Bulletin of the American Physical Society
APS April Meeting 2019
Volume 64, Number 3
Saturday–Tuesday, April 13–16, 2019; Denver, Colorado
Session R10: WIMP Dark Matter IV |
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Sponsoring Units: DPF Chair: Reina Maruyama, Yale University Room: Sheraton Governor's Square 12 |
Monday, April 15, 2019 1:30PM - 1:42PM |
R10.00001: Purification of Ge ingots for growing crystals in developing dark matter detectors Pramod Acharya, Gang Yang, Rajendra Panth, Kyler T Kooi, Alex Kirkvold, Yangyang Li, Dongming Mei High-purity germanium (HPGe) single crystals having the net impurity level of ~ 109 to 1010 cm-3 can be used for the fabrication of high-resolution dark matter detectors. We implement a zone refining technique as a primary purification procedure for growing crystals. The raw germanium ingots in our laboratory have four main impurities, such as Aluminum, Boron, Phosphorous, and Gallium, identified through photothermal ionization spectroscopy (PTIS), with a net impurity level of (1013-1014) cm-3.The process of zone-refining included a two-step strategy, which included initial purification of the raw germanium ingots in a graphite boat, and then further purification of the zone-refined ingots from the first step in a carbon-coated quartz boat. We have optimized the parameter, zone length, zone travel speed and number of passes for impurities ingots having distinguished the segregation coefficient. Using the Van der Pauw Hall method, we were able to measure the electrical properties of zone refined ingots and analyze the distribution of impurities. Results obtained from graphite boat Ingots have the impurity level of (1011-1012) cm-3, and from quartz boat Ingots (1010-1011) cm-3. This work is supported by NSF OISE-1743790 and NSF PHY-1902577. |
Monday, April 15, 2019 1:42PM - 1:54PM |
R10.00002: Crystal growth for developing germanium detectors with internal charge amplification for low-mass dark matter searches. Hao Mei, Guojian Wang, Gang Yang, Sanjay Bhattarai, Mathbar Singh, Dongming Mei High-purity germanium crystals with diameters up to 12 cm were grown by the Czochralski method at the University of South Dakota. The impurity level, charge drift mobility, and resistivity in the crystals were measured by the Hall Effect. The mechanical properties such as hardness and Young’s modulus were measured to calculate the speed of sound in the Ge sample and thus estimate the energy of longitudinal and transverse phonons as well as the number of phonons generated during the creation of e-h pairs. The results were used to further investigate the avalanche effect for germanium detectors with respect to electric fields under different temperatures. This work is supported by NSF OISE-1743790 and NSF PHY-1902577. |
Monday, April 15, 2019 1:54PM - 2:06PM |
R10.00003: Investigation of Amorphous Germanium Contact Properties with Planar Detectors Made from USD-Grown Germanium Crystals Wenzhao Wei, Yangyang Li, Xianghua Meng, Jing Liu, Guojian Wang, Hao Mei, Gang Yang, Dongming Mei, Chao Zhang The characterization of detectors fabricated from home-grown crystals is the most direct way to study crystal properties. We fabricated planar detectors from high-purity germanium (HPGe) crystals grown at the University of South Dakota (USD). In the fabrication process, a HPGe crystal slice cut from a USD-grown crystal was coated with a high resistivity thin film of amorphous Ge (a-Ge) followed by depositing a thin layer of aluminum on top of the a-Ge film to define the physical area of the contacts. We investigated the detector performance including the I-V characteristics, C-V characteristics and spectroscopy measurements for a few detectors. The results document the good quality of the USD-grown crystals and electrical contacts. This work is supported by NSF OISE-1743790 and NSF PHY-1902577. |
Monday, April 15, 2019 2:06PM - 2:18PM |
R10.00004: The Production and Testing of Electrode Grids for the LUX-ZEPLIN (LZ) Experiment Randy G White The LUX-ZEPLIN (LZ) experiment, looking for the elastic scattering of dark matter WIMPs, is a liquid xenon time projection chamber (TPC) that crucially relies upon strong and uniform electric fields throughout its volume. These fields are created by grids of stainless steel wire acting as electrodes at four locations in the vessel. The production of these grids has presented several engineering challenges due to the demanding design requirements, including their high voltage performance, need for high optical transparency and low electron emission, and their impressive size at nearly six feet across. After production, each grid is evaluated for high voltage performance through a testing procedure. This talk will review the status of the production and testing of the grids. |
Monday, April 15, 2019 2:18PM - 2:30PM |
R10.00005: The XENONnT Liquid Purification System and Its Test System Yun Zhang The XENONnT experiment, currently under construction at the Laboratori Nazionali del Gran Sasso in Italy, aims at detecting dark matter weakly interacting massive particles using a dual-phase (liquid/gas) xenon time projection chamber with 1.5m drift length which is filled with about 8400kg of liquid xenon. The liquid purification system of the experiment allows the removal of electronegative impurities from liquid xenon at a much higher speed. A liquid purification test system has been built and operated at the Columbia University Astrophysics Lab. In this talk we will describe the XENONnT liquid purification system and present results from its test system. |
Monday, April 15, 2019 2:30PM - 2:42PM |
R10.00006: Measurement of the nuclear recoil ionization yield in liquid xenon from 0.3 keV to 6 keV Brian Lenardo We present a new measurement of the ionization yield in liquid xenon for nuclear recoils between 0.3 - 6 keV, directly targeting the region of interest for low-mass WIMP dark matter and coherent scattering of solar or reactor neutrinos. A collimated, pulsed source of 579 keV neutrons was generated via the Li(p,n)Be reaction using the tandem accelerator at Triangle Universities Nuclear Laboratory. Neutrons scattering elastically in the xenon target were tagged by liquid scintillator detectors at fixed angles between 15 and 70 degrees. Time-of-flight and pulse shape information in the liquid scintillators provide substantial background rejection, allowing us to measure signals with a threshold at a single ionization electron. Data are taken with three different electric fields applied to the liquid target. This is the lowest energy nuclear recoil calibration in liquid xenon reported to date, and can be used to interpret data and compute sensitivities of liquid-xenon-based experiments searching for coherent neutrino-nucleus scattering or low-mass WIMP dark matter. |
Monday, April 15, 2019 2:42PM - 2:54PM |
R10.00007: Charge-to-light signatures of inner-shell vacancies in xenon time projection chambers Dylan J Temples Dark matter searches using dual-phase xenon time projection chambers (LXe-TPC) rely on the discrimination between electronic recoils (background) and nuclear recoils (signal) based on the ratio of ionization electrons to scintillation photons produced by the interaction. This discrimination is calibrated at low energies using β-decays of tritium. Neutrino and Compton scatters from inner-shell electrons of Xe atoms can result in the emission of Auger electrons and x-rays in addition to the primary recoiling electron, and thus have a different event topology than β-decays. Due to their low energy and large numbers, these secondary particles can deposit large amounts of energy within a small radius, which is uncharacteristic of valence electron recoils and is more akin to nuclear recoils. This effects the profile of the ν-e scattering background in a way that is unaccounted for in LXe-TPC dark matter searches, and presents the possibility of a false discovery claim. The XELDA detector has been constructed to study the electron capture decay of 127Xe to produce a high-purity sample of inner-shell vacancies accompanied by an Auger cascade. In this talk, I present an overview of the XELDA detector system, calibrations, and preliminary results of the 127Xe analysis. |
Monday, April 15, 2019 2:54PM - 3:06PM |
R10.00008: An Improved Nuclear Recoil Calibration in the LUX Detector Using a Pulsed D-D Neutron Generator Dongqing Huang The LUX dark matter experiment has measured the nuclear recoil charge (Qy) and light (Ly) yields in liquid xenon (LXe) down to 0.7 keVnr and 1.1 keVnr, respectively, in situ using a D-D neutron calibration source. Improvements in the D-D calibration have been possible by incorporating pulsing technique with narrow pulses (20 us / 250 Hz). This technique allows the suppression of accidental backgrounds in D-D neutron data and also provides increased sensitivity for the lower energy NR calibrations. I will report the improved NR absolute Qy and Ly measurements using the pulsed D-D calibration technique performed in situ in the LUX detector. This in situ energy calibration, using pulsed D-D neutron, represents the lowest energy NR that has been explored in liquid Xe and are accompanied by a significant improvement in calibration uncertainty. |
Monday, April 15, 2019 3:06PM - 3:18PM |
R10.00009: The LZ Photoneutron Calibration Source Andreas Biekert The LUX-ZEPLIN (LZ) detector will be a two-phase time projection chamber with a seven ton liquid xenon target used to search for dark matter. I will present the design of the LZ photoneutron source, which will produce neutrons with recoil energies in xenon of just a few keV. This source will calibrate the LZ detector response to these low-energy nuclear recoils (NRs). The light and charge yields of xenon at these NR energies are currently not well measured and could have significant impact on the detector efficiency--and therefore the sensitivity of LZ to low-mass (< 20 GeV) dark matter. This calibration will also constrain the detector efficiency for coherently scattering astrophysical 8B neutrinos, a background in low-mass dark matter searches and an interesting physics topic in its own right. I will focus on both the mechanical design and deployment strategy of the source and simulations used to predict the effectiveness of the calibration. |
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