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The Study of Beta-Delayed Neutron Decay Near the Neutron Drip Line

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THE STUDY OF BETA-DELAYED NEUTRON DECAY NEAR THE NEUTRON DRIP LINE By Chandana Sujeewa Sumithrarachchi A DISSERTATION Submitted to Michigan State University in partial ful¯llment of the requirements for the degree of DOCTOR OF PHILOSOPHY Department of Chemistry 2007 ABSTRACT The Study of Beta-delayed Neutron Decay Near The Neutron Drip Line By Chandana Sujeewa Sumithrarachchi The study of neutron-rich oxygen and uorine isotopes can provide important information on the evolution of nuclear shell structure close to the neutron drip line. The structural changes in this region are reected with observations of the rapid change in the location of the drip line at uorine and appearance of a new shell closure at N = 14. The recent experiments along with the shell model calculations provide evidence for the doubly magic nature of 22O. The negative parity states in 22O rooted in the neutron pf orbitals are not experimentally known. The knowledge of nuclear structure in 23F, which has the structure of a single proton outside the doubly magic 22O, is also important as it should be sensitive to the proton s and d orbital splitting. The present work focused on the beta-delayed neutron and gamma- ray spectroscopes from 22N and 23O beta decay. The measurements of 22N and 23O were carried out at the NSCL using fragments from the reaction of 48Ca beam in a Be target. The desired isotopes were stopped in the implantation detector and then monitored for beta-delayed neutrons and gamma- rays using a neutron spectroscopic array and eight detectors from SeGA, respectively. The half-lives and the total neutron emission probabilities were determined to be 20(2) and 97(8) ms, and 57(5)% and 7(2)%, respectively, for 22N and 23O. Single and two beta-delayed neutron decay of 22N was observed and ¯ve new negative parity states identi¯ed in 22O. The measurement also revealed three gamma-ray transitions in 22O and a single gamma-ray for each in 21O and 20O associated with the beta- delayed neutron decay of 22N. Ten gamma-ray decays in 23F and a single gamma-ray in 22F were observed from 23O beta decay. The tentative decay schemes of 22N and 23O were established and compared with shell model calculations. The beta decays of the major contaminants in the beam from the fragment separator 25F, 24O and 26Ne have been investigated to establish decay schemes as well. The observation of a relatively high energy for the ¯rst 2+ state in 22O supports the shell closure at N = 14. The observation of large beta decay strength at high excitation energies in 22O indicates the indirect evidence for the halo structure of 22N. The experimental results of the 22N beta decay are in poor agreement with the shell model calculations suggesting the evolution of single particle structure in this region. However, the overall beta decay results of 23O are in reasonable agreement with shell model calculations. The excitation energies of the ¯rst 1/2+ and 3/2+ states in 23F have been determined at 2243 and 3866 keV and show a widening of the 5/2+ - 1/2+ state gap indicating the appearance of the N=14 shell closure in the uorine isotopes. To my mother, my wife Indikani, my daughter Manekya all my teachers and friends iv ACKNOWLEDGMENTS It is impossible in words to really express the gratitude that I have to the people who I have worked with and met along this road. Without the support, patience and guidance of the following people, this study would not have been completed. First I owe my deepest gratitude to my advisor, Professor Dave J. Morrissey, for his guidance, supervision, and encouragement during this work. His patience, sense of humor, and understanding has been greatly appreciated and are acknowledged. I must especially thank him for his e®orts during my experiment, for the experience I gained from working alongside him and allowing me to work independently, while always being available to help. I thank Professor Paul Mantica for being my second reader and giv- ing additional guidance. His support during experiment and suggestions to complete this work have been especially acknowledged. The contributions of my committee to my education will always be remembered. I would like to thank Professors Michael Thoennessen and Hendrik Schatz for working as members of my dissertation com- mittee and for giving advice through this work. The NSCL technical sta® is totally outstanding and helped make the project real. I really need to thank Mr. Len Morris for his design work, the NSCL Machine Shop for producing parts for our setup and all cyclotron operators. I whish to thank Dr. Mauricio Portillo for support through A1900 work and Professor Thomas Glasmacher and the gamma group for providing the support in use of SeGA. I am grateful to Professor Alex Brown for providing shell model calculations for this work. There are multiple people I would like to acknowledge for their contributions during the experiment. I would like to thank Andrew, Debbie, Elaine, Josh, Weerasiri, Dr. Fancina and Dr. Shimbara giving support thorough the experiment. I especially want to thank Elain Kwan for all her help. Our long discussions were inspiring to me. I have appreciated your friendship and willingness to help along the way. I wish to thank the Morrissey group members, Dr. Folden and Greg Pang for discussions v about various projects. My fellow graduate students at the lab were also invaluable during my time at the lab. I o®er special thanks to my o±cemates Wes, Greg, Josh and Philip, to great friends from the accelerator group, Susan and from the theory group, Jeremy and former graduate students Bryan, Sean and Tao for the interesting conversations to make time in graduate school enjoyable. I wish to thank Jeremy for reading this. I am grateful to the Department of Chemistry, Michigan State University and the National Science Foundation for their ¯nancial support during this work. I wish to thank especially the Chemistry Graduate O±ce, Lisa and Debbie for their kind help throughout my studies. The people that heard me grip the most over the past three years have been my family. I cannot imagine being where I am today without them, and I am blessed to have had their unconditional love and support throughout my academic career and my life. Ariyawathei Sumithrarachchi, my mother, who has always supported, encouraged and believed in me, in all my endeavors. Inikani, my wife, without whom this e®ort would have been worth nothing. Your love, support and constant patience have taught me so much about sacri¯ce, discipline and compromise. I want to mention here about my loving daughter, Manekya. She is everything for me. Her love is always a gripe to my life. vi Contents 1 Introduction 1 1.1 Location of The Neutron Drip Line . 5 1.2 Disappearance of Traditional Magic Numbers . 6 1.3 Shell closure at N = 14 and N = 16 . 7 1.4 Halo structures near neutron drip line . 9 1.5 Motivation . 11 1.5.1 Knowledge of the Structure of 22O and 22N . 11 1.5.2 Knowledge on the Structure of 23F and 23O . 14 1.6 Present Work . 16 2 Overview of Beta Decay 17 2.1 Theory of Beta Decay . 18 2.1.1 Selection Rules . 21 2.2 Beta-delayed Neutron Decay . 23 2.3 Gamma-ray decay . 23 2.4 Gamow-Teller Transition strength . 24 2.5 Application . 26 3 Experimental Setup 28 3.1 Fragment Production . 28 3.2 Implantation Detector System . 31 3.2.1 Data Acquisition System . 33 3.3 Neutron Spectroscopic Array . 36 3.3.1 Detail of the Neutron Spectroscopic Array . 36 3.3.2 Neutron Energy Calibration . 38 3.3.3 Neutron Peak Shape and The Background . 44 3.3.4 Time Walk Correction . 46 3.3.5 Neutron E±ciency Calibration . 48 3.4 Segmented Germanium Detector Array . 51 3.4.1 SeGA detectors . 51 3.4.2 SeGA Energy Calibration . 52 3.4.3 SeGA E±ciency Calibration . 54 4 Data Analysis and Results 59 4.1 Anaysis of The 22N Beta Decay Experiment . 59 4.1.1 Total Number of Beta Decay Events . 61 vii 4.1.2 The analysis of the neutron time-of-ight spectrum from 22N cocktail beam . 64 4.1.3 Gamma-ray Identi¯cation . 66 4.1.4 Beta Decay Scheme of 22N . 71 4.2 Beta Decay of 25F . 76 4.3 Beta Decay of 24O . 83 4.4 Analysis of 23O Deta Decay Experiment . 89 4.4.1 Total Number of Beta Decay Events . 92 4.4.2 Beta-delayed neutron time-of-ight measurement . 94 4.4.3 Beta-delayed gamma-ray measurement . 97 4.4.4 Beta Decay Scheme of 23O . 99 4.5 Beta Decay of 26Ne . 102 5 Discussion 108 5.1 Beta Decay of 22N . 108 5.2 Beta Decay of 23O and Energy States in 23F . 112 5.3 The 1+ Excited States in 26Na . 117 5.4 The Half-lives and Total Neutron Emission Probabilities of Neutron- rich Light Nuclei . 119 5.5 Conclusion and Summary . 120 A Beta Decay Fitting Model 122 B Gamma-ray Spectrum Analysis 128 Bibliography . 135 viii List of Figures 1.1 The di®erential single neutron separation energy . 1 1.2 Schematic diagram of the single-particle states . 3 1.3 Part of the chart of nuclides . 4 1.4 Single neutron separation energies for nuclides of di®erent isospin . 6 1.5 The excitation energy of the lowest 2+ states of even-even oxygen iso- topes .
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