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Double-Beta Decay of 96Zr and Double-Electron Capture of 156Dy to Excited Final States

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Double-Beta Decay of 96Zr and Double-Electron Capture of 156Dy to Excited Final States Double-Beta Decay of 96Zr and Double-Electron Capture of 156Dy to Excited Final States by Sean W. Finch Department of Physics Duke University Date: Approved: Werner Tornow, Supervisor Calvin Howell Kate Scholberg Berndt Mueller Albert Chang 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 2015 Abstract Double-Beta Decay of 96Zr and Double-Electron Capture of 156Dy to Excited Final States by Sean W. Finch Department of Physics Duke University Date: Approved: Werner Tornow, Supervisor Calvin Howell Kate Scholberg Berndt Mueller Albert Chang 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 2015 Copyright c 2015 by Sean W. Finch All rights reserved except the rights granted by the Creative Commons Attribution-Noncommercial License Abstract Two separate experimental searches for second-order weak nuclear decays to excited final states were conducted. Both experiments were carried out at the Kimballton Underground Research Facility to provide shielding from cosmic rays. The first search is for the 2νββ decay of 96Zr to excited final states of the daughter nucleus, 96Mo. As a byproduct of this experiment, the β decay of 96Zr was also investigated. Two coaxial high-purity germanium detectors were used in coincidence to detect γ rays produced by the daughter nucleus as it de-excited to the ground state. After collecting 1.92 years of data with 17.91 g of enriched 96Zr, half-life limits at the level of 1020 yr were produced. Measurements of this decay are important to test 0νββ-decay nuclear matrix element calculations, which are necessary to extract the neutrino mass from a measurement of the 0νββ decay half-life. The second experiment is a search for the resonantly-enhanced neutrinoless double- electron capture decay of 156Dy to excited states in 156Gd. Double-electron capture is a possible experimental alternative to 0νββ decay, which could distinguish the Dirac or Majorana nature of the neutrino. Two clover high-purity germanium detectors were used in coincidence to investigate the decay. A 213.5 mg enriched 156Dy sample was observed for 0.635 year, producing half-life limits of 1017 yr. The limits pro- duced by both of these experiments are currently the most stringent limits available for these decays. iv Contents Abstract iv List of Tables ix List of Figures xi List of Abbreviations and Symbols xiv Acknowledgements xvi 1 Introduction1 1.1 Neutrinos and the Standard Model...................2 1.1.1 Majorana and Dirac neutrinos..................4 1.2 Weak nuclear decays...........................7 1.3 ββ decay..................................9 1.3.1 Excited-state decays....................... 13 1.4 Double-electron capture......................... 15 1.5 Experimental technique.......................... 18 2 Theory 20 2.1 β-decay theory.............................. 20 2.1.1 Decay-rate calculation...................... 21 2.2 ββ decay theory.............................. 25 2.2.1 2νββ decay-rate calculation................... 26 2.2.2 0νββ decay-rate calculation................... 28 v 2.2.3 ββ Nuclear Matrix Elements................... 32 2.3 ECEC theory............................... 39 2.3.1 EC decay-rate calculation.................... 39 2.3.2 ECEC decay-rate calculation.................. 40 2.3.3 0νECEC NME calculations................... 43 3 Experimental Technique 45 3.1 γ-ray detection.............................. 45 3.1.1 HPGe detectors.......................... 47 3.1.2 NaI annulus............................ 50 3.2 Two-coaxial HPGe apparatus...................... 52 3.2.1 Electronics............................. 53 3.3 Two-clover HPGe apparatus....................... 54 3.3.1 Clover HPGe detectors...................... 55 3.3.2 Active shielding.......................... 56 3.3.3 Passive shielding......................... 58 3.3.4 Electronics............................. 59 3.4 KURF................................... 62 3.5 Samples.................................. 62 3.5.1 96Zr sample............................ 64 3.5.2 156Dy sample........................... 66 3.6 Previous measurements.......................... 66 3.6.1 On 96Zr.............................. 67 3.6.2 On 156Dy............................. 68 4 96Zr analysis 70 4.1 Data collection and quality control................... 70 vi 4.2 Data processing.............................. 73 4.3 ββ-decay results.............................. 77 4.4 ββ-decay efficiency calculation...................... 82 4.4.1 Energy dependence........................ 83 4.4.2 Target attenuation........................ 84 4.4.3 Detector separation........................ 86 4.4.4 Z dependence of the efficiency.................. 87 4.4.5 Angular correlation of coincident γs............... 87 4.4.6 Final efficiency.......................... 88 4.5 β-decay results.............................. 90 4.6 β-decay efficiency calculation...................... 94 5 156Dy analysis 99 5.1 Data collection.............................. 99 5.2 Data processing.............................. 100 5.3 Data analysis............................... 102 5.3.1 To the 1946.4 keV state..................... 103 5.3.2 To the 1952.4 keV state..................... 104 5.3.3 To the 1988.5 keV state..................... 106 5.3.4 To the 2003.7 keV state..................... 106 5.4 Efficiency measurements......................... 108 5.4.1 Energy dependence........................ 112 5.4.2 Addback factor.......................... 114 5.4.3 Target attenuation........................ 116 5.4.4 Detector separation and source geometry............ 116 5.4.5 Angular dependence of coincident γ rays............ 117 vii 5.4.6 Final efficiency.......................... 118 5.4.7 Contributions from three-γ decays................ 118 6 Conclusion 121 6.1 Limit setting............................... 121 6.2 ββ decays of 96Zr to excited final states................. 122 6.3 β decay of 96Zr.............................. 125 6.4 ECEC decays of 156Dy.......................... 127 6.5 Concluding remarks............................ 130 Bibliography 132 Biography 137 viii List of Tables 1.1 ββ-decay phase-space factors....................... 12 1.2 ββ-decay experimental half-lifes..................... 13 + 1.3 ββ decay to 01 phase space....................... 15 2.1 β-decay transitions............................ 24 2.2 Resonant-ECEC decays in 156Dy.................... 43 2.3 0νECEC NMEs for 156Dy........................ 44 3.1 Isotopic enrichment for ZrO2 samples.................. 65 3.2 Isotopic enrichment for Dy2O3 samples................. 66 3.3 Limits on ββ decay of 96Zr to excited states at CL = 90%....... 68 3.4 Limits on ECEC decay of 156Dy to excited states at CL = 90%.... 69 4.1 Efficiency and background in the region of interest........... 74 4.2 Count rate for the 96Zr ROI during background and 150Nd runs... 82 4.3 Efficiency correction ratios for different coincidences in 96Mo..... 86 4.4 Systematic error budget for ββ decay of 96Zr.............. 90 4.5 Three most probably decay sequences for the 5+ state of 96Mo.... 91 4.6 Results of search for 96Zr's β decay................... 94 4.7 Systematic error budget for the β decay of 96Zr............ 98 5.1 Coincident efficiency of the two-clover apparatus as measured with 102Rh.................................... 112 5.2 Efficiency correction ratios for different coincidences in 156Dy..... 117 ix 5.3 Efficiency of the two-clover apparatus for different angular distribution 118 5.4 Corrected efficiency for the 156Dy ROIs.................. 118 5.5 Systematic error budget for the ECEC decay of 156Dy........ 119 5.6 Contributions to the ROI from the three-γ decays........... 120 6.1 Sensitivity of ββ decay to excited states in 96Zr............ 123 6.2 Half-life limits on the ββ decay of 96Zr to excited states........ 124 6.3 Half-life limits for 96Zr's β decay..................... 126 6.4 Combined statistical limits for 96Zr's β decay.............. 127 6.5 Statistical sensitivity for the 156Dy ECEC ROIs............ 128 6.6 Final half-life limits for the ECEC of 156Dy to excited states..... 129 6.7 Combined statistical limits for the ECEC decay of 156Dy to the 2003.7 keV state.................................. 129 x List of Figures 1.1 The four states of the Dirac neutrino..................5 1.2 The two states of the Majorana neutrino................5 1.3 The A = 156 isobar............................9 1.4 2νββ decay................................9 1.5 0νββ decay................................ 11 1.6 96Zr ββ decay to excited states..................... 14 1.7 Resonant 0νECEC in 156Dy to an excited state in 156Gd........ 16 1.8 0νECEC decay mediated by exchange of a virtual neutrino..... 17 1.9 Coincidence detection principle..................... 19 2.1 The single-β decay of 96Zr........................ 25 2.2 NME for 0νββ .............................. 34 2.3 NME for 2νββ .............................. 38 2.4 Ratio of ground-state to excited-state NME.............. 39 3.1 Spectra for coaxial HPGe........................ 48 3.2 Coaxial-HPGe schematic......................... 48 3.3 Two-coaxial HPGe apparatus...................... 53 3.4 Two-clover HPGe apparatus....................... 56 3.5 Clover segments.............................. 57 3.6 NaI Annulus................................ 58 3.7 Lead shielding..............................
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