The Development of Active Water Scintillator for the T2K Experiment Marieke Navin Department of Physics and Astronomy University of Sheffield Thesis submitted for the Degree of Doctor of Philosophy in the University of Sheffield · March 2010 · To Jon and Tilly Abstract The Tokai to Kamioka (T2K) experiment started taking data in January 2010. Using the most intense source of man-made neutrinos the experiment will study oscillations of an off-axis muon neutrino beam between the JPARC accelerator complex and the Super-Kamiokande Detector 295 km distant. The unknown mixing angle θ13 will be measured by observing the νµ → νe oscillation. The neutrino energy spectrum, flavour content and interaction rates of the unoscillated beam will be measured by a set of detectors located 280 m (ND280) from the neutrino production target, which is where the UK effort is focussed. Currently some of the ND280 sub-detectors contain passive water lay- ers that are added to provide an oxygen rich target to reduce the system- atic uncertainties that will arise by comparing signals between the Super- Kamiokande water Cherenkov detector and the primarily plastic ND280. This thesis presents the motivation for and development of active water- based scintillator layers to replace the passive layers thereby providing extra kinematic information of neutrino interactions. It is found to be a worthwhile endeavour if certain experimental challenges can be overcome. Contents Acknowledgements xv 1 Neutrino Physics 1 1.1 Neutrinos in the Standard Model . 1 1.2 A brief history of the neutrino . 3 1.2.1 Postulation . 3 1.2.2 Experimental Detection of Neutrinos . 3 1.3 Neutrino Mass Measurements . 4 1.4 Neutrino Oscillation . 5 1.4.1 Theory . 5 1.5 Evidence for Neutrino Oscillation . 7 2 2 1.5.1 Measurements of sin θ12 and ∆m12 ........... 7 2 2 1.5.2 Measurements of sin θ23 and |∆m23| .......... 12 2 1.5.3 Measurements of sin θ13 ................. 17 1.5.4 LSND and MiniBooNE Results . 18 1.5.5 Neutrino Oscillation Parameters - Summary . 19 1.6 Conclusions . 20 2 The T2K Experiment 21 2.1 Introduction . 21 2.2 Aims of the T2K Experiment . 21 i 2.2.1 νµ Disappearance Measurement . 21 2.2.2 νe Appearance Measurement . 22 2.2.3 νµ → ντ oscillation and search for sterile neutrinos . 23 2.2.4 Future Extension: CP Violation . 25 2.3 T2K Experimental Overview . 25 2.3.1 Tokai to Kamioka . 25 2.3.2 The νµ Beam ....................... 26 2.3.3 Off-Axis Beam Configuration . 27 2.3.4 Eν Reconstruction . 28 2.3.5 Far/Near Correction . 28 2.4 The T2K Near Detector . 29 2.4.1 On-Axis Detector: INGRID . 30 2.4.2 Off-Axis Detector: ND280 . 31 2.4.3 Magnet . 31 2.4.4 Pi-Zero Detector (P0D) . 31 2.4.5 Tracker: TPC . 33 2.4.6 Tracker: FGD . 33 2.4.7 Electromagnetic Calorimeter (Ecal) . 34 2.4.8 Side Muon Range Detector (SMRD) . 35 2.4.9 Current Status of the ND280 . 36 2.4.10 Intermediate Detector . 37 2.4.11 The Far Detector: Super-Kamiokande . 37 2.4.12 Future Extension: Hyper-Kamiokande . 38 2.4.13 Conclusions . 39 3 Motivation for Active Water in the T2K Near Detector 40 3.1 Introduction . 40 3.2 Water in the T2K Near Detector . 40 ii 3.3 Simulation of Sub-Detector with Active Water Component . 41 3.3.1 Neutrino Interaction Monte Carlo Generation . 42 3.3.2 Construction of a ND280 Sub-Detector . 43 3.3.3 Physics List . 44 3.3.4 Method . 44 3.4 Results . 45 3.4.1 Validation . 45 3.4.2 Minimum Ionising Particles in the simulation . 46 3.4.3 Threshold for water-based scintillator . 48 3.5 Conclusions . 49 4 Water-Based Scintillator Development 50 4.1 Introduction . 50 4.2 Water-based liquid scintillators . 53 4.3 Liquid Scintillator Quicksafe A . 54 4.4 Initial Investigations . 55 4.5 TRIUMF M11 Beam . 56 4.5.1 A Typical Spectrum - Muon Beam . 59 4.5.2 A Typical Spectrum - Proton Beam . 60 4.6 Beam Tests 1: Improvements to Quicksafe A . 61 4.6.1 Method . 61 4.6.2 Results . 62 4.6.3 Investigation into Experimental Uncertainties . 63 4.6.4 Conclusion . 63 4.7 Beam Tests 2: Alternative Solvents and Fluors . 63 4.7.1 Method . 65 4.7.2 Results . 65 4.7.3 Conclusion . 65 iii 4.8 Beam Tests 3: Alternative Solutions . 66 4.8.1 Method . 68 4.8.2 Results . 68 4.8.3 Conclusion . 69 4.9 Beam Tests 4: An Aqueous Scintillator with Fluor-Containing Nanosuspensions . 70 4.9.1 Introduction . 70 4.9.2 Emission Spectrum of the Nanosuspension . 70 4.9.3 Transparency of Nanosuspension . 70 4.9.4 Beam Tests of Nanosuspension . 71 4.9.5 Summary of Nanosuspension Results . 72 4.9.6 Conclusion . 74 4.10 Conclusion . 74 5 Design Tests of Active Water Prototype Detector 75 5.1 Observations of Quicksafe A and DIPN . 75 5.2 Observations of Default Scintillating Mixture . 75 5.3 Mould Inhibitor Tests . 76 5.3.1 Results . 77 5.3.2 TRIUMF Beam Tests of Mould Inhibitors . 77 5.3.3 Conclusion . 77 5.4 Cloud Point of the Scintillating Mixture . 79 5.4.1 Method . 80 5.4.2 Results - Cloud Point Test . 80 5.4.3 Results - Beam Test . 81 5.4.4 Conclusion . 81 5.5 Cloud Point of the Scintillating Mixture with added Biological Inhibitors . 82 iv 5.5.1 Results . 82 5.5.2 Conclusion . 82 5.6 Mechanical Design of Prototype Detector . 83 5.7 Accelerated Ageing Tests of Plastics . 84 5.8 Long Term Tests of Correx and Polycarbonate . 85 5.8.1 Results . 85 5.8.2 Conclusion . 86 5.9 Adhesive Tests . 86 5.9.1 Sample Preparation . 87 5.9.2 Results . 88 5.9.3 Conclusion . 88 5.10 Reflectance Measurements . 91 5.10.1 Method . 91 5.10.2 Results . 92 5.10.3 Conclusion . 94 5.11 Long Term Tests of Painted Correx . 95 5.11.1 Results . 95 5.11.2 Long Term Tests of Correx Painted with Anti-Fouling paint . 95 5.11.3 Conclusion . 96 5.12 Strength Tests of Correx . 96 5.12.1 Results . 96 5.12.2 Conclusion . 97 5.13 Discussion . 97 6 Detector Prototype 102 6.1 Construction of a Detector Prototype . 102 6.1.1 Early Designs of Prototype . 102 v 6.1.2 Final Design of Prototype . 104 6.1.3 Final Fibre Feedthrough . 106 6.1.4 Filling and Draining . 107 6.2 Comparison Between Active Water and Plastic Scintillator Detector Prototypes . 107 6.2.1 The Muon Telescope . 107 6.2.2 PMT Tests: Single Photoelectron Measurements . 110 6.2.3 Results . 110 6.2.4 Conclusion . 111 7 Conclusions 113 Bibliography 117 vi List of Figures 1.1 Survival probability of muon neutrinos as a function of the neutrino energy . 7 1.2 Flux of solar neutrinos measured by SNO . 9 1.3 The ratio of measured to expected νe flux from reactor exper- iments . 11 1.4 Ratio of the observed anti-neutrino spectrum to the expecta- tion for no oscillation versus L0/E for KamLAND . 12 1.5 Plot to show the determination of oscillation parameters from solar and reactor neutrino sources . 13 1.6 Zenith angle distribution for the atmospheric neutrinos used in SK oscillation analysis . 14 1.7 L/E distributions observed in the Super Kamiokande experiment 15 1.8 Reconstructed neutrino charged current energy spectrum from the MINOS experiment . 16 1.9 Plot to show the determination of oscillation parameters from atmospheric and long baseline accelerator neutrino sources . 17 1.10 Plot of constraints on θ13 from the global analysis of neutrino data ................................ 18 1.11 Exclusion contours for MiniBooNE and KARMEN compared to the LSND allowed region . 19 vii 2.1 Plot of the ratio of the reconstructed neutrino energy distribu- tion with oscillations to one without oscillations for the T2K experiment . ..