Bulletin of the American Physical Society
APS April Meeting 2011
Volume 56, Number 4
Saturday–Tuesday, April 30–May 3 2011; Anaheim, California
Session T11: Background Supression in Noble Gas Dark Matter Detectors |
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Sponsoring Units: GPMFC DAP DPF Chair: Laura Baudis, University of Zurich Room: Garden 2 |
Monday, May 2, 2011 3:30PM - 3:42PM |
T11.00001: New Measurement of $^{39}$Ar in Underground Argon for Dark Matter Experiments Jingke Xu Liquid argon detectors are currently being developed for the direct detection of dark matter WIMPs. Scintillation and ionization signals of argon allow nuclear recoil signals produced by WIMP interactions to be separated from backgrounds due to beta and gamma radiations. However, the low level of radioactive $^{39}$Ar that is produced by cosmic rays in atmospheric argon constrains the size of such detectors to one ton or less. The discovery of sources of underground argon with low levels of $^{39}$Ar makes ton-scale argon detectors a possibility. The first measurements on small samples of underground argon gas showed no sign of $^{39}$Ar with an upper limit of 5{\%} that of atmospheric argon. I will report on the development of a low background liquid argon detector designed to improve the limit on residual $^{39}$Ar. Preliminary measurements made with the detector above ground show no evidence of $^{39}$Ar with higher sensitivity than the initial gas phase measurements [Preview Abstract] |
Monday, May 2, 2011 3:42PM - 3:54PM |
T11.00002: Suppression of Rn-daughters in the DarkSide Dark Matter Search Huajie Cao Alpha-emitting activity from radon daughters will be an important source of background for the next generation of direct dark matter searches. A vacuum swing adsorption (VSA) system with a radon suppression factor better than 100 was constructed and operated to purify the make-up air to the clean room that was used for the construction of the Borexino nylon vessels. The system was recently refurbished and upgraded for use in the construction and assembly of the DarkSide-50 dark matter detector. The VSA system consists of two charcoal-filled tanks cycled between atmospheric pressure and 10 mbar. I will discuss the design and operation of the system and detail its performance. Results from this test may inform the development of radon filters dedicated to support the next generation of dark matter and double beta decay detectors. [Preview Abstract] |
Monday, May 2, 2011 3:54PM - 4:06PM |
T11.00003: Background Characterization in the DarkSide-50 Dark Matter Detector Pablo Mosteiro One of the most active areas of interest in the field of particle astrophysics is the search for dark matter in the form of Weakly Interacting Massive Particles (WIMPs). The DarkSide program consists of a series of incrementally larger and more sensitive two- phase argon Time Projection Chambers (TPCs) that aim to discover WIMPs through their interactions with nuclei. One of the main challenges in the development of such detectors is the characterization and reduction of external and internal backgrounds. I will present a detailed summary of expected sources and rates of radioactive and cosmogenic background in DarkSide-50, a 50kg detector that will use argon derived from underground sources, with a low level of $^{39}$Ar. Background calculations are based on recent studies done with the Monte Carlo simulation toolkit Geant4. Emphasis will be placed on the boron-loaded liquid scintillator neutron veto, one of the key technologies of DarkSide-50. [Preview Abstract] |
Monday, May 2, 2011 4:06PM - 4:18PM |
T11.00004: Gamma background discrimination in the XENON100 experiment Antonio Melgarejo Direct dark matter detection experiments rely on the ability to have an expected background close to 0 in order to be able to identify possible WIMP signals. Among the multiple strategies to achieve this goal, most of the experiments use background reduction techniques which exploit the difference between electron-like signal (most radioactive backgrounds) and neutron-like signals (neutrons and WIMPs). In this talk we will show the studies and measurements within the XENON100 experiment to distinguish signals from electrons and neutrons by comparing their light to signal ratio. A straightforward prediction of this work is the amount of events expected in the dark matter region in this experiment [Preview Abstract] |
Monday, May 2, 2011 4:18PM - 4:30PM |
T11.00005: Suppression of Anomalous Events Using PMT Pattern Information in XENON100 Data Yuan Mei Background discrimination in the XENON100 Dark Matter experiment relies on the identification of events with a single interaction in a selected fiducial volume. The yield difference in primary scintillation light (S1) and ionization charge (S2) distinguishes nuclear recoils from interactions with electrons. Anomalous events with apparent single S1 and single S2, but which are truly multiple scatter events, constitute a substantial fraction of background in the Dark Matter region of interest. Such events have additional scattering sites, yielding S1 PMT patterns different from those of true single scatters. We have developed a method to discriminate against these anomalous events, using the Log Likelihood Ratio of measured PMT patterns over expected single scatter patterns obtained from calibration data. Since the method directly compares the information from background data with calibration data, it is robust against systematic uncertainties in, e.g., PMT quantum efficiencies, position reconstruction, or light collection efficiency. [Preview Abstract] |
Monday, May 2, 2011 4:30PM - 4:42PM |
T11.00006: Estimation of Anomalous Single Scatter Events in XENON100 Data Kyungeun Lim Anomalous single scatter events in XENON100 are events that have only one scintillation pulse (S1) and one ionization pulse (S2), but are multiple scatters in nature. Only one scatter takes place inside the detector's charge and light sensitive volume, resulting in a S2/S1 ratio that is lower than that of true single scatter events and typical of that expected from a WIMP interaction. The identification and suppression of these anomalous events is therefore essential for a sensitive dark matter search. I present results from a Monte Carlo (MC) study that was carried out to estimate the expected number of anomalous single scatter events in the XENON100 WIMP search data. The MC was validated with a comparison with Co-60 gamma-calibration data. [Preview Abstract] |
Monday, May 2, 2011 4:42PM - 4:54PM |
T11.00007: MiniCLEAN surface backgrounds Boqian Wang, Richard Schnee MiniCLEAN is a dark matter experiment using 150kg fiducial mass of liquid cryogen (argon or neon) to search for Weakly Interacting Massive Particles (WIMPs). MiniCLEAN seeks to detect scintillation photons from WIMP-induced argon recoils. A potentially dominant background is from alpha decays on the inner surfaces of the containment vessel. Such events can mimic the prompt signal characteristic of nuclear recoils. This talk will show the expected background rates, methods of background discrimination, and their expected effectiveness. [Preview Abstract] |
Monday, May 2, 2011 4:54PM - 5:06PM |
T11.00008: Neutron Calibration System for the MiniCLEAN Experiment Lu Feng MiniCLEAN is a dark matter experiment using liquid argon (LAr) to detect nuclear recoils from weakly interacting massive particles. In order to characterize the detector response to these events, a neutron calibration system operating a pulsed deuterium-deuterium neutron source will be deployed. This calibration system will also be used to benchmark simulation physics crucial to understanding the neutron background, test neutron tagging techniques, and monitor detector health. Here we describe the hardware and software integration of the calibration system. [Preview Abstract] |
Monday, May 2, 2011 5:06PM - 5:18PM |
T11.00009: Detection of residual krypton in xenon gas for WIMP dark matter searches Attila Dobi The next generation of WIMP dark matter searches using liquid xenon as a target medium will require unprecedented rejection of residual krypton contamination. Krypton contains the beta emitting isotope $^{85}$Kr, with a relative abundance of about $10^{-11}$ ($^{85}$Kr/$^{nat}$Kr), and this beta decay can be an important source of background for these experiments. Krypton is typically present in commercially produced xenon at the level of tens of parts-per-billion, about four orders of magnitude too large for present day dark matter experiments such as XENON, LUX, and XMASS. Additional processing via gas chromatography and distillation are used to separate krypton from xenon, but measuring the remaining krypton level at the part-per-trillion (ppt) level is challenging. Recently we have developed a highly sensitive and simple technique to measure residual krypton contamination in xenon gas using an RGA mass spectrometer and a liquid nitrogen cold trap. We describe here the results of our calibration experiments to determine the ultimate limit of detection of the method, and we discuss the implications for the next generation of WIMP dark matter experiments. [Preview Abstract] |
Monday, May 2, 2011 5:18PM - 5:30PM |
T11.00010: Argon Depletion for a Large Scale Dark Matter Detector Dana Byram, Jason Spaans, Dongming Mei, Yongchen Sun, Christina Keller Argon depleted of $^{39}$Ar utilizing thermal diffusion methods is being implemented in 20 columns at USD.~ Thermal diffusion~is a well-known technique in isotope separation. The columns are 3 meters long and 0.5 inch in diameter.~ This test system will allow us to deplete $^{39}$Ar by a factor of 10 for 1 kg of natural argon over two months. This would lead to the full-scale production of depleted argon by using more columns that are longer in length. This depleted argon can then be used as a target material for next generation dark matter detectors. Using an advanced automation~system,~we can run our test system continuously to extract the enriched $^{39}$Ar for eventual measuring at ANL. The interim results of the test system will be reported utilizing the more abundant isotope $^{36}$Ar. In addition, the current effort for obtaining $^{39}$Ar results will be presented. [Preview Abstract] |
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