Quad: a Millimeter-Wave Polarimeter for Observation of the Cosmic Microwave Background Radiation
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QUAD: A MILLIMETER-WAVE POLARIMETER FOR OBSERVATION OF THE COSMIC MICROWAVE BACKGROUND RADIATION A DISSERTATION SUBMITTED TO THE DEPARTMENT OF PHYSICS AND THE COMMITTEE ON GRADUATE STUDIES OF STANFORD UNIVERSITY IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY James R. Hinderks August 2005 c Copyright by James R. Hinderks 2005 All Rights Reserved ii I certify that I have read this dissertation and that, in my opinion, it is fully adequate in scope and quality as a dissertation for the degree of Doctor of Philosophy. Prof. Sarah E. Church Principal Adviser I certify that I have read this dissertation and that, in my opinion, it is fully adequate in scope and quality as a dissertation for the degree of Doctor of Philosophy. Prof. Giorgio Gratta I certify that I have read this dissertation and that, in my opinion, it is fully adequate in scope and quality as a dissertation for the degree of Doctor of Philosophy. Prof. Steven Kahn Approved for the University Committee on Graduate Studies. iii iv Abstract This thesis describes the design and performance of the QUaD experiment and presents some of its earliest results. QUaD is a millimeter-wavelength polarime- ter designed for observing the Cosmic Microwave Background (CMB). QUaD was commissioned at the MAPO observatory at the Amundsen-Scott South Pole Station in the Austral Summer of 2004/2005, achieved first light in Feb 2005, and began science observation in May. QUaD observes the CMB with an array of 31 polarization-sensitive Neutron Transmutation Doped (NTD) germanium bolometers split between two frequency bands centered at 100 and 150 GHz. The telescope is a 2.6 m on-axis Cassegrain design with beam sizes of 6.3 and 4.2 at the two respective observing frequencies. The resolution and scan strategy are optimized to probe the CMB E-mode power spectrum over a multipole range of 100 to 2500. The performance of the system has been characterized with commissioning observations and a high signal-to-noise map of the CMB temperature anisotropy has been made over a ∼ 50 square degree area. CMB polarization anisotropies, only recently detected, promise a wealth of new cosmological information. Their observation complements the many successful tem- perature anisotropy measurements already performed, confirming our basic under- standing of the early universe and leading to tighter constraints on cosmological parameters. Furthermore, polarization observations provide a probe of structure since the last scattering surface and promise unique constraints on inflation through the imprint of relict gravitational radiation. vi Acknowledgements QUaD has been a collaborative effort since the beginning and I feel privileged to have spent the past six years working with the talented group of scientists listed in Table 1.3 – I have learned a great deal from each one of you. This holds especially true for the Stanford QUaD team with whom I’ve spent innumerable hours in “the lab.” And of course, building a telescope isn’t much good if nobody puts it to use and keeps it in shape. So two gold stars to Robert “Do the Dew” Schwartz and Alan “No Worries” Day for being the best winter-over crew imaginable. Grad school has certainly been a long process. Throughout, Sarah has been a terrific advisor and mentor, and has made so many opportunities available to me. Thanks to the postdocs I worked with in my first two years at Stanford – Brian and Byron – for teaching me so much about doing physics. Thanks to Keith for teaching me a great deal about electronics, careful experimental technique, and about knowing when to just kludge something together. Thanks to Mel for all the great discussions on cosmology. Thanks to Brad, who was my fellow Church lab grad student for many years. Working with you was always fun – whether we were in the lab or on the summit of Mauna Kea. I’ll never forget learning how to install snow chains at 14,000’ or the night we nearly starved to death at South Point. And thanks to Ben – who has been my partner in crime working on QUaD for the past four years – for doing so much to make QUaD a success, and for being a great officemate and friend. Over the years, our lab has been fortunate enough to have a steady supply of top-quality undergraduates. Seebany, Judy, Marteen, Evan, Kapil, Elizabeth, Ali, and Tess: thanks for all your hard work and for making lab so much fun. Particular thanks go to Kapil for his invaluable CAD work on the focal plane and to Evan vii for his excellent senior thesis work on QUaD optical testing (and for his equally excellent shirts). Also QUaD never would have made it to the Pole on time without all the packing and shipping help from Tess, in the form of carpentry, manual labor, chocolate chip cookies, and of course exploding foam. Acknowledgement is also due to the great people of the Stanford Physics De- partment who helped make QUaD a success. Thanks to Dana for her indispensable organizational and administrative help. The extremely talented group of machinists who actually built much of the QUaD receiver – John, Mehmet, Matt, and Karl- heinz – deserve special recognition. And finally, thanks to Stewart, Joel, and Khoi for keeping the Varian building running smoothly, dealing with our endless stream of deliveries, and keeping the stock room filled with the assorted cable ties, resistors, and other random bits that saved the day on many occasions. A few more random words before wrapping this up. To Mike Z, thanks for the inspirational music and all the cowbell – it was hot. To Angiola, thanks for letting me try “one more test,” and for not leaving me on the floor by the water fountain. I wish you hadn’t stolen that towel though. To Ben, thanks for your awesome Dremel skills. To Cara, thanks for the jokes, the laughter, the offbeat news, and the stories about Speedy. Thanks to Dave and Becky for introducing me to the Earthquakes, to Hilton for always knowing where the free food was and to Phil for sending presents to cheer us up at the South Pole. Special thanks to Sarah, Mel, and Ben for all your helpful comments on this document. Best of luck to everyone continuing to work on QUaD, especially Ed, the team’s newest member. Finally, I arrive at the most important people – my family. Mom and Dad, thanks for instilling in me a love of science and of learning. I owe everything to the two of you. To my four amazing grandparents, thanks for all your love over the years. To the Suns, thank you for welcoming me into your wonderful family, and in particular thanks to Christina for providing me with a place to live over the last half year while I wrote this dissertation. And finally to Stephanie: it must be true love when you’re willing to spend weekend afternoons in the machine shop wearing safety goggles or in the lab inhaling solder fumes just to be with your husband. Thanks for being the best thing in my life. viii Contents Acknowledgements vii 1 Introduction 1 1.1CosmologyandtheCosmicMicrowaveBackground.......... 2 1.1.1 OriginandHistoryoftheCMB................. 2 1.1.2 TheModernCosmologicalPicture................ 3 1.1.3 CMBTemperatureAnisotropies................. 7 1.2ThePolarizationoftheCMB...................... 12 1.2.1 StokesParameters........................ 12 1.2.2 OriginsandCharacterizationofCMBPolarization....... 14 1.2.3 ExistingMeasurements...................... 22 1.3TheQUaDExperiment.......................... 24 1.3.1 Overview............................. 24 1.3.2 ScienceGoals........................... 26 1.4ThesisOutline............................... 28 2 Experiment Description 31 2.1TheSouthPoleObservingSite..................... 31 2.2TheDASIMount............................. 32 2.3Optics................................... 33 2.3.1 Overview............................. 33 2.3.2 LensesandColdStop....................... 34 ix 2.3.3 Filtering.............................. 36 2.3.4 CorrugatedFeeds......................... 39 2.3.5 PolarizationSensitiveBolometers................ 40 2.3.6 TheFocalPlane.......................... 42 2.4Cryogenics................................. 46 2.4.1 Cryostat.............................. 46 2.4.2 Sub-Kelvin Refrigerator ..................... 48 2.4.3 TheScienceCore......................... 52 2.4.4 Thermometry........................... 54 3 Readout Electronics 57 3.1Description................................ 57 3.1.1 BiasGenerator.......................... 59 3.1.2 LoadResistorBoxes....................... 61 3.1.3 FocalPlaneWiring........................ 63 3.1.4 JFETBoxes............................ 65 3.1.5 LockinAmplifiers......................... 67 3.1.6 TheDataAcquisitionSystem.................. 70 3.2Performance................................ 71 3.2.1 FunctionalityTests........................ 71 3.2.2 NoisePerformance........................ 77 3.2.3 ChannelCapacitance....................... 85 3.2.4 Microphonics........................... 89 3.2.5 RadioFrequencyInterference.................. 91 4 Receiver Characterization 93 4.1BolometerCharacterization....................... 93 4.1.1 BolometerModel......................... 93 4.1.2 LoadCurves............................ 95 4.2OpticalCharacterization.........................100 x 4.2.1 OpticalTestbed..........................100 4.2.2 SpectralBands..........................102 4.2.3 OpticalEfficiency.........................106 4.2.4 ResponsivityandTimeConstants................110 4.2.5 CosmicRaysandImpulseResponse...............113 4.3PolarizationProperties..........................116 4.3.1 Formalism.............................117