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Development of Phonon-Mediated Cryogenic DEVELOPMENT OF PHONON-MEDIATED CRYOGENIC PARTICLE DETECTORS WITH ELECTRON AND NUCLEAR RECOIL DISCRIMINATION 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 Sae Woo Nam December, 1998 c Copyright 1999 by Sae Woo Nam All Rights Reserved ii I certify that I have read this dissertation and that in my opinion it is fully adequate, in scope and in quality, as a dissertation for the degree of Doctor of Philosophy. Blas Cabrera (Principal Advisor) I certify that I have read this dissertation and that in my opinion it is fully adequate, in scope and in quality, as a dissertation for the degree of Doctor of Philosophy. Douglas Osheroff I certify that I have read this dissertation and that in my opinion it is fully adequate, in scope and in quality, as a dissertation for the degree of Doctor of Philosophy. Roger Romani Approved for the University Committee on Graduate Studies: iii Abstract Observations have shown that galaxies, including our own, are surrounded by halos of "dark matter". One possibility is that this may be an undiscovered form of matter, weakly interacting massive particls (WIMPs). This thesis describes the development of silicon based cryogenic particle detectors designed to directly detect interactions with these WIMPs. These detectors are part of a new class of detectors which are able to reject background events by simultane- ously measuring energy deposited into phonons versus electron hole pairs. By using the phonon sensors with the ionization sensors to compare the partitioning of energy between phonons and ionizations we can discriminate betweeen electron recoil events (background radiation) and nuclear recoil events (dark matter events). These detec- tors with built-in background rejection are a major advance in background rejection over previous searches. 1 Much of this thesis will describe work in scaling the detectors from 4 g proto- type devices to a fully functional prototype 100 g dark matter detector. In partic- ular, many sensors were fabricated and tested to understand the behavior of our phonon sensors, Quasipartice trapping assisted Electrothermal feedback Transition edge sensors (QETs). The QET sensors utilize aluminum quasiparticle traps at- tached to tungsten superconducting transition edge sensors patterned on a silicon substrate. The tungsten lines are voltage biased and self-regulate in the transition region. Phonons from particle interations within the silicon propogate to the surface iv where they are absorbed by the aluminum generating quasiparticles in the aluminum. The quasiparticles diffuse into the tungsten and couple energy into the tungsten elec- tron system. Consequently, the tungsten increases in resistance and causes a current pulse which is measured with a high bandwidth SQUID system. With this advanced sensor technology, we were able to demonstrate detectors with xy position sensitivity with electron and nuclear recoil discrimination. Furthermore, early results from running the 100g detector in the Stanford Underground Facility (SUF) indicate that competitive dark matter results are achievable with the current detector design. Much of the design and testing of the experimental apparatus and instrumenta- tion is described as well. v Acknowledgements There are at least a hundred people I need to thank. At the top of the list is my advisor Blas Cabrera. I am grateful for his tolerance of my often reckless attitude regarding proper scientific procedure . I am also thankful for the amazing amount of latitude he has given me regarding the direction of my thesis. Furthermore, his breadth of knowledge in physics and related disciplines continually amazes me. I feel that I have greatly benefited from being under his guidance. Over the years it has been enjoyable working with the various members of the Cabrera group. In the beginning, it was George Park who was the last person to write a thesis with Laser Switching and truly thought I had the potential to understand how to use SQUIDs. After George graduated, I felt like the odd man out. Everyone left in the group was married, Barron Chugg, Kent Irwin, and Michael Penn. I am greatly indebted to these three. Without their pioneering work, there would be nothing for me to write about. In many ways, I am enjoying the fruits of their labor. More recently, I've felt like an old man interacting with the newer members of the group. Roland Clark was the first person to join Blas' group after me. He is a truly talented individual. His questions always make me realize how little I know. Andrea Davies is the latest person to join. Her diligence for writing things down is a God-send. Then, there are the past members of the Cabrera group who were before my time, Adrian Lee and Betty Young. Both of whom, I had the privilege of interacting with vi scientifically. I am also grateful for their conversation especially in regards to what my future plans are. Within the Stanford Physics department, I am eternally grateful to the machine shop. Wolfgang Jung, Karlheinz Mehlke, and Dan Semnides are incredibly skilled machinists who were always willing to help me in my quest to make a better sample holder. In the main office, I wish to thank Marcia for making sure I always got paid, to Cindy Mendel for not yelling at me when I botched a purchase order, to Lori Jung and what's her name for entertaining conversation whenever I stole food from the main office. I would like to thank the late Barbara Dillard for teaching me how to be a good TA and to Olivia Martinez for good conversations regarding all aspects of life. Outside of the Stanford Physics community, I have been fortunate to be involved in CDMS. I feel very lucky to have had the experience of working with people who think so differently than me yet want the same goal. I've enjoyed interacting with the crew from Berkeley and Santa Barbara which includes Bernard, Dan Akerib, Tom, Rick, Sunil, Walter, Angela, Josef, Andrew, and Dan Bauer. More recently (post thesis work), I have had a truly great time working with the people from Fermilab on CDMS, Mike Crisler, Steve Eichblatt, and Merle Haldeman. In particular, I must thank Steve for distracting me from writing. In the process, we've worked on things which have made reanalysis of a lot of the data in my thesis really easy. Before CDMS and before QET phonon sensors, I was lucky enough to interact with John Martinis, Rick Welty, and Masoud Radparvar on the DC SQUID arrays. Their willingness to help me learn about SQUID arrays and their advantages really is one of the key technologies which made the work in my thesis possible. I also need to thank Mike Hennessey. His help with maintenence of lab equipment and supplying me with vacuum equipment was indispensible. I also enjoyed our conversations at 5:30 in the morning when he arrived to work and when I would be vii leaving. Outside of my research group, I am thankful for all my friends in the Alpine Road House, Stu, Dan, Anneli, Andrei, Matthias, and Tony (honorary), the original Applied Physics 3pm frisbee crew, skirt frisbee (Lori, Betsy, Monica, Erin, et. al.), and the new working stiff's frisbee club at 3pm on Sundays. John, Catherine, and Oscar, thanks for your support. And, finally, I owe everything I've accomplished to my parents and sister for their unquestioned support throughout my whole `formal' education process. viii Contents Abstract iv Acknowledgements vi 1 Introduction 1 1.1EvidenceforDarkMatter........................ 1 1.2NatureofDarkMatter.......................... 3 1.2.1 Cosmology............................. 3 1.2.2 Ω.................................. 4 1.2.3 BaryonicDarkMatter...................... 5 1.2.4 NonbaryonicDarkMatter.................... 5 1.2.5 HotDarkMatter......................... 5 1.2.6 ColdDarkMatter......................... 6 1.3WIMPdetection............................. 7 1.3.1 ExpectedRates.......................... 7 1.3.2 DetectorRequirements...................... 8 1.4Breakdownofthisthesis......................... 9 2 Detector Description 11 2.1Introduction................................ 11 2.1.1 PhononSensors.......................... 11 ix 2.1.2 Ionization............................. 14 2.2PhononSensor.............................. 14 2.2.1 Electro-ThermalFeedbackTransitionEdgeSensor....... 14 2.2.2 Quasi-particleTraps....................... 16 2.2.3 Noise................................ 17 2.2.4 DiscussionofTimeConstants.................. 20 2.2.5 ETF Stability ........................... 24 2.3IonizationSensors............................. 27 2.3.1 Circuit............................... 27 2.3.2 Noise................................ 28 2.3.3 ChargeTrapping......................... 30 2.4Fabrication................................ 31 3 Experimental Apparatus 32 3.1Electronics................................. 34 3.1.1 SQUIDElectronics........................ 45 3.1.2 DetectorBiasing......................... 46 3.1.3 SRSAmplifiers.......................... 48 3.1.4 Qamplifiers............................ 49 3.2DataAcquisition............................. 49 3.2.1 Hardware............................. 49 3.2.2 Software.............................. 52 4 Detector Evolution 53 4.1QuasiparticleTrapDemonstration.................... 53 4.1.1 W/Al2-channeldevice...................... 53 4.1.2 W/Al2-channeldevicewithAuheatsinks........... 59 4.2FullSurfacecoverage........................... 68 x 4.2.1 1cmx1cmx1mmPhonononlysensors...........
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