Helium Burning of 22Ne: Targets, Detectors, and Initial Measurements
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HELIUM BURNING OF 22NE: TARGETS, DETECTORS, AND INITIAL MEASUREMENTS Sean Hunt A dissertation submitted to the faculty of the University of North Carolina at Chapel Hill in partial fulfillment of the requirements for the degree of Doctor of Philosophy in the Department of Physics and Astronomy. Chapel Hill 2019 Approved by: Christian Iliadis Art Champagne Tom Clegg Richard Longland Fabian Heitsch c 2019 Sean Hunt ALL RIGHTS RESERVED ii ABSTRACT SEAN HUNT: Helium Burning of 22Ne: Targets, Detectors, and Initial Measurements. (Under the direction of Christian Iliadis) The goal of the work in this dissertation is to advance the understanding of nuclear astrophysics by remeasuring a reaction vital to the stellar synthesis of heavy elements. The 22Ne(↵,γ)26Mg and 22Ne(↵,n)25Mg reactions are of great importance to the s-process which takes place in asymptotic giant branch and massive stars, and produces half of all elements heavier than iron. The low energy resonances in these reactions have the greatest impact on the s-process, but have poorly understood resonance strengths. The ↵-particle beam necessary to measure the 22Ne(↵,n)25Mg and 22Ne(↵,γ)26Mg reac- tions are very damaging to solid targets. In this study, three types of targets made of glass, sintered metal, and evaporated metal were designed and fabricated. All three targets are capable of withstanding the effects of ↵-particle bombardment. Previous measurements of the 22Ne(↵,n)25Mg reaction saw significant contamination from the 13C(↵,n)16O reaction. A significant effort was made in this dissertation to de- velop new capture-gated detector technologies to differentiate between low energy neu- trons of interest and high energy contaminant neutrons. Structures in the signal pulses were identified, correcting a misunderstanding in the literature, and never-before-made measurements of detector resolution and efficiency were performed. Finally, a measurement of the resonance strength of the 828 keV resonance in the 22Ne(↵,γ)26Mg reaction was made using one of the new target designs. This measurement represents the first of this reaction using a coincidence detector system, the first time this reaction has been measured using a fraction fitting analysis technique, and the first time a nuclear reaction has been measured using blister-resistant solid targets. The value of the resonance strength for the 828 keV resonance determined in this work is 0.047(12) meV. iii iv TABLE OF CONTENTS LIST OF TABLES . viii LIST OF FIGURES . ix LIST OF ABBREVIATIONS . xii 1 Introduction . 1 1.1 Astrophysical Motivation . 1 1.2 The s-Process . 2 1.2.1 The Weak s-Process in High Mass Stars . 3 1.3 The Main s-Process in AGB Stars . 4 1.4 The Importance of 22Ne(↵,n)25Mg......................... 7 1.5 This Study . 8 2 Nuclear Reactions . 10 2.1 Thermonuclear Reaction Rates . 10 2.2 Resonant Contributions . 12 2.2.1 Narrow Resonances . 13 2.2.2 Resonances in 22Ne(↵,n)25Mg and 22Ne(↵,γ)26Mg . 15 2.3 Non-Resonant Contributions . 18 3 Accelerators . 20 3.1 JN Van de Graaff Accelerator . 20 3.1.1 JN Test Stand . 22 3.1.2 JN Modifications . 23 3.2 Eaton Ion Implanter . 24 3.3 Electron Cyclotron Resonance (ECR) Source . 26 4 Targets . 28 v 4.1 Preparation of Target Backings . 28 4.1.1 Wet Etching . 29 4.1.2 Resistive Heating . 30 4.1.3 Dose Calculations . 30 4.2 22Ne Implantation . 31 4.3 Carbon Contamination . 32 4.3.1 Piranha Solution Cleaning . 33 4.3.2 Pentane Cleaning . 34 4.3.3 O2 Plasma Cleaning . 35 4.3.4 Carbon Comparison . 35 4.4 Target Blistering . 37 4.5 Blister-Resistant Targets . 40 4.5.1 Fused Silica Targets . 43 4.5.2 Sintered Metal Targets . 46 4.5.3 Porous Metal Targets . 49 4.5.4 Atomic Force Microscopy . 50 4.6 Target for 22Ne(↵,γ)26Mg Measurement . 52 5 Detectors . 54 5.1 γ-ray Detector System . 54 5.2 Coincidence Electronics . 56 5.3 HPGe Efficiency and Resolution . 57 5.4 BC-523A Detector . 58 5.4.1 Experimental Setup . 61 5.4.2 PSD and the Capture Event . 62 5.4.3 Timing and Gating Tests . 64 vi 5.4.4 Energy Resolution Measurements . 67 5.4.5 Efficiency . 69 5.5 INVS Detector . 71 5.5.1 INVS Energy Resolution . 72 5.5.2 INVS Simulated and Previously Measured Efficiency . 74 5.5.3 INVS Electronics . 75 6 Measurement of the 828 keV Resonance in the 22Ne(↵,γ)26Mg Reaction . 77 6.1 Preface . 77 6.2 Experimental Procedure . 79 6.3 Fraction Fitting . 80 6.4 Targets and Target Degradation . 81 6.5 Coincidence Efficiency . 86 6.6 Analysis . 88 6.7 Resonance Strength Results . 90 6.8 Excitation Energy . 92 7 Conclusions . 94 APPENDIX A: DOPPLER EFFECTS . 97 APPENDIX B: EXCITATION ENERGIES . 100 APPENDIX C: BRANCHING RATIOS . 101 BIBLIOGRAPHY . 103 vii LIST OF TABLES Table 2.1 Past Results . 17 Table 3.1 Analyzing Magnet Calibration Reactions . 22 Table 3.2 JN Hydrogen/Helium Parameters . 24 Table 4.1 Implantation Settings . 32 Table 4.2 Blister-Resistant Target Summary . 50 Table 5.1 HPGe Dimensions . 55 Table 5.2 INVS Dimensions . 72 Table 6.1 Level Density Near Excitation . 78 Table 6.2 Past Results . 82 Table 6.3 Reference Resonance Parameters . 84 Table 6.4 Stoichiometries and Effective Stopping Powers . 85 Table 6.5 ↵ Stopping Powers . 85 Table 6.6 Branching Ratio Comparison . 88 Table 6.7 Resonance Strength Parameters . 90 Table 6.8 831 keV Resonance Measurement Results . 91 Table 6.9 Parameters for Excitation Energy . 92 Table 7.1 Parameters for Excitation Energy of Primary 1 . 100 Table 7.2 479 keV Resonance Branching Ratios . 102 viii LIST OF FIGURES Figure 1.1 Binding Energy . 1 Figure 1.2 Solar Abundances . 2 Figure 1.3 S-Process Network . 3 Figure 1.4 Betelgeuse . 4 Figure 1.5 Mira . 4 Figure 1.6 Hertzsprung-Russell Diagram . 5 Figure 1.7 Thermal Pulse . 6 Figure 2.1 Cross Section Illustration . 12 Figure 2.2 22Ne(↵,n)25Mg and 22Ne(↵,n)25Mg Excitation Functions . 15 Figure 2.3 22Ne(↵,n)25Mg and 22Ne(↵,n)25Mg Contributions . ..