Technique for direct detection of weakly interacting massive particles using scintillation time discrimination in liquid argon

Discrimination between electron and nuclear recoil events in a liquid argon scintillation detector has been demonstrated with simulations by using the differences in the scintillation photon time distribution between these classes of events. A discrimination power greater than 108 is predicted for a liquid argon experiment with a 10 keV threshold, which would mitigate electron and γ-ray backgrounds, including β decays of 39Ar and 42Ar in atmospheric argon. A dark matter search using a ∼2 kg argon target viewed by immersed photomultiplier tubes would allow a sensitivity to a spin-independent WIMP-nucleon cross-section of ∼10-43 cm2 for a 100 GeV WIMP, assuming a one-year exposure. This technique could be used to scale the target mass to the tonne scale, allowing a sensitivity of ∼10-46 cm2

Technique for surface background rejection in liquid argon dark matter detectors using layered wavelength-shifting and scintillating thin films

A technique using layered wavelength shifting, scintillating and non-scintillating films is presented to achieve discrimination of surface α events from low-energy nuclear recoils in liquid argon detectors. A discrimination power greater than 108, similar to the discrimination possible for electronic recoils in argon, can be achieved by adding a 50μm layer of scintillator with a suitably slow decay time, approximately 300 ns or greater, to a wavelength-shifter coated surface. The technique would allow suppression of surface α events in a very large next-generation argon dark matter experiment (with hundreds of square meters of surface area) without the requirement for position reconstruction, thus allowing utilization of more of the instrumented mass in the dark matter search. The technique could also be used to suppress surface backgrounds in compact argon detectors of low-energy nuclear recoils, for example in measurements of coherent neutrino–nucleus scattering or for sensitive measurements of neutron fluxes

Surface background rejection technique for liquid argon dark matter detectors using a thin scintillating layer

Future large liquid argon direct dark matter detectors can benefit greatly from an efficient surface background rejection technique. To aid the development of these large scale detectors a test stand, Argon-1, has been constructed at Carleton University, Ottawa, Canada, in the noble liquid detector development lab. It aims to test a novel surface background rejection technique using a thin layer of slow scintillating material at the surface of the vessel. Through pulse-shape discrimination of the slow light from the scintillating layer, events from the surface of the detector can be discriminated from liquid argon events. The detector will be implemented with high-granularity SiPMs for light detection which will be used to accurately identify surface events to characterize the proposed technique. An overview of the technique and the status of the experiment are discussed here

Radiopurity measurement of acrylic for DEAP-3600

The spherical acrylic vessel that contains the liquid argon target is the most critical detector component in the DEAP-3600 dark matter experiment. Alpha decays near the inner surface of the acrylic vessel are one of the main sources of background in the detector. A fraction of the alpha energy, or the recoiling nucleus from the alpha decay, could misreconstruct in the fiducial volume and result in a false candidate dark matter event. Acrylic has low levels of inherent contamination from 238U and 232Th. Another background of particular concern is diffusion of 222Rn during manufacturing, leading to 210Pb contamination. The maximum acceptable concentrations in the DEAP-3600 acrylic vessel are ppt levels of 238U and 232Th equivalent, and 10-8 ppt 210Pb. The impurities in the bulk acrylic will be measured by vaporizing a large quantity of acrylic and counting the concentrated residue with ultra-low background HPGe detectors and a low background alpha spectrometer. An overview of the acrylic assay technique is presented

Surface backgrounds in the DEAP-3600 dark matter experiment

DEAP-3600 is a dark matter experiment using 3.6 tons of liquid argon to search for Weakly Interacting Massive Particles (WIMPs), with a target sensitivity to the spin-independent WIMP-nucleon cross-section of 10 -46 cm2. The detector is designed to allow for a three year background-free run with a 1-ton fiducial volume. We identify in this paper the potential sources of surface contamination. We require 238U and 232Th contaminations on the order of 10-12 g/g or less, a level achieved by the SNO experiment, and 210Pb not significantly out of equilibrium with 238U, i.e., 10-20 g/g or less 210Pb in the acrylic vessel or TPB wavelength shifter, which should be achievable with appropriate control of exposure to radon

Beta-asymmetry studies on polarized 82Rb atoms in a TOP trap

Atoms of 82Rb (t1/2 = 76 s) confined in a time-orbiting-potential (TOP) magnetic trap make a favorable source for β-asymmetry studies by providing an essentially massless source of highly polarized atoms. An offline mass separator is coupled to a double magneto-optical trap (MOT) and TOP trap system. Once in the TOP trap the magnetic trap's rotating bias field defines the polarization axis and allows one to measure the correlation between the nuclear spin direction and the β emission direction using a single positron detector. A proof-of-principle experiment using this method has demonstrated that the parity violating (β→ · J→) correlation can be studied. Here we outline improvements to the experiment with the goal of a 1% measurement of the β-asymmetry correlation parameter A in the Gamow-Teller decay of 82Rb which in the semi-leptonic sector would pose a competitive test of the Standard Model

Application of the TPB Wavelength Shifter to the DEAP-3600 Spherical Acrylic Vessel Inner Surface

DEAP-3600 uses liquid argon contained in a spherical acrylic vessel as a target medium to perform a sensitive spin-independent dark matter search. Argon scintillates in the vacuum ultraviolet spectrum, which requires wavelength shifting to convert the VUV photons to visible so they can be transmitted through the acrylic light guides and detected by the surrounding photomultiplier tubes. The wavelength shifter 1,1,4,4-tetraphenyl-1,3-butadiene was evaporatively deposited to the inner surface of the acrylic vessel under vacuum. Two evaporations were performed on the DEAP-3600 acrylic vessel with an estimated coating thickness of 3.00 ± 0.02 μm which is successfully wavelength shifting with liquid argon in the detector. Details on the wavelength shifter coating requirements, deposition source, testing, and final performance are presented

Temperature dependence of alpha-induced scintillation in the 1,1,4,4-tetraphenyl-1,3-butadiene wavelength shifter

Liquid noble based particle detectors often use the organic wavelength shifter 1,1,4,4-tetraphenyl-1,3-butadiene (TPB) which shifts UV scintillation light to the visible regime, facilitating its detection, but which also can scintillate on its own. Dark matter searches based on this type of detector commonly rely on pulse-shape discrimination (PSD) for background mitigation. Alpha-induced scintillation therefore represents a possible background source in dark matter searches. The timing characteristics of this scintillation determine whether this background can be mitigated through PSD. We have therefore characterized the pulse shape and light yield of alpha induced TPB scintillation at temperatures ranging from 300 K down to 4 K, with special attention given to liquid noble gas temperatures. We find that the pulse shapes and light yield depend strongly on temperature. In addition, the significant contribution of long time constants above ∼50 K provides an avenue for discrimination between alpha decay events in TPB and nuclear-recoil events in noble liquid detectors