Neutrons are a particularly dangerous background for direct WIMP dark matter searches; their nucl... more Neutrons are a particularly dangerous background for direct WIMP dark matter searches; their nuclear recoils with the target nucleus are often indistinguishable from nuclear recoils produced by WIMP-nuclear collisions. In this study, we explore the concept of a liquid scintillator neutron veto detector that would allow direct dark matter detectors to potentially reject neutrons with greater than 99% efficiency. Here we outline the construction and testing of a small prototype detector and the potential implications of this technology for future dark matter detectors.
Study of Light Collection Efficiency for a Neutron Veto for the DarkSide-50 Dark Matter Detector
DarkSide-50 is a 50-kg liquid argon dark matter detector that will use a borated liquid scintilla... more DarkSide-50 is a 50-kg liquid argon dark matter detector that will use a borated liquid scintillator to veto neutron-induced background events. The liquid scintillator consists of a mixture of tri-methyl-borate and pseudocumene with wavelength shifters. The scintillator is ...
The dark matter interpretation of the DAMA modulation signal depends on the NaI(Tl) scintillation... more The dark matter interpretation of the DAMA modulation signal depends on the NaI(Tl) scintillation efficiency of nuclear recoils. Previous measurements for Na recoils have large discrepancies, especially in the DAMA/LIBRA modulation energy region. We report a quenching effect measurement of Na recoils in NaI(Tl) from 3 keVnr to 52 keVnr, covering the whole DAMA/LIBRA energy region for light WIMP interpretations. By using a low-energy, pulsed neutron beam, a double time-of-flight technique, and pulse-shape discrimination methods, we obtained the most accurate measurement of this kind for NaI(Tl) to date. The results differ significantly from the DAMA reported values at low energies, but fall between the other previous measurements. We present the implications of the new quenching results for the dark matter interpretation of the DAMA modulation signal.
Microanalytical Investigations of Presolar SiC of Type AB
We report the results of a coordinated NanoSIMS and TEM study, including isotopic, structural, ch... more We report the results of a coordinated NanoSIMS and TEM study, including isotopic, structural, chemical, and subgrain data, on four SiC AB grains. The stellar origin of these grains is believed to be both J stars and born-again AGB stars.
Liquid-nitrogen cryogenic system for noble liquid detectors
The next generation of noble liquid dark-matter detectors will require a powerful, stable, and ad... more The next generation of noble liquid dark-matter detectors will require a powerful, stable, and adjustable cryogenic cooling system that will not introduce backgrounds into the detector. I will discuss the design and operation of a novel liquid-nitrogen based heat exchanger capable of delivering a widely varying degree of cooling power and maintaining a range of stable temperatures. I will present measurements from our successful integration of this cooling system with our liquid argon time-projection-chamber, Darkside-10.
Combined TEM and NanoSIMS Analysis of Subgrains in a SiC AB Grain
We report the results of NanoSIMS and TEM analysis, including isotopic, structural, chemical, and... more We report the results of NanoSIMS and TEM analysis, including isotopic, structural, chemical, and subgrain data, on a SiC AB grain. This grain contains the first oldhamite subgrains observed in a presolar grain, as well as TiC- and Fe-rich subgrains.
Pristine SiC Candidates: Spectral Imaging and Auger Investigations
We describe a new spectral imaging method to locate pristine SiCs (those prepared without acid di... more We describe a new spectral imaging method to locate pristine SiCs (those prepared without acid dissolution) from within Murchison matrix material. We present images,X-ray and Auger electron spectra from pristine SiCs, which show carbonaceous surface coatings.
Efficiency Studies and Simulations of a Neutron Background Veto for Dark Matter Detectors
In direct WIMP dark matter detection experiments, neutrons from cosmogenic sources and nuclear re... more In direct WIMP dark matter detection experiments, neutrons from cosmogenic sources and nuclear reactions in detector materials can provide backgrounds indistinguishable from WIMP signals. To reduce this background, an active neutron veto filled with a boron-loaded scintillator is being developed. The scintillator used will be pseudocumene, mixed with trimethyl borate as a boron source, and a PPO wavelength shifter. Such a veto would detect neutrons in the volume surrounding the detector, allowing coincident background events in the detector to be rejected. Neutrons are captured by the ^10B with a high cross section, resulting in an α and ^7Li. The scintillation from the nuclear products is heavily quenched to an equivalent electron energy as low as 50 keV. To detect this, it is necessary to have high light collection efficiency. To model the neutron veto concept, light yield measurements were taken for a small prototype filled with the scintillator mixture and lined with a Lumirror ...
We report the first results of DarkSide-50, a direct search for dark matter operating in the unde... more We report the first results of DarkSide-50, a direct search for dark matter operating in the underground Laboratori Nazionali del Gran Sasso (LNGS) and searching for the rare nuclear recoils possibly induced by weakly interacting massive particles (WIMPs). The dark matter detector is a Liquid Argon Time Projection Chamber with a (46.4 ± 0.7) kg active mass, operated inside a 30 t organic liquid scintillator neutron veto, which is in turn installed at the center of a 1 kt water Cherenkov veto for the residual flux of cosmic rays. We report here the null results of a dark matter search for a (1422 ± 67) kg d exposure with an atmospheric argon fill. This is the most sensitive dark matter search performed with an argon target, corresponding to a 90% CL upper limit on the WIMP-nucleon spin-independent cross section of 6.1 × 10 −44 cm 2 for a WIMP mass of 100 GeV/c 2 .
SABRE (Sodium-iodide with Active Background REjection) is a new NaI(Tl) experiment designed to te... more SABRE (Sodium-iodide with Active Background REjection) is a new NaI(Tl) experiment designed to test the DAMA/LIBRA claim for a positive WIMP-dark matter annual modulation signal. SABRE will consist of highly pure NaI(Tl) crystals in an active liquid scintillator veto that will be placed deep underground. The scintillator vessel will provide a veto against external backgrounds and those arising from detector components, especially the 3 keV signature from the decay of 40 K in the crystal. Through the use of crystal purification techniques and the veto, we aim for a 40 K background significantly lower than that of the DAMA/LIBRA experiment. We present our work developing low-background NaI(Tl) crystals using a highly pure NaI powder and the development of the veto.
Study of Light Collection Efficiency for a Neutron Veto for the DarkSide-50 Dark Matter Detector
Bulletin of the American Physical Society, 2012
DarkSide-50 is a 50-kg liquid argon dark matter detector that will use a borated liquid scintilla... more DarkSide-50 is a 50-kg liquid argon dark matter detector that will use a borated liquid scintillator to veto neutron-induced background events. The liquid scintillator consists of a mixture of tri-methyl-borate and pseudocumene with wavelength shifters. The scintillator is ...
As part of the DarkSide program of direct dark matter searches using two-phase argon TPCs, a prot... more As part of the DarkSide program of direct dark matter searches using two-phase argon TPCs, a prototype detector with an active volume containing 10 kg of liquid argon, DarkSide-10, was built and operated underground in the Gran Sasso National Laboratory in Italy. A critically important parameter for such devices is the scintillation light yield, as photon statistics limits the rejection of electron-recoil backgrounds by pulse shape discrimination. We have measured the light yield of DarkSide-10 using the readily-identifiable full-absorption peaks from gamma ray sources combined with single-photoelectron calibrations using low-occupancy laser pulses. For gamma lines of energies in the range 122-1275 keV, we get light yields averaging 8.887±0.003(stat)±0.444(sys) p.e./keV ee. With additional purification, the light yield measured at 511 keV increased to 9.142±0.006(stat) p.e./keV ee .
Microstructural differences among the isotopic groups of low-density Orgueil graphites
We find difference among the isotopic groups of low-density Orgueil graphites in terms of their s... more We find difference among the isotopic groups of low-density Orgueil graphites in terms of their structure and in the types of internal grains contained. TEM results are presented from one 13C-rich graphite, one nearly solar graphite, and two SN graphites.
A Unique Supernova Graphite: Contemporaneous Condensation of All Things Carbonaceous
We report a supernova graphite that contains internal subgrains of TiC, SiC, Fe and Ni silicides,... more We report a supernova graphite that contains internal subgrains of TiC, SiC, Fe and Ni silicides, and iron metal. These phases comprise a complete list of the phases predicted by equilibrium calculations to condense from C-rich supernova zones.
Raman and isotopic studies of large presolar SiC grains
Raman spectra correlate with grain morphology: euhedral grains are cubic, grains with smooth, blo... more Raman spectra correlate with grain morphology: euhedral grains are cubic, grains with smooth, blocky morphology are hexagonal. An unusually large (30 µm) polycrystalline cubic X grain has wide Raman peaks, indicative of crystallographic disorder.
Low-density presolar graphite spherules from the Orgueil meteorite
TEM studies of two Orgueil low-density SN graphites reveal internal SiCs and TiAl-rich grains, ph... more TEM studies of two Orgueil low-density SN graphites reveal internal SiCs and TiAl-rich grains, phases not seen in Murchison SN graphites. Metallic RuOs-rich grains in a 13C-rich graphite indicate that the source of 13C-rich graphites is s-process enriched.
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Papers by Emily Shields