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The Beta Decay of 79,80,81Zn and Nuclear Structure Around the N=50 Shell Closure

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The Beta Decay of 79,80,81Zn and Nuclear Structure Around the N=50 Shell Closure University of Tennessee, Knoxville TRACE: Tennessee Research and Creative Exchange Doctoral Dissertations Graduate School 12-2011 The Beta Decay of 79,80,81Zn and Nuclear Structure around the N=50 Shell Closure Stephen William Padgett [email protected] Follow this and additional works at: https://trace.tennessee.edu/utk_graddiss Part of the Nuclear Commons Recommended Citation Padgett, Stephen William, "The Beta Decay of 79,80,81Zn and Nuclear Structure around the N=50 Shell Closure. " PhD diss., University of Tennessee, 2011. https://trace.tennessee.edu/utk_graddiss/1212 This Dissertation is brought to you for free and open access by the Graduate School at TRACE: Tennessee Research and Creative Exchange. It has been accepted for inclusion in Doctoral Dissertations by an authorized administrator of TRACE: Tennessee Research and Creative Exchange. For more information, please contact [email protected]. To the Graduate Council: I am submitting herewith a dissertation written by Stephen William Padgett entitled "The Beta Decay of 79,80,81Zn and Nuclear Structure around the N=50 Shell Closure." I have examined the final electronic copy of this dissertation for form and content and recommend that it be accepted in partial fulfillment of the equirr ements for the degree of Doctor of Philosophy, with a major in Physics. Robert Grzywacz, Major Professor We have read this dissertation and recommend its acceptance: Witek Nazarewicz, Carrol Bingham, Jason Hayward Accepted for the Council: Carolyn R. Hodges Vice Provost and Dean of the Graduate School (Original signatures are on file with official studentecor r ds.) The Beta Decay of 79;80;81Zn and Nuclear Structure around the N=50 Shell Closure A Dissertation Presented for the Doctor of Philosophy Degree The University of Tennessee, Knoxville Stephen William Padgett December 2011 Dedication To my father, who has always given me support in my academic pursuits and has never doubted my abilities. Abstract This dissertation reports on new information in the β−[beta minus] decay of the neutron- rich nucleus 81Zn, which populates states in its daughter nucleus 81Ga. This includes new γ[gamma]-ray transitions in the daughter nucleus, 81Ga, as well as a β[beta]-delayed neutron branching ratio. This isotope was produced at the Holifield Radioactive Ion Beam Facility of Oak Ridge National Laboratory through the Isotope Separation Online technique. They are fission fragments from proton-induced fission on a uranium carbide target. These fission fragments are ionized and both mass and isotopically separated before arriving at the Low Energy Radioactive Ion Beam Spectroscopy Station (LeRIBSS). The γ[gamma]-ray and β[beta] electron emissions from the decays are measured and analyzed in this work. A new β[beta]-delayed neutron branching ratio is reported for this decay, which is in agree- ment with recent theoretical values. The core excited states in the daughter nucleus, 81Ga, populated through allowed Gamow-Teller decays are analyzed. A trend in core excited states with other N=50 isotones indicates an increasing gap between a deeply bound neu- tron hole and the valence neutron above the N=50 gap upon moving towards doubly magic 78Ni. This dissertation also reports on additions to the decay schemes of 79Zn and 80Zn decays. Their decay level schemes have been expanded upon and an improved picture of the total allowed Gamow-Teller decay strength is known from 79Zn to 81Zn. This work presents an improved, albeit still incomplete, picture of the energy of states populated through Gamow-Teller decays from below to above the N=50 shell gap in zinc isotopes. iii Contents 1 Introduction 1 2 Theoretical Background 3 2.1 Shell model of the atomic nucleus . 3 2.1.1 Central potential . 3 2.1.2 Spin-orbit splitting . 5 2.2 Experimental evidence of shell gaps and magic numbers . 8 2.3 Microscopic description of β decay . 8 2.4 Fermi and Gamow-Teller operators . 12 2.5 Decay lifetimes, energetics, and selection rules . 14 2.6 β decay branching ratios . 18 3 Experimental studies of the neutron-rich isotopes of Zn 20 3.1 Introduction . 20 3.2 Experimental details . 21 3.2.1 ISOL technique at the HRIBF . 21 3.2.2 Charge exchange cell . 22 3.2.3 High resolution isobar magnets . 23 3.2.4 Charge state contaminants . 26 3.3 LeRIBSS . 26 3.3.1 Electrostatic deflector and tape system . 27 3.3.2 Detector system . 29 3.3.3 HPGe energy and efficiency calibrations . 31 3.3.4 Plastic scintillators for β decay detection . 35 3.3.5 Microchannel plate detector . 36 3.3.6 Digital data acquisition . 39 4 Experimental Analysis and Results: 81Zn 45 4.1 Data reduction methods . 45 4.2 Construction of the decay level scheme . 47 4.2.1 Intensity balance . 49 4.2.2 The β − γ − γ coincidences . 49 4.2.3 Half-life measurements . 50 4.2.4 β efficiency calculation . 51 iv 4.2.5 Determination of isobaric contamination . 54 4.3 Level scheme for 81Zn decay . 57 4.3.1 The 802 keV transition and M1 hindrance . 58 4.3.2 Coincidences and crossover transitions . 59 4.3.3 β decay branching ratios . 60 4.3.4 Neutron emission branching ratio . 60 4.3.5 Neutron branching ratio: Compare to theoretical predictions and prior values from literature . 64 4.4 Pandemonium effect and ground state branching ratio . 65 4.4.1 Pandemonium: Basic information . 66 4.4.2 Estimation of missing β decay intensity . 67 5 Interpretations and Calculations for 81Zn Decay 71 5.1 Theoretical shell model calculations . 71 5.2 N=51 systematics . 72 5.2.1 Previous experiments on N=51 systematics . 73 5.2.2 81Zn ground state Jπ .............................. 73 5.2.3 Calculations on N=51 isotones . 75 5.3 Core excitations of 78Ni ................................ 79 5.3.1 Core excited states in N=50 isotones . 79 5.3.2 Possible N=50 and N=40 shell gap changes . 81 5.4 Predictions for 79Ni and 79Cu............................. 82 6 The Decay Scheme of 79Zn 85 6.1 Verification of original 79Zn decay scheme . 85 6.2 Placement of new transitions in the 79Zn decay scheme . 87 6.3 79Zn decay scheme discussion . 90 6.4 Isomers in N=49 nuclei . 92 79 6.5 Measured Sβ and calculated B(GT) values in Zn decay . 94 6.6 79Zn decay conclusions . 97 7 The Decay Scheme of 80Zn 98 7.1 Verification of original 80Zn decay scheme . 98 7.2 Placement of new transitions in the 80Zn decay scheme . 100 7.3 Isomeric state in 80Ga . 101 v 7.4 Discussion on ground state branching ratio . 103 80 7.5 Measured Sβ and calculated B(GT) values in Zn decay . 103 7.6 80Zn decay conclusions . 105 8 Conclusions and Summary 106 9 My Contributions 109 10 VITA 115 vi List of Figures 1 Woods-Saxon potential . 4 2 Shell model of the atomic nucleus . 7 3 First excited 2+ states and shell gaps . 9 4 Feynman diagram of β− decay . 10 5 Log ft values for allowed decays . 16 6 Log ft values for forbidden decays . 16 7 Nuclear structure and energetics determination of β decay intensity distribution . 19 8 Vertical focusing in the high resolution isobar magnet . 24 9 Horizontal focusing in the high resolution isobar magnet . 25 10 LeRIBSS experimental setup . 27 11 MTC cycles . 28 12 The CARDS with plastic scintillators . 30 13 Efficiency vs. energy for the germanium detectors . 34 14 Microchannel plate detector . 37 15 Pixie16 board layout . 40 16 Trapezoidal filter . 41 17 Trigger threshold vs. data acquisition efficiency . 42 18 Integration time vs. trigger threshold . 42 19 Integration time and source detection example . 43 20 The two dimensional spectrum of energy vs. time with grow-in of activity . 46 21 The γ-ray spectrum for 81Zn decay . 48 22 The intensity balance . 50 23 Efficiency of β electron detection in the scintillators . 53 24 Fit to zinc activity grow-in for isobaric purity determination . 55 25 Fit to gallium activity grow-in for isobaric purity determination . 56 26 The β − γ − γ coincidences in 81Zn decay . 59 27 The level scheme for the decay of 81Zn to 81Ga . 61 28 Neutron branching ratio determination for 81Zn decay . 62 29 Predictions for zinc neutron branching ratios . 65 30 Pandemonium . 66 31 N=51 systematics, including shell model calculations . 77 32 Auxiliary N=51 isotone calculations . 78 33 First forbidden vs. Gamow-Teller decays in the 78Ni region . 80 vii 34 Core excited states in N=50 isotones . 81 35 Predictions for 79Ni and 79Cu............................. 84 36 The γ-ray spectrum for 79Zn decay . 86 37 The level scheme for the decay of 79Zn to 79Ga . 89 38 Systematics for gallium isotopes . 93 79 39 The experimental Sβ values in Ga . 95 40 The theoretical B(GT) values in 79Ga . 96 41 The γ-ray.
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