β-DELAYED NEUTRON EMISSION STUDIES OF NEUTRON-RICH PALLADIUM AND SILVER ISOTOPES A Dissertation Submitted to the Graduate School of the University of Notre Dame in Partial Fulfillment of the Requirements for the Degree of Doctor of Philosophy by Karl Isaac Smith Michael C.F. Wiescher, Director Graduate Program in Physics Notre Dame, Indiana April 2014 © Copyright by Karl Isaac Smith 2014 All Rights Reserved β-DELAYED NEUTRON EMISSION STUDIES OF NEUTRON-RICH PALLADIUM AND SILVER ISOTOPES Abstract by Karl Isaac Smith The rapid-neutron capture process (r-process) is attributed as the source of nearly half the elements heavier than iron in the solar system. The astrophysical scenario responsible is still unknown, making the specific astrophysical conditions uncertain. To gain insight into the r-process nucleosynthesis, uncertainties in the nuclear prop- erties of the involved isotopes must be minimized. The r-process path traverses nuclei far from stability through many isotopes which have not yet been produced in the laboratory. This forces astrophysical models to rely heavily upon theoretical models of the involved nuclear physics. To help constrain parameters used in both astro- physical and nuclear models, an experiment was performed to measure properties of neutron-rich nuclei just below the N = 82 shell closure. These nuclei are believed to be responsible for production of the A = 130 peak in the solar r-process abundance pattern. The first measurements of β-decay half-lives and neutron branching ratios, Pn values, were performed for neutron-rich isotopes of palladium and silver. To Kristen. I’d be nowhere without you. ii CONTENTS FIGURES . .v TABLES . vii ACKNOWLEDGMENTS . viii CHAPTER 1: INTRODUCTION . .1 1.1 Nuclear Astrophysics Overview . .1 1.2 Big Bang Nucleosynthesis . .3 1.3 Stellar Fusion . .3 1.3.1 Hydrogen Burning . .4 1.3.1.1 p-p Chain . .4 1.3.1.2 CNO Cycle . .4 1.3.2 Helium Burning . .5 1.3.3 Heavy Element Fusion . .7 1.4 Neutron Capture Processes . .8 1.4.1 Slow Neutron Capture Process (s-Process) . .9 1.4.2 Rapid Neutron Capture Process (r-Process) . 15 1.4.2.1 Classical r-Process . 16 1.4.2.2 Astrophysical Site of the r-Process . 21 1.4.2.3 Nuclear Parameters Relevent to the r-Process . 25 1.5 Other Processes . 28 1.5.1 p-Process . 28 1.5.2 Rapid Proton Capture Process . 29 CHAPTER 2: INVESTIGATION OF β-DELAYED NEUTRON EMISSION PROBABILITIES AND HALF-LIVES OF PD AND AG ISOTOPES NEAR N=82 ...................................... 31 2.1 Introduction . 31 iii 2.2 Experimental Setup . 32 2.2.1 FRagment Separator (FRS) . 33 2.2.1.1 Atomic Charge Determination . 37 2.2.1.2 Mass Determination . 43 2.2.1.3 Particle Identification . 48 2.2.1.4 Isomer Tagging . 53 2.2.2 Silicon IMplantation Beta Absorber (SIMBA) . 56 2.2.3 BEta-deLayEd Neutron (BELEN) Detector . 64 2.2.4 Triggering Scheme . 69 CHAPTER 3: ANALYSIS . 73 3.1 Event Identification . 73 3.2 Implant-Decay Correlation . 76 3.2.1 β-Decay Neutron Emission Correlation . 78 3.3 Background Estimation . 79 3.4 Kernel Density Estimation . 82 3.5 Differential Decay Solution . 86 3.6 Half Life Analysis . 91 3.6.1 124Pd Half-Life . 103 3.6.2 129Ag Half-Life . 106 3.7 Detection Limits . 106 3.8 Determination of Neutron Branching Probabilities . 108 CHAPTER 4: R-PROCESS IMPLICATIONS . 118 4.1 High Entropy Wind Model . 118 4.2 Conclusion . 125 BIBLIOGRAPHY . 127 iv FIGURES 1.1 Solar Abundances . .2 1.2 Nuclei Binding Energy . .7 1.3 s-Process Path . 10 1.4 Neutron Separation Energies as Function of Neutron Number . 11 1.5 r-Process Path . 20 1.6 Effect of Shell Quenching on r-Process Abundances . 26 1.7 Calculated Half-Life as a Function of Quadrupole Deformation . 27 2.1 Layout of the Helmholtzzentrum für Schwerionenforschung (GSI) . 33 2.2 Fragment Separator (FRS) Schematic . 35 2.3 Energy Loss as a Function of Fragment Velocity . 39 2.4 Resolution of the Atomic Charge Determination . 41 2.5 Energy Loss after Final Focal Plane Degrader . 42 2.6 Time Projection Chamber Calibration . 44 2.7 Time of Flight Calibration . 47 2.8 Particle ID for 126Pd Setting . 50 2.9 Particle ID for 127Pd Setting . 51 2.10 Particle ID for 128Ag Setting . 52 2.11 Isomer TAGging (ITAG) System Schematic . 54 2.12 Isomer Tagger Particle ID . 55 2.13 SIMBA Schematic . 57 2.14 SIMBA Tracking Energy Dependence Calibration . 59 2.15 Linear-Logarithmic Pre-amplifier Gain Matching . 61 v 2.16 Tracking and DSSD Comparison . 62 2.17 Particle ID of Implanted Ions . 64 2.18 BELEN-30 Schematic . 65 2.19 Neutron Moderation Time Distribution . 67 2.20 Wall Effect . 68 2.21 BELEN-30 Neutron Detection Efficiency . 69 2.22 Triggering Scheme . 70 3.1 DSSD Event Energy . 75 3.2 Correlation Probability . 77 3.3 Variation in Background Estimation . 81 3.4 Kernel Density Estimator . 83 123 125 3.5 Decay Curve Fits for − Pd...................... 93 126 128 3.6 Decay Curve Fits for − Pd...................... 94 124 126 3.7 Decay Curve Fits for − Ag...................... 95 127 129 3.8 Decay Curve Fits for − Ag...................... 96 3.9 β-Detection Efficiency . 97 3.10 Half-Life Literature Comparison . 100 3.11 Even-Even Excitation Energies in the Pd Region . 104 3.12 Level Scheme of 126Sn and 128Sn..................... 105 3.13 Histogram of Deposited Energy within Correlation Volume for various Q-values . 112 3.14 Palladium Neutron Emission Probability Correction Factor . 113 3.15 Silver Neutron Emission Probability Correction Factor . 114 3.16 Neutron Branching Ratio Literature Comparison . 117 4.1 Comparison of a High Entropy Wind Model to Solar r-Process Abun- dances . 120 4.2 Impact of New Half-Life Values on r-Process Abundances . 122 4.3 Impact of 128Pd Half-Life Upper Limit on r-Process Abundances . 123 4.4 Impact of Pn Values on r-Process Abundances . 124 vi TABLES 2.1 MUSIC VELOCITY CORRECTION PARAMETERS . 40 2.2 FRS SETTING PARAMETERS AND DURATION . 49 2.3 SIMBA TRACKING ENERGY DEPENDANCE CALIBRATION PA- RAMETERS . 60 2.4 SIMBA TRACKING AND DSSD POSITION COMPARISON . 63 3.1 COMPARISON OF MEASURED HALF-LIVES TO PREVIOUS EX- PERIMENTAL VALUES . 99 3.2 COMPARISON OF MEASURED HALF-LIVES TO THEORETI- CAL CALCULATIONS . 102 3.3 NUMBER OF DETECTED EVENTS AND RESULTING NEUTRON BRANCHING RATIOS . 115 3.4 RESULTING NEUTRON BRANCHING RATIOS . 116 vii ACKNOWLEDGMENTS I would like to thank the many people who were involved in my educational career. First, Fernando Montes who was willing to entertain our long distance discussions and encouraged me throughout the difficult steps in the analysis. My advisors: Hendrik Schatz who continued to offer support throughout my educational career and provided many opportunities to continue my career, and Michael Wiescher, who facilitated the final steps in the the long analysis process of this work, your advice was always useful and appreciated. I’d like to thank the members of both Hendrik Schatz’s group, including Zach Meisel, for the many discussions around the world, Matt Amthor, for the guid- ance early in my career, and Giuseppe Lorusso, for the interesting situations the he would get us into. I’d also like to thank those participating in the JINA journal club for the unique conversations about recent publications. Also, those in Michael Wiescher’s group including James deBoer for the many lunch time discussions and Ethan Uberseder for spending time talking about technical aspects of our research. As well as the those at Notre Dame who were willing to lend a hand around the lab. I’d like to thank the members of my committee for the occasional guidance and, Ani Aprahiman, Xiao-Dong Tang, Jonathan Sapirstein, and Manoel Couder who was willing to sit on my committee on short notice. viii I’d like to thank my collaborators involved with the experimental efforts at GSI, Roger Caballero Folch for the many days spent slaving away during those hot days in Germany, Chiara Nociforo for allowing me to shadow her around the lab and showing me the ropes of the FRS, Alfredo Estrade for the assistance during the experiment, as well as the weekend adventures around Germany, and César Domingo Pardo and Iris Dillmann. Finally, I’d like to thank my parents who provided an environment which encour- aged my curiosity and allowed me to continue my studies. Last, but not least I would like to thank my wife, Kristen, who has been supportive and patient through it all. ix CHAPTER 1 INTRODUCTION 1.1 Nuclear Astrophysics Overview Nuclear astrophysics seeks to explain the formation of the elements observed in the universe. Specifically, the abundance distribution observed in our solar system. The study of the content of our sun has a long history starting with the quantum mechanical description of stellar fusion by Atkinson & Houtermans [1] in 1929 and the introduction of nucleosynthesis within the stellar evolution by Hoyle [2] in 1946. The knowledge of the period was then summarized in the seminal papers by Burbidge et al. [3] and Cameron [4] in 1957. These two papers outlined the possible stellar processes and provides the modern basis for current research in the field of nuclear astrophysics. The general trends of the solar abundance distribution, as shown in Figure 1.1, can be explained by a number of cosmic processes. During the Big Bang, the large fraction of observed hydrogen and helium was produced. Beryllium and boron were produced through cosmic spallation, while the heavier metals were produced through stellar processes.