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Double-Beta Decay of 150Nd to Excited Final States

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Double-Beta Decay of 150Nd to Excited Final States by M. F. Kidd Department of Physics Duke University Date: Approved: Werner Tornow, Advisor Kate Scholberg Steffen Bass Calvin Howell Ying Wu Dissertation submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy in the Department of Physics in the Graduate School of Duke University 2010 Abstract (Double-Beta Decay) Double-Beta Decay of 150Nd to Excited Final States by M. F. Kidd Department of Physics Duke University Date: Approved: Werner Tornow, Advisor Kate Scholberg Steffen Bass Calvin Howell Ying Wu An abstract of a dissertation submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy in the Department of Physics in the Graduate School of Duke University 2010 Copyright c 2010 by M. F. Kidd All rights reserved except the rights granted by the Creative Commons Attribution-Noncommercial Licence Abstract An experimental study of the two-neutrino double-beta (2νββ) decay of 150Nd to various excited final states of 150Sm was performed at Triangle Universities Nuclear Laboratory (TUNL). Such data provide important checks for theoretical models used to predict 0νββ decay half lives. The measurement was performed at the recently established Kimballton Under- ground Research Facility (KURF) in Ripplemeade, Virginia using the TUNL-ITEP double-beta decay setup. In this setup, two high-purity germanium detectors were operated in coincidence to detect the deexcitation gamma rays of the daughter nu- cleus. This coincidence technique, along with the location underground, provides a considerable reduction in background in the regions of interest. This study yields the first results from KURF and the first detection of the + 150 coincidence gamma rays from the 01 excited state of Sm. These gamma rays have energies of 334.0 keV and 406.5 keV, and are emitted in coincidence through + + + a 01 →21 →0gs transition. The enriched Nd2O3 sample obtained from Oak Ridge National Laboratory consists of 40.13 g 150Nd. This sample was observed for 391 days, producing 29 raw events in the region of interest. This count rate gives a half +0.36 20 life of T1/2 = (0.72−0.18 ± 0.04(syst.)) × 10 years, which agrees within error with another recent measurement, in which only the single deexcitation gamma rays were detected (i.e., no coincidence was employed). Lower limits were also obtained for decays to higher excited final states. iv To Kenneth P. Kidd v Contents Abstract iv List of Tables ix List of Figures x List of Abbreviations and Symbols xii Acknowledgements xiv 1 Introduction 1 1.1 Background ................................ 2 1.1.1 Neutrino History ......................... 2 1.1.2 The Standard Model and the Neutrino ............. 5 1.1.3 Mass Searches ........................... 8 1.1.4 The Majorana Question ..................... 9 1.2 Double-Beta Decay ............................ 12 1.2.1 Double-Beta Decay Modes .................... 14 1.2.2 Double-Beta Decay Candidates ................. 15 2 Theory 20 2.1 Beta-Decay Rate ............................. 20 2.2 Two Neutrino Double-Beta Decay Rate ................. 24 2.3 Neutrinoless Double-Beta Decay Rate .................. 26 2.3.1 Phase-Space Factors ....................... 29 vi 2.4 Nuclear Matrix Elements ......................... 30 2.4.1 NME Evaluation Methods .................... 31 3 Experimental Methods and the TUNL-ITEP Double-Beta Decay Setup 36 3.1 Previous Measurements .......................... 36 3.2 Experimental Method ........................... 39 3.2.1 High-Purity Germanium Detectors ............... 41 3.2.2 Gamma-Ray Detection ...................... 44 3.2.3 Double-Beta Decay Source .................... 45 3.2.4 Passive Shielding ......................... 46 3.2.5 Active Shielding .......................... 47 3.2.6 Kimballton Underground Research Facility ........... 47 3.2.7 Electronics ............................. 53 3.2.8 Computer Interface ........................ 56 4 Analysis 57 4.1 Calibration of Gamma-ray Spectra ................... 57 4.2 Coincidence-Efficiency Measurement .................. 62 4.3 Rejection of Background Candidates .................. 73 4.3.1 Background Sources ....................... 74 4.3.2 Identification of Natural Background in Data .......... 79 4.3.3 Cosmic-Ray Backgrounds .................... 82 5 Results and Conclusions 87 150 + 5.1 Double-Beta Decay of Nd to 01 .................... 87 5.2 Double-Beta Decay of 150Nd to Higher Excited States ......... 90 5.3 Conclusions ................................ 92 vii Bibliography 98 Biography 102 viii List of Tables 1.1 Summary of particles in the Standard Model .............. 6 1.2 The nuclei which undergo double-beta decay. .............. 16 3.1 Previously measured values for 150Nd 2νββ half lives. ......... 39 3.2 Background contamination in 150Nd regions of interest ........ 39 3.3 Background-reduction factors ...................... 53 4.1 Summary of systematic error contributions. .............. 71 4.2 Typical concentrations of naturally occurring radioactive isotopes in limestone. [45] .............................. 74 4.3 Potential coincidences near regions of interest. ............. 78 5.1 Summary of results. ............................ 96 ix List of Figures 1.1 The four Dirac neutrino states. ..................... 10 1.2 The two Majorana neutrino states. ................... 11 1.3 The mass parabola. ............................ 13 1.4 Schematic of 0ν mode of double-beta decay. .............. 15 1.5 Generic double-beta decay scheme to excited states. .......... 18 2.1 2νββ schematic. .............................. 24 2.2 0νββ schematic. .............................. 27 3.1 Higher excited states of 150Nd. ...................... 38 3.2 A diagram of the TUNL-ITEP double-beta decay setup. ....... 43 3.3 A picture of the Kimballton Underground Research Facility before and after the infrastructure was completed. ................. 48 3.4 The TUNL-ITEP double-beta decay setup installed at Kimballton Underground Research Facility ...................... 50 3.5 A comparison of data taken at ground level and at Kimballton Un- derground Research Facility ....................... 51 3.6 A comparison of data taken at ground level and at Kimballton Un- derground Research Facility ....................... 52 3.7 Electronics diagram for the HPGe detectors. .............. 54 3.8 Electronics diagram for the veto counters. ............... 55 4.1 An example of a singles spectrum. .................... 59 4.2 An example of a two-dimensional spectrum. .............. 60 x 4.3 The projected and summed event histograms. ............. 61 4.4 Decay scheme for 102Rh. ......................... 63 4.5 Two-dimensional energy spectrum for efficiency measurement ..... 65 4.6 Projection and fit for efficiency measurement. ............. 66 4.7 Singles efficiency measurement for detector 1. ............. 66 4.8 Singles efficiency measurement for detector 2. ............. 67 4.9 Radially dependent coincidence efficiency. ................ 69 4.10 A comparison between the previously measured efficiency and the newer measurement. ........................... 71 4.11 Angular correlations for E2 and M1+E2. ................ 73 4.12 The radon contamination measured in decays of 214Bi starting from April 2008. The LN purge was installed in October 2008. The seasonal dependence is especially noticeable. ................... 77 4.13 Comparison of 742.8 keV to surrounding peaks. ............ 80 4.14 The projected and summed event histograms with Compton scattering peaks labeled. ............................... 81 4.15 Reduction in muon flux by going underground. ............. 84 4.16 Beta-decay scheme of 150Pm. ....................... 85 5.1 Final accumulated event spectrum. ................... 88 5.2 Distribution of time intervals between events. ............. 91 5.3 Coincidence data for higher excited states. ............... 93 5.4 Results from Barabash et al. ....................... 94 xi List of Abbreviations and Symbols Symbols β beta, as in beta decay ββ double-beta (decay) 2νββ two-neutrino double-beta (decay) 0νββ neutrinoless double-beta (decay) ECEC double electron capture Z number of protons in a nucleus N number of neutrons in a nucleus A mass number of nucleus corresponding to Z + N Abbreviations ADC Analog-to-Digital Converter DAQ Data-Acquisition HPGe High-Purity Germanium ITEP Institute for Theoretical and Experimental Physics KURF Kimballton Underground Research Facility LBCF Low Background Counting Facility LN Liquid Nitrogen m.w.e. Meters Water Equivalent NaI Sodium Iodide xii Nd2O3 Neodymium Oxide NME Nuclear Matrix Elements NSM Nuclear Shell Model OFHC Oxygen Free High Conductivity PMT Photomultiplier Tube QRPA Quasi-Random Phase Approximation ROI Region of Interest SNO Sudbury Neutrino Observatory SNO+ The next phase of the Sudbury Neutrino Observatory TAC Time-to-Amplitude Converter TUNL Triangle Univerisities Nuclear Laboratory UPS Uninterruptible Power Supply xiii Acknowledgements I would like to thank several people for their assistance with this experiment: Bret Carlin, Richard O’Quinn, Chris Westerfeldt, John Dunham and Patrick Mulkey. Without their input, especially on the planning and execution of the move to Kim- ballton, my life would have been much harder. James Esterline has put in valu- able time helping me with both theory and experimental questions. Thank you, Brenda West, for handling the multitude of trips I have taken to Blacksburg, VA. I’m also grateful for Dr. Alexander Barabash and Dr. Vladimir Umatov
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