Precise Measurement of Reactor Antineutrino Oscillation Parameters and Fuel-dependent Variation of Antineutrino Yield and Spectrum

Abstract

학위논문 (박사)-- 서울대학교 대학원 : 자연과학대학 물리·천문학부(물리학전공), 2019. 2. 김수봉.The reactor experiment for neutrino oscillation (RENO) 는 2011년 8월부터 영광 원자력발전소 근처에 위치한 두개의 동일한 검출기를 사용하여 전자 반중성미자의 데이터를 받기 시작했다. 대략 2,200일의 데이터를 사용하여, 근거리에서는 850\,666 원거리에서는 103\,212 개의 중성미자 이벤트 후보를 골라내었으며, 그 중 근거리에서는 2.0\%, 원거리에서는 4.7\%의 비율으로 백그라운드가 포함되어있다. 5MeV 부근에서 측정된 중성미자의 선행이벤트 스펙트럼과 최신의 이론을 사용하여 예측한 선행 이벤트 스펙트럼에 차이가 있는 것을 발견하였다. 원거리와 근거리의 비율을 기준으로 분석하여 거리와 중성미자의 에너지에 따라 중성미자가 사라지는 것을 확인하였고, 그 에너지 스펙트럼과 양으로부터 $\sin^2 2 \theta_{13} = 0.0896 \pm 0.0048({\rm stat.}) \pm 0.0047({\rm syst.})$ and $\Delta m_{ee}^2= [2.68\pm0.12({\rm stat.})\pm0.07({\rm syst.})]\times 10^{-3}$~eV$^2$ 를 측정하였다. 한편, 6개 원자로가 작동하는 사이클의 기간이 서로 조금씩 달라 원자로의 연료인 $^{235}$U, $^{238}$U, $^{239}$Pu 그리고 $^{235}$Pu의 비율이 계속적으로 변하고 있으며, 이를 통하여 연료 비율에 따라 IBD 생산량의 변화를 살펴보았다. 이로부터 $^{235}$U 에 대한 IBD 생산량 (6.15 $\pm$ 0.19) $\times$ $10^{43}$ $cm^{2}$/fission, $^{239}$Pu 에 대한 IBD 생산량 (4.18 $\pm$ 0.26) $\times$ $10^{43}$ $cm^{2}$/fission, 그리고 전체 IBD 생산량 (5.84 $\pm$ 0.13) $\times$ $10^{43}$ $cm^{2}$/fission 을 측정하였다. 이러한 측정으로부터, 4가지 연료의 베타 붕괴로부터 발생하는 중성미자의 스펙트럼이 서로 같지 않음을 6.6 $\sigma$로서 확인하고 있다.측정된 각 4가지 연료의 IBD 생산량중 $^{235}$U의 경우가 이론으로부터 예상된 것에 비해 가장 많이 적게 나타나고 있다. 따라서 4가지 연료중 $^{235}$U의 IBD 생산량을 다시 계산하는 것이 원자로 반중성미자의 변칙을 가장 잘 해결할 수 있을 것이다. 또한, 5MeV 부근에서 측정된 중성미자 이벤트의 스펙트럼과 이론으로부터 예상한 스펙트럼의 차이가 원자로 내부의 $^{235}$U의 비율과도 상관관계가 있는 것을 확인하였다.The reactor experiment for neutrino oscillation (RENO) since August 2011 has been extracting electron antineutrino (\nueb) data from two identical detectors located near the Yonggwang nuclear reactors in Korea. Using roughly 2,200 live days of data, we observed 850\,666 (103\,212) reactor \nuebS candidate events with 2.0\%(4.7)\% background in the near (far) detector. A discrepancy of approximately 5 MeV between the measured positron spectra of the reactor \nuebS events and the predicted positron spectra of the current reactor \nuebS model was observed. A far-to-near ratio measurement was conducted using the spectral and rate information, which gave $\sin^2 2 \theta_{13} = 0.0896 \pm 0.0048({\rm stat.}) \pm 0.0047({\rm syst.})$ and $= [2.68\pm0.12({\rm stat.})\pm0.07({\rm syst.})]\times 10^{-3}$~eV$^2$. On the other hand, we observed from the multiple fuel cycles a fuel-dependent variation in an inverse beta decay (IBD) yield of (6.15 $\pm$ 0.19) $\times$ $10^{43}$ $cm^{2}$/fission for $^{235}$U and (4.18 $\pm$ 0.26) $\times$ $10^{43}$ $cm^{2}$/fission for $^{239}$Pu, and measured a total average IBD yield per fission of (5.84 $\pm$ 0.13) $\times$ $10^{43}$ $cm^{2}$/fission. This observation rejects the hypothesis of fuel-independent IBD yield or identical fuel-isotope spectra at 6.6 $\sigma$. The measured IBD yield per fission for $^{235}$U shows the largest deficit relative to a reactor model prediction. Re-evaluation of the $^{235}$U IBD yield per fission could solve the reactor antineutrino anomaly. We also report a correlation between the 5 MeV discrepancy in the observed IBD spectrum and $^{235}$U reactor fuel isotope fraction.List of Figures v List of Tables xi 1 Introduction 1 1.1 Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 1.2 Neutrino Oscillation . . . . . . . . . . . . . . . . . . . . . . . . . . 2 1.3 Reactor Neutrino Experiment . . . . . . . . . . . . . . . . . . . . . 3 1.3.1 Reactor Neutrino Production . . . . . . . . . . . . . . . . . 5 1.3.2 Reactor Neutrino Detection . . . . . . . . . . . . . . . . . . 6 1.3.3 Neutrino Oscillation in Reactor Experiments . . . . . . . . 8 1.3.4 Determination of Mixing Angle theta13 . . . . . . . . . . . . . . 10 1.3.5 Determination of Mass Squared Difference 1.4 The RENO Experiment . . . . . . . . . . . . . . . . . . . . . . . . 12 1.5 Fuel-dependent Variation of Antinetrino Yield and Spectrum . . . 12 2 Setup of the RENO Experiment 15 2.1 Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15 2.2 Experimental Arrangement . . . . . . . . . . . . . . . . . . . . . . 18 2.2.1 Hanbit Nuclear Power Plant . . . . . . . . . . . . . . . . . . 18 2.2.2 Near and Far detectors . . . . . . . . . . . . . . . . . . . . 18 2.2.3 Underground Facility and Experiment Halls . . . . . . . . . 20 2.3 Detector Setup . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21 2.3.1 Target and Gamma Catcher . . . . . . . . . . . . . . . . . . 21 2.3.2 Buffer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24 2.3.3 Veto . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25 2.3.4 PMT . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26 2.4 Liquid Scintillator . . . . . . . . . . . . . . . . . . . . . . . . . . . 29 2.4.1 Optimization for Liquid Scintillator . . . . . . . . . . . . . 30 2.4.2 Gd-loaded Liquid Scintillator . . . . . . . . . . . . . . . . . 32 2.4.3 Long-term Stability of the Liquid Scintillator . . . . . . . . 34 2.5 DAQ and Monitoring System Setup . . . . . . . . . . . . . . . . . 35 2.5.1 Front-End Electronics . . . . . . . . . . . . . . . . . . . . . 35 2.5.2 Qbee Board . . . . . . . . . . . . . . . . . . . . . . . . . . . 37 2.5.3 DAQ System . . . . . . . . . . . . . . . . . . . . . . . . . . 38 2.5.4 Slow Control and Monitoring system . . . . . . . . . . . . . 42 3 Expected Reactor Antineutrino Flux and Spectrum 47 3.1 Production of Reactor Neutrino . . . . . . . . . . . . . . . . . . . . 47 3.2 Calculation of Reactor Neutrino Flux . . . . . . . . . . . . . . . . 50 3.3 Expected Interaction Antineutrino Spectrum . . . . . . . . . . . . 56 3.4 Systematic Uncertainties of Expected Reactor Neutrino Flux and Spectrum . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 58 3.5 Monte Carlo Simulation . . . . . . . . . . . . . . . . . . . . . . . . 60 3.5.1 Detector Simulation . . . . . . . . . . . . . . . . . . . . . . 60 3.5.2 Monte-Carlo Event Reconstruction . . . . . . . . . . . . . . 64 4 Event Reconstruction 71 4.1 Energy Reconstruction . . . . . . . . . . . . . . . . . . . . . . . . . 71 4.2 Muon Energy Reconstruction . . . . . . . . . . . . . . . . . . . . . 73 4.3 Vertex Reconstruction . . . . . . . . . . . . . . . . . . . . . . . . . 73 5 Energy Calibration 81 5.1 Radioactive Sources . . . . . . . . . . . . . . . . . . . . . . . . . . 81 5.2 Source Driving System . . . . . . . . . . . . . . . . . . . . . . . . . 83 5.3 Energy Conversion Function . . . . . . . . . . . . . . . . . . . . . . 85 5.4 Energy Scale . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 89 5.5 Energy Resolution . . . . . . . . . . . . . . . . . . . . . . . . . . . 90 6 Event Selection for IBD Candidates 93 6.1 Data Sample . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 93 6.2 Backgrounds . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 94 6.2.1 Accidental Background . . . . . . . . . . . . . . . . . . . . 95 6.2.2 Fast Neutron Background . . . . . . . . . . . . . . . . . . . 95 6.2.3 Cosmogenic 9Li=8He Background . . . . . . . . . . . . . . 96 6.2.4 252Cf Contamination Background . . . . . . . . . . . . . . 97 6.3 IBD Selection Requirements . . . . . . . . . . . . . . . . . . . . . . 98 6.3.1 Removal of -Rays from Radioactivity, Noise and Flashers . 99 6.3.2 Removal of Accidental Background . . . . . . . . . . . . . . 103 6.3.3 Removal of Cosmogenic 9Li=8He Background . . . . . . . . 108 6.3.4 Removal of Fast Neutron Background . . . . . . . . . . . . 113 6.3.5 Further Removal of 252Cf Contamination Background . . . 118 6.4 Signal Loss due to Selection Requirements . . . . . . . . . . . . . . 122 6.4.1 Timing Veto with Muon or Trigger Information . . . . . . . 122 6.4.2 Remval of Flasher Events . . . . . . . . . . . . . . . . . . . 122 6.4.3 Removal of 252Cf Contamination Background . . . . . . . . 123 6.5 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 123 7 Estimation of Remaining Backgrounds 127 7.1 Accidental Background . . . . . . . . . . . . . . . . . . . . . . . . . 127 7.2 Fast Neutron Background . . . . . . . . . . . . . . . . . . . . . . . 130 7.3 Cosmogenic 9Li=8He Background . . . . . . . . . . . . . . . . . . . 133 7.4 252Cf Contamination Background . . . . . . . . . . . . . . . . . . 137 7.5 Summary of Backgrounds and Background Reduction since 500 live-day Result . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 139 7.5.1 Background Reduction since 500 live-day Result . . . . . . 140 8 Systematic Uncertainty 143 8.1 Detector Related Uncertainties . . . . . . . . . . . . . . . . . . . . 143 8.1.1 Detection Efficiecny . . . . . . . . . . . . . . . . . . . . . . 143 8.1.2 IBD Selection Efficiency . . . . . . . . . . . . . . . . . . . . 147 8.1.3 Summary of Detection and IBD Selection Efficiencies . . . 154 8.2 Reactor Related Uncertainty . . . . . . . . . . . . . . . . . . . . . 155 8.3 Energy Scale Uncertainty . . . . . . . . . . . . . . . . . . . . . . . 156 8.4 Background Uncertainty . . . . . . . . . . . . . . . . . . . . . . . . 157 8.5 Summary of Systematic Uncertainty . . . . . . . . . . . . . . . . . 159 9 Results of theta13 and Measurement 161 9.1 Observed and Expected IBD Rates . . . . . . . . . . . . . . . . . . 161 9.2 Comparison of Observed and Expected IBD Spectra . . . . . . . . 163 9.3 Rate Only Analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . 163 9.3.1 2 Fitting of Rate Only Analysis . . . . . . . . . . . . . . . 165 9.4 Rate and Spectrum Analysis . . . . . . . . . . . . . . . . . . . . . 167 9.4.1 2 Fitting of Rate and Spectrum Analysis . . . . . . . . . . 167 9.5 Energy and Baseline-dependent Reactor neutrino Disappearance . . . . . 169 10 Fuel-dependent Variation of Antineutrino Yield and Spectrum 173 11 Summary and Discussion 179 iii A Muon Energy Correction 183 A.1 Motivation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 183 A.2 New Muon Energy Correction . . . . . . . . . . . . . . . . . . . . . 183 A.3 Result of New Muon Energy Correction . . . . . . . . . . . . . . . 185 B Charge Correction 189 B.1 Temporal and Spatial Variation of Raw Charge . . . . . . . . . . . 189 B.2 Making Charge Correction . . . . . . . . . . . . . . . . . . . . . . . 189 B.3 Stability Check after Applying Charge Correction . . . . . . . . . . 193 C Development of Flasher Cut 197 C.1 Finding highly flashing QmaxPMT . . . . . . . . . . . . . . . . . . 197 C.1.1 Condition on Qmax/Qtot and Qave/Qmax . . . . . . . . . . . 197 C.1.2 Condition on Large R . . . . . . . . . . . . . . . . . . . . 198 C.1.3 Condition on Accidental Background Rate . . . . . . . . . . 199 C.1.4 Development of Flasher Cut . . . . . . . . . . . . . . . . . . 200 D More Details of Signal Loss due to IBD Selection Requirements203 D.1 Timing Veto with Muon or Trigger Information . . . . . . . . . . . 203 D.2 Removal of Adjacent IBD Pairs . . . . . . . . . . . . . . . . . . . . 207 D.3 Removal of Flasher Events . . . . . . . . . . . . . . . . . . . . . . . 208 D.4 Removal of 252Cf Contamination Background . . . . . . . . . . . . 209 D.4.1 Removal of Hotspot . . . . . . . . . . . . . . . . . . . . . . 209 D.4.2 252Cf background Removal by Temporal and Spatial Cor- relation with Prompt Candidates . . . . . . . . . . . . . . . 209 Bibliography 213Docto