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PRECISION MEASUREMENTS of DEUTERON PHOTODISINTEGRATION USING LINEARLY POLARIZED PHOTONS of 14 and 16 MEV by Matthew Blackston

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PRECISION MEASUREMENTS of DEUTERON PHOTODISINTEGRATION USING LINEARLY POLARIZED PHOTONS of 14 and 16 MEV by Matthew Blackston PRECISION MEASUREMENTS OF DEUTERON PHOTODISINTEGRATION USING LINEARLY POLARIZED PHOTONS OF 14 AND 16 MEV by Matthew Blackston Department of Physics Duke University Date: Approved: Henry R. Weller, Supervisor Calvin R. Howell Mark C. Kruse Roxanne P. Springer Ying Wu Dissertation submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy in the Department of Physics in the Graduate School of Duke University 2007 ABSTRACT PRECISION MEASUREMENTS OF DEUTERON PHOTODISINTEGRATION USING LINEARLY POLARIZED PHOTONS OF 14 AND 16 MEV by Matthew Blackston Department of Physics Duke University Date: Approved: Henry R. Weller, Supervisor Calvin R. Howell Mark C. Kruse Roxanne P. Springer Ying Wu An abstract of a dissertation submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy in the Department of Physics in the Graduate School of Duke University 2007 Copyright c 2007 by Matthew Blackston All rights reserved Abstract A precision measurement of the d(~γ, n)p reaction was performed at the High Intensity γ-ray Source (HIγS), which is located at the Duke Free Electron Laser Laboratory on the campus of Duke University. The γ-ray beams were nearly 100% linearly polarized, allowing the angular distributions of both the analyzing power and unpolarized cross section to be measured at 14 and 16 MeV. The photons were incident on a heavy water target and the neutrons from the photodisintegration reaction were detected using the Blowfish detector array, which consists of 88 liquid scintillator detectors with large angular coverage. A transition matrix element (TME) analysis was performed on the data which allowed the amplitudes of the TMEs which contribute to the reaction at these energies to be extracted. This was done by invoking Watson’s theorem, which fixes the relative TME phases using the n-p scattering phase shifts, leaving the TME amplitudes as free parameters in fits to the data. The results indicated very good agreement with a recent potential model calculation for the amplitudes of the three electric dipole (E1) p-waves, which account for over 90% of the cross section at these energies. The extracted TME amplitudes were then used to construct the observable which enters into the Gerasimov-Drell-Hearn (GDH) sum rule integrand. The results are the first experimental indication of a positive value of the GDH integrand in the region near photodisintegration threshold. A positive value at these energies has been shown by theory to be due to relativistic contributions. iv Acknowledgements As I look back over these last few years of my life, I am keenly aware of how indebted I am to so many people. This list of people to thank is long, but it is worth the effort to express my appreciation. I could not ask for more from my advisor, Henry Weller. When your door is open you have always been willing to endure me coming in to bombard you with questions or ideas. I have probably learned just as much physics through our conversations together as I have from many textbooks. I want to thank those who have been part of the Capture group during my time here: Mohammad, Brent, Seth, Sean, Paul, Dick, Mark, Ralph, Moshe, Amanda, and Yannis. You are the ones I have rubbed elbows with in the lab and it is from you that I have learned how to do experimental physics. I also want to thank Matt and Alex for your friendship, both personal and professional, over these years. Your presence at the lab has made coming to work something I look forward to. In addition, I grateful to the technical staff at TUNL: John, Richard, Chris, Bret, and Patrick. You have always been willing to assist me for just about any reason I’ve come up with. This experiment would not have been possible without the help and expertise of the scientists from the University of Saskatchewan and the University of Virginia who are primarily responsible for building and maintaining the Blowfish array here at HIγS: Blaine, Rob, Brad, Ru, Brian, and Ward. The same is true of the scientists and staff at the FEL who were responsible for the great beams used in this experiment: Ying, Stepon, Jingyi, Vern, and Maurice. I would also like to thank Paolo, through whom I have learned a great deal of physics (not to mention world history) which is very relevant to the work presented v in this dissertation. I have appreciated your friendship just as much as I have appre- ciated how much you have taught me. Thank you to Michael Schwamb for sharing your theoretical calculations with us and for making yourself available to answer our many questions. There are many outside of work who have played a role in my life that has been no less valuable to my education here. I want to thank Sam, Matt, Tim, Eric, James, Joe, David, and Michael for always pushing me to follow Christ in every area of my life and for accepting me because of and in spite of all my strengths and weaknesses. A special thanks goes to Todd, who has modeled the love of Christ to me in countless ways. You have taught me how to see life in a way that has allowed me to do my work with greater joy. I want to thank our church community, which is made up of people who have prayed for and encouraged me and my family through this process as well as the various events of our lives over these years. So many come to mind: the Wardles, Gerbers, Bakers, Carters, Kirks, Fleenors, and Kapurs. In particular, what can I say about how much the friendship of the Jacksons has meant to us? Words don’t do justice. All of you have been our “family away from family” and it has been a joy to share life with you. It would be impossible for me to be writing this if it were not for the love, sup- port, and encouragement of my parents over many years. You have put me in the circumstances that have allowed me to succeed. I’ll always be grateful to you and be proud to be called your son. To my sister and brothers, Amy, Andy, and Michael I also say thanks. You too have loved and supported me for many years and have played an important role in shaping who I am today. In addition, I thank my wife’s family for the ways in which you have helped and supported us during my education. And finally, the greatest thank you is reserved for my family. Daniel and Kate, vi you have brought more joy into my life than I could have ever imagined. Just a few moments with you reminds me about what life is really about, and in this way, you have both made me a better physicist. Cortney, none of this would be possible without your presence in my life both as a companion and as the one who helps me to persevere. It is amazing to see where we are now compared to where we were when we arrived in Durham. You have been the heart and soul of our family and you have sacrificed much in order for me to reach this goal. You have been a constant in the midst of several years filled with considerable change. It is with deep love and gratitude that I dedicate this dissertation to you. vii Contents Abstract iv Acknowledgements v List of Figures xv List of Tables xx List of Acronyms xxiii 1 Introduction 1 1.1 Background . 1 1.2 Goals and Motivation . 4 1.2.1 The Observables . 4 1.2.2 Testing Theoretical Calculations . 6 1.2.3 Transition Matrix Elements . 7 1.2.4 The GDH sum rule . 8 1.3 Experimental Tools . 11 2 Theory 13 2.1 Introduction . 13 2.2 Kinematics . 13 2.3 Transition Matrix Elements . 15 2.4 Potential Model Calculation . 17 2.4.1 NN Potential . 17 2.4.2 One-body Currents with Siegert Operators (N) . 18 2.4.3 Meson Exchange Currents (MEC) . 20 viii 2.4.4 Isobar Configurations (IC) . 21 2.4.5 Relativistic Currents (RC) . 22 2.4.6 SAPM Predictions . 23 2.5 Effective Field Theory . 23 2.6 The GDH Sum Rule . 26 2.6.1 Overview of the Derivation . 27 2.6.2 GDH for the Deuteron . 28 2.6.3 SAPM Prediction . 29 3 Description of the Experiment 31 3.1 Introduction . 31 3.2 The High Intensity ~γ-ray Source (HIγS) . 31 3.2.1 The Duke OK-4 Free Electron Laser (FEL) . 32 3.2.2 γ-ray Production . 35 3.2.3 Beam Properties . 37 3.2.4 The Recent HIγS Upgrade . 42 3.3 HIγS Collimator Hut and Target Room . 44 3.3.1 Collimator Hut . 44 3.3.2 Target Room . 46 3.3.3 Axial Alignment . 47 3.4 Beam Monitoring . 48 3.4.1 HPGe: Energy, Energy Spread, and Flux . 48 3.4.2 Paddle System: Relative Flux Monitor . 52 3.4.3 Blowfish: Beam Polarization Axis . 54 3.5 The Blowfish Detector Array . 55 ix 3.5.1 Array Design . 55 3.5.2 Blowfish Detectors . 56 3.5.3 Gain Monitoring System . 58 3.5.4 Targets . 60 3.6 Electronics . 61 3.6.1 Storage Ring Bunch Signal . 62 3.6.2 Acquisition Windows . 62 3.6.3 Blowfish or “Neutron” Trigger . 64 3.6.4 Pedestal Trigger . 65 3.6.5 HPGe or Flux Trigger . 66 3.6.6 LED Flasher Trigger . 67 3.6.7 Flasher Monitor Trigger .
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