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Enrico Fermi Institute the University of Chicago COMPOSITION AND SPECTRUM OF COSMIC RAYS AT THE K. EE MEASURED BY THE CASA-BLANCA EXPERIMENT BY JOSEPH WESTBROOK FO\VLER MARCH 2000 Enrico Fermi Institute The University of Chicago Dissertation THE UNIVERSITY OF CHICAGO COMPOSITION AND SPECTRUM OF COSMIC RAYS AT THE KNEE MEASURED BY THE CASA-BLANCA EXPERIMENT A DISSERTATION SUBMITTED TO THE FACULTY OF THE DIVISION OF THE PHYSICAL SCIENCES IN CANDIDACY FOR THE DEGREE OF DOCTOR OF PHILOSOPHY DEPARTMENT OF PHYSICS. BY JOSEPH WESTBROOK FOWLER CHICAGO, ILLINOIS MARCH 2000 To my family ABSTRACT The energy spectrum and primary composition of cosmic rays with energy between 3 x 10 14 and 3 x 10 16 eV have been studied using the CASA-BLANCA detector. In this energy range, a "knee" in the spectrum has been recognized for over 40 years. The astrophysical origins of the knee remain unknown. Various models propose that the knee indicates an inherent feature of cosmic ray sources, a new type of source, or a change in the propagation of cosmic rays through the Galaxy. Measuring the spectrum and elemental composition of cosmic rays near the knee can help to address the problem. The favored model of acceleration in supernova shock waves predicts that cosmic rays with energy above,....., 1016 eV consist entirely of heavy nuclei. The measurements were made by BLANCA, a new array of 144 angle-integrating Cherenkov light detectors located at the CASA-MIA site in Utah. CASA data on particle density are used to find the core and direction of air showers, while BLANCA measures the lateral distribution of Cherenkov light about the core. The Cherenkov detectors receive light emitted throughout the air shower rather than relying only on the few particles which reach the ground. This advantage over air shower arrays makes it possible for BLANCA to measure shower energy with little composition bias and to estimate accurately the depth of shower maximum. The differential flux of cosmic rays measured by BLANCA exhibits a knee in the range of 2-3 Pe V with a width of approximately 0.5 decades in primary energy. The power law indices of the differential flux above and below the knee are - 2. 72 ± 0. 02 and -2.95 ± 0.02. The data on mean shower depth indicate that the composition is lighter at 3 Pe V than below the knee and that it becomes heavier with increasing en- ergy above 3 PeV. Cherenkov measurements are interpreted using the predictions of the CORSIKA Monte Carlo air shower simulation coupled with each of four hadronic interaction codes (QGSJET, VENUS, SIBYLL, and HDPM). The distribution of air shower depths can be reproduced well at all energies by the QGSJET and VENUS models, and these distributions suggest the same composition trends exhibited by the mean shower depth. lll ACKNOWLEDGMENTS CASA-BLANCA was a small project compared to many particle astrophysics ex- periments, and it was possible only because of the dedicated efforts of a small core group of researchers. I have been fortunate to work on an experiment from its design through construction, operation, and analysis. Three physicists above all guided me through the project and shared the myriad duties. Without them, BLANCA would not have happened. Clem Pryke steered me through the calibration and analysis of the instrument and led the enormous CORSIKA-for-BLANCA simulation effort. My advisor Rene Ong offered me a hard and exciting thesis topic and led me wisely through the minefields but always gave me the independence to decide issues both big and small. His larger view of astrophysics has kept me anchored, while his excel- lent example as a writer and spea~er (along with hours and hours worth of detailed suggestions) has led me to make huge improvements in this thesis. Crucially, Lucy Fortson dared to take on a huge project nearly alone with the full-time help of only one green graduate student. Her leadership, hard work, and friendship over the past four years have made BLANCA not only possible but also exceptionally fun. I am grateful to the entire CASA-MIA collaboration for supporting the BLANCA effort always and for their invaluable assistance in the design and construction of BLANCA. The combined knowledge of the CASA group was staggering and inspir- ing. Many members of the High Resolution Fly's Eye (HiRes) group also shared their expertise, their tools, camaraderie, the A-1 steak sauce out of their refrigerator, and above all the complete set of old Fly's Eye PMTs, without which BLANCA would have been too expensive to build. Stan Thomas was especially helpful in suggest- ing simple solutions to our weirdest design problems. We tested optics in the Utah labs of Werner Gellermann and Di Li. Michigan's Ande Glasmacher wrote the first CASA-MIA thesis on cosmic ray composition. I thank her for the guidance it pro- vided (and for teaching me to drive the CASA four-wheel ATVs). I also thank the command and staff of the U.S. Army Dugway Proving Ground for their cooperation. Many people helped to build BLANCA in 1996. They include Matt Pritchard and Katherine Riley, who sweated in the desert in August. Jeremy Meyer aluminized 150 lV v Winston cones in Salt Lake City. A great many physicists from the CASA, STACEE, and HiRes groups volunteered their time to construct and install detectors in the field, including Jim Cronin, Mark Chantell, Scott Oser, Charlie Jui, Dave Kieda, Jon Matthews, and Paul Sommers. In addition, Kevin Green, Ken Gibbs, Brian Fick and Brian Newport spent desert days "swapping lead" to prepare CASA for the addition of BLANCA. Megan McClellan and Caleb Cassidy helped in 1997 to get ready for the second season. Clara Eberhardy patiently pioneered the BLANCA laboratory measurements, using a beautiful dark box and computer-controlled neutral density filter graciously loaned us by Kelby Anderson. Above all, our talented on-site tech- nician Mike Cassidy made BLANCA materialize. His practical knowledge turned the BLANCA concept into an huge array of sturdy, water-tight, baby blue detectors. He kept the trailers cool in the summer, warm in the winter, and always friendly. For making life in Chicago fun (even in Hyde Park) I thank the friends who did problem sets, studied for the candidacy exam, threw the frisbee, watched Bulls games, and went to the Pub with me so many nights, especially Jeff Berryhill, James Geddes, Jordan Koss, and Ted Quinn. Life in the CASA/STACEE group was fun and stimulating thanks to folks like Corbin Covault, Richard Scalzo, Dan Schuette, Jaci Conrad, and especially my long-time fellow student Scott Oser. I am grateful ... to Barbara Schubert for her superb orchestra program and for conducting Strauss, Stravinsky, Bruckner, and Mahler; to my other conductor, Antoinette Arnold; and to my musician pals Sylvie Anglin and Martin Pergler. Nobuko McNeill, Aspasia Sotir-Plutis, and Marty Dippel helped my stay in Chicago be as swift and painless as they could conveniently make it. I thank the National Science Foundation and the McCormick and Grainger Foundations for my financial support at Chicago. Like anyone who completes the 22nd grade, I have been blessed with many won- derful teachers and mentors over the years. Dozens prepared me for this thesis and more generally for a life of the mind. I am grateful to them all but especially to the two most inspiring, Carolyn Huff and Pat Freeman. Finally, I thank my parents, my sister Amy, and my wife Jennifer for their love, patience, and faith in me over many years. They made this worthwhile. TABLE OF CONTENTS ABSTRACT lll ACKNOWLEDGMENTS IV LIST OF FIGURES IX LIST OF TABLES XI 1 COSMIC RAY PHYSICS 1 1.1 What we do know about cosmic rays . 1 1.1.1 The discovery of cosmic rays and air showers . 2 1.1.2 The cosmic ray energy spectrum ....... 4 1.1.3 The age of cosmic rays and other observations 7 1.2 How cosmic rays are created and transported. 8 1.2.1 Supernovae ................. 8 1.2.2 Fermi acceleration ........... 9 1.2.3 Propagation and escape from the Galaxy 11 1.2.4 Current models of composition ..... 12 1.3 Existing measurements of the cosmic ray composition 17 1.3.1 Direct measurements at energies below the knee 17 1.3.2 Indirect measurements at the knee 17 1.3.3 Ultra-high energy composition . 19 2 EXTENSIVE AIR SHOWERS 20 2.1 The anatomy of an air shower 20 2.2 The Cherenkov effect ..... 23 2.3 Composition and air showers . 26 2.3.1 Atmospheric depth of shower maximum . 27 2.3.2 Cherenkov light ............. 31 2.3.3 Other composition-dependent properties 34 3 THE CHICAGO AIR SHOWER ARRAY 36 3.1 The CASA Instrument . 37 3.1.1 The array trigger . 40 3.1.2 Measuring particle densities 41 3.1.3 Surface array timing . 41 3.2 CASA Offiine Reconstruction . 43 3.2.1 Detector calibration .... 43 3.2.2 Fitting the air shower core 46 3.2.3 Fitting the air shower direction 47 VI Vll 4 THE BLANCA CHERENKOV ARRAY 49 4.1 Array overview . 49 4.1.1 Use of the CASA trigger .. 51 4.1. 2 Operating the Cherenkov array 51 4.2 BLANCA detectors ...... 52 4.2.1 Optical design ......... 55 4.2.2 Simulations of the optics ... 59 4.2.3 Laboratory studies of the optics 61 4.3 BLANCA electronics . 64 4.3.1 Preamplifiers . 64 4.3.2 Station electronics 66 4.4 Operating BLANCA 68 5 BLANCA CALIBRATION 71 5.1 Determining the BLANCA constants 71 5.1.1 Pedestals .....
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