Electron Antineutrinos in the Water Phase of the SNO+ Experiment by Pawel Mekarski A thesis submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy Department of Physics University of Alberta c Pawel Mekarski, 2018 Abstract The SNO+ experiment will soon complete its commissioning and begin searching for the neutrinoless double beta decay of tellurium, loaded within its liquid scintillator. As a large-scale (780 tonne) liquid scintillator detector, SNO+ will also be well positioned to make a precision measurement of antineu- trinos, produced from nearby nuclear reactors. Measuring these antineutrinos would provide direct information about the cores of these reactors and enable a study of neutrino properties. In anticipation, a first search for antineutrinos was performed over 69.7 live days of data collected while the SNO+ detec- tor was filled with water, an intermediate commissioning phase. A combina- tion of Monte Carlo simulations and measurements with radioactive calibration sources were used to determine what the antineutrino signal (characterized by a coincident positron and neutron) would look like in the detector. The neutron modeling was first validated by performing a series of measure- ments of an americium-beryllium (AmBe) neutron source at the University of Alberta. The neutron interactions were detected by irradiating various tar- gets and measuring the γ rays of the resulting reactions using a High Purity Germanium detector. Following Monte Carlo simulations of antineutrinos in the SNO+ detector, a search algorithm was developed to distinguish this signal ii from naturally occurring backgrounds. Lastly, another AmBe source was placed within the SNO+ detector to exactly characterize its neutron detection capa- bilities. Searching the detector-collected data yielded a total of 5 antineutrino candidate events in the region of interest. This was in agreement with expec- tations from another Monte Carlo simulation that was developed to model the detector backgrounds for this specific signal. From this, an upper limit at 90% confidence was determined for the flux of antineutrinos from nuclear reactors passing through the SNO+ detector of (1.45 0.23) x 107 ν¯/(cm2 s). ± iii Preface Much of the work performed in this thesis was done in contribution to the multi-national SNO+ collaboration. As such, some of the work was performed using tools developed by the researchers involved. The majority of the work in Chapter 4 has been published as Duke, M. Hallin, A. Krauss, C. Mekarski, P. and Sibley, L. (2016) “A precise method to determine the activity of a weak neutron source using a germanium detec- tor.” Applied Radiation and Isotopes 116:51–56. I performed the experimental measurements using the equipment of Krauss, C. with the assistance of Sibley, L. I was solely responsible for the development of the simulation. I also per- formed the data analysis and created the manuscript for submission. Duke, M. Hallin, A. Krauss, C. and Sibley, L. were involved in scientific discussion and manuscript revision. This work was performed under the supervision of Hallin, A. and Krauss, C. The simulations of the SNO+ detector, described in Chapters 5, were per- formed by myself using the software programs developed by the SNO+ collab- oration. The background simulation, also detailed in Chapter 5 was my own work. The various data presented in Chapters 5–8, collected using the SNO+ detector, were the result of a collaborative effort in detector operation. The design of an antineutrino search algorithm (Chapter 5), its application on the data and simulations (Chapters 5–8), and its optimization (Chapter 6) were my own work. I designed and created the analysis algorithms to determine both the an- tineutrino signal expectation (Chapter 7) and subsequently the limit on the antineutrino flux (Chapter 8). I executed these algorithms on data and simu- iv lation using the computing resources, provided by Compute Canada, that were allocated to Krauss, C. All analyses, on the simulations and data in Chapters 4–8, are my own work that was performed under the supervision of Hallin, A. and Krauss, C. v Acknowledgments Over the five years during which I performed my PhD research, there were innumerable people who supported me on this journey. I’d first like to greatly thank my supervisor Dr. Carsten Krauss, who’s guidance and knowledge have immensely helped me develop as a researcher. Additionally, I’d like to thank Dr. Aksel Hallin for the direction and insight he provided as I progressed through my many projects on the way to this dis- sertation. This work was also made possible by many agencies who’s support allowed me to dedicate a much larger amount of my efforts to research. A big thank you to the Natural Sciences and Engineering Research Council, Alberta Innovates, and the Killam Trust for this support. Moreover, I am very appre- ciative of the computing resources provided by Compute Canada that made my data analyses possible. This work was only possible due to the collaborative effort of over a hundred members of the SNO+ experiment. I am profoundly grateful for everyone’s con- tribution to make this experiment a reality and a success. I’d like to specifically thank Dr. Christine Kraus and Dr. Erica Caden for their roles in enabling me to help locally in Sudbury with multiple aspects of the detector. I’d like to thank the Department of Physics at the University of Alberta for hosting me and providing the resources with which I have performed my research, and specifically to Sarah MacKinnon who guided me through the many years of my program. My time on the SNO+ experiment was greatly enhanced by my fellow re- searchers at the University of Alberta: thank you Kalpana, David, Aleksandra, Karin, and Juan Pablo. A large thanks goes to my office mates, with who I vi would spend the day, sharing many discussions scientific or otherwise: Court- ney, Tom, Jie, Pooja, and Joe. You made my time as a graduate student even more enjoyable. A special thanks goes to Dr. Logan Sibley, who’s impact on my life I cannot begin to describe. Lastly, I’d like to thank my dear wife Michelle, who embarked on this adven- ture with me, and Sarge, who is there everyday for us. I’m excited to continue and see where life takes us all next! vii Contents List of Tables xiii List of Figures xiv 1 Introduction 1 1.1 HistoricalBackground . 2 1.1.1 FirstDiscovery ....................... 2 1.1.2 OtherEarlyExperiments. 3 1.1.3 NeutrinoOscillationDiscovery. 3 1.2 CurrentStatusoftheField. 4 2 Antineutrino Theory 6 2.1 AntineutrinoSources ........................ 6 2.1.1 Atmosphericν ¯ ........................ 6 2.1.2 Supernovaν ¯ ......................... 7 2.1.3 Reactorν ¯ .......................... 8 2.1.4 Geoneutrinos ........................ 10 2.2 InverseBetaDecay ......................... 11 2.3 NeutrinoOscillation . 13 2.3.1 MatterEffects........................ 16 2.4 MajoranaNeutrino ......................... 17 2.4.1 NeutrinolessDoubleBetaDecay . 17 viii 3 The SNO+ Detector 20 3.1 SNO+Detector........................... 20 3.1.1 DetectorOverview . 20 3.1.2 PhasesofOperation . 21 3.1.3 DifferencesfromSNO. 22 3.1.4 CurrentStatus ....................... 23 3.2 PhysicsGoals ............................ 24 3.3 ParticleDetection. 26 3.3.1 Interaction.......................... 26 3.3.2 DetectorTriggerLogic . 27 3.3.3 DataCollection . 28 3.3.4 EventReconstruction. 29 3.4 AntineutrinosinSNO+. 31 3.4.1 LocalSources ........................ 31 3.4.2 InteractionExpectations . 33 3.4.3 InverseBetaDecayinSNO+ . 37 3.5 Current Reactor Antineutrino Experiments . 38 3.5.1 DetectionStrategies . 38 3.5.2 CurrentExperiments . 39 4 Neutron Measurements and Modeling 41 4.1 NeutronMeasurements . 41 4.1.1 NeutronSource ....................... 41 4.1.2 NeutronReactions . 43 4.1.3 ExperimentalSetup. 44 4.1.4 MeasurementDetails . 46 4.1.5 BackgroundMeasurements. 48 4.1.6 PeakActivityDetermination. 50 4.2 Simulation.............................. 52 ix 4.2.1 GeometryDefinition . 52 4.2.2 PhysicsProcesses . 53 4.2.3 InelasticScatteringProcess . 53 4.2.4 DataCollection . 54 4.3 Results................................ 55 27 27 4.3.1 Al(n,n′) and Al(n,p)Reactions. 55 4.3.2 27Al(n,γ)Reaction ..................... 57 4.3.3 1H(n,γ)Reaction ...................... 58 4.4 ModelComparison ......................... 59 4.4.1 NeutronActivityDetermination . 59 4.4.2 ConsiderationsandCorrections . 59 4.4.3 NeutronSourceActivity . 63 4.5 InelasticScatteringEvaluation. 66 4.5.1 CrossSections........................ 66 4.5.2 Measurements........................ 68 4.5.3 Simulation.......................... 69 4.5.4 ResultsandDiscussion . 69 4.6 Conclusions ............................. 72 5 Simulations and the Antineutrino Search Algorithm 74 5.1 SNO+MonteCarloSimulation . 75 5.1.1 AntineutrinoSimulation . 75 5.2 AntineutrinoEventSelection. 78 5.2.1 TimeCoincidence. 78 5.2.2 PositionDifference . 79 5.2.3 NumberofPMTs...................... 81 5.3 SearchAlgorithm .......................... 82 5.3.1 AlgorithmOverview . 82 5.4 BackgroundMonteCarlo. 84 x 5.4.1 DrawbacksandAlternative . 85 5.4.2 ToyMonteCarloApproach . 85 5.4.3 EventGeneration . 86 5.4.4 ComparisontoData . 92 5.4.5 Conclusions ......................... 96 5.5 WavelengthShifterImprovements . 96 5.5.1 Overview .......................... 96 5.5.2 WavelengthShifterDetails . 97 5.5.3 ReactorAntineutrinoEffect . 98 5.5.4 Outlook ........................... 99 6 Optimizing the Search for Antineutrinos 102 6.1 AmBeSourceCalibration . 102 6.1.1 AmBeSource