### Session W12: Dark Matter Searches III

 Tuesday, May 5, 2009 10:45AM - 10:57AM W12.00001: Current Status of the XENON100 Dark Matter Experiment Uwe Oberlack Non-baryonic Dark Matter makes up $\sim$85{\%} of all matter in the universe. A plausible theoretical class of candidates are Weakly Interacting Massive Particles (WIMPs). XENON100, located at the Gran Sasso National Laboratory in Italy, is a liquid/gas xenon time projection chamber for direct detection of WIMP-nucleon recoils. XENON100 is the successor of the highly successful XENON10 experiment, featuring 10 times greater sensitive mass ($\sim$50 kg fiducial) and 100 times lower background. The expected sensitivity is $\sim$10$^{-45}$ cm$^{2}$ for spin-independent interactions. XENON100 has been installed and has started operating. I will report on the present status of XENON100 and its expected physics reach. Tuesday, May 5, 2009 10:57AM - 11:09AM W12.00002: Calibration of the XENON100 Time Projection Chamber Kyungeun Lim The XENON Dark Matter Experiment aims at the direct detection of dark matter Weakly Interacting Massive Particles (WIMPs) with dual phase (liquid/gas) xenon time projection chambers. Following the successful performance of the XENON10, we have designed and built a new detector with a total Xe mass of 170 kg, and with 100 times less background. The XENON100 detector is currently undergoing commissioning at the Gran Sasso Underground Laboratory. The calibration of the detector with gamma sources and with low energy neutrons is essential to determine the response to electron and nuclear recoils, and their discrimination based on the ratio of ionization to scintillation, as well as on event positioning and scattering-multiplicity within the active liquid volume. External gamma sources used for the XENON100 calibration include Cs-137, Co-57 and Co-60. An external Am-Be source is used for irradiation by neutrons. Additionally, we have been testing mixing Kr-83m (tau=12.6 hr, 18 and 32 keV electrons and 13 keV x-rays) into the LXe target, as internal source of low energy electron recoils. We discuss how different detector performance parameters such as light yield and electron lifetime can be inferred from these calibrations and show how the spatial dependence of some other parameters (light collection efficiency for example) can be obtained. We also present comparisons of results from calibrations with Monte Carlo simulations. Tuesday, May 5, 2009 11:09AM - 11:21AM W12.00003: Position reconstruction with the XENON100 TPC based on least-squares fitting Yuan Mei The determination of interaction locations is a key feature of dual-phase Time Projection Chambers (TPCs) for Dark Matter search. XENON100 is a liquid/gas xenon TPC, searching for Weakly Interacting Massive Particles at the Gran Sasso National Laboratory. While the z-coordinate of an event is determined by the drift time of electrons with regard to a scintillation light trigger, the x/y position is reconstructed using the distribution of proportional light on the top PMT array. Previously for the relatively small TPC of Xenon10, a exhaustive search procedure was developed to perform the least-squares fitting. However, for the larger TPC of current detector Xenon100, the very procedure becomes unpractically slow. In this work, we present the development of a new procedure which searches the minimum of $\chi^2$ between real signal and simulated data efficiently. The procedure performed on a Monte Carlo generated data-set yields millimeter resolution of x-y position. The performance on real signals employing proper uncertainties from various sources is also discussed. Tuesday, May 5, 2009 11:21AM - 11:33AM W12.00004: Relative scintillation efficiency of Xenon for low energy nuclear recoils. Angel Manzur , Alessandro Curioni , Louis Kastens , Daniel McKinsey , Kaixuan Ni , Taritree Wongjirad In the past few years, experiments using liquid xenon as a medium for detecting Cold Dark Matter have given competitive upper limits on the elastic WIMP-nucleon cross section. However, the dominant uncertainty in these limits is due to the uncertainty in the nuclear scintillation efficiency for xenon ($\mathcal{L}$$_{eff}). The \mathcal{L}$$_{eff}$ is defined as the amount of scintillation produced by nuclear recoils, divided by the amount of scintillation produced by electron recoils of the same energy. Previous experiments measuring the $\mathcal{L}$$_{eff} gave inconsistent extrapolations at recoil energies below 20 keV, an energy window crucial for dark matter searches. In this talk we report a new \mathcal{L}$$_{eff}$ measurement for energies below 10 keV, done with monoenergetic neutron scattering of a liquid xenon detector. Tuesday, May 5, 2009 11:33AM - 11:45AM W12.00005: The Cryogenic Dark Matter Search and Carrier Transport in 40 MilliKelvin Germanium $<100>$ Kyle Sundqvist The Cryogenic Dark Matter Search (CDMS) is searching for Weakly Interacting Massive Particles (WIMPs) via their interactions in Ge and Si detectors at a temperature of $40 ~mK$. Measuring the ionized charge and non-thermalized phonons from particle interactions enables CDMS to discriminate candidate WIMP interactions from electromagnetic background. Operation at such low temperature represents a unique regime for electron and hole transport processes. As these carriers are always hot, typical assumptions of thermal equilibrium are no longer valid. We have simulated transport processes of charge carriers in germanium $<100>$ at a temperature of $40 ~mK$. We will present how this new understanding is beneficial to future detector development. Tuesday, May 5, 2009 11:45AM - 11:57AM W12.00006: Active Neutron Shielding for Dark Matter Searches Jocelyn Monroe , Richard Yamamoto , Peter Fisher , Brett Cornell , Mareena Robinson , Dianna Cowern , Richard Eyers , Shawn Henderson Neutrons are a dangerous background to direct dark matter detection searches because they can mimic exactly the signal signature. Recent studies find that the few existing underground measurements of the fast, muon-induced neutron flux disagree at the 30\%-50\% level with predictions. Given this level of uncertainty, it is desirable to measure the neutron flux in-situ, as well as to reduce the number of neutrons incident on a dark matter detector. Towards these ends, we are developing a neutron veto system for both active and passive shielding. The goals of this R\&D are (i) a measurement of the neutron energy spectrum underground above 10 MeV neutron kinetic energies, and (ii) measurements of the attenuation vs. energy of these neutrons in 1 meter of water, concrete, and liquid scintillator. These measurements will provide valuable input for simulation and design of shields for low-background experiments underground. Tuesday, May 5, 2009 11:57AM - 12:09PM W12.00007: ABSTRACT WITHDRAWN Tuesday, May 5, 2009 12:09PM - 12:21PM W12.00008: A radon daughter deposition model for low background experiments K. Rielage , V.E. Guiseppe , A. Mastbaum , S.R. Elliott , A. Hime The next generation low-background detectors operating underground, such as dark matter searches and neutrinoless double-beta decay, aim for unprecedented low levels of radioactive backgrounds. Although the radioactive decays of airborne radon (particularly $^{222}$Rn) and its subsequent daughters present in an experiment are potential backgrounds, more troublesome is the deposition of radon daughters on detector materials. Exposure to radon at any stage of assembly of an experiment can result in surface contamination by daughters supported by the long half life (22 y) of $^{210}$Pb on sensitive locations of a detector. An understanding of the potential surface contamination will enable requirements of radon-reduced air and clean room environments for the assembly of low background experiments. It is known that there are a number of environmental factors that govern the deposition of daughters onto surfaces. However, existing models have not explored the impact of some environmental factors important for low background experiments. A test stand has been constructed to deposit radon daughters on various surfaces under a controlled environment in order to develop a deposition model. Results from this test stand and the resulting deposition model will be presented. Tuesday, May 5, 2009 12:21PM - 12:33PM W12.00009: Radon as a Source of External Background at Homestake Mine Keenan Thomas , Dongming Mei , Chao Zhang , Fred Gray , Richard Gaitskell , Simon Fiorucci External sources of radioactivity are important concerns for experiments planned for DUSEL at the Homestake Mine in Lead, South Dakota. Radon emanation and deposition is a major threat to the targeted sensitivity of low background experimentation such as double beta decay detection and dark matter searches. Methods to reduce and mitigate these measured levels will need to be developed to prevent experimental signals from contamination through airborne radon decays as well as the deposition of radon daughters. Radon levels were measured at various depths at the Homestake Mine in December of 2008, January and March of 2009. These measurements will be useful in the development of an underground ventilation system to dilute radon concentrations in the air and subsequent systems to provide radon-free air to clean rooms, as well as preparing researchers for the hazards they pose to their experiments. In addition, the measured radon level will be used to understand the radon emanation from different types of rock.