SPECTROSCOPY of NEUTRON-UNBOUND FLUORINE by Gregory Arthur Christian a DISSERTATION Submitted to Michigan State University in Pa
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SPECTROSCOPY OF NEUTRON-UNBOUND FLUORINE by Gregory Arthur Christian A DISSERTATION Submitted to Michigan State University in partial fulfillment of the requirements for the degree of DOCTOR OF PHILOSOPHY PHYSICS 2011 ABSTRACT SPECTROSCOPY OF NEUTRON-UNBOUND FLUORINE by Gregory Arthur Christian Neutron-unbound states in 27F and 28F have been measured using the technique of invariant mass spectroscopy, with the unbound states populated from nucleon knock out reactions, 29Ne(9Be, X). Neutrons resulting from the decay of unbound states were detected in the Modular Neutron Array (MoNA), which recorded their positions and times of flight. Residual charged fragments were deflected by the dipole Sweeper magnet and passed through a series of charged particle detectors that provided position, energy loss, and total kinetic energy measurements. The charged particle measurements were sufficient for isotope separation and identification as well as reconstruction of momentum vectors at the target. In addition to the neutron and charged particle detectors, a CsI(Na) array (CAESAR) surrounded the target, allowing a unique determination of the decay path of the unbound states. In 27F, a resonance was observed to decay to the ground state of 26F with 380 60 keV relative ± energy, corresponding to an excited level in 27F at 2500 220 keV. The 28F relative energy ± spectrum indicates the presence of multiple, unresolved resonances; however, it was possible to +50 27 determine the location of the ground state resonance as 210 60 keV above the ground state of F. − This translates to a 28F binding energy of 186.47 0.20 MeV. ± Comparison of the 28F binding energy to USDA/USDB shell model predictions provides in- sight into the role of intruder configurations in the ground state structure of 28F and the low-Z limit of the “island of inversion” around N = 20. The USDA/USDB calculations are in good agreement with the present measurement, in sharp contrast to other neutron rich, N = 19 nuclei (29Ne, 30Na, and 31Mg). This proves that configurations lying outside of the sd model space are not necessary to obtain a good description of the ground state binding energy of 28F and suggests that 28F does not exhibit inverted single particle structure in its ground state. ACKNOWLEDGEMENTS As with any project, a large number of people have made contributions to the work presented in this dissertation, and each one of them deserves recognition and thanks. First, I thank my guidance committee members—Artemis Spyrou, Michael Thoennessen, Alex Brown, Carl Schmidt, Carlo Piermarocchi, and Hendrik Schatz—for their time and effort in reviewing my work. In particular, Michael and Artemis have both served as top-notch academic advisors at various points in my time as a graduate student, giving useful insight, advice, assistance, and motivation. A number of people made direct contributions to the experiment and its analysis, including the entire MoNA Collaboration, the NSCL Gamma Group, and NSCL operations and design staff. I will single a few of them out, and I hope I do not miss anyone. Nathan Frank wrote the original experiment proposal and was a close collaborator during the preparation, running, and ongoing analysis. The MoNA graduate students—Michelle Mosby, Shea Mosby, Jenna Smith, Jesse Sny- der, and Michael Strongman—were all expert shift-takers during the run. In particular, Jesse took every night shift, which I think we all appreciated. Thomas Baumann and Paul DeYoung designed the electronics logic for the timestamp setup, without which the experiment would not have run. Alexandra Gade, Geoff Grinyer, and Dirk Weisshaar provided much assistance in the assembly and running of CAESAR, and Alissa Wersal, NSCL REU student, performed much of the hard labor necessary for CAESAR calibration and analysis. Daniel Bazin kept the Sweeper magnet and its associated detectors tuned and running throughout the experiment, and Craig Snow and Renan Fontus put a lot of work into designing and assembling a magnetic shield for CAESAR. On the theory side, Angelo Signoracci provided the matrix elements and binding energy corrections for the IOI shell model calculations. For each person who made direct contributions to this work, there are many more who con- tributed indirectly. First among these are my parents, Paul and Jane, who have always provided support and encouragement. All of my extended family and friends at various stages in life have iii contributed in some way, but they are too numerous to list individually. I should also thank my teachers at all levels of education, in particular the late Owen Pool, my first physics teacher, and John Wood, who introduced me to nuclear physics. I am also grateful to everyone in the NSCL and the MSU Department of Physics and Astronomy for creating an environment that is, by and large, a pleasure to work in as a graduate student. There are plenty of horror stories out there regarding graduate student life, but I have found little of this to be true in my time at MSU and the NSCL. Along these lines, I am very grateful that I was able to take a two year break from my graduate studies and be allowed to return and pick up right where I left off, with no negative impact to my progress. Finally, I must acknowledge the funding agency which made this work possible, the National Science Foundation, under grants PHY-05-55488, PHY-05-55439, PHY-06-51627, and PHY-06- 06007. iv TABLE OF CONTENTS List of Tables .......................................... vii List of Figures .........................................viii Chapter 1 Introduction ................................... 1 Chapter 2 Motivation and Theory ............................. 4 2.1 Evolution of Nuclear Shell Structure ......................... 4 2.2 Theoretical Explanation ................................ 9 2.3 Correlation Energy .................................. 11 2.4 The Neutron Dripline ................................. 13 2.5 Previous Experiments ................................. 15 Chapter 3 Experimental Technique ............................ 22 3.1 Invariant Mass Spectroscopy ............................. 22 3.2 Beam Production ................................... 24 3.3 Experimental Setup .................................. 26 3.3.1 Beam Detectors ................................ 29 3.3.2 Sweeper .................................... 31 3.3.3 MoNA ..................................... 33 3.3.4 CAESAR ................................... 34 3.3.5 Electronics and Data Acquisition ....................... 36 Chapter 4 Data Analysis ................................... 39 4.1 Calibration and Corrections .............................. 39 4.1.1 Sweeper .................................... 39 4.1.1.1 Timing Detectors .......................... 39 4.1.1.2 CRDCs ............................... 41 4.1.1.3 Ion Chamber ............................ 51 4.1.1.4 Scintillator Energies ........................ 52 4.1.2 MoNA ..................................... 57 4.1.2.1 Time Calibrations ......................... 57 4.1.2.2 Position Calibrations ........................ 63 4.1.2.3 Energy Calibrations ........................ 65 4.1.3 CAESAR ................................... 65 4.2 Event Selection .................................... 67 4.2.1 Beam Identification .............................. 68 4.2.2 CRDC Quality Gates ............................. 69 4.2.3 Charged Fragment Identification ....................... 70 4.2.3.1 Element Selection ......................... 70 4.2.3.2 Isotope Selection .......................... 71 v 4.2.4 MoNACuts .................................. 80 4.2.5 CAESAR Cuts ................................ 80 4.3 Physics Analysis ................................... 81 4.3.1 Inverse Tracking ............................... 82 4.3.1.1 Mapping Cuts ........................... 85 4.3.2 CAESAR ................................... 87 4.4 Consistency Checks .................................. 89 4.4.1 23O Decay Energy .............................. 89 4.4.2 Singles Gamma-Ray Measurements ..................... 89 4.5 Modeling and Simulation ............................... 91 4.5.1 Resonant Decay Modeling .......................... 91 4.5.2 Non-Resonant Decay Modeling ....................... 94 4.5.3 Monte Carlo Simulation ........................... 97 4.5.3.1 Verification .............................100 4.5.4 Maximum Likelihood Fitting .........................101 Chapter 5 Results ......................................106 5.1 Resolution and Acceptance ..............................106 5.2 27F Decay Energy ...................................106 5.3 28F Decay Energy ...................................114 5.4 Cross Sections .....................................120 Chapter 6 Discussion ....................................123 6.1 Shell Model Calculations ...............................123 6.2 27F Excited State ...................................126 6.3 28F Binding Energy ..................................128 Chapter 7 Summary and Conclusions ...........................136 Appendix A Cross Section Calculations ...........................140 A.1 27F Excited State ...................................140 A.2 28F Ground State ...................................144 References ...........................................150 vi LIST OF TABLES 2.1 Bound state gamma transitions observed in [45]. ..................... 18 3.1 List of charged particle detectors and their names. Detectors are listed in order from furthest upstream to furthest downstream. ........................ 32 3.2 Numeric designations for the