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Improvements to the Resolution and Efficiency of the DEAP-3600 Dark Matter Detector and Their Effects on Background Studies

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Improvements to the Resolution and Efficiency of the DEAP-3600 Dark Matter Detector and Their Effects on Background Studies University of Alberta Improvements to the resolution and efficiency of the DEAP-3600 dark matter detector and their effects on background studies by Kevin S. Olsen A thesis submitted to the Faculty of Graduate Studies and Research in partial fulfillment of the requirements for the degree of Master of Science Department of Physics c Kevin S. Olsen Fall 2010 Edmonton, Alberta Permission is hereby granted to the University of Alberta Libraries to reproduce single copies of this thesis and to lend or sell such copies for private, scholarly or scientific research purposes only. Where the thesis is converted to, or otherwise made available in digital form, the University of Alberta will advise potential users of the thesis of these terms. The author reserves all other publication and other rights in association with the copyright in the thesis and, except as herein before provided, neither the thesis nor any substantial portion thereof may be printed or otherwise reproduced in any material form whatsoever without the author’s prior written permission. Examining Committee Dr. Aksel Hallin, Physics Dr. Carsten Krauss, Physics Dr. Craig Heinke, Physics Dr. Steve McQuarrie, Oncology “I’ve always wanted,” he said dreamily, “to know exactly what would happen when an irresistible force meets an im– movable object.” Paul Sarath, Emeritus Professor of Archaeology, University of Taprobane The Fountains of Paradise Arthur C Clarke Abstract The Dark matter Experiment using Argon Pulse-shape discrimination will be a tonne scale liquid argon experiment to detect scintillation light produced by interactions with weakly interacting massive particles, leading dark matter candidates. The detector will be constructed out of acrylic and use a spherical array of 266 photo-multiplier tubes (PMTs) to count photons produced by an event and will use properties of liquid argon to discriminate signals from background α and γ events. There is currently a smaller prototype, with a cylindrical geometry and using two PMTs, in operation underground at SNOLAB an underground laboratory in eastern Canada. The goal of the prototype detector is to understand the sources of background signals in a detector of our design and to validate our method of distinguishing different types of background radiation. The work presented herein is a series of studies with the common goal of understanding the source of background signals, and improving the resolution and efficiency of the detector. Acknowledgements The road to completing this thesis was long and difficult, but I was aided along the way by so many people who were a part of my life in some way. They helped me prepare for the longer and more difficult paths that lie ahead. My supervisor, Aksel Hallin, was very supportive and encouraging throughout. He always asked the right questions and helped me understand why they were the right ones to ask. He also helped me believe that we were truly capable of doing or building anything (usually when going over the engineering requirements for DEAP). Thank you, Aksel, for all the time and support over the last couple years, thanks for helping me better understand our work, and helping me learn to do research. Thanks, mom, thanks dad. It’s been so nice having you nearby, supporting me, helping me when I need it, and letting me visit Floydie all the time. I got to visit fairly regularly, doing laundry, making you dinner, playing Halo with my brothers, and watching Sunday night HBO. And thank you Fraser and Trevor. We have been on so many adventures together, and I know we have many more to look forward too. Heather, you have been a very special part of my life, we’ve achieved so much together, and have plenty to look forward to in Toronto. Here’s to us and our quest for PhDs. Between getting mad at the trains and hiding from the humidity, we can check out some strange shows that avoid Edmonton, and discover some new places to eat well at. A special thanks goes out to all the people I’ve worked closely with. Thank you to those people at UofA who I go to with silly questions all the time: Carsten Krauss and Chris Howard (good luck in S. Carolina). Chris Jillings, you taught me more about DEAP and the dark matter field than anyone. Thanks to all of those who I worked underground with: Paradorn, Corina, and Luke. Thank you to the rest of the group at Alberta, whatever experiments you work on: Rafael, Berta, Pierre, Alexandra, Rob, Shanoor, Logan, Zach, Mohammed, and Darren. The summer students whose projects overlapped with me have been a tremendous help to my work, and to the collaboration, so thank you Carson Mincheng and Rhys Chouinard. To all the other students on various projects, it’s been a pleasure working with you: Ward, Brad, Sarah, Stephen, Tania, Tom, and Andrei. To all the collaboration members who I’ve worked closely with, it has been a great experience meeting you all and learning from you: Hugh, Tina, Bei, Mark, Peter, Fabrice, Eoin, and Marcin. To the rest of the collaboration members it was great to meet you and to get together for all the meetings, dinners, drinks. Good luck building 3600! Finally, I’d like to thank all my friends at UofA, all those new friends I met along the way, all of those friends from my past. Thanks to every person who ever went to Dewey’s, Sugarbowl, Remedy or RATT with me, and thanks to those that came to the lake, learned to sail or went to the mountains with me to play in the snow. To the University of Alberta Sailing League, we had a number of adventures together, either freezing and wet, or getting way too much sun, thanks for keeping the sport going through our university years: Ian Elliot, Fraz, Chris and Amanda Shafenaker, and Mark Bugiak. I hope to see as many of you as possible at future races, thanks for all those we’ve already had. A very special thanks goes out to everyone who made the I-14 go scary fast with me, Zak, Fraz, Mike Leitch, Ian E., and Ian Chisholm. I’ve always loved the ski trips the most, to Kicking Horse, Revelstoke, Fernie and Marmot. Thanks Josh and Liz, Denis, Tyler, Mark and Mir (congratulations), Fraz and Trev. We’ll get a few more in before I leave. Thanks to those that still need to learn how to ski, your talents at apr`es ski are formidable: Allison and John, Ian and Negar, Tyrone and Cheryl, and Kells. Joe, you have been a great friend over the years, good luck finishing your PhD, and I wish you the best on your future endeavours, wherever they guide you. Denis, it was great making beer and playing chess with you. I had fun watching your skiing improve, take care of Kells. Charlie, it’s been fun watching you grow, thanks for being my bud for so many years. The potlucks were a blast, cooking for you guys was fun, hats off to the hosts, Josh, Ian and Kells. John, thanks for all the Axis, I can hardly wait for the next chance to beat your ass at that game. It’s been a pleasure to also have the company of so many cousins at the UofA for the last couple years, hopefully I will get to see enough of you in the future, thanks for the pintings Cam, Brett, Scott and Lisa (and Fraser, Trevor and Carmen). Table of Contents 1 Introduction 1 1.1 Observational Evidence of Dark Matter ............. 3 1.1.1 The Coma Cluster .................... 3 1.1.2 Rotation Curves of Galaxies ............... 4 1.1.3 Gravitational Lensing ................... 5 1.1.4 The Cosmic Microwave Background Radiation ..... 8 1.1.5 The Bullet Cluster .................... 11 1.2 The Distribution of Dark Matter ................ 14 1.3 Candidates for Dark Matter ................... 17 1.3.1 WIMPs .......................... 17 1.3.2 Axions ........................... 19 1.4 Detection of WIMP Dark Matter ................ 20 1.4.1 Indirect Detection ..................... 22 1.4.2 Direct Detection ..................... 23 1.4.3 Producing WIMPs at Accelerator Labs ......... 26 1.5 Current Dark Matter Searches .................. 26 1.5.1 CDMS ........................... 27 1.5.2 PICASSO ......................... 30 1.5.3 XENON .......................... 31 1.5.4 Controversial Results: DAMA, CoGeNT and XENON100 ........................ 32 2 The DEAP Programme 34 2.1 DEAP-1 .............................. 34 2.2 Detecting Dark Matter with Liquid Argon ........... 36 2.2.1 Scintillation from Noble Gases .............. 36 2.2.2 Argon ........................... 38 2.3 Pulse-shape Discrimination and Analysis ............ 40 2.3.1 Scintillation Efficiency .................. 43 2.3.2 22Na Calibration ..................... 45 2.3.3 PSD Studies ........................ 45 2.3.4 Additional Variables and Data Cuts ........... 46 2.4 Detector Operations ....................... 47 2.5 DEAP-3600 ............................ 49 3 Analysis of Alpha Backgrounds in the DEAP-1 Detector 53 3.0.1 Alpha Particles ...................... 53 3.0.2 Beta and Gamma Radiation ............... 55 3.0.3 Atmospheric Muons .................... 57 3.0.4 Neutrons .......................... 58 3.1 Analysis of High-Energy α Backgrounds in DEAP-1 ...... 59 3.2 Closing Remarks ......................... 67 4 Relative energy corrections for the DEAP-1 detector 69 4.1 Fitting Mechanism ........................ 70 4.2 Limitations ............................ 73 4.3 Applications and Results ..................... 74 4.3.1 Correction Factors .................... 74 4.3.2 Calibration of 2008 Data ................. 77 4.3.3 Analysis of α Backgrounds After Corrections ...... 78 4.3.4 Calibration of 2009 Data ................. 81 4.4 Closing Remarks ......................... 85 5 Measurement of High Efficiency Hamamatsu R877-100 PMT 86 5.1 Experimental Setup and Procedure ............... 87 5.2 Measurements and Results .................... 92 5.2.1 Dark Pulse Height Spectrum ..............
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