Decay Spectroscopy of Neutron-Rich Cadmium Around the N = 82 Shell Closure

Decay Spectroscopy of Neutron-Rich Cadmium Around the N = 82 Shell Closure by Nikita Bernier B.Sc., UniversitéLaval, 2011 M.Sc., UniversitéLaval, 2013 A THESIS SUBMITTED IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY in The Faculty of Graduate and Postdoctoral Studies (Physics) THE UNIVERSITY OF BRITISH COLUMBIA (Vancouver) December 2018 c Nikita Bernier 2018 The following individuals certify that they have read, and recommend to the Faculty of Graduate and Postdoctoral Studies for acceptance, the dissertation entitled: Decay Spectroscopy of Neutron-Rich Cadmium Around the N = 82 Shell Closure submitted by Nikita Bernier in partial fulfillment of the requirements for the degree of Doctor of Philosophy in Physics. Examining Committee: Dr Reiner Krücken, Physics Supervisor Dr Colin Gay, Physics Supervisory Committee Member Dr Janis McKenna, Physics University Examiner Dr Chris Orvig, Chemistry University Examiner Additional Supervisory Committee Members: Dr Sonia Bacca, Physics Supervisory Committee Member Dr Robert Kiefl, Physics Supervisory Committee Member ii Abstract The neutron-rich cadmium isotopes (Z = 49) near the well-known magic numbers at Z = 50 and N = 82 are prime candidates to study the evolving shell structure observed in exotic nuclei. Additionally, nuclei around the doubly-magic 132Sn have been demonstrated to have direct implications for astrophysical models, leading to the r-process abundance peak at A ≈ 130 and the corresponding waiting-point nuclei around N = 82. The β-decay of the N = 82 isotope 130Cd into 130In was investigated in 2002 [1], but the information for states of the lighter indium isotope 128In is still limited. Detailed β-γ-spectroscopy of 128;131;132Cd was accomplished using the GRIFFIN [2] facility at TRIUMF. In 128In, 32 new transitions and 11 new states have been observed in addition to the four previously observed excited states [3]. The 128Cd half-life has also been remeasured via the time distribu- tion of the strongest γ-rays in the decay scheme with a higher precision [4]. For the decay of 131;132Cd, results are compared with the recent EURICA data [5, 6]. These new results are compared with recent shell model and IMSRG [7, 8, 9] calculations, which highlight the necessity to re-investigate even \well-known" decay schemes for missing transitions. iii Lay Summary The discovery of radioactivity (1896) and the atomic nucleus (1911) are fairly recent in the history of mankind, but our understanding of the nucleus has advanced rapidly through numerous experiments. The Earth and its inhabi- tants are composed of various elements, such as gold and uranium, which are not produced in our Solar system but in massive stars and are transferred into the Solar System via the Interstellar Medium. Thus, every atom around us is made of previous stardust. Such radioactive nuclei could not be studied until we produced them with particle accelerators. These new experiments push the limits of our theories on the nuclear structure: how neutrons and protons work together to make up matter. Nuclear astrophysics works on explaining how these elements are created in stars. This work highlights results from experiments at TRIUMF with radioactive cadmium nuclei, which bring important information on the structure of neutron-rich nuclei. iv Preface Chapter 5 is based on work conducted at the TRIUMF laboratory under the supervision of Professor Reiner Krücken [TRIUMF/UBC] and Dr Iris Dill- mann [TRIUMF/University of Victoria, Canada]. I was responsible for the analysis of the data sets for the β-decay of 128;131;132Cd collected in August 2015. The data files were sorted using the analysis framework GRSISort [10], a code written in ROOT [11]. The figures of level schemes for this thesis have been created using the SciDraw scientific figure preparation system [12]. Chapter 6 is based on work conducted at the TRIUMF laboratory under the supervision Professor Reiner Krücken and Dr Jason Holt [TRIUMF]. I was responsible for running the NuShellX@MSU [13] code provided by Professor Alex Brown [Michigan State University/National Superconduct- ing Cyclotron Laboratory, USA] for 128;131In. I was also responsible for running the NuShellX@MSU code with the IMSRG interaction provided by Dr Jason Holt for 127;128;129;130;131Sn, 127;128;129;131In, 125;126;127;128;129Cd and 125;127Ag. Finally, I was responsible for running the effective single particle energy (ESPE) code provided by Dr Jason Holt using the calculations previously mentioned. Section 5.1.5 presents the analysis of the half-life of 128Cd, which was independently extracted from the same data set by Ryan Dunlop [University of Guelph, Canada] and published in 2016 [4]. A manuscript describing the current work on the nuclear structure of 128In is in preparation for submission to Physical Review C. v Table of Contents Abstract ................................. iii Lay Summary .............................. iv Preface .................................. v Table of Contents ............................ vi List of Tables .............................. ix List of Figures .............................. x List of Symbols and Acronyms . xiii Acknowledgements ........................... xv Dedication ................................xvi 1 Introduction ............................. 1 2 Motivation and Theory ...................... 3 2.1 Nuclear Structure . 3 2.1.1 Non-Interacting Shell Model . 4 2.1.2 Interacting Shell Model . 8 2.1.3 Recent Developments . 9 2.2 Nuclear Astrophysics . 9 2.3 Nuclear Decay . 12 2.3.1 Decay Law . 12 2.3.2 Beta Decay . 14 2.3.3 Gamma Decay . 19 vi 3 Review of Literature ........................ 24 3.1 128Cd ............................... 24 3.2 131Cd ............................... 28 3.3 132Cd ............................... 33 4 Experiment .............................. 36 4.1 Beam Production . 36 4.2 Detectors . 38 4.3 Data Processing . 41 5 Data Analysis and Results .................... 47 5.1 128Cd ............................... 47 5.1.1 β-Gated γ-Singles Measurements . 47 5.1.2 β-Gated γ-γ Coincidence Measurements . 52 5.1.3 Decay Scheme . 60 5.1.4 Spin Assignments . 67 5.1.5 Half-Life . 70 5.1.6 248-keV Isomer . 72 5.2 131Cd ............................... 77 5.2.1 β-Gated γ-Singles Measurements . 77 5.2.2 β-Gated γ-γ Coincidence Measurements . 82 5.2.3 Decay Scheme . 85 5.3 132Cd ............................... 94 5.3.1 β-Gated γ-Singles Measurements . 94 5.3.2 β-Gated γ-γ Coincidence Measurements . 97 6 Shell Model Calculations . 101 6.1 128In . 102 6.1.1 Level Energies . 102 6.1.2 Configurations . 104 6.1.3 Effective Single-Particle Energies . 110 6.2 131In . 112 6.2.1 Level Energies . 112 6.2.2 Configurations . 112 7 Conclusions and Outlook . 114 Bibliography ...............................117 vii Appendices A Data Calibration and Processing . 123 B Data Analysis ............................126 viii List of Tables 2.1 Selection rules for β-decay angular momentum and parity . 18 2.2 Selection rules for γ-decay angular momentum and parity . 21 128 5.1 γ-ray energies in In, their intensities relative to Iγ(247.96) = 100 % and the initial energy levels are compared to previous work [14]. 66 5.2 Level energies in 128In, their β-feeding intensities per 100 decays and the log(ft) values . 68 131 5.3 γ-ray energies in In, their intensities relative to Iγ(988) = 100 %, absolute intensities per 100 decays, and the initial energy levels are compared to previous work Ref. [6]. 91 5.4 Level energies in 131In, their β-feeding intensities per 100 decays and the log(ft) values . 93 6.1 Single-Particle Energies for the jj45pn model space . 102 6.2 Comparison of proton-neutron coupling configurations in 128In 105 6.3 Orbitals occupancy and configuration in 131In . 113 ix List of Figures 2.1 Nuclear shell structure with various potentials . 6 2.2 Proton (π) and neutron (ν) valence orbitals for 128In (Z = 49, N = 79) and single-particle energies (SPE) [in MeV] . 7 2.3 Nuclide chart with one potential rapid neutron capture (r-) process path and r-process solar abundances . 12 2.4 N = 82 region of the nuclide chart close to Z = 50 . 13 2.5 Number of β-decays as a function of time for 128Cd and 128In 15 2.6 β-decay and β-delayed neutron decay processes . 17 2.7 Examples of γ-γ angular correlations . 22 3.1 Published decay schemes of 128Cd . 26 3.2 Evolution of the ground state, first 1+ and isomeric state(s) in even-mass 122−130In . 27 3.3 Published decay schemes of 131Cd . 29 3.4 Evolution of the 1/2{9/2 states in odd-mass 123−131In . 31 3.5 Single-particle orbitals in the 132Sn region [6] . 32 3.6 Published decay schemes of 132Cd. 34 3.7 Tentative levels energies [in keV] for 132In . 35 4.1 TRIUMF ISAC experimental hall layout . 37 4.2 Concept of the Ion Guide Laser Ion source (IG-LIS) . 38 4.3 124−130Cd yields at ISAC using the Ion Guide Laser Ion source 39 4.4 GRIFFIN γ-ray spectrometer . 40 4.5 SCEPTAR scintillator array and moving-tape collector . 40 4.6 Comparison of spectra observed for a 60Co source with and without crosstalk correction . 42 4.7 Comparison of clover addback [blue] and γ-singles [red] spectra observed for a 60Co source . 44 4.8 Time difference between consecutive triggers as a function of crystal number for a 152Eu source . 45 4.9 Absolute γ-ray detection efficiency for the GRIFFIN spectrometer . 46 x 5.1 Difference between time stamps of β-particles and γ-rays . 48 5.2 Comparison of β-gated γ-singles [blue] and γ-singles [red] spectra observed for the decay of 128Cd . 49 5.3 Comparison of β-gated γ-ray spectra observed for the decay of 128Cd in addback mode with lasers on [blue] and laser blocked [red] .

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