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Neutron Activation Analyses and Half-Life Measurements at the Usgs Triga Reactor

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Neutron Activation Analyses and Half-Life Measurements at the Usgs Triga Reactor NEUTRON ACTIVATION ANALYSES AND HALF-LIFE MEASUREMENTS AT THE USGS TRIGA REACTOR by Robert E. Larson A thesis submitted to the Faculty and the Board of Trustees of the Colorado School of Mines in partial fulfillment of the requirements for the degree of Doctor of Philosophy (Applied Physics). Golden, Colorado Date Signed: Robert E. Larson Signed: Dr. Uwe Greife Thesis Advisor Golden, Colorado Date Signed: Dr. Thomas E. Furtak Professor and Head Department of Physics ii ABSTRACT Neutron activation of materials followed by gamma spectroscopy using high-purity ger- manium detectors is an e↵ective method for making measurements of nuclear beta decay half-lives and for detecting trace amounts of elements present in materials. This research explores applications of neutron activation analysis (NAA) in two parts. Part 1. High Precision Methods for Measuring Decay Half-Lives, Chapters 1 through 8 Part one develops research methods and data analysis techniques for making high pre- cision measurements of nuclear beta decay half-lives. The change in the electron capture half-life of 51Cr in pure chromium versus chromium mixed in a gold lattice structure is ex- plored, and the 97Ru electron capture decay half-life are compared for ruthenium in a pure crystal versus ruthenium in a rutile oxide state, RuO2.Inaddition,thebeta-minusdecay half-life of 71mZn is measured and compared with new high precision findings. Density Func- tional Theory is used to explain the measured magnitude of changes in electron capture half-life from changes in the surrounding lattice electron configuration. Part 2. Debris Collection Nuclear Diagnostic at the National Ignition Facility, Chapters 9 through 11 Part two explores the design and development of a solid debris collector for use as a diagnostic tool at the National Ignition Facility (NIF). NAA measurements are performed on NIF post-shot debris collected on witness plates in the NIF chamber. In this application NAA is used to detect and quantify the amount of trace amounts of gold from the hohlraum and germanium from the pellet present in the debris collected after a NIF shot. The design of asoliddebriscollectorbasedonmaterialx-rayablationpropertiesisgiven,andcalculations are done to predict performance and results for the collection and measurements of trace amounts of gold and germanium from dissociated hohlraum debris. iii TABLE OF CONTENTS ABSTRACT ......................................... iii LIST OF FIGURES . xii LISTOFTABLES......................................xix LISTOFSYMBOLS.....................................xx LIST OF ABBREVIATIONS . xxi ACKNOWLEDGMENTS . xxii DEDICATION . xxiii CHAPTER 1 THE DEVELOPMENT OF HIGH PRECISION METHODS FOR MEASURING BETA DECAY HALF-LIFE . 1 1.1 Nuclear Beta Decay . 2 1.1.1 Electron Capture Beta Decay . 2 1.2 Measurement Campaigns . 3 1.3 Theoretical Descriptions . 4 1.4 NeutronActivationLaboratory . .4 1.5 Summary of Chapters for Part 1 . 5 CHAPTER 2 PREVIOUS RESEARCH INTO CHANGES IN ELECTRON CAPTURE HALF-LIFE . 6 2.1 Earliest Research . 6 2.2 Later Research . 8 2.3 Recent Measurements . 9 2.4 Recent High Z Research, a Motivation for this Research . 10 iv CHAPTER 3 DATA COLLECTION METHODS . 12 3.1 Decay Half-life Measurement Apparatus . 12 3.1.1 The USGS TRIGA Reactor . 12 3.1.2 ORTEC HPGe Detectors . 13 3.1.3 MAESTRO Software . 15 3.1.4 PC Clock Synchronization . 15 3.2 Pure Chromium Campaign . 15 3.2.1 51Cr Decay Scheme . 15 3.2.2 Chromium Sample Preparation . 17 3.2.3 Irradiation and Decay Count Spectrum . 17 3.3 Gold/Chromium Campaign . 17 3.3.1 Sample Preparation for Sputtering . 17 3.3.2 Irradiation and Decay Spectrum . 23 3.3.3 Mass from Activity Analysis . 26 3.4 Ruthenium Crystal and Ruthenium Oxide Campaigns . 26 3.4.1 Electron Capture Decay Scheme of 97Ru . 26 3.4.2 Irradiation and Decay Count Spectra . 26 3.4.3 Ruthenium Crystal . 28 3.4.4 Ruthenium Oxide . 30 3.5 Zinc Campaign . 32 3.5.1 Decay Scheme of 71Zn ...........................33 3.5.2 Irradiation and Decay Count Spectrum . 33 CHAPTER 4 HALF-LIFE ANALYSIS METHODS . 37 v 4.1 Radioactive Decay Equation . 37 4.1.1 Linearization Method Not Adequate for High Precision . 37 4.2 Protocol for Uncertainty Assessment of Half-lives . 38 4.2.1 Uncertainty Deviation Models . 38 4.3 Data Processing Methods . 39 4.3.1 Half-life Data Collection using ORTEC MAESTRO Software . 39 4.3.2 Method for Analyzing Gamma-ray Spectra Using gf3 . 41 4.3.3 RStudio, a Statistical Computational Tool . 44 4.4 Data Analysis Methods . 44 4.4.1 Autocorrelation as a Data Acceptance Criterion . 45 4.4.2 The Breusch-Godfrey Autocorrelation Test . 46 4.4.3 Simulated Data Used as a Reference for Statistical Consistency . 47 4.4.4 Half-life Measurements and Uncertainty Analysis . 49 4.4.5 Examining the Data for Possible Medium Term Periodic Patterns . 50 4.4.6 Systematic Error Estimation . 50 4.5 Dead Time Measurement Studies Using a Precision Pulse Generator . 54 CHAPTER 5 RESULTS OF ELECTRON CAPTURE HALF-LIFE MEASUREMENTS . 58 5.1 Pure Chromium 51Cr Results . 58 5.1.1 Pure Chromium 51Cr Decay Count Plot . 58 5.1.2 Half-life Values for Full Pure Chromium Campaign . 59 5.1.3 Breusch-Godfrey Plot for Pure Chromium Data . 59 5.1.4 Half-life Results for Pure Chromium for the Accepted Data . 61 vi 5.1.5 Estimate of Systematic Standard Error for the Pure Chromium Measurement . 64 5.1.6 Pure Chromium Campaign Data Judged to be Spurious . 64 5.2 51Cr in a Gold Lattice: AuCr Results . 66 5.2.1 AuCr 51Cr Decay Count Plot . 66 5.2.2 Breusch-Godfrey Plot for Gold Chromium . 67 5.2.3 Half-life Results for AuCr for the Accepted Data . 67 5.2.4 Estimate of Systematic Standard Error for the Gold Chromium Measurement . 70 5.2.5 198Au Half-life Measurement . 72 5.2.6 Comparison with Established Chromium Results . 74 5.2.7 Comparison to Previous 51Cr Measurements . 76 5.3 Ruthenium Crystal Results . 78 5.3.1 Ruthenium Crystal 97Ru Decay Count Plot . 78 5.3.2 Ru Crystal Half-life Values . 79 5.3.3 Breusch-Godfrey Plot for the Ruthenium Crystal . 79 5.3.4 Half-life Results for the Ruthenium Crystal Campaign . 79 5.3.5 Estimate of Systematic Standard Error: Ruthenium Crystal Measurement . 82 5.3.6 Comparison to Previous 97Ru Measurements . 84 5.3.7 Results for 103Ru 497 keV Line . 84 5.4 Ruthenium Oxide Results . 89 5.4.1 Ruthenium Oxide 97Ru Decay Count Plot . 89 5.4.2 Ru Oxide Half-life Values . 91 vii 5.4.3 Breusch-Godfrey Plot for Ruthenium Oxide . 91 5.4.4 Half-life Results for the Ruthenium Oxide Campaign . 91 5.4.5 Estimate of Systematic Standard Error for the Ruthenium Oxide Measurement . 95 5.4.6 Comparison with Ruthenium Crystal Results . 95 CHAPTER 6 RESULTS OF ZINC MEASUREMENTS . 98 6.1 Enriched 71mZn Decay Count Plots . 99 6.2 Half-life Plots for 71mZn Lines . 99 6.3 Breusch-Godfrey 71mZn Plots . 102 6.4 Half-life Results for the Zinc Campaign . 104 6.4.1 Residual and Autocorrelation Plots . 107 6.4.2 Estimate of Systematic Error for the Zinc Data . 111 6.4.3 WeightedAverage............................. 112 6.5 Comparison to Previous 71mZn Measurements . 114 CHAPTER 7 THEORETICAL ANALYSIS OF CHANGES IN ELECTRON CAPTURE DECAY HALF-LIFE . 116 7.1 Theoretical Approaches . 116 7.1.1 Thomas-Fermi Theory . 116 7.1.2 Hartree-Fock-Slater Method . 117 7.1.3 Density Functional Theory . 118 7.2 Density Functional Theory Calculations . 119 7.2.1 The WIEN2k Program Package for DFT Calculations of Solids . 119 7.2.2 Linear Relationship Between Nuclear Electron Density and Half-life . 120 7.2.3 The Location of the Chromium Atoms in the Gold Lattice . 121 viii 7.2.4 Chromium in Gold Lattice Calculation . 122 7.2.5 AuCr Comparison to Theoretical Analysis . 123 7.2.6 Ruthenium Oxide Calculation . 124 7.2.7 Ruthenium Comparison to Theoretical Analysis . 125 7.2.8 WIEN2k Results . 125 CHAPTER 8 PART 1 SUMMARY AND CONCLUSIONS HIGH PRECISION HALF-LIFE MEASUREMENTS ....................................... 126 8.1 Research Motivation . 126 8.2 Detector Dead Time . 127 8.3 51Cr in Gold Lattice . 129 8.4 97Ru in Ruthenium Oxide . 131 8.5 71mZn Beta-minus Decay Half-life Measurement . 131 8.6 Overall Conclusions, Part 1 . 132 8.7 Areas of Continuing Research . 133 8.7.1 Changes in Electron Capture Half-life from Electronic Structure Compression.
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