Measuring the 7Be Neutrino Flux From the Sun: Calibration of the Borexino Solar Neutrino Detector Steven E. Hardy Dissertation submitted to the Faculty of the Virginia Polytechnic Institute and State University in partial fulfillment of the requirements for the degree of Doctor of Philosophy in Physics R. Bruce Vogelaar, Chair Mark Pitt Ramaswamy Raghavan Tatsu Takeuchi March 31, 2010 Blacksburg, Virginia Keywords: Neutrino physics, calibration, source development Copyright 2010, Steven E. Hardy Measuring the 7Be Neutrino Flux From the Sun: Calibration of the Borexino Solar Neutrino Detector Steven E. Hardy (ABSTRACT) The Borexino solar neutrino detector is a real-time liquid scintillator detector designed to measure sub-MeV neutrinos. With its unprecedented level of radio-purity, Borexino is poised to provide the most precise measurements to-date of solar neutrino and geo-antineutrino fluxes. However, in order to reduce the systematic errors to sub-5% levels, the detector must be care- fully calibrated to understand, among other things, the position and energy reconstructions. To that end, the Virginia Tech component of the Borexino collaboration has constructed a system for deploying and locating calibration sources within the detector. The system was used in four separate calibration campaigns and deployed numerous sources in almost 300 locations throughout the detector. The data from the calibrations have already resulted in the reduction of several sources of systematic error by a factor of two or more. With the results from the calibration, the Borexino detector has entered a new era of low- energy, high-precision, neutrino detection. This work was supported by NSF Grant 0802114 Dedication To my wife Beth, and parents, Don and Lori. I love you all and will never forget all that you have done for me in my life. iii Acknowledgments I owe a tremendous debt of gratitude to all those that have given their help and support during my graduate career. The work presented in this dissertation would not have been possible without the involvement of a great number of people. I would first like to thank my advisor, Prof. Bruce Vogelaar for his support and guidance over the last five years. His willingness to let his students explore their own ideas and take on new projects has made my graduate career truly a pleasure. I would also like to thank Prof. Raju Raghavan for the interest he has taken in my professional development and for being a wonderful resource for any physics or chemistry questions. None of this work could have been accomplished without the staff of the Virginia Tech Physics department. Tina, Betty, Chris, Diane, Tammy, Judy, Nora, and Lisa have always been willing to help, and even forfeit their lunch hour when my plane tickets to New Zealand were mysteriously cancelled. I would also like to thank Roger & Travis for a truly endless stream of computing support over the last six years and for not getting mad when I went through three laptop keyboards in the course of writing this dissertation. I also owe a great deal of thanks to John, Melvin, Scott, Ron, Fred and Norm in the electronics and machine shops for their help and humor. Thanks also to Tom Wertalik in the glass shop for sealing every one of our source vials, and for not losing faith when they started exploding. iv I have had the pleasure of working with many wonderful and hilarious students in Room 10 over the years. Thank you to: Brad Williams and Mike Sperry for annoying each other in the most hilarious ways, Matt Joyce for sharing an appreciation of Olympic curling and for always wanting a coffee, Derek Rountree for breaking a radioactive source inside of my lab book, and Russ Mammei for always knowing where to find whatever I was looking for; you have all helped to make days in the lab go by a lot easier. I also owe a great deal of thanks to Henning Back for his help and guidance throughout both my undergraduate and graduate career. A special thank you also to Szymon Manecki for taking the reigns over the past year while I have been writing; you are a pleasure to work with and your ability to develop new ways of ruining rental cars is unparalleled. I also would like to thank the Mach4 analysis group, particularly: Richard Saldanha, Ben Loer, and Alvaro Chavarria for always being willing to help and for remaining calm when bugs were pointed out. Thank you also to Kevin McCarty, Prof. Cristiano Galbiati, and Prof. Frank Calaprice for inviting me to join the analysis effort several years ago. I owe a tremendous thank you to the entire Borexino collaboration for their efforts and help. In particular thank you to Andrea and Aldo Ianni, Augusto Goretti, Yury Suvorov, George Korga, Paolo Lombardi, Sergio Parmeggiano, and Augusto Brigatti for their help during the fabrication and installation of the insertion system. I also would like to express my gratitude to Steve Kidner and Laszlo Papp, without their help, none of this work would have been possible; my stays in Italy would have been a lot more difficult if not for their wives, Cecile and Anita, who helped make Assergi feel more like home. Thank you to my parents and brother for their support in everything I have done in my life. Many of the figures in this dissertation would look much worse if not for my brother and his Photoshop prowess. v Finally, thank you to my wife Beth for her help and support throughout this entire process. Your patience throughout the many trips to Italy and late nights in the lab have made the last six years so much easier. vi Contents 1 Introduction 1 2 Neutrino Physics 3 2.1 Neutrino Masses and Mixing . 3 2.1.1 Vacuum Oscillations . 4 2.1.2 Neutrino Oscillations in Matter . 12 2.2 Solar Neutrinos . 18 2.2.1 PP chain . 21 2.2.2 CNO cycle . 25 2.3 Outlook . 29 2.3.1 Neutrino Mass . 29 2.3.2 Neutrino-less Double Beta Decay . 32 2.3.3 The value of θ13 .............................. 35 2.3.4 Elemental Abundances . 36 vii 3 The Borexino Detector 41 3.1 Neutrino Detection & Their Sources in Borexino . 42 3.1.1 7Be Neutrinos . 44 3.1.2 CNO and pep neutrinos . 50 3.1.3 8B Neutrinos . 51 3.1.4 Geo and supernova neutrinos . 53 3.2 Detector Design . 61 3.2.1 Structural components . 61 3.2.2 Scintillator . 67 3.2.3 Photomultiplier Tubes . 79 3.2.4 Purification Plants . 82 3.3 Data Acquisition and Software . 85 3.3.1 DAQ . 85 3.3.2 Software Reconstruction . 90 3.4 Backgrounds . 104 3.4.1 Decay Chains . 105 3.4.2 Other isotopes . 112 3.4.3 Muons and Cosmogenics . 116 3.4.4 CTF . 120 viii 4 Motivation For A Calibration 125 4.1 Fiducial Volume Determination . 126 4.2 Energy Response . 129 4.2.1 Spectral Impacts . 131 4.2.2 Electronics Saturation . 132 4.3 Other Effects . 135 4.3.1 Trigger Threshold . 135 4.3.2 Photomultiplier Tube Timing . 137 4.4 Design Considerations for a Calibration System . 139 4.4.1 Maintaining Purity . 139 4.4.2 Accuracy of Position Determination . 142 4.4.3 Mechanical Considerations . 143 4.4.4 Adopted Design Philosophy . 147 5 A Source Imaging System for Borexino 149 5.1 Source Location System Hardware . 150 5.1.1 Digital Cameras . 150 5.1.2 Camera Housing . 152 5.1.3 Cabling . 158 5.1.4 Camera System Control . 161 ix 5.2 Software Reconstruction of Images . 163 5.2.1 Camera System Geometry . 164 5.2.2 Transforming Between Simulated and Real Camera Geometries . 165 5.2.3 Position Reconstruction From Images . 170 5.2.4 Camera Calibration . 174 5.3 Analysis of the Vessel Volumes . 179 5.3.1 Obtaining the Vessel Shape . 180 6 A Calibration Source Deployment System For Borexino 199 6.1 In-Detector Hardware . 200 6.1.1 Insertion Rods . 201 6.1.2 Tether & Fiber Optic . 205 6.1.3 Source Coupler . 208 6.2 Glovebox, Cross, and Associated Hardware . 210 6.2.1 Glovebox . 212 6.2.2 Cross . 217 7 Calibration Source Production 225 7.1 Calibration Source Design . 225 7.1.1 Containment Vials . 226 7.1.2 Design Variations For Different Particle Types . 234 x 7.2 Preparation of the Sources . 244 7.2.1 Source Loading Station . 244 7.2.2 Production of 14C Sources . 249 7.2.3 Production of Rn Sources . 255 7.2.4 Gamma Source Production . 259 7.3 Characterization of Calibration Sources . 262 7.3.1 Quality Control Detector . 263 7.3.2 Relative Light Yield Measurement . 269 7.3.3 Measurements of Sources Used In Calibrations . 278 8 Results From The Calibrations 287 8.1 Performance of The Calibration System . 287 8.1.1 On-Axis Calibration . 288 8.1.2 Off-Axis Calibrations . 291 8.2 Radioactive Contamination Introduced During Calibrations . 296 8.2.1 238U Contamination . 296 8.2.2 232Th Contamination . 304 8.2.3 85Kr Contamination Measurement . 309 8.2.4 Contamination In The 7Be-11C Valley . 318 8.3 Reconstruction of Calibration Data . 324 xi 8.3.1 Energy Reconstruction Performance . 325 8.3.2 Position Reconstruction Performance . 327 8.3.3 Attempts to Resolve the Z offset . 332 8.4 Systematic Errors After Calibration . 345 8.4.1 Impact on the 7Be Neutrino Analysis . ..