded to a ceramic chip carrier with a 10 nm Au wire...... 131 Figure 63. The measurement setup used for elevated temperature alpha spectroscopy measurements in a vacuum chamber at pressures <3mTorr and temperatures above 500oC...... 134 Figure 64. Alpha spectra from a 241Am alpha source measured with a 4H-SiC detector heated from 20oC to 500oC...... 135 Figure 65. A selection of alpha spectra from a 4H-SiC detector heated from 20oC to 500oC. The centroid position of the alpha spectra increases in channel number with increasing temperature...... 136 Figure 66. Previous detector design energy resolution from 20oC to 500oC showing an energy resolution of ~16% at 500oC [23] [20]...... 137 Figure 67. Increase in centroid position from the 20oC centroid position versus temperature of the detector...... 138 Figure 68. The W-value for alpha irradiation of 4H-SiC measured versus temperature. As the temperature of the detector increases, the measured W-value decreases...... 140 Figure 69. Current versus voltage response curves in the forward bias direction for a 4H- SiC detector at various temperatures. As the temperature increases, the turn-on voltage of the detector in forward bias decreases...... 142 Figure 70. The turn-on voltage of the detector versus the detector temperature. As the temperature increases, the turn-on voltage decreases linearly...... 143 Figure 71. The argon atmosphere glovebox...... 145 Figure 72. The Kerr Auto Electro-melt furnace installed in the argon atmosphere glovebox. The PID controller is visible in the bottom left...... 146

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Figure 73. Pre-salt immersion 241Am alpha spectrum measured with a 4H-SiC alpha detector at -200V bias ...... 149 Figure 74. The reverse bias leakage current for a 4H-SiC detector pre-and post- submersion in the molten salt...... 150 Figure 75. 241Am spectrum from a -100V biased 4H-SiC detector after a 24-hour submersion in a 550oC molten LiCl-KCl salt...... 151 Figure 76. RIV curves before and after submersion of a 4H-SiC detector in 550oC molten salt for 24 hours...... 152 Figure 77. The stainless-steel outer casing. Left: an unmodified outer casing. Right: a modified casing with a 3mm aperture for the alpha spectroscopy measurements...... 155 Figure 78. Cotronic Resbond 940HT two-stage ceramic adhesive. The ceramic powder is mixed with the activator to form the adhesive...... 156 Figure 79. Left: A ceramic adhesive disc cured under <3 mTorr pressure at room temperature for 24 hours. Middle: A ceramic adhesive disc cured at room temperature and pressure for 24 hours. Right: A 200oC and atmospheric pressure cured adhesive disc after 4 hours of submersion in a 500oC molten salt...... 158 Figure 80. From the top, the stainless-steel case forms the outer boundary, separating the electrical connections from the molten salt. The ceramic insulation forms a ring which both mounts the detector to the stainless-steel case and seals the aperture from salt ingress...... 163 Figure 81. View of the bottom face of the detector (to be submerged in the molten salt). The detector epitaxial contact is connected to the outer casing by silver solder and grounded...... 164 Figure 82. The side view of the assembled detector. The MI cable inner conductor provides positive bias to the bulk-side contact of the detector. The dashed line represents the joint between the two steel outer casing halves, joined and sealed with silver solder...... 165 Figure 83. Packaged Detector. From Left to Right: The stainless-steel casing and detector, 5mm diameter stainless-steel MI cable, MI to BNC adapter, BNC connector. 166 Figure 84. The leakage current of the packaged 4H-SiC alpha detector...... 167 Figure 85. An 241Am alpha spectrum measured with a packaged 4H-SiC alpha detector. The detector resolution degraded because of the increased leakage current...... 168 Figure 86 Electrode furnace setup at University of Utah...... 169 Figure 87. The packaged detector following return to The Ohio State University after electrodeposition was performed at the University of Utah...... 170 Figure 88. The collected alpha spectrum from the electrodeposited detector following background subtraction...... 171 Figure 89. Alpha particle spectrum from an electroplated 232Th source collected using a 4H-SiC detector with a 21 μm depleted region. Daughter nuclides of the thorium decay series are evident. However, due to the limited depleted region thickness, only those nuclei with initial emission energies at or below approximately 6.4 MeV can be fully attenuated...... 177

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Figure 90. Alpha particle spectrum from an electroplated depleted uranium source collected using a 4H-SiC detector with a 21 μm depleted region over a 72-hour collection period. The off-equilibrium 234U is visible as a distinct peak...... 178 Figure 91. The computed alpha spectrum of a thin depleted uranium source with a 234U enrichment of 0.00286 at.% measured with a 0.94% energy resolution detector ...... 180 Figure 92. Calculated relationship between 234U and 235U enrichment in product stream from gaseous diffusion and gaseous centrifugation, respectively. As 235U enrichment increases, the enrichment of 234U also increases in a linear manner and the enrichments from the two methods diverge...... 181

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Chapter 1. Introduction

Wide band-gap semiconductor radiation detectors possess the vital characteristics required for alpha detection in a molten salt environment. Such operation is desirable as there is an expressed need for on-line monitoring of actinide concentrations in the pyroprocessing environment. Idaho National Laboratory has expressed an interest in monitoring the actinide concentration in their electro-refiner system [1].The motivation for monitoring the concentration of actinides extends beyond process monitoring for optimal electrodeposition. The ability to monitor the concentrations of special nuclear material is of interest [2]. Unlike methods for determining actinide concentrations which rely solely on potentials or current flow, such as potentiometry or chronoamperometry, measurement of the alpha emission spectra of a representative sample of the molten salt provides isotopic concentrations in addition to the elemental concentration in the salt. Additional applications of wide band-gap radiation detectors include nuclear forensic applications where traditional semiconductor radiation detectors are ill-suited, such as in high temperature environments.

The Department of Energy is currently funding research on actinide concentration measurement in molten salt. The Mark VI electrorefiner at INL is representative of the current state of pyroprocessing technology [3]. Used nuclear fuel is dissolved in a basket submerged in a LiCl-KCl eutectic molten salt. Dissolution efficiencies of 98% for uranium

1 and approximately 99% for plutonium have been achieved [3]. Thus, there is considerable special nuclear material (SNM) present in the electrorefiner during operation. As previously stated, multiple methods of determining the actinide concentrations are in development including square wave voltammetry and chronoamperometry [4] [5].

However, these methods have not yet been shown to be sufficiently accurate to be used for safeguarding against diversion of SNM.

Figure 1. The Mark IV Idaho National Laboratory Electrorefiner The method of actinide concentration determination in a molten salt in near-real time depends on acceptable detector resolution capable of discerning between multiple alpha spectra at elevated temperatures. Of interest is the ability to delineate between the uranium species alpha emission and plutonium emissions [6]. By taking advantage of the characteristic electronegativity of each element and its electrochemical potential when

2 dissolved in the salt, actinides can be deposited onto the surface of an electrode [1]. For alpha spectrometry, this electrode would be the protective layer on the surface of a radiation detector. However, there is not currently a simple method to deposit only plutonium using this electrochemical deposition. Currently, deposition of only a mixture of plutonium and uranium is achievable, if plutonium deposition is desired [6]. Therefore, to determine plutonium concentrations, a detector of sufficient resolution to resolve the characteristic alpha emissions of uranium and plutonium is required. To achieve multiple measurement capability and reuse of the detector, the removal of a measured source after counting is desired. After measuring the alpha spectra of the deposited actinide source, the electric current is reversed and the actinides are driven from the surface of the detector so that the detector can be reused. This method allows for repeated near-real time measurements of actinide concentrations in the electrorefiner molten salt.

1.1 The Nuclear Fuel Cycle

At the time of this work there are 99 operating commercial nuclear power plants in the

United States split between roughly 2/3 pressurized water reactors and 1/3 boiling water reactors [7]. These power plants operate light water reactors (LWRs) which make use of low-enriched uranium (LEU) for fuel. A typical operating cycle for a pressurized water reactor is 18 months of operation, followed by a roughly 1 month refueling outage. The

LEU fuel is then removed from the reactor and placed in a spent fuel pool, where the residual decay heat from the fission products is removed for several years. This is known as the once-through or open fuel cycle [3]. Only a small fraction of the energy stored in the 3 used nuclear fuel (UNF) is released in the once-through fuel cycle. However, for many reasons including structural concerns and fission product build-up in the UNF, the fuel is not readily usable after being off-loaded from LWRs. To be usable as fuel in an LWR, the fission products with large neutron absorption cross-sections in the UNF must be removed.

The fissile and fissionable isotopes such as uranium and plutonium in the UNF can then be fabricated into new fuel for LWRs. This process of removing neutron absorbing fission fragments and fabricating new fuel is known as reprocessing [3]. A fuel cycle which uses reprocessed UNF as fuel for continued power generation is known as a closed fuel cycle.

A fuel cycle which makes use of reprocessing of UNF can be either partially or fully closed depending on the total amount of reprocessing which is performed versus the addition of newly fabricated fuel.

A closed fuel cycle has several advantages over a once-through or open fuel cycle. The first is the increased extraction of energy from a unit mass of nuclear fuel. Since a once- through cycle in an LWR releases less than 10% of the total energy stored in the fissionable isotopes of the fuel, reprocessing allows for more energy per unit mass to be released [8].

Additionally, since less fuel is required to generate a given amount of power, the total mass of UNF that is generated per unit power is less for a closed fuel cycle. However, a closed fuel cycle has several drawbacks as well. Energy is required to separate the fission products and fabricate new fuel, thus reducing the total available energy gained from reprocessing.

Also, since UNF is highly radioactive, special handling and processing steps must be taken to safely reprocess the fuel. After fabrication, reprocessed fuel is much more radioactive

4 than fresh once-through nuclear fuel. Fresh once-through fuel is only slightly more radioactive than natural uranium due to the enrichment process and can be handled safely by hand, while reprocessed fuel must be handled remotely to avoid dangerous exposure to elevated radiation levels. Additionally, it is less costly to operate a once-through fuel cycle given the cost of uranium as of the time of this work, thus making reprocessing economically less appealing from a front-end fuel cost analysis.

In addition to the economic aspects of implementing reprocessing, a major obstacle to a closed fuel cycle is the issue of non-proliferation [1]. The technology which can be used to separate fission products from the fissionable isotopes in UNF can potentially be used to fabricate nuclear weapons. Chemical separation such as the Plutonium Uranium Redox

Extraction (PUREX) process are descendants of processes designed specifically to separate uranium and plutonium from irradiated nuclear fuel with the objective of fabricating a nuclear weapon [9]. The fundamental physics behind chemical separation methods results in high purity output streams of plutonium and uranium. While this is useful for fabricating nuclear fuel by separating out fission products, it poses a proliferation concern. High purity plutonium, especially that which is produced in short-duration irradiations where the weapons-usable 239Pu is in highest abundance relative to the less weapons-usable isotopes of plutonium, presents a particularly large proliferation concern. In principle, any entity capable of producing high purity plutonium from UNF has overcome the largest barrier to constructing a nuclear weapon [6].

Though also initially investigated as a potential method to separate plutonium from uranium for nuclear weapon production, electrochemical separation known as pyroprocessing proved ill-suited for proliferation activities and is therefore of greater interest for reprocessing of UNF for closed fuel cycle applications [10].

1.2 Pyroprocessing and Molten Salts

Electrochemical separation of fissionable isotopes from fission products in UNF, specifically at elevated temperatures typically in the range of 400oC to 600oC, is known as pyroprocessing. Pyroprocessing was first developed as a possible method of separating plutonium from uranium in irradiated fuel from the Hanford reactor during the Manhattan

Project in the early 1940’s [1]. The method of separation relies on the solubility of actinides in a molten salt held above its melting point and the unique electrochemical potential of each element when dissolved in the salt [6]. Attempts were made to form pure plutonium deposits on an electrode in the salt. The separation method was found to be ill-suited for making a nuclear weapon since there was always some amount of contaminating uranium in the plutonium deposit. This is due to the less negative electrochemical potential of uranium than plutonium in the salt which results in uranium depositing before plutonium on the working electrode and thus requiring that all the uranium be extracted from the salt before a pure plutonium deposit could be created [10]. This was found to be impractical and the method was abandoned for chemical extraction methods.

The operating principle of electrodeposition, which pyroprocessing makes use of, is the exploitation of the unique electronegativity of each element of the periodic table. When dissolved in an ionic solution such as a molten salt, the electrochemical potential of each dissolved chemical species is unique. At minimum, an anode and cathode are placed in the solution along with a reference electrode. A potential which is referenced to the reference electrode is applied across the anode and cathode. The potential which is applied determines which elements in the solution are deposited onto the surface of the cathode, also known as the working electrode [3].

An example of a pyroprocessing electrorefiner has been demonstrated by the Idaho

National Laboratory (INL). The Mark V Electrorefiner has demonstrated the capability to dissolve both driver and breeder UNF from the Experimental Breeder Reactor II (EBR II)

[3]. The fuel, which consisted of metallic uranium and plutonium, was dissolved from the anode basket of the electrorefiner into the molten salt. The uranium and plutonium were dissolved into the salt while the noble fission products were left in the anode basket. A deposition potential was applied across the anode and cathode and uranium and plutonium co-deposits formed on the working cathodic electrode.

Molten salts, such as the eutectic mixture of LiCl and KCl found in the INL Mark V

Electrorefiner, have been pursued as the electrodeposition cell solution as they provide suitable solubility for actinides while limiting the solubility of fission products [11].

However, to maintain a liquid solution the salt must be held above its melting point of

7 approximately 350oC. With the inclusion of a margin to account for temperature distributions and avoid partial freezing of the salt, the nominal operating temperature of

500oC was chosen [2]. Given the high solubility of metal oxides in molten salts, the electrorefiner environment imposes stringent corrosion resistance requirements on materials to be used in the salt.

1.3 Alpha Spectrometry

Actinides, along with most nuclei beyond the atomic number of lead, emit alpha radiation.

Alpha radiation is the emission of a fully ionized helium nucleus possessing a kinetic energy on the order of a few MeV. These helium nuclei decelerate as they interact with the electron cloud of the material they pass through [8]. The half-life of alpha emission is inversely proportional to the energy of the alpha emission; thus a short lived alpha emitter will emit high energy alpha particles, while long lived alpha emitters will emit lower energy alpha particles [9].

The energy of an alpha particle upon emission from the parent nucleus is unique to the parent nucleus and is monoenergetic. However, multiple discreet energy alpha emissions can occur for a single radionuclide [9]. These discreet alpha emissions follow a measurable branching ratio and give rise to a discreet spectrum of observed energies occurring with a specific frequency based on the branching ratio for the radionuclide. Additionally, since alpha particles are doubly ionized upon emission and relatively massive compared with neutron, beta, and gamma radiation they possess a short range despite their large kinetic energy. Because of their double ionization, alpha particles interact strongly with the 8 electron cloud of materials and possess a large linear energy transfer (LET) coefficient [8].

This large LET results in strong self-shielding effects when an alpha emitter source thickness is on the order of microns. This self-shielding results in a broadening of the observed energy spectrum; thus, for high energy resolution, alpha emitting sources must be kept to a minimal thickness. This can introduce complications as the total alpha emitter source area may grow very large for a desired alpha activity for low specific activity alpha emitters.

1.4 Nuclear Forensics

Nuclear weapon proliferation concerns have given rise to the field of nuclear forensics.

This field of research concerns itself with the identification of useful information about processes related to nuclear material fabrication processes, age, history, origin, trafficking, and other information useful to the monitoring of nuclear materials of interest [12]. Nuclear forensic studies may be performed on nuclear material to determine atomic enrichment, country of origin, age of the sample or time since enrichment, casting or machining techniques used on the material, and material composition analysis for trace elements.

Along with mass spectroscopy and chemical analysis, nuclear forensic assays make use of alpha and gamma spectroscopy techniques. Gamma spectroscopy provides potential identification of isotopes of interest in a nuclear material sample nondestructively because the low LET of gamma rays with material enables gamma spectroscopy to be performed at atmospheric pressure. However, the gamma spectra can be complicated due to multiple interaction modes of gamma rays with materials such as Compton scattering, pair

9 production, and photoelectric effect contributions [8]. This can result in difficulty positively identifying isotopes in a nuclear material source. In contrast, alpha spectroscopy can provide readily identifiable energy peaks which can be quickly correlated back to their parent nuclei [12]. This simplified spectral analysis comes at a heavy cost, however. Alpha spectroscopy is often a destructive analysis technique, as dissolution of nuclear material is often necessary to prepare a suitably thin alpha emitting source to avoid self-shielding.

Additionally, alpha spectroscopy is often performed in a vacuum chamber to avoid energy attenuation, as the alpha particles pass through the air separating the source from the detector.

Chapter 2. Background

To provide understanding of the motivation for this work some background on semiconductor radiation detectors, molten salt chemistry, and electrodeposition will be provided.

2.1 Diode Principles

Semiconductor radiation detectors provide a compact form factor for radiation detection when compared to gas and scintillating detector types. The most common semiconductor based radiation detectors are based on silicon diodes. Diodes fall under two basic categories; those making use of a sharp doping concentration gradient within the detector material such as P-N and P-I-N diodes, and those which make use of a rectifying contact formed at the interface of a metal and semiconductor known as Schottky diodes [13]. In either case, a semiconductor material is used to create non-linear electric current behavior, i.e., current flows through a conductor in one direction through a circuit but encounters a large resistant when flowing the opposite direction through the circuit. This behavior is known as rectifying behavior and is the essential characteristic of a diode.

Semiconductors are typically crystalline solids. The overlapping probability density functions for the electrons in the atoms of the crystal results in the creation of bands of 11 allowable energy levels within the crystal which charge carriers can occupy [13]. As a direct result of the implications of the Schrodinger equation to describe particles bound in a potential well, energy levels exist which correspond to the energies available to electrons in a bound system. Such a system exists in a solid comprised of a collection of neutrally charged atoms. In a solid, electrons can be broadly described as existing below the valence band, in the valence band, or in the conduction band. In the case of conductors, the difference in energy between the conduction and valence bands is negligible or zero because the Fermi level of conductors overlaps the conduction band [14].

Because of the quantum mechanical effect known as the Pauli Exclusion Principle, no two electrons can possess identical quantum numbers. As a result, even at 0 Kelvin, where there is no available thermal energy to promote electrons to higher energy levels, there is a minimum number of occupied energy levels. The highest energy level which is occupied by electrons at 0 Kelvin is known as the Fermi level [13]. In materials at temperatures above 0 Kelvin electrons possess thermal energy and occupy energy levels above the Fermi level. This uppermost populated energy level is known as the valence band. In the case of insulating materials, the energy difference between the valence band and the energy required for an electron to be promoted to an empty energy level and enter the conduction band is too large to be overcome by random thermal excitation. In the case of a pure semiconductor above 0 Kelvin, a small fraction of charge carriers will be thermally excited into the conduction band. As a result, pure or intrinsic semiconductors exhibit non- negligible conductivity.

In perfectly pure semiconductor materials without any impurities a small amount of free charge carriers) due to thermal excitation is present (on the order of 1010/cm3 charge carriers [8]. However, in practice even highly pure semiconductor crystals typically contain impurity levels on the order of 1013 atoms/cm3.

Impurities introduce additional charge carriers in one of two ways. Taking silicon as an illustrative example, consider adding a single phosphorus atom to a silicon crystal lattice.

Phosphorus has five electrons in the valence shell and is in column five of the periodic table. Silicon, meanwhile, has four electrons in the valence shell and is in column four of the periodic table. Thus, the phosphorus creates a covalent bond with each of its neighboring silicon atoms but has a remaining electron with no remaining nearby silicon atoms to bond with. Thus, the phosphorus electron is effectively free in the conduction band, as the valence band for the phosphorus atom is full. The fixed phosphorus atom is then left as a fixed positively charged atom in the crystal lattice [8].

Similarly, for impurities which are deficient in electrons, such as impurities with only three electrons in the valence shell as found in column three of the periodic table, covalent bonds are made between the silicon neighbors. However, the column three element will capture a thermally freed electron from a silicon atom in the lattice, resulting in a net negative charge on the column three atom and a mobile positive charge known as a hole. Movement

13 of electrons to fill a bond vacancy can accurately be modeled as the movement of a positively charged carrier in the opposite direction.

The concentration of negatively charged charge carriers (electrons) in a semiconductor crystal is denoted as n, while the positive charge carrier (hole) density is denoted as p.

2.1.1 P-N and P-I-N Diodes

The resistivity of a semiconductor material is strongly dependent on the dopant or impurity concentration of the material. As stated earlier, resistivity of a pure intrinsic semiconductor material depends on the band-gap of the material and would possess a resistivity dominated by free charge carrier densities on the order of 1010 carriers/cm3 [13]. However, in practice, intrinsic semiconductor materials typically possess impurity concentrations roughly three orders of magnitude larger than the intrinsic carrier density. Thus, in order to achieve an increased resistivity, an intrinsic semiconductor must make use of some other effect. One way to increase the resistivity of a semiconductor crystal is to couple it with some other material which will create a charge depleted region at the junction of the materials. When two semiconductor materials with dissimilar impurity concentrations are brought into close proximity, i.e., physically bonded, an electric field due to the difference in impurity concentrations is produced [13].

Consider the case of a semiconductor crystal unintentionally doped to an n-type semiconductor material. In the n-type material, mobile negatively charged electrons are the 14 primary charge carriers and exist in some density n within the material. Additionally, positively charged impurity atoms within the crystal create fixed positive charges within the crystal lattice. Additionally, consider an otherwise identical semiconductor crystal which has been doped with p-type impurities. In the p-type semiconductor crystal, positively charged holes are the primary charge carriers and exist in some density p in the semiconductor crystal. The impurity atoms in the p-type material act as fixed negative point charges within the p-type crystal lattice. When the two materials, the n-type and p-type crystals, are brought into close contact such that charge carriers can flow, diffusion of the negatively charged electrons from the n-type material into the p-type material and diffusion of the positively charged holes into the n-type material results in recombination of the electrons with holes [14].

An electric field is set up by the fixed point charges which are effectively uncovered by the now recombined electron and hole diffusion behavior. The electric field lines point from the positively charged fixed atoms in the n-type material to the negatively charged atoms in the p-type material. With sufficient time, which is determined by the drift velocity of the charge carriers and the diffusion length of the charge carriers in the interface region, a fully charge carrier depleted region forms [8]. As a result of the electric field established by the uncovered fixed-point charges, negatively charged electrons which are freed from the valence band by thermal excitation or ionizing radiation are swept from the charge depleted region toward the fixed positively charged atoms in the n-type material. Similarly, any freed holes in the charge depleted region are swept out of the charge depleted region toward the fixed negative atoms in the p-type region.

The electric field which is established by the fixed atoms acting as point charges is known as the built-in electric field. The magnitude of the built-in electric field in a pn junction is determined by the band gap of the material, while the depleted region dimensions are determined by the doping concentrations of the p and n-type semiconductors [15].

Because of the charge depleted region within a pn diode, when a positive voltage is applied to the n-type side of the diode and a negative voltage applied across the p-type material, the electric field is reinforced by the external applied voltage. This is known as reverse- biasing the diode [13]. As the external bias increases (the positive applied voltage growing more positive or the negative voltage becoming more negative) the charge depleted region inside the diode increases. This is due to the increased electric field allowing more charge carriers to recombine across the charge depleted region, thus uncovering more fixed charges in the form of the dopant atoms near the junction. Additionally, some amount of charge which is freed due to random thermal excitation in the charge depleted region will be swept out. This charge is steadily created and removed, resulting in a constant observed current under reverse bias known as leakage current or dark current [13].

If an external negative voltage is instead applied to the n-type material and a positive voltage applied at the p-type material, the electric field resulting from the charge recombination in the junction is reduced. If the external bias voltage exceeds the built-in voltage of the junction, the charge depleted region is entirely removed as positive charge carriers are driven into the p-type material and cover the fixed negative charges by the

16 electric field set up by the external bias voltage, while the electrons once more cover the fixed positive charges in the n-type material. Once the charge depleted region is negated, charge carriers are free to flow through the diode. This is known as forward biasing the diode.

For some applications of diodes, such as photovoltaic cells and radiation sensors, it is desirable to have as large a depleted region as possible within the diode [13]. To accomplish this, in certain cases a large external reverse bias voltage may be applied to the diode.

However, this is often not feasible for many reasons. In the case of photovoltaic cells, where the aim is to generate electricity from incident light radiation on the diode, any external bias voltage results in an inefficiency of the cell. For radiation sensors, it may not be possible in an application to have access to the necessary bias voltage, or the required bias voltage to achieve a particular depleted region dimension would exceed the breakdown voltage of the diode. The breakdown voltage being the reverse bias voltage beyond which the diode experiences a sudden and exponential increase in current flow through the device

[13]. This behavior is due to the electric field exceeding the dielectric capability of the semiconductor material. In these situations, a pn diode can be altered by the inclusion of an intrinsic semiconductor region between the p and n type semiconductor crystals. In this arrangement, a nearly intrinsic semiconductor crystal is doped with the opposite impurity

(e.g. p-type dopant for an n-type crystal) such that the effective dopant concentration is equivalent to an intrinsic semiconductor. Since intrinsic semiconductors have free charge carrier concentrations at room temperature of only approximately 1010/cm3, the free charge

17 carriers are rapidly recombined with the charge carriers present in the p or n type material.

This structure is called a P-i-N diode, where i stands for the intrinsic layer, with a charge carrier concentration of ni. The intrinsic region can be grown to greater than 300 microns in semiconductor radiation sensors and require little or no applied bias voltage to form a charge depleted region of the same magnitude [14].

2.1.2 Schottky Diode

An alternative to a pn diode where the doping concentrations of two materials determine the charge depleted region at the junction is a Schottky diode [16]. In a Schottky diode, a metal layer is placed in contact with either a p- or n-type material. Referring to the previous discussion of energy bands, the potential barrier, known as a Schottky barrier, is formed through a mismatch between the electron affinity of the semiconductor and the work function of the metal [17].

The electron affinity of a semiconductor is the energy required to remove a charge carrier from the valence band of the semiconductor to infinitely far away. The work function of a metal is the energy required to remove a charge carrier from the Fermi level of the metal atom to vacuum. Since the Fermi level of a metal and a semiconductor is different- but must be equivalent at the metal-semiconductor junction, charge flows from the higher potential material into the lower potential material. Thus, one of the materials will become slightly positively charged, as some number of electrons were removed from the material

[17].

The charge depleted region in a Schottky diode is formed in a manner similar to that of a pn diode. The metal layer in a Schottky diode functions similarly to the p-type layer in a pn diode when coupled with an n-type semiconductor. Electrons from the n-type region experience a potential difference due to the difference in potential between the metal work function and semiconductor electron affinity and diffuse across the junction into the metal, leaving behind exposed positive charges in the form of the dopant atoms in the n-type material. These exposed charges set up an electric field which points from the n-type material into the metal layer [16]. As a result, a charge depleted region is formed across the junction, extending into the metal layer and semiconductor. The charge depleted region extends primarily into the semiconductor material, as the metal serves as a strong charge sink for electrons. This allows for a charge depleted region close to the surface of the diode, a useful feature for radiation sensors. The magnitude of the Schottky barrier is to first order determined by the mismatch between the Fermi level of the metal and that of the semiconductor, or equivalently, the difference between the metal work function and the semiconductor electron affinity.

To higher order however, the Schottky barrier height undergoes Fermi level “pinning”, where the Schottky barrier height is limited to roughly ½ the bandgap of the semiconductor and is insensitive to the metal used [17]. This phenomenon occurs due to a high density of imaginary k value (thus exponentially decaying wave equation) energy states present at the junction of the metal and semiconductor due to the band overlap, metals having many available energy states in the bandgap region of semiconductors. Fermi pinning occurs at the crossover point between conduction band and valence band behavior of the energy

19 states at the junction. The Schottky barrier height is pinned to the Fermi level due to this splitting of the bandgap by available states in the bandgap, induced by the overlap of conduction band energy states. A Schottky barrier thus forms whenever a metal is deposited onto a semiconductor surface. For applications where a barrier is desired, uniformity of the barrier throughout the interface is desirable [18]. This can be achieved by annealing the metal-semiconductor interface at a temperature which produces minor diffusion of the metal into the semiconductor. In the case that a small Schottky barrier is desired, such as in the case of establishing an ohmic contact for electrical connections to a semiconductor, the metal-semiconductor interface may be annealed at a higher temperature to encourage diffusion of the metal into the semiconductor such that a smooth transition in metal concentration versus depth into the semiconductor is achieved. This has the effect of reducing the Schottky barrier height by decreasing the effective dopant concentration gradient across the interface [18].

2.2 Wide Band-Gap Semiconductors

Semiconductors have varying bandgaps, determined by the chemical composition and the crystal structure of the semiconductor. Semiconductors can be either elemental, such as Si and Ge, or compound semiconductors such as SiC, GaAs, or GaN. Compound semiconductors can be made from columns 3 and 5 of the periodic table, known as III-IV semiconductors, or columns 4 and 5, known as IV-V semiconductors [19]. Both elemental semiconductors are found in column 4. Silicon has a bandgap of 1.14 eV while germanium has a bandgap of 0.67 eV [20]. The relatively narrow bandgap of Ge means that it must be held at low temperatures, often cooled with liquid nitrogen, to minimize leakage current 20 from thermal excitation. Semiconductors with a bandgap larger than that of silicon are referred to as wide bandgap semiconductors. Some examples include 4H-SiC which has a bandgap of 3.23 eV, GaN which has a bandgap of 3.4 eV, Ga2O3 and AlN which has a bandgap of 6.0 eV [19]. As the bandgap increases, the number of thermally generated charge carriers decreases.

2.3 Elevated Temperature Diode Operation

As the temperature of a diode increases, more energy is available for thermal excitation of charge carriers. The number of charge carriers present at a given temperature for a diode with a constant dopant concentration is dependent on the bandgap of the semiconductor material from which the diode is made. As the bandgap increases, more energy is required to thermally excite charge carriers from the valence band into the conduction band of the diode.

When a forward bias voltage is applied across a diode, the charge depleted region at the junction decreases with increasing forward bias voltage [16]. Beyond a forward bias of approximately the built-in voltage potential in magnitude, the charge depleted region is completely removed by charge carriers which are forced across the depleted region by the external electric field. Once the charge depleted region has been removed the diode acts approximately as a resistor, allowing current to flow through the diode [13]. The applied bias voltage at which the current begins to increase exponentially is known as turn-on voltage. Equation (1) shows the ideal diode equation which describes this behavior for an ideal diode [13]. IS is the dark saturation current which describes recombination effects in the diode. The voltage V is the applied external voltage, q is the charge of an electron, kB

21 is the Boltzmann constant, and T is temperature in Kelvin. The turn-on voltage is the voltage at which the dark saturation current is overcome (i.e. the exponential term exceeds

1).

푞푉 푘 푇 퐼 = 퐼푠 (푒 퐵 − 1) (1)

This voltage is temperature dependent, as charge carriers in the diode will increase in energy as the temperature increases, resulting in a smaller turn-on voltage value for increasing temperatures.

When a reverse bias voltage is applied, the charge depleted region increases in size. Charge carriers in a semiconductor not under irradiation exist in thermal equilibrium with the surrounding lattice. As the temperature of the semiconductor increases, the average thermal energy of the bound electrons increases. Due to the nature of the energy distribution associated with thermal excitation, some fraction of the bound electrons will possess enough thermal energy to be promoted spontaneously to the conduction band per unit time.

Thus, as temperature increases, the leakage current through the device increases as more energy is available for charge carriers to be created in the charge depleted region and thus a larger number of charge carriers are produced and removed by the electric field at steady state to contribute to the leakage current.

Another parameter of interest that is temperature dependent is the amount of charge released per unit energy lost by incident radiation in a semiconductor when operating as a radiation sensor. This is often referred to as the W value for the material and is an extension 22 of the same concept in gas radiation detectors [8]. Semiconductors in general have a larger

W-value than their gas detector counterparts, due largely to the increased physical density of the semiconductor material. This behavior manifests itself in the measured radiation spectra by a shift to a higher channel in a multichannel analyzer with increasing detector temperature, given a constant gain.

2.4 4H-Silicon Carbide

In addition to knowledge concerning the physical properties of semiconductor diodes, an understanding of the properties of the substrate on which they are formed is necessary.

In this work, 4H-Silicon Carbide (4H-SiC) is employed due to its large band gap (3.23 eV at room temperature), high melting point, and availability as relatively large, high purity, low doped samples [19]. SiC occurs in a crystalline structure; the exact orientation of the crystal can vary based on the crystal designation, known as a polytype [13]. As the name implies, 4H-SiC is a polytype of the chemical compound formed by the combination of silicon and carbon in a 1:1 ratio. Typical crystals are grown by the Lely method, with doping levels on the magnitude of 1014 cm-3. The “4H” designation indicates that the polytype is a pure crystal with a hexagonal atomic stacking pattern [21]. In the case of 4H-

SiC, the atoms are arranged in an alternating hexagonal and cubic crystal structure as indicated in Figure 2. In the case of 4H-SiC when compared to 6H-SiC, the two polytypes vary in the vertical axis. Where 4H-SiC follows a pattern of “ABCB”, 6H-SiC follows a pattern of “ABCACB”- thus the respective designations.

Figure 2. The Ramsdell classification scheme representation of the stacking order of 4H- SiC [22]. To use a semiconductor for radiation spectroscopy, a sufficient depleted region must be created, thus the desire for a suitably thick crystal. A crystal thickness which is less than the range of the radiation of interest in the crystalline material is not well suited for many radiation detection applications, where knowledge of the total energy of the incident radiation is desired. Additionally, enough free charge must be created through ionization to produce a measurable pulse, which is distinguishable above the background noise of the detector signal. The charge that is created in the radiation sensor volume must be predictably collectable with minimal losses to preserve energy resolution and linearity between charge creation and collection. The most important material characteristic for free charge creation is the energy required to create one electron-hole pair (ehp). This value was reported by Garcia to be 7.82±0.02 eV/ehp [23]. In comparison, the reported alpha particle energy per electron-hole pair in silicon is approximately 3.58 eV/ehp [15]. By this comparison silicon appears to be the better material, as more free charge is liberated by a given energy deposition, allowing for improved counting statistics and a potential finer energy resolution. 24

2.5 Semiconductor Radiation Detection Principles

Semiconductor radiation detectors can be analogously compared to gas chamber radiation detectors [8]. Both operate by creating an electric field between two electrodes and collecting the charge freed by ionizing radiation passing through the intermediary space between electrodes. Semiconductor radiation detection primarily operates on the principle of free charge creation in a charge carrier depleted region of a semiconductor material. The charge is freed by interaction with ionizing radiation, which transfers energy to bound charge carriers in the detector volume that is sufficient to promote the carriers to the conduction band of the detector. After being promoted to the conduction band, the charge is removed from the charge depleted region and collected at electrodes by an electric field which is created by the fixed charge distribution in the material, commonly reinforced with an external electric field from an applied voltage across the electrodes of the detector [8].

Considering the special case of a pn diode operating in reverse bias as a radiation detector is illustrative of the operation of semiconductor radiation detectors.

When ionizing radiation passes through the charge depleted region of the pn diode detector, charge is freed and transported by the present electric field to the n-type region in the case of negative charge carriers, and the p-type region in the case of positive charge carriers.

Since the positive charge carriers tend to move much more slowly than the negative charge carriers, the resulting pulse often consists only of the negative charge carrier contribution.

Equation (2) shows the approximate relationship between the dopant acceptor and donor concentrations within the semiconductor, NA and ND respectively, the potential difference 25 due to the recombination of charge across the interface VBi, the externally applied voltage

(if any) Vapplied, the electrical permittivity of the material 휖, and the resulting thickness of the charge depleted region Xd [8].

1 1 푋푑 = √2휖(푉푎푝푝푙𝑖푒푑 + 푉퐵𝑖) ( + ) (2) 푁퐴 푁퐷

This charge depleted region functions as the sensitive volume of the detector and depends on both the external bias voltage applied and intrinsic properties of the semiconductor material.

Since the potential formed at the diode junction interface due to the dopant concentrations alone is on the order of a few volts or less and results in a small depleted region, which subsequently results in a small amount of charge being freed by incident radiation in the sensitive region of the detector, it is helpful to apply an external voltage to reinforce the built-in potential. By applying an external biasing voltage, the charge depleted region can be increased, allowing for more energy to be deposited by incident ionizing radiation. The amount of charge freed by an ionizing particle of a given energy can be increased up to the point that either the charge depleted region extends throughout the entire semiconductor substrate, or the charge depleted region is large enough that the radiation can deposit the entirety of its energy in the charge depleted region. In the former case, the detector is said to be fully depleted. As can be seen from Equation (2), the charge depleted region behavior will change at the border between a low doped region and a highly doped region; thus for this work the depleted region that is considered to be reasonably possible to form is limited

26 to the region consisting of the low-doped epitaxial region, because a highly doped region results in a small depleted region when compared with a low doped region.

The W-value, defined earlier as the energy required to produce a single free electron-hole pair (ehp) in the semiconductor material, determines the total charge produced from an ionizing particle passing through the detector. This parameter is dependent on the bandgap of the material, given that the energy required to produce an ehp is equivalent to the energy required to break a chemical bond in the crystal lattice of the semiconductor material. This energy is subsequently equivalent to the energy required to promote an electron from the valence band into the conduction band; thus, the W-value depends on the bandgap of the semiconductor material. As a result, wide band-gap materials possess a larger W-value, resulting in fewer ehps per unit energy lost by an ionizing particle, than for narrow bandgap materials such as silicon. As an example, the reported W-value for silicon at 300K is 3.62 eV/ehp. Meanwhile, the reported W-value for 4H-SiC at 300K is approximately 8.4 eV/ehp

[23]. This is a ratio of 0.43 for WSi/W4H-SiC, while the bandgap ratio GapSi/Gap4H-SiC is

0.345; thus there are factors independent of the bandgap alone contributing to the W-value of a semiconductor material. The greater the number of ehps created by an ionizing particle within the semiconductor detector, the greater the statistics for energy resolution. This is due to the statistical contribution to the energy spectrum broadening produced by radiation incident on a semiconductor detector. This statistical broadening is shown in Equation (3).

Equation (3) holds for a large number n of nuclei present during the counting, and a small probability of decay of any one nucleus during the count time. These are necessary requirements for the mathematical simplification of the binomial distribution of a counted

27 source to the Gaussian distribution where Equation (3) holds true and K is a proportionality constant [8]. The full width at half maximum (FWHM) as shown in Equation (4) describes the spread of the distribution and is a useful metric for determining the energy resolution of radiation detectors.

휎푠푡푎푡 = 퐾√푛 (3)

퐹푊퐻푀푠푡푎푡 = 2.355휎푠푡푎푡 (4)

The statistical contribution to the FWHM of the detector is dependent on the standard deviation of the number of charge carriers σstat, which is in turn dependent on the total number of charge carriers produced in the detector.

To detect ionizing radiation, an incident particle must enter the detector sensing volume.

Immediately after emission from the parent nucleus, the particle begins to lose energy through interaction with the surrounding volume. As the particle reaches the detector, it has lost some energy; however, in the case of radiation traveling through vacuum or for radiation with a low linear energy transfer coefficient this may be a negligible loss compared to other effects. Some of these other effects are interaction between the incident radiation and the non-sensitive regions of the detector, such as the electrical contacts or bulk structural material of the detector. For a semiconductor detector then, it is beneficial to limit the amount of material the ionizing particle travels through before interacting with the sensitive volume of the detector. One way to do this is to minimize the thickness of the insensitive region, known as a dead layer, in the detector. Detectors which form a rectifying

28 potential barrier at the surface of the detector are known as surface barrier detectors. These detectors can be made from either pn, pin, or Schottky diodes. In the case of pn and pin diodes, the non-native semiconductor material (the p-type material on an n-type semiconductor for instance) is kept to a minimal thickness. Because of the requirement for charge neutrality in the charge depleted region, the non-native material dopant concentration must be greater than the native dopant concentration. A typical p-type thickness on an n-type substrate is on the order of 0.1 micron and can represent a major contribution to the energy resolution degradation in a pn or pin detector [14]. Schottky diodes also possess a dead layer. In the case of a Schottky diode, since the rectifying barrier is made at the junction between a metal and a semiconductor, the dead layer is primarily in the metal. This is due to the metal acting as a strong sink for charge carriers, like a heavily doped p-type material on an n-type substrate, and thus the depleted region extends primarily into the semiconductor [17]. In the case of Schottky diode detectors, the thickness of the metal is the primary contributor to the dead layer thickness but can often be minimized to only a few nanometers in thickness.

Since semiconductor radiation detectors are operated in reverse bias to form a charge depleted region, ideally at zero temperature no current would flow through the detector beyond what is excited by incident ionizing radiation. However, since detectors must operate at some nonzero temperature, free charge carriers will exist in the charge depleted region due to thermal excitations. These carriers contribute to the observed leakage current in the detector. This leakage current is minimized in diode-based semiconductor detectors by the potential barrier formed by a blocking contact across the detector. A second form of

29 leakage current, which is dependent on the voltage gradient across the detector contacts and the surface area of the detector exists [13]. This leakage current can be mitigated by inserting a sacrificial guard ring which can be held at a potential equal to that of a contact

[24]. This results in, ideally, no voltage gradient between the guard ring and the signal electrode it encircles. With no voltage gradient, there is no electrostatic force to drive charge from the signal electrode to the guard ring. Any charge which flows to or from the guard ring to the non-signal electrode can be compensated for by the power supply used to establish the guard ring potential. Another method of reducing surface leakage current is the introduction of a passivation layer to the surface of the detector [24]. A passivation layer usually consists of a material with insulating properties which is added to the detector surface. Commonly for silicon detectors, this passivation layer consists of oxidizing the silicon surface to form SiO2. Both bulk and surface leakage current must be held to a minimum to provide a reduced noise floor. The leakage current for a typical semiconductor radiation detector in reverse bias is on the order of pico or nanoamperes, while a signal generated by an incident charge might be on the order of a microampere [24].

The fixed charges in a diode are well modeled as a parallel plate capacitor and semiconductor detectors demonstrate similar behavior [8] [25]. When ionizing radiation is incident on a detector and generates free charge carriers some amount of charge, based on the capacitance of the detector, is briefly stored in the detector volume and discharged similarly to a discharging capacitor. For an RC circuit, the discharge time constant τ is dependent on the capacitance of the capacitor and the resistance of the resistor. For a discharging detector, τ is representative of the time required for the detector to generate a

30 current pulse for an incident particle. The response time of a detector must be small in relation to the average time interval between events measured by the detector to avoid overlap of signals. To achieve high count rates and good energy resolution, it is advantageous to minimize detector capacitance. Since the detector capacitance follows

Equation (5), the surface area, distance of separation of the charges, and dielectric permittivity must be considered.

휖퐴 퐶 = (5) 퐷

Since the dielectric permittivity is constant for a given semiconductor detector material, it can be considered a fixed parameter in detector design restrained to a single material. From

Equation (5), it is evident that minimizing the area A will reduce the capacitance of the detector as will increasing the separation distance D. In a diode detector, D is identically the depleted region depth of the detector volume, which increases with increasing bias voltage as calculated in Equation (2) [25].

Thus, it is evident that the capacitance of the detector can be reduced by increasing the applied bias voltage, up to the point that the detector is fully depleted or experiences breakdown. As a result, detector resolution is expected to improve with increasing bias voltage. Thus, the ability to fully deplete a detector is desirable.

2.6 Detector Characterization and Testing Equipment

To characterize the diode and radiation detection capabilities of a semiconductor radiation sensor, both electrical and spectroscopic measurements are informative.

2.6.2 Electrical Characterization Equipment

To measure the leakage current of the fabricated detectors at varying biasing voltages, a

Keithley 2410 Sourcemeter was used as shown in Figure 3

Figure 3. Keithley 2410 Sourcemeter used for electrical characterization. The Sourcemeter has both 2 and 4-wire measurement capability. These measurements were performed by applying an external voltage and measuring the electrical current which flows through the device and subsequently through the measuring circuit in the absence of ionizing radiation. The Sourcemeter has the capability to supply either a sweeping current or sweeping voltage. The Keithley also contains an internal current limiter which prevents greater than 10-6 amps from flowing through the detector

[26]. The Keithley 2410 communicated with a Labview program via an SCPI GPIB communication port at a rate of 1700 readings per second. The Keithley Sourcemeter has

2, 4, and 6 wire operation options, of which the two-wire system was implemented.

2.6.1 Electrical Characterization Methods

To determine the diode related performance of a detector, multiple techniques may be employed. These measurement techniques include forward current versus voltage (FIV) measurements, reverse current versus voltage (RIV), and capacitance versus voltage (CV) measurements.

To characterize the detectors’ diode properties, FIV and RIV measurements were taken using a Keithley 2400 Sourcemeter. A varying electric voltage was applied in reverse bias across the Schottky diode. The resulting current was measured by the Keithley Sourcemeter and recorded. The FIV relationship can be described by Equation (6) for voltages near forward bias [13].

푞푒푉 퐼 = 퐼푠(exp (− − 1)) (6) 푛푘퐵푇

As before, I is the observed current in amperes, IS is the saturation current in amperes, qe is the magnitude of the fundamental unit of charge in units of e, n is the semiconductor ideality factor, kB is the Boltzmann constant in eV/K, V is the applied voltage in units of volts, and T is the temperature in Kelvin. The ideality factor n of a diode is a useful

33 parameter for determining the goodness of a diode. An ideal diode would have an n of 1, while a value of less than 1 would be a non-physical result. The ideality of a detector indicates how closely the diode follows the ideal diode equation shown in Equation (6).

For voltages much more negative than considered above (e.g. a fully biased detector voltage of -200 volts) the saturation current-voltage relationship proves more useful, and is listed in Equation (7). A voltage consideration known as Schottky barrier lowering is given in Equation (8) [13].

2 푞푒휙퐵 퐼 = −퐴퐴푅푇 exp (− ) (7) 푘퐵푇

푞 퐹 Δ휙 = √ 푒 푀푎푥 (8) 퐵 4휋휖

In Equation (7), I is again the observed current at an applied voltage, A is the cross sectional area of the Schottky contact, AR is a constant known as the Richardson constant, qe is again the magnitude of the fundamental charge in units of e, 휙퐵 is the

Schottky barrier height, kB is the Boltzmann constant as previously defined, and T is the temperature in Kelvin [21]. The Richardson constant is a parameter depending on the interfacial properties between the semiconductor and metal of the Schottky contact.

Equation (8) shows the dependence of Schottky barrier height on the maximum electric field, and thus applied voltage. As applied voltage increases, the Schottky barrier height is decreased due to the presence of image charges in the metal contact. These image charges build up as the voltage is increased, caused by the approach of charge carriers toward the metal contact. Accounting for potential Schottky barrier height lowering because of the 34 maximum electric field surrounding the Schottky barrier interface (Fmax) yields a more accurate voltage dependent equation, shown in Equation (9) [21].

푞푒퐵푉 휙퐵 − √ 2 4휋휖 퐼 = −퐴퐴푅푇 exp⁡(−푞푒 (9) 푘퐵푇 ( )

In Equation (9), B is a proportionality constant, and the remaining terms are as defined above. In this work, the Richardson constant is assumed to be the value found in literature, approximately 146. For measurements near zero voltage, the Schottky barrier height lowering is negligible. For this work, the detector RIV behavior was evaluated from 0 to greater than -200V. FIV behavior was evaluated from 0 to 2V to determine parameters including 휙퐵, the ideality factor n, and the turn-on voltage.

In addition to FIV and RIV measurements, CV measurements are also used to characterize diodes and semiconductor radiation detectors. To perform the capacitance versus voltage measurements, a Boonton Model 7200 Capacitance Meter was used at room temperature

(300 Kelvin). An internally supplied voltage varying from -60 to 0 volts was applied and the capacitance measured and recorded. Additionally, a Keithley 4200A-SCS measurement system was used for both CV, FIV, and RIV measurements. By plotting the behavior of the capacitance of the detectors against voltage, the quality (in terms of qualitative defect presence) of the internal structure of the SiC substrate could be confirmed [25].

Additionally, once the range of the incident alpha particles used for spectroscopic characterization is known, the required depleted region thickness is established. Since the 35 depleted region thickness depends on applied voltage, CV measurements are useful for determining if the depletion region is increasing with increasing applied voltage following

Equation (2).

2.6.2 Alpha Spectroscopy Equipment

Alpha spectroscopy was performed in a glass bell jar held below 3 mTorr. The bell jar was placed on a stainless-steel platform and sealed with a rubber gasket. The vacuum was drawn using a Pfeiffer HiCube. The HiCube consists of a turbopump backed by a mechanical roughing pump with an integrated pressure gauge.

Figure 4. The vacuum was established using a Pfeiffer HiCube turbopump and mechanical pump unit. 36

The test stand consists of a glass bell jar mounted on a stainless-steel stand with BNC vacuum feedthroughs as shown in Figure 5.

Figure 5. Vacuum bell jar on top of a stainless-steel stand. A metal platform inside houses the heater and serves as the measurement stand. To obtain alpha spectroscopy data, a preamplifier was implemented in conjunction with a

DSA-2000 Multichannel Analyzer and the Genie 2000 Spectroscopy software package.

The DSA-2000 is shown in Figure 6. The DSA-2000 serves as the analog to digital converter, amplifier, and high voltage power supply for the spectroscopy system.

Figure 6. Canberra DSA-2000 Multichannel Analyzer. An Ortec 142B preamplifier, as shown in Figure 7, was used to transduce the charge pulse generated and collected within the detector by an incident alpha particle to a voltage pulse. The primary resistor of the preamplifier has a charge sensitivity of

20mV/MeV in silicon and a rise time of less than 25 nanoseconds [27]. This is well below the reciprocal of the source rate of the 1.88 μCi 241Am alpha source used for irradiation.

Figure 7. Charge sensitive Ortec 142B preamplifier

To perform heated alpha spectroscopy measurements, a Bluewave substrate heater was installed in the bell jar on a metal stand and is shown in Figure 8.

Figure 8. Blue Wave substrate heater with a detector mounted on the surface. To control the temperature of the Blue Wave substrate heater, a Blue Wave heater controller was employed. Figure 9 shows the heater controller and pressure gauge which was used with a mechanical roughing pump. The heater controller has a dead-band which allows for measurements to be taken while the heater is not energized, reducing noise.

Figure 9. The Blue Wave heater controller, which was interfaced with a Labview program and laptop computer. To provide an interface between the Blue Wave heater controller and the laptop computer, a Labjack microcontroller was used. The Labjack was connected by a USB to serial connection to the laptop computer and to the Blue Wave heater controller by a power relay.

To prevent the DSA-2000 from collecting data while the heater is energized, the Labjack generates an inhibit signal which is sent to the DSA-2000 inhibit input while the heater is energized. The Labjack microcontroller is shown in Figure 10.

Figure 10. Labjack Microcontroller used to interface between the data acquisition system, heater controller, and laptop computer.

2.7 The Molten Salt Environment

For this work, a LiCl-KCl eutectic salt was chosen to study the behavior of 4H-SiC detectors in molten salt. LiCl-KCl salt is being studied as a solvent for the INL electrorefiner. In the electrorefiner, the salt is heated to approximately 500oC in an inert argon atmosphere [3]. Used nuclear fuel is inserted into the salt and dissolved in an anode basket. The used fuel is either metallic breeder or driver fuel from the Experimental

Breeder Reactor II (EBR II), or oxide fuel which has been converted into metallic fuel. The metallic fuel is dissolved at the anodic basket and a potential difference is established across the anodic basket and a working cathode submerged in the salt. The potential difference, established in relation to a reference electrode in the salt, sets up an electric field which drives the electrodeposition of elements onto the cathode. 41

LiCl-KCl salt is a mixture comprising 55.7 mol% LiCl and 44.3 mol% KCl. The salt is being explored as a solvent for nuclear fuel reprocessing. The salt is a hard, white solid at room temperature with a melting point of approximately 350oC. The salt is highly hygroscopic, thus handling of the salt is commonly performed in an inert atmosphere, such as the argon atmosphere established in the INL electrorefiner. The salt is highly corrosive to some materials at the elevated temperatures required to maintain liquidity of the salt. As a result, materials chosen for structural components of the electrorefiner must be resistant to chemical attack by the salt. Nickel and nickel-bearing steel alloys have demonstrated good corrosion resistance to 500oC molten LiCl-KCl [3]. Additionally, noble metals such as gold and platinum have demonstrated good chemical resistance.

Highly radioactive used nuclear fuel containing alpha, beta, gamma, and neutron emitting isotopes is dissolved into the salt. As a result, the environment in the salt contains a mixed radiation field in addition to the corrosive environment established by the molten salt’s reactivity with many materials. Because of the harsh characteristics of this environment, a radiation sensor intended to operate in or near the molten salt must be insensitive to many types of radiation or have some way of discriminating between them, be chemically resistant to the salt, and be thermally stable at 500oC. Such a sensor would also require sufficient energy resolution to distinguish between various isotopic emissions if spectroscopic measurements were desired. The INL electrorefiner operates in a hot cell due to the large amount of radioactivity present from the dissolved used nuclear fuel. Because

42 of the practical aspects of hot cell operation and the strong radiation field in and around the electrorefiner, a detector which is intended to operate in or near the salt must be radiation hard to ensure long operational life.

2.8 Electrodeposition and Electronegativity

Each element possesses a unique electronegativity when dissolved into a solvent. These electronegativities have been explored for many elements dissolved in molten LiCl-KCl salts. The unique electronegativity of several elements dissolved into a molten LiCl-KCl solvent are shown in Figure 11.

Figure 11. The electronegativity of selected elements when dissolved into a molten LiCl- KCl eutectic salt [10]. 43

Electrodeposition makes use of the valence of a dissolved species in solution by either reducing or oxidizing the species onto an electrode. For metals, which form cations in solution, a potential is applied from an anode to a cathode. As result of this potential, the metals are reduced onto the surface of the cathode. For cations in solution and a negative deposition voltage, less negative species will reduce first, depositing on the cathode.

Due to the near continuum of electronegativities of dissolved elements in a solution and the behavior of less negative cations to deposit first, it can prove difficult to deposit a single element onto a cathode from a mixed species solution [10]. In the INL electrorefiner, the primary goal is to extract actinides from the used fuel, separating out the fission products which are left behind in the molten salt for further processing into a suitable waste form.

2.9 Thick Film Source Geometry

The energy of alpha particles is readily attenuated in solid matter. As a result of this, alpha particle source thickness must be minimized, if a monoenergetic energy is desired from a particular alpha emitter. The energy attenuation of alpha particle energy in a non-zero thickness source due to alpha particle interaction with the source material results in spectral broadening as shown in Figure 12 and is known as self-shielding [28].

Figure 12. As the source thickness increases, the minimum incident alpha energy decreases and the spectrum broadens. Spectral broadening refers to the increased standard deviation of the distribution of observed energies from the alpha source. The spectral broadening increases with increasing source thickness to a maximum value. The maximum spectral broadening for a source composed of a single isotope which emits a monoenergetic alpha particle can be found through an analysis of extreme cases, where measurements are performed in vacuum for simplification. In the case where the source is infinitely thin, there is no energy spectrum broadening due to self-shielding as alpha particles emitted from the source do not interact with the source material, interacting only with the detector material. The other extreme is when the source thickness exceeds the range of the alpha particle emitted by the source

45 nuclei in the source material. Alpha particles emitted from the “back” of the source but toward the detector will deposit the entirety of their energy in the source material and result in no energy deposited in the detector. In addition to this effect, alpha particles emitted from the surface of the detector will possess the full alpha emission energy, when they are incident on the detector. The alpha particles emitted from depths between these two extremes will create a continuous energy distribution from 0 to the full alpha emission energy. The shape of the spectrum resulting from this continuous distribution of energies is determined by the stopping power of the source material and the initial energy of the alpha emission [9].

Additionally, for sources with multiple alpha emission energies the energy spectrum becomes more convoluted. The interaction behavior of alpha particles complicates the prediction of the spectrum distribution and it becomes necessary to have some understanding of alpha particle energy loss behavior as a function of energy. Initially, at near full emission energy the energy loss is due to interaction with the electron cloud of material through which the alpha particle passes. These interactions result in small energy losses for the alpha particle. This can be explained through the conservation of momentum considerations that limit the total energy transfer from the alpha particle to an electron, the alpha particle being substantially more massive than the electron [9]. Because of this mass difference, only a small amount of energy can be given to an electron by a collision with an alpha particle. As the alpha particle slows down, however, the probability of interaction with a nucleus increases. Because of the closer ratio of masses between the target nucleus and the alpha particle compared to the ratio of an alpha particle with an electron, the

46 maximum amount of energy which can be transferred in an interaction increases. The ratio of masses of an alpha particle to even a heavy nucleus such as 238U is roughly 4/238 or

1.68%, while the ratio of alpha particle mass to electron mass is 0.0137%. [9] For the case of alpha particles interacting with 238U, this represents a 43 times greater disparity in mass between the nucleus and an electron. However, it should be noted that this analysis is classical in nature and does not take quantum mechanical interactions into account, such as the interaction of the alpha particle with multiple electrons in the electron cloud simultaneously. This classical analysis does however serve as an instructive model to explain the alpha particle linear energy transfer behavior. Figure 13 shows this behavior for a 241Am particle interacting with 4H-SiC.

Figure 13. SiC stopping power for 5485 keV alpha particles. As the energy of the alpha particle decreases, the stopping power rapidly increases.

To reduce complicated spectral analysis of sources containing multiple alpha emitting nuclides it is therefore desirable to minimize source thickness and thus self-shielding effects.

2.10 Nuclear Forensic Applications of Alpha Spectrometry

Nuclear forensics involves the identification of source isotopes and related information from samples taken from various environments. These environments range from the benign environmental survey to identify man-made radionuclides to the potential post-blast analysis of a detonated nuclear weapon. The goal of nuclear forensic studies is the identification of parameters including the source of the material and the history of fabrication or manipulation of the material including enrichment. Common sources for nuclear forensic material include environmental samples near nuclear reactors and enrichment facilities. Possible sources for nuclear forensic material, which is commonly studied using surrogates, is the debris from nuclear weapons explosions.

Isotopes which may be of interest for nuclear forensic study include the fissile isotopes

235U, 233U, 239Pu, 241Pu and fertile materials such as 232Th and 238U [29]. More exotic isotopes such as 239Np or 233Pa may also be of interest.

Analysis of nuclear material for nuclear forensic study makes use of many techniques.

These techniques include mass spectrometry, which is employed to determine the relative atomic abundance of various isotopes in a forensic sample. This information can provide evidence of the sample enrichment in the case of 235U and 238U disparities from natural uranium concentrations. This technique can also provide information about non-fissionable

48 material of use in nuclear proliferation such as the 6Li to 7Li enrichment, 6Li being a useful material for nuclear fusion weapon development.

For isotopes with a decay chain leading to radioactive daughter products, the equilibrium of the parent nuclide to daughter nuclides in the nuclear material can be used to determine the age of the nuclear material [29]. As an example, a newly enriched uranium fuel rod will possess a low concentration of uranium decay chain daughters, the concentration of these daughters providing the age at which the uranium for the fuel rod was purified of the decay chain daughters. This is deduced from the fact that the decay chain daughters will have been separated out during the purification process required for fuel rod fabrication.

Chapter 3. Detector Fabrication and Simulation

3.1 Initial Detector Design

Early work with 4H-SiC wafers for radiation detection applications made use of a low doped epitaxial layer of 4H-SiC grown on the surface of a bulk 4H-SiC crystal. The epitaxial wafer had a Si terminated epitaxial face. The 4H-SiC wafer used in this work was grown at Cree Labs with a 300 μm thick bulk layer doped with 1x1018 atoms/cm3 of nitrogen. Grown on top of the bulk is a 0.5 μm thick epitaxial layer, with a doping of 1x1018 atoms/cm3 of nitrogen. Finally, a second epitaxial layer is grown on top with a thickness of 21 microns and doped with 5x1014 atoms/cm3 of nitrogen [30]. These detectors made use of a 100-nm nickel layer deposited on the bulk face of the 4H-SiC crystal and annealed to 900oC to form an ohmic contact. After creation of the ohmic contact a 100 nm nickel layer was also deposited on the epitaxial face, followed by a 10 nm titanium layer and 10 nm gold layer. The detector was then annealed to 650oC to form a Schottky barrier at the junction between the nickel and epitaxial SiC surface. The titanium layer was intended as a diffusion barrier to prevent diffusion of gold atoms from the topmost layer into contact with the 4H-SiC epitaxial surface. The gold layer was intended as a corrosion barrier and electrical contact, as it does not oxidize in air and is otherwise chemically inert in most environments [30].

The detectors were characterized using FIV, RIV, and CV measurements. The detectors were also used to perform alpha spectrometric measurements of a 241Am alpha source in vacuum. The alpha spectroscopy characterization was performed from room temperature

(20oC) up to 500oC. The energy resolution of the detectors versus temperature was measured by collecting an alpha spectrum over 15-minute counting intervals at increasing temperatures. Additionally, the long-term temperature behavior of the detectors was observed. The detectors were heated to 500oC in the evacuated bell jar. After the detectors had reached an equilibrium temperature with the substrate heater, spectra from an 241Am source were collected in 15-minute counting intervals. The detectors were held at 500oC for increasing intervals of time, initially for 24 hours. The detectors were held at 500oC for increasing time intervals while collecting alpha spectra from the 241Am source, until the

FWHM increased beyond 5% for the 5.48 MeV alpha particles [31]. The longest time under temperature before detector failure was found to be 168 hours. After failure, the detectors were removed from the bell jar. The surface composition of the detectors was measured using energy dispersive x-ray spectroscopy (EDS). The EDS measurements showed an increased presence of both titanium and nickel on the surface of the detectors. The titanium and nickel had formed oxides when exposed to air. The presence of the titanium and nickel at the surface of the detector indicated that diffusion of the metal layers had been driven by the elevated temperature operation and had contributed to the eventual detector failure over time [31]. The diffusion of titanium and nickel to the surface which then oxidized resulted in degraded electrical connections, as neither titanium or nickel oxides are as conductive as a pure metal contact. Additionally, the diffusion of metals to the surface indicates that the gold and titanium layers could diffuse into the surface boundary layer between the SiC and the nickel. Titanium and gold both form Schottky barriers with SiC, but neither is predicted to form a barrier height which is as large as the barrier formed at a nickel-SiC

51 interface. Thus, the observed decrease in Schottky barrier height, evidenced by the increased leakage current and degraded energy resolution, could be explained by diffusion of titanium or gold into contact with SiC [31].

To reduce the effects of metal diffusion in the thin metal layers of the detector contact, the titanium and gold were removed from the detector design. By removing the titanium and gold, only nickel could form a Schottky barrier with SiC, ensuring physical stability at elevated temperatures of the metal-semiconductor junction. However, since the intended application of the detector involves submersion into a molten LiCl-KCl salt, which can oxidize nickel, a protective metal layer which provides good electrical contact is required.

A metal which is chemically inert in the molten salt and is highly electrically conductive would offer protection for the nickel layer from oxidation. Similarly, in the case of diffusion of the protective metal layer, it should form a Schottky barrier height as close to that formed by nickel as possible. Platinum is inert in molten LiCl-KCl, is a good electrical conductor, and is predicted to form a Schottky barrier with SiC like that of nickel. An additional gain in energy resolution is made by reducing the metal thickness, which contributes to the dead layer of the detector. The platinum layer was chosen to be 10 nm in thickness to provide physical robustness of the metal layer and to provide some resistance to diffusion of ions in the salt into the nickel layer, while minimizing thickness to prevent energy loss in the dead layer of the detector.

3.2 Silicon Carbide Wafer Preparation

To fabricate the detectors a 4H-SiC wafer was diced into 5 mm x 5 mm dies. These pieces were then taken to Nanotech West Lab to undergo clean room preparations. The method of cleaning and preparing the samples consists of removing organic contaminants, removal of oxide layers, and the removal of ionic compounds and contamination. To accomplish this, an organic solvent was used to strip the photoresist. Then a standard RCA cleaning process was used [32].

In more detail, as the first step, the diced dies were placed in a 50 mL N-Methyl-2- pyrrolidone (NMP) solution. NMP is an organic solvent which dissolves photoresist. The photoresist present on the dies was added by Cree to protect the surface of the dies during storage. The NMP was heated to 80oC on an electric hot plate before the dies were submerged. After initial submersion for 5 minutes, the detector dies were removed and placed in a clean, room temperature 50 mL NMP solution for 5 minutes. The detector dies were then removed from the NMP solution and washed for 2 minutes with deionized water

(DI) and dried with an N2 gun. After this organic cleaning, the detector dies were subjected to an organic and oxide contamination cleaning step; i.e. the RCA cleaning process [32].

The RCA cleaning process begins with a process to remove the organic contamination.

This was accomplished by immersing the SiC samples in a hydrogen peroxide (H2O2), ammonium hydroxide (NH4OH), and distilled DI water solution in ratio of 1:1:5. The solution was heated to 80oC on an electric hot plate and allowed to soak for 15 minutes in a 150-mL beaker. After the 15-minute soak in solution, the samples were removed to a DI water bath and rinsed for 2 minutes. Following the organic solvent removal, the samples 53 were placed in a hydrofluoric acid (HF) and DI water bath in a ratio of 1:50 [32]. The samples were washed in the solution for 15-20 seconds. The purpose of this step is to remove oxides which may have formed during previous cleaning steps and to remove any residual photoresist which had been deposited by Cree to protect the epitaxial surface of the SiC wafer.

After the oxide removal, the samples were rinsed with DI water and placed in a hydrochloric acid (HCl), H2O2, and DI water solution in a 1:1:5 ratio. The purpose of this step is to remove the ionic contaminants on the surface of the samples. The samples were held in this solution at 80oC for 15 minutes, followed by a 2-minute rinse in DI water. As the final steps in the RCA cleaning process, the samples were washed once again. In the final washing, prior to evaporation of the metals to the SiC surface, the samples were placed in a HF and DI water bath in a ratio of 1:50 for 15-20 seconds to remove any oxides from the previous ionic cleaning step. Following this, the detectors were rinsed in DI water and dried with an N2 gun.

After cleaning, the samples were placed in a CHA Solutions Electron Beam Evaporator.

The evaporator was evacuated to less than 2E-6 Torr. A 99.99% pure nickel source was heated by the electron gun and a nickel ohmic contact of 100 nm was applied to the bulk side of the dies. The deposition rate was 3 angstroms per second. Once the nickel ohmic contact was applied, the metal source was switched to a platinum source which was heated by the electron beam. A 10-nm thick platinum layer was deposited on the back of the detector. The samples were then moved to a rapid thermal annealer (RTA). The temperature inside the annealer was ramped to 700oC, followed by a 900oC and 950oC

54 stage. The samples were held at 950oC for 30 seconds. The samples were cooled for 900 seconds before being removed from the RTA. After removal from the RTA, the detectors were cleaned in an isopropyl alcohol wash and dried with an N2 gun [30]. After drying, the detectors were transferred back to the electron beam evaporator. Finally, the Schottky contact was applied by evaporating 100 nm of nickel, followed by 10 nm of platinum to the surface of the samples. The detectors were then annealed at a temperature of 650 degrees Celsius for 30 seconds and allowed to cool for approximately 600 seconds, or until the temperature dropped to approximately 150oC. The higher annealing temperature allows the formation of the nickel ohmic contact, while the lower annealing temperature allows for the formation of the Schottky contact [30].

3.3 Shadow masking metal deposition

Circular contacts are desirable for detector applications. Electrical contacts with straight edges result in electric field crowding at corners [8]. To ensure a more uniform electric field with a weaker gradient across the detector contact geometry circular Schottky contacts were deposited on the epitaxial surface. These circular contacts were deposited in the electron beam evaporator using shadow masking, also known as stencil lithography.

Shadow masking to form circular contacts is performed by placing a cover with a circular hole over the detector surface. This cover prevents the deposition of metal onto much of the detector, allowing only the exposed detector surface to be metallized. Shadow masking is limited in its applications to features which can be represented using contiguous segments. Patterns such as rings cannot therefore be produced using a single shadow mask 55 deposition and single mask layer. Shadow masking has several advantages over photolithography techniques, as it requires no additional chemical additions such as photoresist, does not require specialty light exposure equipment, or additional chemical development or heating processes.

3.4 Annealing for Schottky Contact

When a metal layer is deposited onto the surface of a semiconductor material a Schottky barrier is formed. The barrier height is predicted to first order by the Schottky-Mott rule which relates the barrier height to the metal work function and the semiconductor electron affinity [17]. However, the Schottky barrier height is more accurately described through the metal-semiconductor surface features. The concentration gradient of metal atoms versus depth into the semiconductor material partially determines the Schottky barrier height, with a weak gradient resulting in a small Schottky barrier height. By annealing a metal-semiconductor junction, the gradient can be reduced as metal atoms diffuse into the semiconductor, resulting in ohmic contact behavior. In contrast, to achieve a uniform

Schottky contact barrier height, some annealing is desirable. An unannealed Schottky contact may have a non-uniform barrier height, due to surface topology of the semiconductor before deposition or metal concentration differences across the surface. By briefly annealing the metal-semiconductor junction, the Schottky barrier can be made more uniform across the surface [18].

3.5 Design Revisions

After fabricating multiple batches of detectors in the manner described above, certain detector qualities were focused upon for improvement. First among these was the desire to reduce leakage current when the detectors were reverse-biased. This was motivated by the requirement that the detectors possess less than 2% energy resolution for 5.48 MeV alpha particles at 500oC. To achieve this reduction in leakage current, the ohmic contact was changed to a Schottky contact in the device fabrication process, a passivation layer was added to insulate the electrical contacts from surface leakage current effects, and the addition of a guard ring was explored.

3.5.1 Double Schottky Contacts

A Schottky contact acts as a blocking contact for charge carriers by establishing a potential barrier across the metal-semiconductor junction. The metal region of a Schottky contact acts analogously to a heavily doped n or p-type semiconductor in a pn diode. The low concentration of minority charge carriers, holes in the case of an n-type semiconductor, causes the Schottky contact to act as a blocking contact. A blocking contact, also known as a noninjecting contact, is employed to reduce the magnitude of current flowing through the bulk of the semiconductor detector. A blocking contact reduces the leakage current flowing through the bulk by preventing the replacement of charge carriers initially removed by an externally applied electric field through the opposite contact. As an example, holes removed from the charge depleted region of the detector cannot easily be reinjected through the n-type ohmic contact. This is due to electrons acting as the majority carrier in the n-

57 type material and the low concentration of holes resulting in recombination of holes at the n-type contact. Similarly, injecting free electrons through the Schottky barrier is difficult due to the large charge sink for electrons presented by the metal conduction band. When freed, electrons are collected at the ohmic contact and holes are collected at the Schottky contact because of the electric field in the detector. When the metal contact deposited on the bulk side of the detector is annealed at a lower temperature, a Schottky barrier contact is formed between the metal and semiconductor rather than an ohmic contact. By annealing the metal to form a Schottky barrier, a second charge depleted region forms between the bulk material and the metal. This charge depleted region is several orders of magnitude smaller in size than the charge depleted region formed with the epitaxial region. This is due to the higher doping concentration of the bulk crystal compared to the epitaxial crystal.

Since the doping in the bulk is higher, and the charge depleted region thickness Xd depends on the doping concentration Nd, the resulting charge depleted region formed at the bulk side junction is smaller. However, with the addition of a second charge depleted region which forms at the bulk n-type semiconductor interface with a metal contact annealed to form a Schottky contact, the leakage current which can flow through the bulk material is further reduced. Because of the combination of the noninjecting behavior and the charge depleted region formation at the metal-semiconductor junction on the bulk side of the crystal the total leakage current through the bulk of the detector is reduced when compared with an ohmic contact.

3.5.2 Epitaxial SiO2 Passivation Layer

Since leakage current through the bulk of the detector can be controlled and limited to some extent by the addition of blocking contacts and charge depleted region formation, surface leakage current effects can become the dominant contribution to measured leakage current in a detector. Surface leakage current arises from the flow of charge across the surface of a detector from one electrode to another. The charge is driven by the potential difference between the two electrodes, which can be hundreds of volts for reverse biased radiation sensors with only moderately low doped crystals. The potential difference between the electrodes, which is applied externally to form a charge depleted region within the detector, establishes an electric field which drives current from the anode to the cathode. The current flows across the surface of the device. The surface of semiconductor materials has a nonzero conductivity, controlled primarily by the dopant concentration at the termination points of the crystal. Since insulating materials exist which have higher resistivity than low doped semiconductor materials, an insulating material can be added to increase the apparent resistivity of the semiconductor surface.

In this work, a silicon dioxide (SiO2) layer was deposited on the detector surface.

Initially, a 1-micron thick SiO2 layer was deposited on the epitaxial surface of the 5mm x

5mm 4H-SiC crystal die. The dies were stripped of photoresist in an 80oC 50 mL NMP solution for 15 minutes. Next, the dies were washed in a 20oC NMP solution for 5 minutes. The dies were then rinsed in isopropyl alcohol for 2 minutes. Finally, the dies were rinsed with DI water for 2 minutes and dried with an N2 gun. Following cleaning 59 the dies were placed in a Plasma-Therm Plasma Enhanced Chemical Vapor Deposition

(PECVD) tool. The chamber was evacuated to below 20 mTorr and the dies were heated

o on the deposition platform to 250 C. A 1-micron SiO2 layer was deposited over the course of 53 minutes using SiH4 and N2O gas at a flow rate of 100 and 300 sccm respectively. The resistivity of SiO2 is nominally 1014 - 1016 Ω-cm. In contrast, the resistivity of even intrinsic silicon is on the order of 105 Ω-cm, with low doped 4H-SiC undoubtedly being less resistive. Thus, the addition of a passivation layer is expected to greatly reduce the surface leakage current. Figure 14 shows the resulting detector surfaces after the SiO2 deposition and before etching and contact deposition.

Figure 14. The 4H-SiC surface is shown in the above figure coated in SiO2. The 1 μm thick SiO2 layer is shown in green and pink from refraction effects.

3.5.3 Guard Ring

While passivation of the surface of the semiconductor die may result in reduced surface leakage current because of increased resistivity over the path between the detector electrodes, for very large bias voltages a non-negligible surface leakage current may persist. This is due to the increasing voltage resulting in an increased leakage current as a direct result of Ohm’s law. The potential difference between the electrodes drives the current flow from the anode to the cathode. If the potential difference were driven to zero, no potential would exist and no electric field would be present to drive current from one contact to the other, thus negating the surface leakage effects. This is not possible if a depleted region is desired inside the detector volume between the electrodes, though the addition of a sacrificial electrode can provide a similar advantage. By depositing a second electrode as a ring around the signal electrode on the epitaxial surface of the detector and biasing the ring to a voltage equivalent to the signal electrode, the electric field between the signal electrode and the ring electrode can be forced to zero. Since the electric field is zero, no current flows from the signal electrode to the ring. The ring can be powered by a separate power supply from the one that is used to create a charge depleted region in the detector. Since the ring electrode is set to a potential equivalent to the signal electrode, the voltage difference between the ring electrode and the bulk side electrode is equivalent to the bias voltage. As a result, current will flow across the surface of the detector from the ring electrode, also known as a guard ring, to the bulk side electrode. However, since the 61 power supply used to bias the guard ring is separate from that used to establish the depleted region and collect the signal pulses, the leakage current from the guard ring to the bulk electrode does not contribute to the noise floor of the detector signal. Figure 15 shows an overview schematic of the guard ring and SiO2 structure. Figure 16 shows the detector after deposition of the SiO2, photolithography, and plasma etching.

Figure 15. An overhead view of the guard ring and SiO2 structure of the 4H-SiC detector.

Figure 16. The guard ring structure is separated from the Schottky contact by a 250 μm thick SiO2 ring. The SiC appears in the above figure in off-white, while the SiO2 appears as green and pink.

3.5.4 Photolithography

To deposit a ring structure, shadow masking is not sufficient. To deposit more complicated features, photolithography can be used. Photolithography consists of the application of a light sensitive chemical known as photoresist to the surface of the semiconductor die. The light sensitive photoresist is selectively exposed to the correct wavelength of light to alter the chemical composition of certain regions of the photoresist. If the photoresist is positive resist, the areas which are exposed to the light will undergo a reaction which makes the 63 photoresist readily dissolvable in a solution known as a developer. The unexposed photoresist remains intact during development, and the exposed photoresist dissolves into the developer, revealing the material beneath the photoresist. For negative photoresist, the light from exposure causes a reaction which makes the exposed photoresist insoluble in the developing solution. In the case of a negative photoresist, only the exposed photoresist remains on the semiconductor die after development.

The method of photolithography and etching is based on previous work [33]. In this work, positive photoresist was used, specifically S1813 photoresist from Microposit. This photoresist is a liquid polymer which can be used to form 1-micron thick layers of photoresist. A uniform coating is formed by spin coating the photoresist onto the 4H-SiC die. Spin coating is accomplished by mounting a semiconductor die on a pedestal, which in the case of the Cost-Effective Equipment Spin-Coater used in this work, uses vacuum to hold the die in place. After mounting, the die is spun at low speed to verify that it is centered, then the rpm is rapidly ramped to a higher speed which is held to ensure a uniform surface thickness. The final film thickness is determined by the balance of the centripetal force of rotation with the surface adhesion of the polymer. For this work, the die was initially spun for 4 seconds at 300 rpm, ramped at a rate of 100 rpm/sec from a stationary position. After the 4 seconds elapsed, the die was accelerated at 5000 rpm/sec to 3000 rpm, where it was held for 60 seconds. After the spin-coating process was completed, the die was placed on a 115oC hotplate for 60 seconds. This is known as a soft bake and helps to encourage adhesion of the photoresist to the substrate and encourage stability of the

64 photoresist during later steps by allowing off-gassing of solvents present in the photoresist solution.

After the softbake, the die is placed in a contact aligner. For this work a Karl Suss MJB3 contact aligner was used. The contact aligner employs a mask, which is typically glass coated in a light absorbing chrome. The chrome is opaque except for the patterns which are to be transferred onto the die. The mask is placed above the die in the contact aligner, which allows the user to line up the patterns on the mask with the desired location on the die with the aid of a microscope viewing setup and micro-positioners. The die is brought into physical contact with the mask once lined up and vacuum is drawn at the interface between the mask and the die. Care must be used when making contact as too much applied contact force may damage the mask or the die. Once the mask and die are in contact with the vacuum drawn, known as hard contact, a 15 mW/cm2 mercury i-line (350 nm wavelength) UV light source exposes the photoresist. The duration of exposure determines the amount of cross-linking which occurs in the photoresist and thus how the photoresist behaves in the developer. Underexposure may result in too little cross-linking and, for a positive resist, excess photoresist left behind on the die after developing. If, however, the photoresist is overexposed, minute details may be lost due to exposure of edges and corners from diffractive phenomena. For this work the ideal exposure time was found to be 5 seconds. While an ideal exposure time for S1813 has in the past been found to be 2.4 seconds, due to the small size of the dies, edge effects resulted in increased photoresist thickness after spin-coating and softbaking. This increased thickness requires a longer exposure time to ensure full removal of the positive resist.

After exposure, the die is immediately placed in Microposit MF-321 photoresist developer. The development time determines the extent to which the exposed photoresist is dissolved. Shorter development times results in excess residual exposed photoresist. Too long of a development time results in dissolution of unexposed photoresist and loss of definition in patterns. The development time for this work was found to be 2 minutes with continuous agitation of the solution. After development, the die is immediately removed from the developer and rinsed with DI water for 2 minutes. This rinse halts the dissolution of photoresist by the developer. The die is then examined under a yellow-light microscope to verify that the pattern from the mask has been transferred to the die. The development and exposure time are correct if the pattern has been transferred to the die without loss of definition of the pattern but with the complete removal of the exposed photoresist down to the uppermost layer of the die. If the pattern has not been transferred with high fidelity from the mask or excess photoresist remains, the die can be further exposed or developed.

Complete removal of the photoresist may be necessary if the pattern is unsatisfactory. This can be done by following the photoresist process using NMP described earlier.

3.5.5 Etching

Once the photoresist pattern has been transferred to the surface of the 4H-SiC die, the pattern can be etched into the passivation layer. If a passivation layer is desired for the reduction of leakage current, then some form of etching must be performed to provide access to the surface of the 4H-SiC die to deposit a metal electrode and form a Schottky barrier. While chemical etching could be employed to selectively etch away the SiO2, the 66 inability of wet etching to produce sharp vertical structures precludes its use, since precise dimensions of the Schottky contact are required. Thus, for this work, plasma etching using a Plasma-Therm plasma etcher was used. Plasma etching uses radio frequency electromagnetic radiation to ionize a gas in a chamber and promote etching of the semiconductor die surface. The ionized gas is further controlled by a magnetic field which is established in the etcher. For this work, CF4 gas was used to etch the SiO2 passivation layer to expose the 4H-SiC die. The passivation layer thickness was 1 micron. The photoresist layer thickness was also approximately 1 micron. The selectivity of an etching process is the ratio of the thickness of photoresist etched per unit thickness of the desired substrate etched. For this work, the selectivity of the CF4 etching process for photoresist to

SiO2 was conveniently approximately 1:1. Since the selectivity of the process was 1:1, etching of the SiO2 1 micron down to expose the 4H-SiC would result in complete etching of the photoresist. The etch rate was found by the Wright Patterson Airforce Institute to be

o 200 nm/min for SiO2 for a CF4 flow rate of 30 sccm and a substrate temperature of 25 C.

Given the observed etch rate, the etch time was set to 6 minutes. This results in an over- etch into the 4H-SiC epitaxial layer of 200 nm. This was done to decrease the likelihood that excess SiO2 remained after etching. Excess SiO2 could otherwise remain due to anisotropy in the deposition rate during PECVD. The results of the etching process were examined with an optical microscope. The pattern from the photolithography mask was verified to have been transferred from the photoresist to the etched SiO2 pattern by optical microscopy.

3.5.6 Liftoff

After the SiO2 passivation layer has been patterned and etched down to the 4H-SiC to expose the central contact and guard ring structure, the metal must be deposited to form the contacts. However, before the metal can be deposited, a layer of photoresist must be applied as before. Any remaining photoresist from the initial coating that may not have been removed in the etching process is stripped off by submerging the etched 4H-SiC dies in an 80oC 50 mL NMP solution for 15 minutes. After the NMP cleaning, the dies are rinsed in isopropyl alcohol and rinsed for 2 minutes with DI water. The dies are then dried with an N2 gun. Once the dies have been cleaned and dried, they are coated in a 1-micron thick S1813 photoresist film using the same process described earlier. The dies are then placed in the Karl Suss MJB3 contact mask aligner as before. Care must be taken to ensure that the mask aligns with the etched pattern on the die surface. The dies are again exposed by a mercury i-line lamp for 5 seconds. The dies are then developed in MF-319 developer for 2 minutes and rinsed in DI water for 2 minutes. After developing and rinsing, the dies are dried with an N2 gun. The dies are then examined with a yellow-light microscope to verify that the pattern from the mask has been transferred to the die and that any remaining photoresist has been removed from the bare 4H-SiC surface. The presence of residual photoresist on the bare 4H-SiC may degrade the Schottky barrier height, so care must be taken that all the photoresist on the exposed region is removed.

Once the pattern has been transferred to the dies, the metal contacts can be deposited. A

CHA Solutions Electron Beam Evaporator is used to deposit a 100-nm layer of nickel on the photoresist coated surface. After this, without breaking vacuum, a 10-nm layer of 68 platinum is deposited. The dies are removed from the evaporator and placed in a 25mL acetone solution for 30 minutes. The process of dissolving the photoresist with an organic solvent and leaving behind the deposited metal on the bare 4H-SiC surface is known as liftoff. After the acetone bath, the dies are placed in an 80oC NMP solution for 1 hour to remove any remaining photoresist. The metal which was deposited on top of the photoresist is removed from the die when the photoresist is dissolved by the organic solvents. If any photoresist remains, the dies are placed in a room temperature NMP solution for 8-24 hours.

Some of the photoresist remained for multiple batches of detectors after the 24-hour NMP soak. To fully remove the remaining photoresist, the detectors which still had residual photoresist were submerged in an NMP solution in an ultrasonic cleaner. The detectors were cleaned in the ultrasonic bath for 5 minutes. However, this method was found to remove some of the metal that was deposited on the bare 4H-SiC and was thus abandoned as a liftoff method.

After the liftoff step is completed, the dies are washed in an isopropyl alcohol bath, rinsed for 2 minutes with DI water, and dried with N2. A metal contact is deposited on the bulk side of the die using a shadow mask in the electron beam evaporator. The metal contact is composed of 100 nm of nickel and 10 nm of platinum. After deposition, the dies are annealed at 650oC as described earlier to form Schottky electrodes from the epitaxial and bulk side contacts. The below figures depict the primary steps of the fabrication process and illustrate the cross-sectional appearance of the detector at each step.

Figure 17. Initially, the 4H-SiC die is cleaned and stripped of any remaining photoresist or oxides.

Figure 18. A 1 μm thick layer of SiO2 is deposited on the epitaxial surface of the SiC die using PECVD.

Figure 19. A 1 μm layer of photoresist is applied to the surface of the detector, spin coated to ensure uniform thickness, masked, and exposed to a mercury i-beam light to create the appropriate pattern in the photoresist.

Figure 20. The photoresist is held for 2 minutes in a developer which dissolves the exposed photoresist leaving behind the pattern shown in the above figure.

Figure 21. The die is etched using a plasma etcher and CF4 gas. The etching process removes the photoresist and SiO2 at a nearly 1:1 preference.

Figure 22. After etching, a layer of photoresist is again applied, and the pattern developed as before the etching process. This is to ensure that the metal contacts are only deposited in contact with the 4H-SiC, not on the SiO2.

Figure 23. The metal contacts are deposited on the epitaxial surface of the detector, and the photoresist is removed with a chemical solvent along with the metal deposited on top of it. This leaves behind only the metal deposited in contact with the 4H-SiC.

Figure 24. Metal contacts are deposited on the bulk side of the die, and the detector is annealed at 650oC to form a Schottky contact on both the epitaxial and bulk side of the detector. 3.7 Electrical Connections and Wire Bonding

To ensure good electrical connections are maintained between the signal, bias, and ground connections to the 4H-SiC detector, a robust connection method must be investigated. Low melting temperature methods used to form connections such as lead or lead-free soldering are not viable options, due to the operating temperature within the electrorefiner where the detector would operate. The average operating temperature of the molten salt in the INL

Mark VI electrorefiner is 500oC. The melting point of typical lead-free solder is approximately 180oC. However, while low temperature soldering methods are not suitable, many elevated temperature soldering or brazing methods are similarly precluded. This is due to the methods, such as brazing, requiring temperatures which are beyond the annealing temperature of the Schottky barrier contact of the detector. The lowest temperature brazing materials such as silver, require temperatures more than 650oC [16]. Since the detector

73 performance is dependent on the Schottky barrier height, these methods are likely to result in degraded detector performance. This is because the Schottky barrier height may be lowered by the brazing process, possibly to the point of behaving as an ohmic contact if heated sufficiently. Annealing temperatures of 850oC have been shown to form ohmic contacts between 4H-SiC and nickel. As a result, a method of electrical connection bonding which does not require temperatures greater than 650oC but is robust in a 500oC molten salt is required.

Gold wire bonding is commonly used to establish electrical connections between microchips and their chip carriers [16]. Wire bonding can be ball bonding, wedge bonding, or compliant bonding. Ball bonding requires the most heat, with the end of the wire becoming a molten ball through the combination of high-voltage electric charge applied through the wire, which results in a discharge to a sacrificial surface to form a molten tip, and the surface tension of the now molten tip of the wire. Wedge bonding operates at a lower pad temperature of approximately 300oC and relies more heavily on the ultrasonic power and contact pressure to form the weld. Compliant wire bonding makes use of an indentable intermediate layer, usually aluminum tape. Wedge bonding is a common method of wire bonding which consists of using a nano or micro scale gold wire to bond through ultrasonic vibration and heating to a gold surface known as a pad. Bonding the wire to the pad is done by making physical contact between the gold wire and pad. The pad is heated to soften the metal and the wire is vibrated at a high frequency while held in contact with the pad. This produces a localized surface melt, which solidifies when the

74 wire vibration ceases, welding the wire to the pad. Since the melting point of gold is

1064oC, well above the 500oC operating temperature of the molten salt, the bonded wire will not melt. Also, since gold is a non-reactive metal, in the salt it is resistant to chemical attack. This ensures physical robustness of an established electrical connection.

3.8 Simulated Source Spectra

To determine the maximum FWHM of the detectors that would allow for identification of the unique isotopes of interest in the electrorefiner environment, simulations were performed to generate possible source conditions. Several isotopes of interest which may occur simultaneously in an electrodeposition were included. Since 239Pu is of greatest interest in terms of monitoring for possible diversion from the pyroprocessing flowsheet, a deposition containing 239Pu was considered [29]. Because of the large fraction of dissolved

UNF which is composed of uranium and the physical mechanism governing the selectivity of the electrodeposition process, namely the unique electronegativity of each element in the molten salt, any 239Pu source will contain some amount of uranium. In the situation where there is no uranium present in the salt, it may be possible to deposit solely Pu; however, such a scenario would be precluded by the difficulty of reducing the uranium concentration through electrochemical means undetected [6].

Electrodeposition is not sensitive to the isotope of the element that is being deposited [1].

All isotopes of a given element have the same electronegativity when dissolved in the salt

[10]. As a result, the isotopic abundances of the actinides that are deposited on the working electrode will mirror the abundances in the salt. 75

There are several isotopes of both uranium and plutonium present in UNF. While 235U and

239Pu present the greatest proliferation concern, the presence of alpha emitting isotopes such as 238U, 241Pu, and 237Np represent confounding alpha sources that will be present in an electrodeposition containing 239Pu [7]. In addition to the overlap of the various alpha energies of the confounding isotopes present in the source, the thickness of the source itself presents a potential spectral broadening effect through self-shielding. Since alpha particles from deeper in the source lose more energy, in the case of a homogeneous source each isotope will have a broadened spectrum corresponding to the LET for that alpha emission.

The LET for each isotope is determined by the initial alpha energy and the material density and can be predicted using deterministic software such as Stopping Range of Ions in Matter

(SRIM) [28].

As an initial approximation, the isotopes in the source are assumed to be uniformly distributed across an infinitely thin surface. For thin sources, where the source thickness is a small fraction of the range of the alpha particle in the source material, this is a good approximation as the LET for alpha particles increases rapidly with decreasing energy; thus, high energy alpha particles emitted in a thin source will experience minimal energy loss. The actinides of interest for this simulation are those which are grouped around the electronegativity of U in the molten LiCl-KCl salt. While Np is not currently considered a special nuclear material requiring stringent monitoring, it appears between U and Pu in the electronegativity series for elements dissolved in LiCl-KCl eutectic salt and may present a confounding alpha source [29]. Only the primary alpha emissions of each isotope of interest is considered. The alpha emissions considered for each isotope make up >90% of

76 the total alpha emissions for each isotope considered. Additionally, the source geometry was assumed to be a point-source geometry to simplify analysis. Figure 25 shows the simulated spectra measured with an idealized 0.01% energy resolution detector with negligible dead layer thickness overlain on a single plot. The FWHM of the detector is assumed to be constant with respect to energy. The total source mass was 1 gram with a count time of 1 second. The elemental mass ratio of the actinides was assumed to be equivalent to the mass ratio in UNF. The mass ratio of Pu in the sample was assumed to be

1.2%, the Np mass ratio was assumed to be 0.2% (the mass ratio of all transuranic actinides in UNF), and the remaining 98.6% was assumed to be U [7].

Figure 25. The simulated alpha spectrum from a mixed nuclide source bearing U, Pu, and Np in ratios representative of used nuclear fuel. The simulation assumes an idealized 0.01% energy resolution. 77

As can be seen in the simulated spectrum in Figure 25 the alpha spectrum of 238U is readily identified and separated from the other isotopes. Similarly, the 235U alpha spectrum is well separated from other alpha emitters for all but the highest energy 235U alpha. The 234U alpha spectrum however overlaps with both 237Np and 235U alpha peaks but has a distinct alpha peak at 4.72 MeV. Similarly, the highest probability 237Np alpha overlaps with a 234U peak but is distinct at the lower and higher energy alpha peaks. 241Pu is well separated from the other alpha emitters. However, it is clear from Figure 25 that 240Pu and 239Pu will overlap for all but an idealized detector resolution making direct measurement of the 239Pu concentration difficult. The Pu isotopic concentration assumed for these calculations is shown below corresponding to burnup of 33 MWd/kg heavy metal [34].

Table 1 Isotopic Abundances of Pu in LWR UNF

Isotope 239Pu 240Pu 241Pu

Abundance (%) 58 23 14

For a real detector, the energy resolution will likely be much poorer than in the above idealized scenario. The energy resolution for 4H-SiC alpha detectors has been shown to be approximately 0.3% at room temperature. Figure 26 shows the resulting combined spectrum using the above simulation methodology for a 0.3% energy resolution detector.

In Figure 26 the contribution of counts of each isotope was normalized to the maximum number of counts. Figure 27 shows the overlain spectra of each isotope considered in the simulated source as measured by a 0.3% FWHM detector. As is evident in Figure 26, while the spectra are substantially intermixed for the various isotopes, many of the distinguishing 78 alpha peaks remain such that the characteristic alpha spectra of isotopes of interest can still be identified. The alpha spectra for 238U is clearly separable from the alpha spectra for 235U except for an overlap between the highest energy 238U alpha and the lowest energy 235U alpha. The highest energy 235U alpha overlaps with the lowest energy 234U alpha peak.

However, the lowest energy 237Np peak is still visible as are the higher energy 234U peaks, though the higher energy 237Np peaks add to the total counts at the upper 234U alpha energies, requiring a subtraction of the activity of 237Np from the upper 234U peaks by determination of the 237Np activity from the visible low energy 237Np peak. The highest energy alpha peaks for 241Pu are visible, while the lowest energy peak contributes to the highest energy 234U peak. The presence of 240Pu completely obscures the 239Pu peaks.

However, since the lowest energy 240Pu alpha peak is well separated from the combined upper energy 239Pu/240Pu alpha peaks, the activity of 239Pu can be determined.

Figure 26. The resulting alpha spectrum from a mixed nuclide actinide alpha source. The source contains the expected ratios of U, Pu, and Np from PWR UNF.

Figure 27. The simulated alpha spectra of each of the actinides of interest for safeguards concerns. Each of the actinide spectra have been plotted separately to illustrate the convolution of the spectra as a result of a mixed source with 0.3% detector resolution The activity of 240Pu must first be determined by calculating the activity required to produce the lower energy 240Pu alpha spectrum, then by using the probability of the higher energy emissions, subtracting the appropriate activity from the measured combined

239Pu/240Pu peak. This will result in only the 239Pu spectrum remaining, allowing for calculation of the 239Pu activity.

Practically, 4H-SiC alpha detector resolution has been shown to be approximately stable at roughly 1% when operating between room temperature and 200oC. The argon atmosphere above the molten salt in the electrorefiner has a minimum temperature of approximately 200oC and represents the lowest temperature in the electrorefiner volume

[3]. Thus, for 4H-SiC detectors the argon headspace is the most likely location for measuring the alpha spectra from an electrodeposited source. A simulated spectrum was 81 generated using a 1% FWHM for the detector resolution and is shown in Figure 28. The overlain spectra of all the isotopes considered is shown in Figure 29.

Figure 28. The simulated combined spectra of a U, Pu, and Np bearing alpha source as measured with a 1% FWHM energy resolution detector. Many of the unique alpha peaks are visible but the 239Pu and 240Pu peaks are indistinguishable.

Figure 29. The individual spectra for a detector with 1% FWHM energy resolution. At 1% resolution, several of the distinct alpha peaks are still visible. From the above figures the spectra begin to become difficult to distinguish as the resolution degrades from an ideal resolution to more practically demonstrable resolutions. The 238U spectrum is clearly visible in the above figures, as is the middle energy of the 235U spectrum. The 237Np spectrum is completely obscured by the 234U and 241Pu spectra and serves only to convolute the spectra. The highest energy 234U peak is still distinguishable from the 241Pu peak which is also still visible. The 240Pu and 239Pu alpha energies are completely overlapping and not readily separable by spectral analysis. From the analysis of an idealized source measurement with a detector operating in the headspace of the electrorefiner, and considering only the most probable alpha emissions for each isotope,

83 determining the 239Pu concentration is not feasible with currently demonstrated detector resolution at 200oC due to the interference of the 240Pu alpha spectrum.

Currently, the best 4H-SiC alpha detector resolution reported at 500oC is 2.2% for 5.48

MeV 241Am alpha particles. Simulation of the spectrum resulting from counting an infinitely thin source as described earlier with a 4H-SiC detector with 2.2% resolution, assumed to be operating at 500oC in the molten salt, presents a current “worst-case” resolution for the detector (neglecting source self-shielding effects). Such a simulated spectrum is shown in Figure 30.

Figure 30. The resulting spectrum from a mixed U, Pu, and Np source as measured with a 2.2% energy resolution alpha detector. Little information is readily available from the spectrum concerning specific actinide concentrations. As is evident in Figure 30, at 500oC with current detector resolution capabilities, determining the isotopic concentrations of even the far-space 238U in the source becomes difficult to determine with reasonable certainty, due to the contribution of overlapping

84 spectra. The results of this analysis seem discouraging, but it should be noted that the more likely temperature of operation is 200oC, where all but the 239Pu/240Pu ratio can be determined.

240Pu undergoes spontaneous fission at a branching ratio of 5.7x10-6, while 239Pu undergoes spontaneous fission at a branching ratio of 3x10-10. Spontaneous fission is the disintegration of the nucleus in the absence of a neutron flux [9]. The energy of the fission fragments for

240Pu are on average approximately 68 MeV for the heavier fragment and 93 MeV for the lighter fragment [35]. These energies are far larger than the alpha decay energy observed for 240Pu or 239Pu. Since 240Pu has a relatively high spontaneous fission rate (1.9x104 larger than 239Pu) the observation of fission energy particles in the 4H-SiC detector could be used to determine the 240Pu mass. By subtracting the 240Pu spectrum from the observed

239Pu/240Pu alpha spectrum, the alpha spectrum of 239Pu in the source spectrum may be resolved. By resolving the 239Pu alpha contribution to the spectrum, the mass of 239Pu on the sample may be determined, and therefore the isotopic concentration of 239Pu in the electrorefiner may be determined.

To determine the 240Pu concentration through fission fragment measurements, the fission fragment LET in 4H-SiC must first be determined. Since fission fragments are heavily ionized, they lose energy rapidly as they pass through material. This gives fission fragments a high LET and a short range. The maximum fission yield for spontaneous 240Pu fission has been shown to be for mass numbers 106 and 134 [36]. The spontaneous fission yield of 240Pu is a double-humped distribution like the thermal neutron fission yield distribution for thermal fission of 235U. From the fission product yield distribution for 240Pu for thermal

85 neutrons, two of the most abundant fission products are 103Ru and 134Xe [35]. These fission products were modeled as the light and heavy fission fragments resulting from a 240Pu spontaneous fission disintegration, due to a lack of data concerning isotopic identification of 240Pu spontaneous fission fragments. The range of each of the fission fragments in 4H-

SiC was calculated using the deterministic SRIM 2013 code package [28]. The range of a

134Xe fragment with the average heavy fragment energy of 68 MeV is predicted to be 7.64

μm. The lighter 103Ru fragment with an energy of 93 MeV is predicted to have a range in

4H-SiC of 10.21 μm. In both cases, the fission fragment energy will not be greatly attenuated by a thin (i.e. 1 μm thick) electrodeposited source. The fission fragment can be identified as a pulse which is significantly larger than the expected alpha particle induced pulses. Figure 31and Figure 32 show the LET curves for the 68 MeV 134Xe fragment and the 93 MeV 103Ru fragment.

1.60E+04 1.40E+04

1.20E+04 m) μ 1.00E+04 8.00E+03 6.00E+03

dE/dX dE/dX (keV/ 4.00E+03 2.00E+03 0.00E+00 0 20000 40000 60000 80000 100000 Energy (keV)

Figure 31. The stopping power of 103Ru in 4H-SiC plotted versus energy. Until the 103Ru particles reach a low energy (a few MeV), the stopping power varies slowly and linearly.

1.60E+04 1.40E+04

1.20E+04 m)

μ 1.00E+04 8.00E+03 6.00E+03

dE/dx (keV/ 4.00E+03 2.00E+03 0.00E+00 0 20000 40000 60000 80000 Energy (keV)

Figure 32. The stopping power of 134Xe. As the heavier fission fragment, 134Xe receives a smaller fraction of the total kinetic energy from fission. As a result, the stopping power of SiC for the 134Xe and the 103Ru are similar.

The range of the fission fragments is greater than the 1 μm thick electrodeposited source, but less than the 20 μm epitaxial thickness which forms the charge sensitive region of the detector. The energy losses of a light and heavy fission fragment emitted from the point farthest from the active volume of the detector on the source (emitted from deepest in the source) were modeled to predict the energy distribution expected solely from fission products. Since the electrodeposited source is composed primarily of uranium, the source material for attenuation was modeled as solid metal U as a “worst-case” scenario, as the actual source material density is likely to be lower than the density of U. Figure 33 shows the paths simulated by the TRIM 2013 code of 134Xe fission fragments passing through a 88

1 μm thick U source. In the electrodeposited source, the U will likely not be 1 micron thick, and some amount of 240Pu will spontaneously fission closer to the detector-source interface.

Figure 33. The simulated path of 134Xe fission fragments from spontaneous 240Pu fission through a 1-micron thick source layer. As can be seen in Figure 33, some of the 134Xe fission fragments do not pass fully through the U layer into the detector. The predicted energies for the light and heavy fission fragments are shown in Table 2. In Table 2 ⟨퐸퐾−푅푒푠.⟩ is the average residual kinetic energy of the fission fragment after passing through the U layer, EMin is the minimum observed residual energy, and EMax is the maximum residual energy observed.

Table 2. Fission Fragment Energies

Fiss. Fragment Initial Energy ⟨퐸퐾−푅푒푠.⟩ EMin EMax

134Xe 68 MeV 41.88 MeV 6.65 MeV 43.95 MeV

103Ru 93 MeV 65.94 MeV 29.27 MeV 67.45 MeV

For each fission fragment, 10,000 particles were simulated to pass through the U layer.

From Table 2, the average kinetic energy of the fission products as they enter the 4H-SiC epitaxial charge sensitive region will far exceed the energy of incident alpha particles; even after the particles have passed through the entire source thickness. For particles which undergo excessive straggling, the energy of the fission products still exceeds the expected alpha energies for the combined Pu-Np-U source. It is unlikely, however, that discrimination between the heavy and light fission products will be possible because of the overlap between the transported light fragment’s minimum observed energy and the transported heavy fragment’s maximum observed energy.

Chapter 4. Detector Characterization Results

4.1 Current-Voltage Behavior under Forward Bias

The FIV and RIV behavior of the detectors was measured as an initial indicator of detector performance. The FIV measurements allowed for determination of the turn-on voltage of the detector, the ideality factor, and the Schottky barrier height. These measurements were performed using a Keithley 2410 Sourcemeter operating as a voltage supply while measuring the voltage in a two-probe configuration. All FIV measurements were performed in a dark Faraday cage to eliminate electromagnetic interference (EMI) and avoid current generation from ambient light incident on the detector. It is important to note that FIV measurements provide diode performance for detectors with a single Schottky contact and an ohmic contact. For detectors with the double Schottky structure (a Schottky contact on the epitaxial side and a Schottky contact for the bulk-side electrode) FIV measurements are not readily attainable. This is due to the nature of the double Schottky behavior. When the epitaxial contact is forward biased, the bulk-side contact is reverse biased. This is shown in Figure 34. When the epitaxial contact is reverse biased, the bulk- side contact is forward biased.

Figure 34. When a negative voltage is applied to the epitaxial Schottky contact to form a charge depleted region, the bulk contact is forward-biased.

4.1.1 Turn-On Voltage

The turn-on voltage of a diode is the voltage at which the forward current overcomes the leakage current in the reverse direction, also known as the cut-in voltage [13]. At the turn- on voltage the diode begins to act as an ohmic resistor in the forward direction, with a rapid increase in current. The turn-on voltage is easily deduced from the semi-log plot of the FIV curve as the minimum current value on the plot. The inflection point of the curve is the turn-on voltage, where the leakage current is negated by the forward current as the diode

“turns on”. Figure 35 shows the plot of a detector under forward bias. Figure 36 shows the semilog plot of a similar FIV curve for a detector with an epitaxial Schottky contact and ohmic bulk-side contact.

5.90E-07 4.90E-07 3.90E-07 2.90E-07 1.90E-07

9.00E-08 Forward Current (A) Current Forward -1.00E-08 0 0.5 1 1.5 Volts

Figure 35. The forward-bias IV curve for an epitaxial Schottky contact detector with an ohmic bulk-side contact.

-5

-10

-15

-20

Ln(Current) (A) Ln(Current) -25

-30 0.00 0.50 1.00 1.50 2.00 2.50 Forward Bias (V)

Figure 36. Forward bias semilog plot of a Ni-Pt 4H-SiC alpha detector. The natural logarithm of the current is plotted against the bias voltage.

4.1.2 Schottky Barrier Calculation

The Schottky barrier height can be calculated using FIV measurements. By applying the ideal diode equation as presented in Equation (1), the Schottky barrier can be determined through measurement of the forward bias current [16]. First, the natural logarithm of the measured forward bias current is taken. Then, the saturation current value is determined.

The saturation current is the current value I extrapolated to zero applied voltage of the linear portion of the semi-logarithmic plot of the FIV curve. The turn-on voltage appears at the beginning of the linear portion of the semi-log plot of a FIV curve. Since Is from

Equation (10) is the intercept of the logarithmic plot of the FIV curve, kB is the Boltzmann constant, A is the area of the Schottky contact, AR is the Richardson constant for 4H-SiC,

o and T is known (for this work ~21 C), 휙퐵 can readily be determined in terms of electron volts (eV).

2 휙퐵 퐼푠 = 퐴퐴푅푇 exp (− ) (10) 푘퐵푇

The results of this calculation are shown in Figure 37 for multiple Schottky contact diameters. One assumption made in this calculation is that the Schottky barrier height is homogenous across the contact surface.

1.6 1.4 1.2 1 0.8 0.6 0.4

0.2 Schottky (eV) Barrier Schottky 0 0 1 2 3 4 5 6 Diameter (mm)

Figure 37. Schottky barrier height 휙퐵 versus Schottky contact diameter. As the diameter of the contact increases, the apparent Schottky barrier height decreases.

4.1.3 Ideality Factor

The ideality factor, n, of a diode is the degree to which a diode obeys the ideal diode equation as shown in Equation (1). Diodes that do not closely match the ideal diode equation will have values for n which are greater than 1. Diodes which exactly match the expected ideal diode behavior will have an n value of 1. Values of n less than 1 are non- physical and suggest errors in the method of measurement. For values of n greater than 1, the current increase with increasing voltage lags the predicted value. This can be due to defects in the crystal structure which result in charge trapping in the diode or resistances imposed by the electrical contacts used to apply the forward bias voltage. The ideality factor can be determined by applying Equation (1) in the region of 50-100 mV where the saturation current correction can be neglected [13]. This results in Equation (11). 95

푞푒푉 퐼 = 퐼푠 exp (− ) (11) 푛푘퐵푇

By determining the slope of the logarithmic plot of the FIV curve the ideality factor can be determined. Figure 38 shows a plot of the ideality factor of several diameter detectors with a Schottky contact and ohmic contact.

2.5

1.5

IdealityFactor 0.5

0 0 1 2 3 4 5 6 Diameter (mm)

Figure 38. Diode ideality versus Schottky contact diameter. As the diameter increases, the ideality factor of the diode increases. This suggests an increase in available charge trapping sites.

1.6 1.4 1.2 1 0.8 0.6 0.4

0.2 Schottky (eV) Barrier Schottky 0 0 0.5 1 1.5 2 2.5 Ideality Factor

Figure 39. Schottky barrier height 휙퐵 versus diode ideality. As the Schottky barrier height decreases, the ideality factor of the diode decreases. It is useful to note that both parameters decrease with increasing contact diameter. The Schottky barrier height and ideality factor were determined for detectors of various epitaxial Schottky contact sizes. Figure 39 shows the relationship between the Schottky contact barrier height and the ideality of the diode.

Figure 37 shows that the Schottky barrier height appears to depend strongly on the contact size of the detector. The variability in Schottky barrier height 휙퐵 with Schottky contact diameter was determined for detectors from the same batch process. The decreasing

Schottky barrier height with increasing diameter may be due to the increased likelihood of unfavorable surface conditions on the 4H-SiC die, leading to a decreased barrier height- as the barrier height has been shown to be highly sensitive to surface topology [17]. The ideality of the diodes was plotted against both Schottky contact size, shown in Figure 38 and Schottky barrier height, shown in Figure 39.

Figure 39 suggests that the ideality of the detectors roughly follows the Schottky barrier height behavior, lending support to the hypothesis that poor surface topology leads to poor device performance and divergence from ideal diode behavior.

4.2 Current-Voltage Behavior under Reverse Bias

While the forward bias region of the detectors is informative for their behavior as diodes, the reverse bias voltage behavior is the most important for detector operation. RIV measurements were performed from 0 volts to greater than -300 volts applied bias. The behavior of the detectors with reverse bias voltage is indicative of their performance due to the dependence of energy resolution on leakage current as a source of noise and the determination of the charge depleted region depth on the measured capacitance.

4.2.1 Leakage Current versus Voltage

Leakage current typically increases with increasing reverse bias voltage. This is due to the strengthening of the electric field inside of the detector as voltage increases which results in more thermally excited charges possessing enough energy to overcome the Schottky barrier height. RIV measurements were performed from 0 to -300v to characterize the detectors. Leakage currents on the nanoampere scale were observed. As the detector bias voltage increases, the leakage current stays nearly constant until the voltage approaches the breakdown voltage of the detector, where thermally excited charges possess enough energy to overcome the Schottky barrier. Figure 40 shows the leakage current measurement for a

98 single Schottky contact detector. Similarly, Figure 41 shows the leakage current for a double Schottky contact detector.

0.00E+00 -1.00E-06 -2.00E-06 -3.00E-06 -4.00E-06 -5.00E-06

Current (A) Current -6.00E-06 -7.00E-06 -8.00E-06 -365 -360 -355 -350 -345 -340 -335 Voltage (V)

Figure 40. Leakage current for a single Schottky contact detector at 20oC. As the bias voltage becomes more negative, the leakage current increases exponentially.

0.00E+00

-1.00E-01 )

nA -2.00E-01

-3.00E-01 Current ( Current -4.00E-01

-5.00E-01 -600 -400 -200 0 Voltage (V)

Figure 41. Semilog plot of leakage current versus voltage for a double Schottky detector. The measured leakage current is notably lower for the double Schottky contact.

100

Figure 42 shows the RIV response curve for a detector with a SiO2 passivation layer. The

SiO2 layer is 1 μm thick and results in reduced leakage current.

0.00E+00

-5.00E-02

-1.00E-01

-1.50E-01

Current (nA) Current -2.00E-01

-2.50E-01

-3.00E-01 -600 -500 -400 -300 -200 -100 0 Voltage (V)

Figure 42. A SiO2 passivated detector. The leakage current is reduced by an order of magnitude with the inclusion of a passivation layer.

4.2.2 Capacitance-Voltage Behavior

The relationship between the applied reverse bias voltage across a diode and the measured capacitance can be determined by applying the equation for the capacitance of a parallel plate capacitor. This is due to the behavior of the fixed charges in the charge depleted

101 region of a reverse biased diode. These fixed charges are analogous to the positively and negatively charged plates of a parallel plate capacitor. The capacitance of the 4H-SiC detectors was measured using a Keithley 4200A-SCS measurement system. By analysis of the parallel plate capacitor equation, shown in Equation (5), as the distance d between the plates grows larger the capacitance C decreases.

Similarly, if the area A of the plates is increased, the capacitance is expected to increase.

Since both the area and relative permittivity ε are fixed for a given 4H-SiC detector, changes in the capacitance with increasingly negative bias voltage depend only on the distance between the effective plates. Figure 43 shows a typical CV measurement performed by linearly increasing the negative bias voltage across a detector from 0 to -

60V.

1200 1000 800 600 400

Capacitance (pF) Capacitance 200 0 -80 -60 -40 -20 0 Voltage (V)

Figure 43. Capacitance versus voltage measurement in reverse bias for a Schottky contact detector. An important consideration is that the distance between the plates depends on the charge depleted region formation equation, Equation (2). The capacitance varies only with the 102 applied voltage in CV measurements of detectors from a single wafer, as the doping concentration and other material properties are fixed for a given detector. Thus, the decreasing capacitance versus increasing magnitude bias voltage is due to the increasing size of the charge depleted region. Since a charge depleted region exists at 0V applied bias, the detector exhibits a finite capacitance even without an applied bias voltage. As the bias voltage becomes positive and approaches the turn-on voltage of the diode, the capacitance approaches infinity. This is due to the elimination of the charge depleted region as the positive bias voltage forces charge carriers into the charge depleted region.

4.3 Charge Depleted Region Calculation

As stated earlier, the charge depleted region thickness depends on the applied bias voltage across the detector and the material properties of the detector. This is shown in Equation

(5). Since the capacitance of the detector is readily measurable, this provides a method for verifying that the charge depleted region is growing as expected with increasing reverse bias voltage. It is important to understand the charge depleted region behavior versus voltage to ensure that the charge depleted region is greater than the range of incident radiation in 4H-SiC. To fully attenuate the ionizing radiation in the charge depleted region of the detector the depleted region thickness must be greater than the range. Due to the

LET curve of alpha particles, a partially depleted detector with a charge depleted region less than the range of the incident alpha particles may produce misleading measurements.

Alpha particles with less energy which are fully attenuated will appear in higher energy

103 channels in the spectrum than higher energy particles which deposit only a fraction of their full energy.

To verify that the charge depleted region is increasing as expected with increasing reverse bias voltage Equation (2) can be substituted into Equation (5). The resulting expression, rearranged to express capacitance C as a function of applied voltage is shown in Equation

(12). Figure 44 shows a plot of predicted capacitance with increasing applied voltage and thus increased depletion depth. The relative permittivity of 4H-SiC and the area of the depleted region are necessary parameters to calculate the expected capacitance. The area of the charge depleted region may be approximated as the area of the Schottky contact on the epitaxial surface, as the depleted region will possess nearly parallel electric field lines and form from the metal-semiconductor junction into the epitaxial semiconductor. Figure

44 shows this behavior.

휖퐴 퐶 = ⁡ 1 1 (12) √2휖(푉푎푝푝푙𝑖푒푑 + 푉퐵𝑖) ( + ) 푁퐴 푁퐷

104

500

400

300

200

100 Capacitance (pF) 0 0 5 10 15 20 25 Depletion Depth (um)

Figure 44. Calculated capacitance for a 1.5 mm diameter detector versus the calculated depleted region depth xd.

4.4 Room Temperature Alpha Spectrometry

Since the goal of this work is to fabricate and test 4H-SiC detectors which function as alpha spectrometry detectors in an elevated temperature molten salt environment, room temperature alpha spectrometry was performed on a 1.88 μCi electroplated Eberline 241Am source in a vacuum bell jar held below 3 mTorr. Measurements were taken over 15-minute intervals to ensure sufficient counts to minimize statistical fluctuations. Electrical connections were established using micro-positioners. A negative voltage was applied to the epitaxial Schottky contact and a positive voltage was applied to the base of the

Bluewave Semiconductor Substrate Heater which served as the platform for testing. The bulk contact was in direct electrical connection with the conductive heater surface by

105 placing the detector directly on the surface of the heater. The 241Am source was collimated with an aluminum collimator and held at 2 cm from the surface of the detector.

Measured alpha spectra were fitted to a Gaussian equation which has three terms, corresponding to the three main alpha peaks of 241Am. These alpha energies are 5.486,

5.443, and 5.388 Mev with relative frequencies of 84.8%, 13.1%, and 1.66% respectively.

The applied voltage was varied from 0V to -250V. At each applied voltage a spectrum from the 241Am source was collected over a 15-minute counting time. Figure 45 shows the resulting spectra on a single plot.

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Figure 45. 241Am spectra collected with a Ni-Pt Schottky contact 4H-SiC detector at 20oC at increasing bias voltage. As the bias voltage increases, the depletion depth increases resulting in a larger pulse and thus a higher centroid channel number for the spectrum. As is evident in Figure 45, the spectrum centroid for each measurement corresponding to an applied voltage increases as the applied voltage increases. The increase in centroid position is due to the increased depletion region thickness in the detector volume. As the applied voltage is increased, the charge depleted region increases in thickness. As the charge depleted region increases in thickness, an increasing fraction of the energy deposited by the incident alpha particles is deposited in the charge depleted region. With an increasing amount of energy deposited in the charge depleted region, more of the freed

107 charge from the alpha particle energy transfer to the electrons of the semiconductor crystal are collected and swept to the electrodes by the electric field in the charge depleted region.

With an increasing amount of charge being collected at the electrodes per incident alpha particle the magnitude of the current per alpha particle increases. This increased current is integrated in the Ortec 142-B charge sensitive pre-amplifier and transmitted to the DSA-

2000 MCA as an increased voltage signal. The DSA-2000 then bins the larger signal into a higher energy channel resulting in the centroid position of the alpha spectrum shifting to higher channels with increased bias voltage.

In addition to the increasing channel position of the measured alpha spectrum centroid position, the FWHM of the spectrum decreases with increasing bias voltage to a minimum value corresponding to a depletion region thickness equivalent to the range of the alpha particle in the detector material. This is in part due to the use of a charge-sensitive preamplifier. As the capacitance of the detector increases, the output signal to noise ratio

QSi/QNo from the pre-amplifier increases through a linear relationship shown in Equation

(13) where vni is the equivalent input noise voltage for the preamplifier [14].

푄 1 푄 푆𝑖 = 푆𝑖⁡⁡⁡ (13) 푄푁표 퐶 푣푛𝑖

The capacitance of the detector varies linearly with the charge depleted region thickness as modeled by a parallel plate capacitor. Since the charge depleted region thickness varies as √푉푎푝푝푙𝑖푒푑 the detector resolution is expected to vary as the inverse of the square root of

108 the applied voltage. Figure 46 shows this behavior in the FWHM for the observed spectra from the 241Am source at varying voltages.

10 500 9 450 8 FWHM(%) 400 7 350 Capacitance 6 300 5 250 4 200

3 150 Capacitance (pF) Capacitance

Energy Resolution (%) Resolution Energy 2 100 1 50 0 0 0 50 100 150 200 250 300 Applied Voltage

Figure 46. As the applied voltage of the detector increases, the energy resolution (FWHM) of the detector decreases linearly to approximately 160V. Beyond this the resolution varies little. Decreasing capacitance improves resolution. The detector energy resolution was found to be approximately 0.94% at room temperature at -200V bias. Figure 47 shows a typical alpha spectrum measured with a 4H-SiC detector.

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Figure 47. A typical 241Am alpha spectrum measured with a 4H-SiC detector. The energy resolution for this detector was 0.94%, which is not sufficient to resolve the secondary and tertiary alpha energies of the 241Am alpha spectrum. 4.4.1 Triple Alpha Source

A thin electroplated alpha source was purchased from Eckert & Ziegler. The source consisted of 237Np, 241Am, and 244Cm with activities of 150 Bq, 100 Bq, and 100 Bq respectively. The primary alpha particle emission energy for each is 4.788 MeV, 5.486

MeV, and 5.805 MeV for the 237Np, 241Am, and 244Cm respectively. This source was chosen as it represents a range of alpha energies that are representative of the energies of actinides which are found in the pyroprocessing environment. Additionally, the separation of the alpha peaks is such that the ability of an alpha detector to distinguish between multiple alpha emission energies can be tested.

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The triple alpha source was measured first using a Canberra silicon surface barrier detector with a charge depleted region of 300 μm at 40V bias voltage counted for 10 hours. The energy resolution of the Canberra detector was approximately 0.3%. The spectrum collected with the Canberra detector is shown in Figure 48. The alpha spectrum peak for each of the isotopes is identified.

Figure 48. The characteristic alpha peaks of a 237Np, 241Am, and 244Cm alpha source from Eckert and Ziegler. The spectra were collected using an off-the-shelf Canberra silicon surface barrier detector. After the triple alpha source was characterized with the Canberra detector, a fully depleted

4H-SiC alpha detector was used to measure the alpha spectrum from the source. The triple alpha source was counted for 15 minutes. Figure 49 shows the resulting spectrum.

111

10 Counts 5

0 4000 4500 5000 5500 6000 Energy (keV)

Figure 49. The alpha spectrum collected using a fully depleted 4H-SiC alpha detector operating at -200V bias voltage for 15 minutes. Since the electrodeposited triple alpha source is thin, the energy resolution degradation contributed by the source thickness was minimal compared to the detector resolution. As evidenced by Figure 49, the detector possesses sufficient energy resolution to separate two of the alpha peaks of the 241Am and 244Cm spectra, along with resolving three of the peaks of the 237Np spectrum.

4.4.2 Thick Film Electrodeposited Source

Uranium and thorium sources fabricated through electrodeposition in a molten salt were deposited by the University of Utah. To fabricate a 232Th source, an alumina crucible containing LiCl-KCl eutectic salt with several weight percent ThCl4 was heated to 500 °C to ensure homogenous distribution of the ThCl4 in the molten salt [37]. A stainless-steel holder was attached to a coupon made of either stainless steel or nickel and a square region of 1 cm × 1 cm was immersed into the salt. A reducing current was applied to the system

112 via a potentiostat utilizing repeated chronoamperometry to deposit a 1 μm thick thorium metal layer onto a coupon. Repeated chronoamperometry consists of the repeated switching of the applied potential to promote uniform growth, by removing less stable dendritic formations, and to prevent dendritic formations [37]. A reference electrode fabricated from a mullite tube loaded with 100% AgCl and a silver wire was used. For deposition onto stainless steel, the potential of the coupon was alternated between -2.38 and -2.28 V relative to the reference electrode. For deposition onto nickel, the potentials alternated between -2.33 and -2.19 V versus the same reference electrode. This method of electrodeposition results in a uniform deposit that is free of dendrites. The same procedure was used to fabricate a depleted uranium source, substituting UCl3 for the solute in the

LiCl-KCl salt and the substitution of nickel as the deposition substrate.

Following the deposition, SEM images were taken to verify that the surface morphology was uniform and that the 232Th or depleted uranium was deposited as expected. The average deposited mass was estimated to be equivalent to 1 μm of 232Th or 238U, respectively, based on the amount of charge passed during the electroplating process. The uranium topology shows improvements in surface regularity, due to the formation of an alloy between the uranium and the nickel substrate [37]. The alloying resulted in the penetration of U metal into the nickel, eliminating the possibility of dendrite formation.

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Figure 50. SEM image of electroplated 232Th sample surface. Thorium, shown in yellow, is abundant on the substrate surface, which is otherwise composed of iron, nickel and chromium – components of the stainless steel. Red dots indicate zirconium deposits from the reference electrode.

Figure 51. SEM image of electroplated depleted uranium (shown in green) on a nickel substrate. The alpha spectra of the thorium and uranium sources were collected using a fully depleted

4H-SiC detector as well as an industry standard Canberra silicon detector as shown in

Figure 52 for the thorium source. The Canberra silicon detector has a 300 μm depleted region at a bias voltage of 40 volts and an energy resolution of <0.3% for 5.48 MeV alpha particles.

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Figure 52. The alpha spectrum from an electrodeposited thorium source using a Canberra silicon surface barrier alpha detector operating at 40V. The daughter isotopes are labeled for the thorium decay chain. Figure 53 and Figure 54 show the thorium and uranium spectra collected with a fully depleted 4H-SiC detector.

115

Figure 53. The alpha spectrum from an electrodeposited thorium source measured with a 4H-SiC alpha detector at -200v bias voltage.

116

100

80 U-238

60 U-234

Counts 40

0 0 500 1000 1500 2000 Channel

Figure 54. The alpha spectrum from an electrodeposited depleted uranium source collected with a 4H-SiC alpha detector operating at -200V.

The silicon detector was used first to provide a reference spectrum for each source and to separate the detector contributions to spectral broadening from the self-shielding effects in the sources.

The thorium source alpha spectrum was also measured with a partially depleted 4H-SiC alpha detector. The combination of the thorium source thickness and the partial energy deposition of the alpha particles in the partially depleted detector volume convoluted the observed alpha spectrum. However, since the alpha emitting isotope (232Th) contributing most of the activity to the source was known, the energy spectrum characteristics can be determined using a partially biased 4H-SiC alpha detector. The Results and Discussion section of a paper titled “Determination of the Thickness of an Electrodeposited Thorium

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Film with SiC Alpha Detectors” published in the Journal of Radioanalytical and Nuclear

Chemistry below details the results of this analysis.

4.4.3 Results from Partially Depleted Detector

The spectrum obtained after counting the electroplated 232Th source for 72 h is shown in

Figure 55. It is clear from the small number of counts for the long count time that the activity of the natural thorium source is much lower than that of the 241Am source, as expected from a naturally occurring radioactive material such as 232Th. However, there is a clear response that takes the shape of a stair step that is slanted towards lower deposited energies. The upper and lower edges of the step correspond to the maximum and minimum energy deposited in the detector during the measurement. The slanting pattern towards lower channels (energies) is expected [38]. If one of the energy values corresponding to the upper or lower edge is known, the digital spectrum analyzer (DSA) channel value can be calibrated to energy to interpret the feature.

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Figure 55. The resulting alpha spectrum from an electroplated thorium source using a partially depleted 4H-SiC alpha detector. Low energy noise below channel 800 is excluded. Source was counted for 72 h to achieve a reasonable signal from the low activity natural sample.

A Canberra silicon surface barrier detector (model PD 300–19–100 AM) was used as a reference method to compare against the partially depleted SiC detector. Figure 52 shows the resultant spectrum recorded over a 24-hour collection period. The measurement was performed approximately 6 months after the SiC detector's 232Th spectrum was acquired.

The daughter products and trace amounts of 228Th and 230Th are identified in the figure below. Additionally, the sharp cutoff of each distinct isotopic peak is indicative of the energy broadening caused by the source thickness. The maximum value of each distinct peak corresponds to the unattenuated energy of that alpha decay. This energy corresponds to an alpha particle emitted from the surface of the source. The presence of 230Th, a daughter of the 238U decay chain, indicates that the thorium used for electrodeposition may

119 have at least in part come from thorium which was chemically separated from a uranium bearing mineral such as uranothorite.

Partially biased detector simulation analysis

The major complication in this study was the inability to fully bias the SiC detector due to diode breakdown and leakage current. The resulting spectra obtained from the measurement procedure therefore did not exhibit a linear response between incident particle energy and deposited energy in the device, and the upper edge of the spectrum cannot be assumed to correspond directly to the nominal alpha particle energy of 232Th

(4.012 MeV). An analysis of stopping power (loss of energy over a unit distance traveled by the particle in a given medium) was performed in order to correctly interpret the results.

120

Figure 56 shows a visualization of the medium thickness source and partially biased dE/dx detector used in this study.

Figure 56. Visualization of energy deposition in a partially depleted detector from a relatively thick source.

Alpha particles are energy attenuated by the source (visualized by red color gradient), and deposit a broadened energy spectrum in the depleted region of the SiC detector. A

4.012 MeV alpha particle from the source surface will be at full emission energy when it hits the detector. However, it will not deposit all of its energy in the detector because its range is larger than the depletion depth.

The partial depletion depth was calculated to be 8.86 μm at −40 V using Equation (14) with the known parameters of the SiC device [31]. This depletion depth is less than the full range of a 4.012 MeV 232Th alpha particle in SiC, which was calculated to be 11.55 μm using

SRIM 2013. The energy deposited in the 8.86 μm biased region of the device was

121 calculated using the stopping power of SiC, also obtained using SRIM 2013 and shown in

Figure 57.

Figure 57. Stopping power of alpha particles in SiC from SRIM. Higher energy alpha particles have lower stopping power than lower energy particles, meaning that energy deposition is not linear for higher energy particles in a partially depleted detector.

1 휖푆𝑖퐶(푉푏𝑖 + 푉푏𝑖푎푠) 푁퐷 2 (14) 푥푑 = [ ] 푞 푁퐷(푁퐴 + 푁퐷)

The amount of energy deposited in the detector by an alpha particle of given energy was calculated by tracking a single particle’s linear path through 8.86 μm of SiC using a step size of 0.01 μm and updating the particle energy using the dE/dx curve from Figure 57. An example of the results of the calculation is shown in Figure 58 for an alpha particle energy of 4.012 MeV. This analysis was performed over a range of energies to obtain a comparison of incident particle energy to deposited energy, shown below in Figure 59. Higher energy particles deposit less energy due to lower stopping power at higher energies. The peak of

122 the plot is at the incident energy that will fully deposit in 8.86 μm. The deposited energy was calculated by summing up ΔE graphs such as the one shown in Figure 58 from 0 to

8.86 μm. The maximum energy deposited in 8.86 μm of SiC was determined to be

3.27 MeV.

Figure 58. Change in energy during particle tracking in SiC for an alpha particle of energy 4.012 MeV through 11.55 μm of SiC. The step changes in ΔE are due to the discrete stopping power data from SRIM.

Figure 59. Deposited energy in partially biased SiC calculated for incident particle energy on SiC with an 8.86 μm depletion depth.

123

Based on the visualization in Figure 56 and the deposited versus incident energy graph in

Figure 59, the expected spectrum from a partially depleted detector was developed and is displayed in Figure 60. The graph on the left of Figure 60 shows the signal from a range of energy attenuated particles impinging on a detector. Energies greater than the 100% deposition cutoff (3.27 MeV) will deposit a fraction of their energy corresponding to the region of the plot shown in Figure 59 for E > 3.27 MeV. The contribution to lower energy counts from energies higher than the cutoff energy can be visualized as a reflection across the 100% deposition energy, and is visualized by the red dashed line. The graph at the right of Figure 60 shows the expected combined spectrum, which includes a double stair step feature.

Figure 60. Expected energy spectral results in a partially biased detector.

The right-hand spectrum in Figure 60 was compared to the features in the 232Th results shown in Figure 55. The major features matched despite the counting noise in the 232Th spectrum. The expected double stair step due to partial deposition of higher energy particles could not be resolved from the noise in the acquired spectrum. Energy calibration was done

124 to determine if the lower channel edge of the measurement corresponds to either: (1) the lowest energy attenuated particle emitted from the back of the source (2) the partial deposition of the full 4.0 MeV unattenuated particle. The deposition value for the 4.0 MeV particle was calculated based on Figure 59, and was determined to be 2.73 MeV. The maximum energy deposition value of 3.27 MeV was used to calibrate the upper edge of the sample spectrum at DSA channel 1448. Assuming that the DSA channel 0 corresponds to 0 MeV and that there is a linear relationship between channel and energy, the lower edge of the slanted step at channel 1105 was calibrated to 2.49 MeV. Because this is below the

2.73 MeV deposited by the unattenuated 4.0 MeV Th-232 alpha particle in the partially depleted SiC, the lower edge of the obtained spectrum must correspond to the attenuated energy of the alpha particles emitted from the back of the source sample. Finally, a similar analysis of stopping power in 232Th was performed to determine the thickness needed to attenuate a 4.0 MeV emission particle to 2.49 MeV. The thickness of the electroplated sample was calculated to be 4.65 μm for an assumed 100% theoretical thorium density

(11.72 g cm−3). The mass thickness estimate is therefore 3.97 × 10−5 g/cm2.

The charge integration from electrodeposition indicates a thickness ~1.0 µm. The alpha spectrum analysis from the fully depleted Si (Figure 52) also yields a similar thickness.

The reason that Figure 55 presents a much wider thickness, i.e., 4.65 µm, might be due to a combination of factors including (1) the globular and dendritic nature of the deposition or porosity in the deposit that leads to non-theoretical densities, (2) the uncertainty associated with sensor materials properties, e.g., carrier concentration that may lead to a large uncertainty in depletion depth calculation, thus affecting the energy calibration scale,

125

(3) more importantly, the broadening of the peak due to the in-growth of 232Th daughter nuclide. However, we believe that the analysis method presented in this paper for calculating the medium thickness of an alpha source with a dE/dx detector is still rational.

4.4.4 Paper 1 Conclusions

The results of the spectrum counting shows that wide band-gap alpha particle detectors can measure emissions from an electroplated, low activity actinide sources on the order of micrometers. Although being as a special case, a dE/dx detector versus a fully depleted detector, measuring thickness directly with alpha spectrum analysis will allow the spectral analysis to better account for peak spreading and overlapping of alpha particles from multiple isotopes by disregarding peak edges that are known to be caused by attenuation in the source thickness. This research can be furthered improved by refining the fabrication technique of wide band-gap semiconductor alpha spectroscopy devices to allow for reverse biasing up to the full range of expected alpha particle energies in SiC, thus eliminating peak edges due to the insufficient thickness of the depletion region. Better deposit quality via different deposition techniques (e.g., repeating chronoamperometry) is also desired, since deposit uniformity (minimization of bald spots and dendrites) can have a large influence on the quality of the resulting spectra. Resolving these issues will make it easier to determine the cause of the discrepancy between alpha spectroscopy thickness estimation and the charge integration estimation used during electrodeposition.

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4.4.5 Energy Loss in Increased Thickness Schottky Contact

To establish a physically robust electrical connection to the Schottky contact of the 4H-

SiC alpha detector, wire bonding was pursued. Since wedge wire bonding does not require temperatures above approximately 300oC the process is unlikely to damage the Schottky contact through elevated temperature annealing effects which may lower the Schottky barrier height. However, to avoid damaging the Schottky contact through the required ultrasonic frequency vibrations and physical pressure between the 10-nm gold wire and the

Schottky contact surface (typically consisting of 10 nm of Pt), an increased Pt thickness was investigated. Since the initial Pt thickness was only 10 nm, followed by a nickel thickness of 100 nm, there was concern that the wire bonding process would penetrate the

Pt layer and damage the interfacial region between the nickel and 4H-SiC epitaxy. To avoid this, a batch of detectors were made which had a modified Pt thickness. The Pt layer on the epitaxial surface was increased from 10 nm to 300 nm to ensure physical robustness of the layer and avoid penetration of the Pt layer during wire bonding. However, after wire bonding was proven effective, as shown in the wire bonded 4H-SiC detector in a ceramic chip carrier in Figure 61, concerns over the energy attenuation of incident alpha particle energies through the thicker Pt layer were investigated.

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Figure 61. A 4H-SiC alpha detector with a 300 nm thick Pt layer wire bonded to a ceramic chip carrier. The connection is made using a 10 nm Au wire. The primary concern was that the increased thickness of the Pt layer would contribute to the energy loss of the alpha particles through attenuation by increasing the thickness of the dead layer from approximately 110 nm to 400 nm. This increase in dead layer would result in increased energy loss of the alpha particles in a non-sensing region of the detector. Since the linear energy attenuation curve for alpha particles does not follow a linear relationship

(see Figure 13 for a typical LET curve), alpha particles of lower energy will lose more energy in the dead layer than higher energy alpha particles. As a result, the spectrum from lower energy alpha emitters will be shifted to lower energies in the measured alpha spectrum to a greater degree than higher energy alpha particle spectra. This would increase the complexity of identifying isotopes in a mixed nuclide source, which would in turn complicate the determination of actinide concentrations in a mixed source. In principle,

128 however, this complication could be accounted for using post-process analysis if the dead layer thickness was well known.

Additionally, the energy loss of the particles as they pass through the dead layer will vary with the path length. For alpha particles which are perpendicularly incident on the detector surface and travel straight through the dead layer (a good approximation for high energy alpha particles far from their Bragg peak) the energy lost will be determined by the integral of the LET curve over the thickness of the dead layer.

To determine the contribution of the 300 nm Pt layer to the detector resolution, the various contributions to the FWHM of the detector must be determined. The total FWHM of the spectroscopy system, here denoted FWHMTotal is comprised primarily of three terms, as shown in Equation (15) [24]. FWHMDetector is the total contribution to the FWHM of the alpha detector from detector properties such as leakage current effects and statistical fluctuations in the total number of charge carriers produced in the detector volume per alpha particle. FWHMElectronic is the contribution to the FWHM from front-end electronic noise and the preamplifier. FWHMDead is the spectrum broadening from the different path lengths available as the dead layer grows and the convolution of the resolution from the variation in LET with energy. FWHMOther is the contribution of miscellaneous terms such as energy loss due to source self-shielding and variations in the entrance angle of alpha particles through the detector window.

2 2 2 퐹푊퐻푀푇표푡푎푙 = 퐹푊퐻푀푎푙푝ℎ푎 + 퐹푊퐻푀퐸푙푒푐푡푟표푛𝑖푐 (15)

2 2 2 2 퐹푊퐻푀푎푙푝ℎ푎 = 퐹푊퐻푀퐷푒푎푑 + 퐹푊퐻푀퐷푒푡푒푐푡표푟 + 퐹푊퐻푀푂푡ℎ푒푟 (16)

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Since the 241Am source used for detector characterization was a thin electroplated source, the contribution of source self-shielding can be neglected. Similarly, since the source was collimated using an aluminum collimator and held at 2 cm from the detector (such that a point source approximation is valid), the variation in alpha particle entrance angle into the detector window can be neglected. FWHMTotal was measured using a 300 nm Pt capping layer 4H-SiC detector biased to -200V with a total dead layer thickness of 400 nm to

241 measure a Am spectrum in a vacuum of <0.3 mTorr. FWHMTotal was found to be 1.396% or 76.57 keV for 5485 keV alpha particles. FWHMElectronic was determined using an Ortec

419 pulser in combination with the front-end electronics and an Ortec 142-B preamplifier.

FWHMElectronic was found to be 0.371%. FWHMElectronic was determined by measuring the

FWHM of the spectrum generated by the pulser while a fully biased 4H-SiC alpha detector was connected to the 142B preamplifier. The FWHMElectronic does contain the FWHM of the pulser electronics as well, but it is negligible compared to the contributions of the front- end electronics and leakage current contributions from the detector. FWHMalpha was calculated by subtracting the measured FWHMElectronic from the observed FWHMTotal and was found to be 1.346%. FWHMDead was determined using SRIM simulations to determine the stopping power of 241Am alpha particles in 4H-SiC and Pt to generate an LET curve.

The LET curve was then used in combination with a TRIM simulation to generate the energy loss distribution of 5.48 MeV alpha particles in 300 nm of Pt. FWHMDead was predicted to be 0.659% or 34.96 keV for initially collimated 5485 keV 241Am alpha particles. This was compared to a 10 nm Pt capping layer detector which had a calculated

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FWHMDead of approximately 0.16%. As a result, the 300 nm Pt capping layer was determined to be too thick for use in a molten salt actinide concentration sensing application.

A 10 nm Pt capping layer 4H-SiC detector was used with wedge wire bonding to connect a 10 nm thick gold wire to the Schottky contact of a 4H-SiC detector. The detector is shown in Figure 62 in a ceramic chip carrier with the wire bonded to an electrode of the chip carrier.

Figure 62. A 4H-SiC detector with a 10 nm Pt capping layer wire bonded to a ceramic chip carrier with a 10 nm Au wire. 4.5 Elevated Temperature Alpha Spectrometry and Leakage Current

One of the intended applications for the detectors developed in this work is submersion in a molten salt containing dissolved nuclear fuel for pyroprocessing applications. The detectors would have source material from the molten salt electrodeposited onto them and

131 an alpha spectrum would be collected. The temperature of the molten salt in the INL electrorefiner, which was treated as a typical system for pyroprocessing of used nuclear fuel (UNF) is approximately 500oC. In the electrorefiner above the molten salt is an argon atmosphere vapor space. This vapor space is cooler than the molten salt, but still typically above 200oC [3]. As a result, any detector which is intended to take measurements while still in the electrorefiner must be able to maintain good energy resolution and physical robustness at elevated temperatures. Additionally, verification of the detector temperature through forward bias behavior was explored. The detectors were heated to increasing temperatures and the forward bias turn-on voltage was measured. This was correlated linearly with temperature to develop a method of verifying the detector operating temperature in the electrorefiner environment.

4.5.1 Elevated Temperature Alpha Spectroscopy

To test the energy resolution versus temperature of the 4H-SiC alpha detectors a 241Am alpha source spectrum as measured in vacuum. Vacuum was maintained during the heated testing for two reasons. The primary reason to maintain vacuum in the bell jar chamber is to reduce the energy loss of the alpha particles as they travel 2 cm from the alpha source to the detector.

Additionally, the detectors were heated on a Bluewave substrate heater. To maintain a constant temperature across the detector convective cooling and air currents must be avoided. By maintaining vacuum, only radiative and conductive cooling can occur. Since the detector is only in physical contact with the heater and the micropositioner probes, conductive cooling is limited; thus, radiative heat transport is the primary heat loss 132 mechanism for the detector and a nearly uniform temperature profile across the detector is maintained. The temperature of the substrate heater was controlled by a Eurotherm

Temperature Controller which interfaced with a LabVIEW program on a laptop computer.

The temperature of the substrate heater was monitored by a k-type thermocouple and controlled by a PID controller. The Eurotherm controller supplied the necessary RF frequency driving current to heat the controller.

As in the room temperature alpha spectroscopy measurements, the detector signal was connected to an Ortec 142-B charge sensitive preamplifier which was in turn connected to a Canberra DSA-2000 MCA which served as an amplifier, an analog to digital converter, and high voltage power supply. Figure 63 shows the setup including the Eurotherm heater controller.

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Figure 63. The measurement setup used for elevated temperature alpha spectroscopy measurements in a vacuum chamber at pressures <3mTorr and temperatures above 500oC. The 4H-SiC alpha detector was biased to -200V to form a charge depleted region greater than the range of the 5.485 MeV alpha particles from the 241Am source, approximately 19

μm. The 241Am alpha spectrum was first collected at room temperature (20oC) and again at increasing temperature. The temperature was increased at 100oC intervals. The detector was held at each temperature for 15 minutes to allow the temperature to equalize between the detector and the substrate heater. After the temperature had stabilized, the alpha spectrum was counted for 15 minutes. Figure 64 shows the spectrum from 20oC to 500oC.

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Figure 64. Alpha spectra from a 241Am alpha source measured with a 4H-SiC detector heated from 20oC to 500oC. Figure 65 shows a selection of temperatures from 20oC to 500oC to more clearly show the behavior of the spectra with increasing temperatures.

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Figure 65. A selection of alpha spectra from a 4H-SiC detector heated from 20oC to 500oC. The centroid position of the alpha spectra increases in channel number with increasing temperature. As is evident in Figure 65 as the temperature of the detector increases the centroid of the alpha spectrum shifts to higher channels. The dependence of centroid position on temperature may be explained by the increasing thermal energy of bound valence electrons.

As the temperature of the 4H-SiC crystal increases, the average thermal energy of the charge carriers in the crystal also increases. This allows the charge carriers in the crystal to exist higher in the valence band, closer to the conduction band of the semiconductor crystal.

As a result, with increasing thermal energy, less energy from the incident ionizing radiation is required to promote a bound electron to the conduction band. Thus, the total number of free charge carriers produced by a given incident alpha particle increases with increasing temperature of the detector and as a result, the magnitude of the signal pulse increases. This 136 results in the measured alpha spectrum of the 241Am source shifting into a higher channel number as shown above in Figure 65.

The centroid and energy resolution of the alpha spectra collected by the heated detector at each temperature were calculated. The energy resolution at 500oC was found to be 2.25%

FWHM. This is an improvement over previous designs using a Ni-Ti-Au design, as shown in Figure 66.

Figure 66. Previous detector design energy resolution from 20oC to 500oC showing an energy resolution of ~16% at 500oC [23] [20]. The measured increase in centroid position of the alpha spectra versus temperature is shown in Figure 67. The increase in centroid position from room temperature measurements is indicated as the channel number increase.

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Figure 67. Increase in centroid position from the 20oC centroid position versus temperature of the detector. As can be seen in Figure 67, the increase in channel number of the observed 241Am spectrum follows a linear relationship with temperature. This follows from the explanation for increased charge carrier production per incident ionizing particle. Equation (17) shows the expression relating average kinetic energy ⟨퐸푘⟩ of a particle in Joules to its measured temperature T in Kelvin, where kB is the Boltzmann constant in J/K.

3 ⟨퐸 ⟩ = 푘 푇 (17) 푘 2 퐵

As can be seen from Equation (17), ⟨퐸푘⟩ increases linearly with increasing temperature.

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4.5.2 4H-SiC W-Value Determination

Part of the work described here was published in the Institute of Electrical and Electronics

Engineering Transactions on Nuclear Science. The W-value is defined as the average number of charge carriers produced per unit energy lost by an incident ionizing particle; e.g. the number of electron-hole pairs (ehp) created per MeV of energy lost by an incident particle (ehp/MeV). As described above, as ⟨퐸푘⟩ increases the energy required to promote a charge carrier from the valence band to the conduction band decreases. As a direct consequence, the W-value of the detector decreases. The decrease of the W-value has been previously described by Klein as shown in Equation (18) where Eg is the band gap energy,

⟨퐸푘⟩ is the average residual kinetic energy of electrons and holes without sufficient energy to create additional ehps, and ⟨퐸푅⟩ is the average energy loss due to energy transfer to optical phonons [20]. As stated earlier, ⟨퐸푘⟩ can be calculated using Equation (18).

Similarly, the change in band gap energy with temperature has been described as shown in

Equation (19), where T is the temperature in Celsius [20].

푊4퐻−푆𝑖퐶 ⁡ = ⁡ 퐸𝑔 ⁡+ ⟨퐸퐾⟩⁡+ ⟨퐸푅⟩⁡ (18)

(푇⁡ + ⁡273)2 퐸 (푇) = ⁡3.265 − ⁡6.5 ×⁡10−4 ⁡[푒푉] (19) 𝑔 푇 + 1573

The theoretical results from previous work show good correlation to the experimental results found in this work [23]. However, given differences between 4H-SiC detector designs in this work and that of previous studies, the model should be modified. The experimental data was fitted using MATLAB and the result is shown in Equation (20).

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−5 푊4퐻−푆𝑖퐶 = 퐸𝑔(푇) − 7 × 10 푇 + 5.15 (20)

By applying the experimental results and Equation (20), Figure 68 can be obtained which presents the relationship between the W-value versus temperature from 20oC to 500oC.

Figure 68. The W-value for alpha irradiation of 4H-SiC measured versus temperature. As the temperature of the detector increases, the measured W-value decreases. Figure 68 shows the approximately linear behavior of the W-value of the 4H-SiC detector from 20oC to 500oC. The negative slope of the W-value equation with increasing temperature follows directly from the increasing thermal energy of the bound charge carriers in the crystal.

4.5.3 Forward Bias Temperature Sensing

By applying a small forward bias voltage and measuring the resulting forward current versus voltage behavior with a Keithley Sourcemeter, the turn-on voltage of the detector can be determined. The detectors were placed on a Bluewave substrate heater in an

140 evacuated bell jar at a pressure below 0.3 mTorr to minimize convective cooling and ensure a uniform temperature of the detector. The substrate heater was controlled by a LabVIEW code run on a laptop computer which communicated with a Eurotherm Temperature

Controller. The Eurotherm controller supplied the necessary RF frequency driving signal and maintained the setpoint temperature with a PID controller. The temperature of the substrate heater was monitored using a k-type thermocouple connected to the Eurotherm controller. The temperature of the 4H-SiC detector was held for 15 minutes at each temperature, starting at room temperature and increasing by 100oC increments to 500oC.

This was done to allow time for the detector to reach a uniform temperature profile and achieve the same temperature as the substrate heater. A forward bias voltage was applied using two micropositioners, one in contact with the epitaxial Schottky contact of the detector and the other in contact with the substrate heater surface. A LabVIEW code was used to ramp the applied forward bias voltage from 0 to 3V. The resulting forward current was measured by a 2410 Keithley Sourcemeter and recorded to a laptop computer for post- processing analysis. Figure 69 shows the resulting FIV measurements at increasing temperatures.

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Figure 69. Current versus voltage response curves in the forward bias direction for a 4H- SiC detector at various temperatures. As the temperature increases, the turn-on voltage of the detector in forward bias decreases. As is evident in Figure 69, as the temperature of the detector increases, the turn-on voltage decreases. The turn-on voltage of the detector is the applied voltage at which the forward bias voltage reduces the charge depleted region sufficiently that charge can flow freely across the diode. Figure 69 shows a semilog plot of the current response to the applied voltage. The voltage at which the linear portion of the FIV curve begins in the semilog plot is the turn-on voltage. From Figure 69 above 500oC the turn-on voltage will approach 0V.

This indicates that there is a temperature at which this detector will fail to accurately indicate the operating temperature.

If the turn-on voltages are plotted against the operating temperature of the detector, Figure

70 can be generated.

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Figure 70. The turn-on voltage of the detector versus the detector temperature. As the temperature increases, the turn-on voltage decreases linearly. Figure 70 shows the temperature versus turn-on voltage response of the detectors. The turn- on or “cut in” current was set to 0.5 μA as a current well above the noise floor which would not damage the detector. A linear fit has been applied to the measured results to allow for interpolation between the measured data points and allow for arbitrary temperatures to be derived from the measured turn-on voltage. From the forward bias behavior of the detector and the linear fit, Equation (21) was generated.

푉푇푂 = −0.0008푇 + 0.7411 (21)

In Equation (21), VTO is to the turn-on voltage corresponding to 0.5 μA and T is the

o temperature measured in Celsius. Solving for a VTO of 0 yields a temperature of is 926 C,

143 indicating an upper limit at which point the turn-on voltage of the detector would be 0V and would not be useful as a temperature sensor.

4.6 Molten Salt Experiments

Since the primary application for the 4H-SiC detectors considered in this work is for actinide concentration determination in a molten salt electrorefiner, the ability of the detectors to operate in a molten salt environment is paramount. To test the detectors capability to operate in a molten salt, both the chemical resistance of the detectors and the thermal response of the detectors was determined. Since separate measurements were carried out to determine the detector characteristics at elevated temperatures, the detector behavior because of the molten salt chemistry could ideally be investigated separately from temperature-related effects. However, since the method of heating the detectors explored earlier consisted of a slow increase in the temperature of the detectors due to the slow ramping capability of the substrate heater used, the detector behavior in response to a thermal shock was a confounding variable. Thus, the detectors were subjected to short duration thermal shock tests to investigate the effects of a rapid thermal transition on the detector characteristics.

The molten salt was formed from a 99.99% pure LiCl-KCl salt. 50 grams of the LiCl-KCl salt was heated in a Kerr Auto Electro-Melt furnace in an inert atmosphere argon glovebox held at slightly (<1 psia) positive pressure. Since LiCl-KCl salt is highly hygroscopic, the humidity of the glovebox was kept to 10 ppm H2O. The positive pressure was maintained to ensure that any gas flow would be from the glovebox to the outside atmosphere, 144 preventing air ingress. The argon atmosphere was used to ensure that no oxidation occurred on the detector from atmospheric oxygen in the glovebox atmosphere or as a result of air inclusion in the salt. The Kerr Auto Electro-Melt furnace has a 50 cm3 graphite crucible, and is capable of a maximum temperature of 1120oC [39]. The furnace is controlled by a digital PID controller. The desired temperature is set using the PID controller and is maintained within an adjustable 5oC dead-band. An alumina crucible was placed within the graphite crucible to contain the molten salt. The furnace, installed in the argon atmosphere glovebox, is shown in Figure 71 and Figure 72.

Figure 71. The argon atmosphere glovebox.

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Figure 72. The Kerr Auto Electro-melt furnace installed in the argon atmosphere glovebox. The PID controller is visible in the bottom left.

4.6.1 Thermal Shock Response from Molten Salt Submersion

To test the detector response to a rapid increase in temperature, a 99.99% pure LiCl-KCl salt was heated to 550oC in an alumina crucible. The salt was heated for 1 hour to ensure that the salt had completely melted. Visual confirmation of the liquid salt was also performed. Since molten LiCl-KCl salt is colorless and transparent, while solid LiCl-KCl is a white solid powder, visual inspection provided an easily performed verification method. A 4H-SiC alpha detector was characterized using FIV, RIV, CV, and alpha

146 spectrometry. The turn-on voltage, leakage current, and capacitance behavior versus voltage was determined. Using a 241Am source in an evacuated bell jar the energy resolution of the detector was determined prior to submersion in the salt. The 4H-SiC alpha detector was held using a stainless-steel alligator clip. Care was taken to ensure that the alligator clip held the detector by a non-metallized edge to ensure that neither the epitaxial or bulk-side Schottky contacts were damage by the spring force of the alligator clip required to hold the detector. The distance from the bottom of the molten salt in the crucible to the arm of a metal ring stand positioned over the furnace was measured to be 15 cm. A stainless-steel wire was connected to the alligator clip to create a 16-cm length assembly and attached to the metal ring stand. The detector was positioned over the open furnace and lowered into the molten salt. The detector was submerged 1 cm into the molten salt for

30 seconds. The detector was then removed from the salt and allowed to cool suspended from the ring stand while held by the alligator clip. After the detector had cooled, a small amount of solidified LiCl-KCl salt which had remained on the detector while it cooled was observed. The detector was removed from the argon atmosphere glovebox and cleaned with isopropyl alcohol and DI water to remove the solidified LiCl-KCl salt. After the salt was removed, the detector was dried with compressed air. The detector was then placed in an evacuated bell jar and characterized using alpha spectroscopy and RIV. The thermal shock measurement showed no degradation in the alpha spectrum resolution or leakage current.

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4.6.2 Long Term Detector Behavior in Molten Salt

The detectors will be submerged in the molten salt of the electrorefiner for extended periods of time. This time will likely be cumulative, however, as the duration of the deposition process which deposits actinides on the detector is on the order of minutes. The

INL electrorefiner resides in a hot cell to limit radiation dose to operators and experimenters. A commercial application of pyroprocessing would likely be similarly isolated. As a result, it is desirable that the detector operating lifetime is long, as the hot cell environment complicates the process of removing and installing equipment. Thus, in addition to radiation hardness to resist the effects of the strong radiation field present in the electrorefiner, the detectors must be thermally and chemically stable in the salt. As has been shown in previous work, the thermal stability of thin metal layers used to form the epitaxial contact on 4H-SiC diodes are susceptible to thermally driven diffusion. This diffusion degrades the rectifying capability of the Schottky contact and potentially reduces conductivity. To test the detectors for thermal stability in the molten salt, the detectors were submerged for increasing time periods in a molten LiCl-KCl salt held at 550oC. The detectors were submerged in 50 grams of molten salt held in an alumina crucible. The salt was heated and held at temperature in a Kerr Auto Electro Melt furnace in an argon atmosphere glovebox. The detectors were held in the salt using a stainless-steel alligator clip which was attached to a stainless-steel wire. The stainless-steel wire was attached to a ring stand over the furnace. Care was taken to ensure that the detector was submerged totally in the salt but was not in contact with the sides of the alumina crucible. The detectors were held initially in the salt for 15 minutes. After the initial submersion, the detectors 148 were removed from the salt and allowed to cool in the argon atmosphere glovebox. The detectors were then removed from the glovebox and the residual salt which had solidified on the detectors was removed using isopropyl alcohol and DI water. The detectors were then dried with compressed air. After the detectors were dried, they were placed in the evacuated bell jar and RIV and alpha spectroscopy measurements were performed.

Figure 73 shows the 241Am alpha spectra measured by the detector before and after submersion in the salt. The initial FWHM of the detector before salt immersion was 69 keV for 5.485 MeV alpha particles at a bias voltage of -200V. After salt submersion, the detector FWHM was determined by again measuring the alpha spectrum of a 241Am source.

The resulting FWHM was 85 keV after salt submersion.

Figure 73. Pre-salt immersion 241Am alpha spectrum measured with a 4H-SiC alpha detector at -200V bias 149

The leakage current was determined from RIV measurements after submersion of the detector in the salt. The detector was allowed to cool to room temperature and the leakage current was measured from 0 to -200V applied bias. Figure 74 shows the leakage current of the detector before and after submersion in the salt. As shown in Figure 74, leakage current increased slightly because of the thermal shock test.

1.00E-10 0.00E+00 -1.00E-10 -2.00E-10 -3.00E-10

Current (A) Current -4.00E-10 Post-Salt -5.00E-10 Pre-Salt -6.00E-10 -250 -200 -150 -100 -50 0 Voltage (V)

Figure 74. The reverse bias leakage current for a 4H-SiC detector pre-and post- submersion in the molten salt. After measuring the effects of a 15-minute submersion on detector performance, a second measurement was performed. A new detector was submerged to explore the effects of time submerged. The detectors were held in the 550oC molten salt for 24 hours for a 96-fold increase in total submersion time. After submersion, as before, the detectors were: removed from the molten salt, allowed to cool in the glovebox, and cleaned of solidified salt using isopropyl alcohol and DI water. Care must be taken when removing the detector from the alligator clip and during salt removal. The metal contacts were observed to strip off of the bulk side contact, if the detector was removed from the alligator clip prior to submersion 150 in solvents. After the salt was completely removed, the detectors were then characterized using RIV and alpha spectroscopy measurements. The results for a submerged detector with a cumulative time in the salt of 24 hours is shown in Figure 75 while the RIV measurement for the detector after salt exposure is shown in Figure 76.

Figure 75. 241Am spectrum from a -100V biased 4H-SiC detector after a 24-hour submersion in a 550oC molten LiCl-KCl salt.

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5.00E-10

0.00E+00

-5.00E-10

Current (A) Current -1.00E-09 Post-I Pre-I -1.50E-09 -250 -200 -150 -100 -50 0 Voltage (V)

Figure 76. RIV curves before and after submersion of a 4H-SiC detector in 550oC molten salt for 24 hours. As is evident when comparing the pre and post salt alpha spectra, the resolution of the detector was degraded. The FWHM of the detector increased from ~0.94% for 5.48 MeV alpha particles at room temperature up to ~3% FWHM at room temperature. Additionally, we can see that the leakage current after the 24 hour submersion in the molten salt increased uniformly from 0 to -200 volts. Possible mechanisms for the detector performance degradation may included damage to the crystal structure from the thermal shock of submersion resulting in increased crystallographic defects and the intrusion of ions from the salt into contact with the epitaxial SiC surface. Alternatively, the long duration at 550oC may have resulted in the diffusion of the Pt layer into electrical contact with the SiC resulting in a decreased Schottky barrier height.

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Chapter 5. Detector Packaging for Molten Salt Applications

5.1 Detector Packaging Design Requirements

To survive the molten salt environment present in the electrorefiner, the alpha detectors must have robust electrical connections, excellent corrosion resistance, and elevated temperature operational stability. The previous sections have discussed the design considerations of the detectors for corrosion resistance and elevated temperature operational stability. These considerations revolved primarily around the properties of 4H-

SiC and Schottky contact behavior. In this chapter, the design constraints imposed by the molten salt environment on the detector packaging are considered. The chemical compatibility of the materials used to construct the housing of the detector, the electrical signal cabling, and the sealant material to prevent salt intrusion into the detector housing were considered. Additionally, the elevated temperature behavior of the housing materials, the electrical signal insulation, and the electrical connection method were also evaluated.

5.2 Molten Salt Material Compatibility

Since molten LiCl-KCl salt can be highly corrosive with common construction materials such as iron and metal oxides, the selection of materials which can be submerged for long periods of time in the salt is important. Noble metals such as gold and platinum are well suited to resist chemical attack but prove cost-prohibitive if used for applications such as structural components which require relatively large quantities of the metals. Nickel has been shown to resist chemical attack in the molten salt environment, and high nickel alloys

153 such as Hastelloy have been pursued for use as structural material in the salt. Stainless steel is an abundant structural material with well-defined characteristics over a broad range of temperatures and environments. The relatively high percentage of nickel in stainless steel provide some corrosion resistance in the molten salt. Additionally, materials with melting points below roughly 500oC are ill-suited to the molten salt environment, as the combination of chemical attack and a component transitioning to a liquid would likely result in detector failure. Since the salt is held in a molten state, highly porous materials are ill-suited for use in the detector packaging, as the uptake of molten salt into the pores of a material may cause swelling of the component and result in detector packaging failure.

Similarly, materials for components which are in intimate contact with each other must share similar coefficients of expansion under increasing temperatures. A mismatch in the coefficient of expansion between two connected components with tight tolerances at room temperature would likely result in the deformation or failure of one or both of the components.

5.3 Detector Housing

The detector housing must serve as a physical support for the detector and provide corrosion resistance to the molten salt. Additionally, the detector housing must serve as a hermetically sealed container to prevent ingress of the molten salt into the detector volume.

The detector housing material must be relatively low cost, as it makes up the bulk of the detector packaging mass. As a result, the detector housing material used was stainless steel.

The detector housing consists of a two-piece assembly. The lower assembly is a 19mm 154 diameter cylinder with an open bottom made from 0.15mm thick 304 stainless steel as shown in Figure 77.

Figure 77. The stainless-steel outer casing. Left: an unmodified outer casing. Right: a modified casing with a 3mm aperture for the alpha spectroscopy measurements.

On the closed surface of the lower assembly a 3mm diameter hole was punched using a hand-operated metal punch. The 3mm diameter opening serves as a window for the detector which is housed inside. The lower assembly doubles as the ground connection for the 4H-SiC alpha detector epitaxial Schottky contact. The upper assembly is an identical open stainless-steel cylinder, with a 5mm diameter hole punched in the closed face. The

5mm diameter hole serves as an opening for electrical connections. The upper and lower 155 housings are sealed together with insulating ceramic adhesive and electrically connected with silver solder.

5.4 Detector Mounting and Hermetic Sealing Requirements

The 4H-SiC detector is mounted to the lower housing assembly using insulating ceramic adhesive. The ceramic adhesive is Cotronic Resbond 940HT. The ceramic adhesive is an

Al2O3 two-stage epoxy. The 940HT was chosen as it has a low coefficient of expansion with temperature.

Figure 78. Cotronic Resbond 940HT two-stage ceramic adhesive. The ceramic powder is mixed with the activator to form the adhesive.

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The ceramic adhesive was tested in a molten LiCl-KCl salt to measure the molten salt uptake of the ceramic. First, the mass of the ceramic adhesive with the resin and hardener was measured. A small disk of the adhesive was left to harden at room temperature and pressure for 24 hours. The disk was then placed in an argon atmosphere glovebox and submerged in a 500oC molten LiCl-KCl eutectic salt. The disk was left submerged in the salt for 1 hour. The disk was then removed from the salt and allowed to cool on a stainless- steel tray for 30 minutes. The disk was then removed from the argon atmosphere glovebox, cleaned using DI water and allowed to dry for 24 hours. The disk was then weighed and the total mass was compared with the initial mass of the unhardened adhesive.

In addition to determining the salt uptake of an adhesive disk that was cured at room temperature and pressure, two more disks were similarly measured. The first disk was weighed, as before, while still in two separate phases (the hardener and the resin), but after mixing was placed in a vacuum chamber at room temperature. A vacuum was drawn to establish a pressure of <3mTorr. The adhesive mixture was left to cure for 24 hours under vacuum.

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Figure 79. Left: A ceramic adhesive disc cured under <3 mTorr pressure at room temperature for 24 hours. Middle: A ceramic adhesive disc cured at room temperature and pressure for 24 hours. Right: A 200oC and atmospheric pressure cured adhesive disc after 4 hours of submersion in a 500oC molten salt. As with the room temperature and pressure cured disk, the vacuum cured disk was submerged in the molten salt, removed, cooled, cleaned, allowed to dry, and weighed. The last disk was weighed as separate components as before and mixed, but placed in a 200oC furnace at atmospheric pressure to cure for 1 hour. The disc was then removed, cooled, and tested in the same way as the previous disks. Table 3 shows the initial and final masses of each of the ceramic discs, and the calculated salt mass which remained in the discs after cleaning.

Table 3. Ceramic Adhesive Disc Masses

Disc Type Initial Mass (g) Final Mass Salt Mass

20oC Cured Disc 14.23 14.31 0.08

Vacuum Cured Disc 12.72 13.43 0.71

200oC Cured Disc 13.94 14.21 0.27

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As can be seen in Table 3 a small amount of salt was entrained into the ceramic adhesive.

The room temperature and pressure cured adhesive had the smallest amount of salt uptake after submersion.

5.5 Signal Cable Considerations

The molten salt environment, consisting of a 500oC corrosive ionic solution, imposes limitations on the types of cables which can be used for signal transport from the detector to the system electronics.

The elevated temperature of the electrorefiner environment precludes the use of polyvinyl or rubber insulated signal cables. These cables are poorly suited to the elevated temperature environment as the insulation would rapidly degrade or combust.

The detector configuration is such that there are at minimum two separate electrodes required to bias the detector to form a charge depleted region, bias the detector surface to drive electrodeposition of actinides on the Schottky contact surface, and acquire the alpha pulse spectrum. The magnitude of the bias voltage required to create a fully charge depleted epitaxial layer in the 4H-SiC detector is approximately -200V; the signal cables which are used to bias the detector also transport the radiation induced charge pulses to the preamplifier and ultimately to the spectral analysis equipment. As a result, the signal cables for the detector pulses must be able to withstand steady state operation with a -200V potential between the ground and positive cable of the signal cable. For coaxial cables this requirement means that the insulation separating the inner conductor from the outer conductor must be physically and electrically stable at 500oC. The leakage current between 159 the central conductor and the outer conductor will contribute to the total leakage current observed in the detector system. Since the leakage current of the detector is on the order of nA, the resistivity of the insulation separating the coaxial conductors must be sufficiently large to limit the current flow between the conductors to no greater than the nA level. Since the voltages applied to bias the detector are steady-state DC voltages, a simple application of Ohm’s law provides the required resistivity of the signal cable insulation. The resistivity of the cable is related to the dimensions as shown in Equation (24).

푉 = 퐼푅 (22)

200푉 푅 = − = 2 × 1011⁡Ω⁡ (23) 1 × 10−9퐴 푅퐴 휌 = (24) 퐿

Resistivity is typically expressed in Ω-cm as the resistivity increases with increasing insulation cross-sectional area but decreases with increasing length L. To determine the necessary dimensions for an insulated coaxial cable, Equation (25) can be rearranged as shown below in cylindrical coordinates. In Equation (25), the required resistance R is determined by the resisitivity ρ, the radius of the inner conductor a, the inner radius of the outer conductor b, and the length of the cable L.

휌 푏 푅 = ln ⁡ (25) 2휋퐿 푎

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As can be seen in the above equation, as the cross-sectional area increases the resistance decreases. Increasing path length between the conductors increases the resistance. Thus, for a fixed resistivity, minimizing the cross-sectional area and maximizing the path length between conductors (and thus the insulation thickness between them), increases resistance and thus decreases leakage current through the insulation. For a typical coaxial cable, the insulation thickness is on the order of a few mm. As a result, the insulation resistivity must be high to allow for long cabling with minimal leakage current as the cross-sectional area will depend on both the length of the cable and the diameter of the central conductor. The electrical resistivity of the Cotronics ceramic adhesive used in this work (Resbond 940HT), is 1011 Ω-cm [40].

An elevated temperature cabling solution for fire-rated signal cables is mineral insulated

(MI) cables. These cables consist of a metal outer conductor separated from a copper central conductor by a compacted powdered mineral insulation. Pure MgO insulation has

15 o a resistivity of 10 Ω-cm at approximately 500 C. Similarly, hafnium oxide, HfO2, has excellent electrical resistance at elevated temperatures. The outer metal cladding can be made from corrosion resistant Hastelloy to withstand chemical attack in the salt.

5.6 Electrical Connections

The bias voltage requirement to form a sufficient charge depleted region within the detector is approximately 200V. Initially, the Schottky contact was biased to -200V while the bulk side contact was held at the ground potential of the power supply. However, since the outer housing described earlier is electrically connected to the Schottky contact surface, a voltage 161 far from the ground potential of the electrorefiner salt is not practical. Operating the detector with the outer casing, which is in contact with the salt, at highly negative voltages may result in electrodeposition from the salt and undesirable electrical complications. As a result, the central conductor of a MI cable was biased to +200V and connected to the bulk side contact of the detector. The epitaxial contact was connected to the outer casing as before, and the casing was connected to the outer conductor, which was held at ground potential, of the MI cable.

The central conductor was connected to the bulk side contact using silver solder two-part epoxy. The silver solder was applied to the back of the detector, and the central conductor was placed in contact with the solder and bulk side electrode. The silver solder was allowed to cure overnight. The upper housing was lowered over the outer conductor and into contact with the lower housing. The upper and lower housings were sealed and bonded using the ceramic adhesive. The adhesive was left to cure overnight. Silver solder was then used to ensure electrical contact between the upper and lower housing. A small amount of silver solder was then used to ensure electrical contact between the epitaxial side electrode and the lower housing. Afterward, the connection between the outer conductor and the upper housing was sealed with ceramic adhesive.

To provide easy connect and disconnect of the detector to industry standard connections, a

BNC adapter was fitted to the end of the MI cable. An adapter which, mounted on the MI cable, was used to convert the bare cable to a BNC connection.

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5.7 Packaged Detector Characterization

A schematic (not to scale) of the packaged detector is shown in Figure 80, Figure 81, and

Figure 82. The packaged detector as assembled is shown in Figure 83.

Figure 80. From the top (SiC bulk side), the stainless-steel case forms the outer boundary, separating the electrical connections from the molten salt. The ceramic insulation forms a ring which both mounts the detector to the stainless-steel case and seals the aperture from salt ingress.

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Figure 81. View of the bottom face of the detector (SiC epitaxial side to be submerged in the molten salt). The detector epitaxial contact is connected to the outer casing by silver solder and grounded.

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Figure 82. The side view of the assembled detector. The MI cable inner conductor provides positive bias to the bulk-side contact of the detector. The dashed line represents the joint between the two steel outer casing halves, joined and sealed with silver solder.

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Figure 83. Packaged Detector. From Left to Right: The stainless-steel casing and detector, 5mm diameter stainless-steel MI cable, MI to BNC adapter, BNC connector.

After the detector was packaged in the upper and lower housing and the electrical connections were established, the leakage current of the detector was measured using a

Keithley 2410 Sourcemeter as before. The measured leakage current is shown in Figure

84.

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5.00E-10 0.00E+00 -5.00E-10 -1.00E-09 -1.50E-09 -2.00E-09

Current (A) Current -2.50E-09 -3.00E-09 -3.50E-09 -250 -200 -150 -100 -50 0 Voltage

Figure 84. The leakage current of the packaged 4H-SiC alpha detector. As is evident in Figure 84 the leakage current of the packaged detector increased over that of the bare detector. The insulation was found to be insufficiently resistive due to prolonged exposure in air which allowed for the intrusion of water into the MgO powder. This reduced the resistivity of the insulation. As a result, the measured leakage current of the detector in the packaging was found to be larger than that of the unpackaged detector.

Following leakage current measurements, the detector was biased to form a charge depleted region (with a grounded epitaxial contact and a +200V bias on the bulk side contact) in a vacuum chamber held below 3 mTorr. A 1.88 μCi 241Am alpha source was positioned 1 cm above the detector and the alpha emissions counted for 15 minutes. The resulting spectrum is shown in Figure 85.

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140 120 FWHM:1.65% 100 80

60 Counts 40 20 0 1000 1500 2000 Channel

Figure 85. An 241Am alpha spectrum measured with a packaged 4H-SiC alpha detector. The detector resolution degraded because of the increased leakage current. The FWHM of the detector was found to have degraded from before packaging. The initial FWHM was approximately 1%, while the packaged detector was found to have a resolution of 1.65%. This is due to the increased leakage current of the detector packaging, resulting in a degraded signal to noise ratio.

5.7 Alpha Spectrum from Surface Deposited Actinide Source

A packaged detector was sent to the University of Utah for electrodeposition in a molten

LiCl-KCl salt of a combined thorium and uranium source. The packaged detector was constructed as previously described in this work. Electrical continuity was verified by the

University of Utah using a hand-held multimeter prior to electrodeposition. The detector was first heated to 500oC for 20 minutes to ensure durability at elevated temperatures.

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The detector was submerged in a LiCl-KCl-ThCl4-UCl3 salt with 2 wt% uranium and thorium respectively in the University of Utah furnace as shown in Figure 86.

Figure 86 Electrode furnace setup at University of Utah. Following electrodeposition, the detector was returned to The Ohio State University and connected to the alpha spectroscopy system. No vacuum was necessary as the actinides were deposited directly to the surface of the detector. The detector could not be biased beyond -50V without experiencing breakdown. It is currently unclear whether this is due to the electrical connectivity measurements performed by the University of Utah in the forward bias direction accidently, or a result of the electrodeposition process. The returned detector is shown in Figure 87. The upper casing is visibly separated from the lower casing.

Electrical connectivity between the upper and lower casing was re-established prior to measurement.

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Figure 87. The packaged detector following return to The Ohio State University after electrodeposition was performed at the University of Utah. The alpha spectrum of the detector was collected for 8.33 hours. The source activity was estimated based on mass deposition to be approximately 1 Bq. Additionally, several 30 second duration measurements were taken to characterize the background noise of the detector. The duration of the background characterization measurements was limited to 30 seconds to minimize the number of source counts which would be collected during this time given the low activity of the source.

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3000 2500 2000 1500

Counts 1000 500 0 0 100 200 300 400 500 Channel

Figure 88. The collected alpha spectrum from the electrodeposited detector following background subtraction. Because of the partial bias of the detector, the source self-shielding energy attenuation, the mixed nuclide source composition, and the inherent detector resolution limits of the

4H-SiC detectors the resulting spectrum in the above figure is difficult to characterize in terms of energy peaks. However, this demonstrates proof of concept for the electrodeposition of actinides to the surface of a 4H-SiC Schottky contact detector in a molten salt.

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Chapter 6. Nuclear Forensic Applications of 4H-SiC Alpha Spectrometry

In this chapter, the work concerning nuclear forensic applications of the alpha spectrometry capabilities of 4H-SiC detectors in combination with electrodeposited sources is discussed.

Much of this chapter concerns work which was published under the title “4H-SiC Alpha

Spectrometry for Nuclear Forensics with Electrodeposited Sources” in the Journal of

Radioanalytical and Nuclear Chemistry.

6.1 Nuclear Forensic Analysis of U and Th Electrodeposited Sources

The alpha emission spectrum of electrodeposited thin film 232Th and depleted uranium sources, including the alpha emitting 232Th decay chain daughters and 238U daughter nuclide 234U, have been measured using a thin film 4H-SiC alpha detector. The energy resolution of this detector proved suitable for the identification of many of the major characteristic alpha emissions associated with the 232Th daughter isotopes. The identification of the lower energy daughter isotopes made from the 4H-SiC detector measurements was confirmed using a silicon surface barrier detector. A depleted uranium source was fabricated using electrodeposition in a molten salt and the alpha spectrum was acquired with a 4H-SiC detector. The 234U/238U activity ratio and atomic enrichment of

234U in the sample were determined. A method of determining the 235U enrichment in the product stream of the enrichment process used to produce the depleted uranium source was developed. This method makes use of the 234U and 235U waste stream enrichments, as indicated by the depleted uranium source, and a mass balance equation to determine the

172 product 235U enrichment. Since the 235U spectrum was obscured in the electrodeposited source by peak broadening of 238U and 234U, a table of common 235U waste enrichment values was used to compute 235U product enrichments, thereby resulting in a plausible identification of the enrichment process as a Department of Energy HEU enrichment process.

6.2 Alpha Spectrometry for Nuclear Forensics

Alpha spectrometry for nuclear forensic applications can provide information about the isotopic composition of actinides in the forensic material. Since source preparation for alpha spectrometry measurements requires destructive techniques, preservation of information contained in the nuclear material is desirable. Electrical characterization such as coulombic measurements to determine source mass can be coupled with source fabrication in molten salt deposition [37]. Electrodeposition of actinides in molten salt provides flexibility in source fabrication for nuclear forensic applications due to the high solubility of many actinidic chlorides in molten salt [11]. Additionally, deposition of an actinide source directly to the surface of an alpha detector eliminates the need for vacuum equipment to minimize alpha particle energy attenuation in air by placing the source directly in contact with the detector. This deposition directly to the surface in molten salt imposes temperature and corrosion resistance requirements on the detector design.

4H-SiC alpha detectors with energy resolution sufficient to identify multiple actinides in a molten salt electrodeposited source have been developed [41]. The wide band-gap of 4H-

SiC of 3.2 eV allows for reduced thermal noise at elevated temperatures. The corrosive

173 environment of a molten salt limits material selections to those which are compatible with the molten LiCl-KCl environment at temperatures ranging from 450 to 600oC [1]. SiC possesses sufficient chemical stability in molten LiCl-KCl salt to withstand prolonged exposure [42]. To measure alpha spectra with sufficient energy resolution for the identification of characteristic alpha energies of a given isotope in a mixed source, energy attenuation must be limited as much as possible [12]. This imposes the restrictions of both a thin source geometry and a requirement to minimize the energy attenuation as the alpha particles travel from the source to the detector. A uniform thin film actinide source with known mass can be deposited onto a conducting surface of the detector using chronoamperometry [4]. During actinide deposition, coulombic measurements can be performed to determine the mass of the actinide of interest in the dissolved salt [37]. The thin film source can then be withdrawn from the salt, measured by the alpha detector on which it is deposited, submerged in the salt, the deposition current polarity reversed, and the actinides driven back into the salt allowing the detector to perform multiple measurements of the salt concentration. To verify the fabricated 4H-SiC alpha detector possesses sufficient energy resolution, the alpha peaks of a multiple isotope alpha source can be measured. A source, which is electrodeposited onto a metal coupon and measured in a vacuum chamber, serves as a surrogate for the final source that has a geometry with the source deposited on the detector surface.

232Th is the most commonly occurring thorium isotope and is the progenitor of the naturally occurring thorium decay chain. This decay chain consists of multiple alpha emitting daughter isotopes, each with a unique alpha emission energy spectrum. The 232Th 174 decay chain consists of a wide range of alpha emission energies and half-lives. By fabricating a thin film 232Th source and measuring the resulting alpha spectrum, the ability of the 4H-SiC detector to identify various isotopes in a molten salt electrodeposited mixed nuclide source can be evaluated.

The isotopic enrichment of natural uranium to low enriched (LEU) or high enriched uranium (HEU) is fundamentally tied to proliferation concerns; thus, a thin film depleted uranium source was fabricated, and the resulting alpha spectrum measured and used to characterize the source history using 4H-SiC [37]. Gaseous diffusion and gas centrifugation uranium enrichment rely on the ratio of masses of lighter uranium isotopes to 238U. The feedstock for the enrichment process can be either natural uranium with a 235U enrichment of 0.72% or can be reprocessed uranium with variable 235U enrichment. The waste stream has a lower-than-natural 235U enrichment, usually at or below 0.2% [43]. The product is subsequently enriched in 235U along with the lighter isotopes of uranium, if they are present in the feedstock. Both natural uranium and reprocessed uranium contain trace amounts of 234U. The 234U enrichment is reduced in the depleted uranium waste product and enriched in the product stream of an enrichment facility.

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6.2 Electrodeposited Source Fabrication of Forensic Samples

The electrodeposited sources used for the nuclear forensic analysis were identical to those used for examination of the effects of a thick source geometry. As described earlier, the electrodeposition process allows for elemental selectivity when depositing actinides onto a substrate to form a source. This is useful for alpha spectrometry measurements as often the daughter actinide alpha activity in a chain decay can obscure the isotopic activities of interest. The sources were fabricated with a 1 μm actinide thickness. As will be discussed later in this chapter, the sources were thicker than optimal thickness for nuclear forensic applications. However, they serve as an initial proof-of-concept. The work in this section includes material published in the Journal of Radioanalytical and Nuclear Chemistry [44].

6.3 Results and Analysis of Electrodeposited U and Th

Thorium Daughter Identification

Figure 89 shows the 232Th source spectrum measured over a 68-hour collection period. The alpha emitting daughter nuclei of the 232Th decay series with initial emission energies below roughly 6.4 MeV, as well as 230Th, are identified in the figure below. The sharp cutoff of the spectrum for each isotope, with emission energies below 6.4 MeV, indicates the energies of alpha particles emitted from the surface of the source. For alpha particles with energies greater than 6.4 MeV, the energy loss is only partial, leading to a lack of high energy counts. The presence of 230Th, a daughter of the 238U decay chain, indicates that the

176 thorium used for electrodeposition may have been partially comprised of thorium which was chemically separated from a uranium bearing mineral such as uranothorite [45].

Figure 89. Alpha particle spectrum from an electroplated 232Th source collected using a 4H-SiC detector with a 21 μm depleted region. Daughter nuclides of the thorium decay series are evident. However, due to the limited depleted region thickness, only those nuclei with initial emission energies at or below approximately 6.4 MeV can be fully attenuated.

Depleted Uranium Source Spectrum

The uranium source spectrum shown in Figure 90 was measured using the same detector as used in the thorium source measurement with a 21 μm charge depletion region. Figure

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90 shows the resulting spectrum, when the uranium source was measured for 72 hours. Due to the enrichment process in which 234U and 235U are separated from 238U, the 234U concentration in the depleted uranium source is out of equilibrium with 238U [8]. The activity ratios of the various isotopes of uranium provide information about the enrichment facility for the depleted uranium. The absence of 232U and 236U indicate that the enrichment feedstock was likely not reprocessed uranium. The overlap of the 234U and 238U peaks is due to the peak broadening caused by self-shielding in the source. This overlap obscures the 235U peak.

The measured activity ratio of 234U to 238U was found to be disrupted from the expected value of AU-234/AU-238 ≅ 0.99 and is shown in Equation (26). The observed disruption from a natural 234U to 238U activity ratio indicates that the enrichment process was not Atomic

Vapor Laser Isotope Separation (AVLIS). Waste depleted uranium from AVLIS enrichment would result in no disruption from the natural 234U to 238U activity ratio, because only the 235U is removed from the feedstock and enriched in the product stream

[46]. This means that the uranium was enriched by either the gaseous diffusion or the gaseous centrifugation process.

퐴234 푈 ≅ 0.52 (26) 퐴238푈

235U Product Enrichment Determination

Energy resolution of 4H-SiC Schottky alpha detectors has been demonstrated below 0.94% energy resolution [47]. For sufficiently thin sources, the 238U and 234U peaks do not obscure the 235U peak. Figure 91 shows the computed alpha spectrum of a depleted uranium source that is thin enough to avoid self-shielding effects.

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Figure 91. The computed alpha spectrum of a thin depleted uranium source with a 234U enrichment of 0.00286 at.% measured with a 0.94% energy resolution detector

235 234 A linear relationship between the U and U enrichment, NP235 and NP234 respectively, in the product can be demonstrated from calculated enrichments using M* code [46].

Figure 92 shows an example of this relationship for gaseous diffusion and centrifugation, respectively, across three enrichments; natural uranium, low enriched uranium (LEU), and high enriched uranium (HEU). Equation (27) shows the relationship between the 235U enrichment and the resulting 234U enrichment for the gaseous diffusion process. Equation

(28) shows the corresponding for the gaseous centrifugation process.

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0.01 ) NP234 = 1.00740E-02NP235 - 2.20760E-05

R² = .999 P234 0.008 Centrifuge

Diffusion 0.006 Linear (Centrifuge) 0.004

NP234= 9.58803E-03NP235 - 1.84397E-05

0.002 R² = .999 U Product Enrichment ProductEnrichment U (N

234 0 0 0.2 0.4 0.6 0.8 1 235 U Product Enrichment (NP235)

Figure 92. Calculated relationship between 234U and 235U enrichment in product stream from gaseous diffusion and gaseous centrifugation, respectively. As 235U enrichment increases, the enrichment of 234U also increases in a linear manner and the enrichments from the two methods diverge.

−2 −5 푁푃234 = 1.0074 × ⁡10 푁푃235 − 2.2076 × 10 (27)

−3 −5 푁푃234 ⁡ = ⁡9.58803 ×⁡10 ⁡푁푃235 ⁡− ⁡1.84397 ×⁡10 (28)

234 235 The observed enrichments NW234 and NW235 for U and U in the depleted uranium, respectively, can be measured. By applying the standard mass balance equations for enrichment for each isotope, as shown in Equation (29), where F, W, and P are the feedstock, waste, and product in number of atoms, respectively, and assuming a natural uranium feedstock, a relationship between NP235, NW234, and NW235 can be determined.

Equation (31) can be determined by substitution of Equation (30) into Equation (29). In

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234 235 Equation 5, NF234, and NF235 are the feedstock U and U enrichments. Equation (32) shows the relationship between NP235, NW234, and NW235.

퐹푁퐹 = 푃푁푃 + 푊푁푊 (29)

푊 = 퐹 − 푃 (30)

푁 − 푁 푁 − 푁 푃235 푊235 = 푃234 푊234 (31) 푁퐹235 − 푁푊235 푁퐹234 − 푁푊234

훽(푁퐹235 − 푁푊235) − (푁푊234)(푁퐹235 − 푁푊235) 푁푃235 = ( + 푁푊235) 푁퐹234 − 푁푊234 (32) 푁 − 푁 ∗ ( 퐹234 푊234 ) (푁퐹234 − 푁푊234) − 훾(푁퐹235 − 푁푊235)(푁퐹234 − 푁푊234)

Equation (31), along with Equation (27) or Equation (28), allow for the calculation of NP235, given known values for NF234, NF235, NW234, NW235, and the constants β and γ for either gaseous diffusion or gaseous centrifuge enrichment. The constant γ is the reciprocal of the slope of the linear relationship shown in Figure 92 while β is the intercept of the same relationship multiplied by γ.

Table 4 and Table 5 were generated by applying Equation 6 to the observed depleted uranium spectrum from Figure 4 and assuming a range of waste enrichments NW235 from

235 natural product enrichment (i.e., no enrichment, NP235 = 0.72%) to a U waste enrichment corresponding to NP235 greater than 90% (NW235 < 0.18% ). The measured NW234 was found to be 0.00286 at.% and was used in Equation (32) to generate Table 4 and Table 5.

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Table 4. Product 235U enrichment from plausible waste stream 235U enrichments from gaseous diffusion with NW234=0.00286 at.%

NW235 (%) Predicted NP235 (%)

0.18 99.40

0.33 54.67

0.52 21.88

0.69 3.34

0.72 0.72

Table 5. Product 235U enrichment from plausible waste stream 235U enrichments from gaseous centrifugation with NW234=0.00286 at.%

NW235 (%) Predicted NP235 (%)

0.18 92.53

0.33 49.54

0.52 19.48

0.69 3.020

0.72 0.720

By analyzing Table 4 and Table 5, it can be determined that the depleted uranium source is likely to have come from an HEU enrichment process. This is supported by the observation that U.S. Department of Energy depleted uranium typically falls below 0.3%

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235U enrichment and is generally found to be closer to 0.2% enrichment. Given the common enrichment levels of products from enrichment facilities in the U.S., Table 4 and

Table 5 indicate that the enrichment product for this depleted uranium was likely greater than 90% enriched with a waste 235U enrichment less than or equaling 0.18%. The gaseous diffusion and gaseous centrifugation model indicate an HEU product.

6.4 Conclusions from Alpha Spectrometry

The ability to resolve multiple isotopes in a mixed nuclide source created using electrodeposition in a molten salt has been shown for a 4H-SiC detector. Due to the limited thickness of the charge depleted region within the detector volume, high energy isotopes above approximately 6.4 MeV initial emission energy are not discernible. Additionally, the detector FWHM of 0.94% limits the capability to discern closely grouped isotopes. The identification of 230Th in the electrodeposited source as an indicator of the material history suggests that 4H-SiC is capable of alpha spectrometry enabled nuclear forensics.

Supporting this conclusion is the ability to detect the deviation from natural uranium composition in the uranium source, specifically the deviation from equilibrium 234U activity. A predictive model of the product 235U concentration of an enrichment process by analysis of the waste 234U and 235U enrichments was developed using mass balance. Due to peak broadening, the 235U waste enrichment was not discernible. By populating a table with common and plausible values for the waste 235U enrichment and comparing with the observed waste 234U enrichment an analysis indicating that the depleted uranium was sourced from a facility that produces HEU was shown. 184

Chapter 7. Discussion and Conclusions

7.1 Discussion

This chapter discusses the results presented in the previous chapters and presents the conclusions drawn from the data collected in the experiments previously described.

The experiments previously described have focused primarily on the development of a 4H-

SiC alpha detector capable of operating in an elevated temperature environment that is corrosive and contains a strong radiation field. The results of the experiments carried out demonstrate the capability of the detectors to operate at elevated temperatures. The leakage current of the devices, which has so far been dominated by the surface leakage current term, has been addressed in several ways. The addition of a double Schottky contact structure has allowed for improved leakage current by minimizing contamination during fabrication which may lead to a lowering of the barrier height and serves as a secondary blocking contact. The addition of a passivation layer has further reduced leakage current by increasing the resistivity of the detector surface.

The detector packaging which was designed and constructed for this work has demonstrated a proof-of-concept level of functionality but needs further refinement. The leakage current between the central and outer conductor of the device needs to be reduced through improved assembly techniques that reduce the exposure of MgO to moisture and the possible substitution of MgO for a higher resistivity insulator. The addition of the passivation layer to the surface of the detector may aid in the reduction of leakage current effects which degrade the current packaged detector energy resolution.

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7.2 Future Work

This section describes the work which would most readily move the sensor technology toward implementation in an industrial scale process. The future work is categorized as dealing with the 4H-SiC detector design itself and how the design might be improved upon for general alpha spectrometry, the developments of the detector required for application to monitoring the pyroprocessing environment, and finally the required detector design alterations and experiments for nuclear forensic applications.

7.2.1 General Detector Design Improvements

Future detectors would likely benefit from the addition of a guard ring structure as explored in this work. An increase in the thickness of the separation layer between the Schottky contact and the guard ring would likely simplify the metal lift-off process. Currently, there is a high probability of the metal lift-off stage to result in a short-circuit between the guard ring and the Schottky contact. This provides at best an increased Schottky contact surface but more probably leads to an elimination of the guard ring leakage current blocking ability.

If a sacrificial power supply is in use, this may result in total degradation of the detector signal. As a result, it is important to prevent electrical contact between the guard ring and

Schottky contact.

Reducing the epitaxial Schottky contact thickness will reduce the energy attenuation of the incident radiation as it passes through the dead layer of the detector. Thinner contact thicknesses have been shown to form sufficient rectifying contacts with 4H-SiC.

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However, for the application of submersion in a molten salt for electrodeposition and alpha spectroscopy measurements, the thickness of the contact must be optimized to prevent diffusion of ions into the metal-semiconductor interface.

The inclusion of an SiO2 passivation layer reduces the leakage current of the device under reverse bias by at least an order of magnitude. The addition of a passivation layer on the bulk side of the detector may further reduce the leakage current of the device by blocking surface leakage current effects. Alternatively, the additional passivation may allow for larger contact size detectors with acceptable (nA to pA range) leakage currents.

Since silicon nitride (SiN) has a greater resistivity than SiO2, the substitution of SiN for

SiO2 for the passivation layer may result in reduced leakage current by more efficiently blocking the surface leakage current.

7.2.3 Pyroprocessing Applications

To develop the 4H-SiC detector for alpha spectroscopy in the pyroprocessing environment, testing with a high gamma background at 500oC in a molten salt is required. This testing will more closely simulate the environment present inside the electrorefiner.

Testing should ideally include RIV measurements to characterize the leakage current of the device at 500oC while in the argon headspace. The RIV testing can be performed in the argon headspace to give an indication of detector leakage current during spectrum collection. A testing procedure consisting of submersion of the detector into a molten salt, followed by withdrawal to an argon headspace and RIV measurements would provide electrical characterization of the detector in a simulated pyroprocessing environment. The 187 headspace would be subject to a similar high intensity gamma background to that found in the salt and should be replicated in testing if possible.

Additionally, the alpha spectroscopy performance under repeated electrodeposition onto the surface of the detector must be investigated. The ability to drive the actinides off of the detector surface between alpha spectrum measurements must be investigated. Any hysteresis resulting from previous electrodepositions should be characterized such that the alpha spectra collected can be corrected for residual electrodeposited material.

7.2.3 Nuclear Forensic Applications

Improving the energy resolution of the detectors would provide many possible new avenues for nuclear forensic applications. Currently, the energy resolution of the 4H-SiC detectors developed in this work is limited to approximately 0.9% FWHM. As a result, the closely spaced alpha peaks from actinides of interest to nuclear forensic applications are currently not fully resolvable (i.e. the alpha spectra from 234U resolved from the 235U alpha emission). With improved energy resolution, the ability for the detectors to resolve the actinide concentrations of closely spaced alpha peaks would provide a means to identify the isotopic concentrations of materials of interest to nuclear forensics.

The deposition of actinides directly to the surface of sensing detectors should be further explored to provide a rapid method of source-on-detector measurement of materials of interest. Given the wide range of materials soluble in molten salt, the rapid fabrication of a 188 measurable source which eliminates the need for vacuum chamber equipment for measurement is of interest for nuclear forensic assays.

Additionally, the exploration of the response of 4H-SiC Schottky detectors to fission fragments should be explored. The response of the detectors to fission fragments has been simplistically modelled in this work but a more rigorous model must be developed.

Determination of the detector response to fission fragment irradiation would provide information concerning the 240Pu concentration in an electrodeposited actinide source and the relative concentration of multiple plutonium isotopes in a material sample.

7.3 Conclusions

A method of detector fabrication has been developed which has demonstrated an order of magnitude improvement of reverse bias leakage current. This method makes use of a SiO2 passivation layer on the epitaxial surface of the detector. Additionally, the application of a guard ring has been explored. Detectors fabricated in this method present energy resolutions below 0.9% FWHM at room temperature and energy resolutions less than 2.2%

FWHM at 500oC.

The spectrum of a mixed nuclide electrodeposited source was modeled using MATLAB code. The concentration of isotopes was modeled after UNF from a PWR as an approximation of the feedstock for a pyroprocessing system. The resulting modeled spectra were generated with an assumed detector energy resolution ranging from an idealized

0.01% to the demonstrated 500oC energy resolution of 2.2%. In combination with fission

189 fragment identification, which was modeled using SRIM 2013, the energy resolution of

1%, corresponding to a 200oC operating temperature was shown to be sufficient to identify the alpha peaks of a mixed nuclide source containing several actinides of interest to nonproliferation and safeguards concerns. Additionally, a method was developed to determine the source thickness of an electrodeposited alpha source using a partially depleted 4H-SiC alpha detector. By using the SRIM 2013 software to generate a differential energy loss curve for a range of alpha particle energies, the thickness of an alpha source was determined based on the detector response of a partially depleted detector.

As has been shown in this work, 4H-SiC Schottky contact alpha detectors have been develop which can survive in a 500oC molten LiCl-KCl salt for extended periods of time with minor performance degradation. Additionally, a packaging has been developed which allows for the electrodeposition of actinides from a molten salt to the surface of a 4H-SiC detector. The detector packaging consists of a stainless-steel housing, ceramic adhesive, and MI cables suitable for use in the molten salt environment. The leakage current from the central conductor of the MI cable to the outer casing was found to be excessive. As a result, a more resistive insulation is required between the central and outer conductor. An alpha spectrum was collected from a mixed nuclide electrodeposited source which was deposited onto the surface of the measuring detector.

A nuclear forensic technique has been developed using electrodeposited sources from a molten salt in combination with a 4H-SiC detector. This technique makes use of the solubility of actinic salts in molten LiCl-KCl eutectic salt and electrodeposition to fabricate

190 alpha sources for spectroscopic analysis. The atomic ratio of 234U and 235U to 238U in a depleted uranium source fabricated in this manner can potentially provide information about the enrichment technique from which the depleted uranium was generated. 235U concentrations were absent in these measurements requiring bounding estimates based on knowledge of common enrichment practices. Similarly, a thorium source was fabricated in the same way and analyzed through alpha spectrometry with a 4H-SiC alpha detector. The resulting analysis provides insight as to the geological source of the thorium, likely uranothorite, due to the presence of 230Th in the alpha spectrum.

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