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Fig.1: (left) Schematic version of a single-phase cryogenic liquid scintillation detector. The UV scintillation light (128 nm) is converted to detectable optical light by a wavelength-shifting layer viewed in 4p by a PMT array. (right) Rejection of internal (39Ar beta decay) is via Particle-ID using pulse-shape discrimination of the triplet-to-singlet light ratio. External backgrounds (surface radon progeny and fast neutrons) are rejected by self-shielding and fiducialization. MiniCLEAN will be the first single-phase LAr detector with 3D event reconstruction.

It is now generally accepted in the scientific community that roughly 85% of the matter in the universe is in a form that neither emits nor absorbs electromagnetic radiation. Multiple lines of evidence from cosmic microwave background probes, measurements of galaxy cluster and galaxy rotation curves, strong and weak gravitational lensing and big bang nucleosynthesis all point toward a cosmological concordance model containing cold dark matter particles as the best explanation for the universe we see. Alternative theories involving modifications to Einstein's theory of gravity have not been able to explain the observations across all scales. A compelling candidate for dark matter is Weakly Interacting Massive Particles (WIMPs) that could be directly detected as they scatter from massive, ultra-pure detector targets operating deep beneath the Earth's surface.

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Fig.2: (Left) Model of the MiniCLEAN central detector with its 4p target viewed by 92 optical cassettes. (Right) The optical cassettes are 30 cm long and consist of a 10 cm thick acrylic plug, the front surface of which is coated with a wavelength-shifting fluor (TPB), and 30 cm light guide leading to the PMT. The inner target defined by the TPB surface contains 500 kg of LAr within a nominal radius of 44 cm.

The challenge to realizing sensitive dark matter detectors is in separating ubiquitous backgrounds from the nuclear-recoil events characteristic of the WIMP signature. Indeed, we face the daunting task of separating a single event in a tonne of target material or more following a year's exposure. Noble liquid detectors exploiting liquid xenon or liquid argon hold great promise in realizing this intimidating goal. LANL scientists are spearheading a novel approach, dubbed CLEAN for Cryogenic Low-Energy Astrophysics with Noble Liquids, to the direct detection of dark matter using the unique capabilities of liquid argon for unprecedented discrimination between electromagnetic backgrounds (electrons and gamma rays) and nuclear-recoil events (ref.[1]). CLEAN is unique in its ability to exchange the liquid argon target with liquid neon in the same detector, offering potential to the discovery of WIMP dark matter in a "beam-on, beam-off" approach and further expanding its scientific portfolio to the detection of low-energy neutrinos.

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Fig.3: A collage of MiniCLEAN detector components under assembly at SNOLAB: (top left) the Outer Vessel on its stand and inside the water shield tank in the Cube-Hall; (top right) a view of light-guides as they are pre-fit into the Inner Vessel; (bottom left) a PMT assembled in its holder along with the light-guide and acrylic plug with wavelength shifter; (bottom right) the Inner Vessel that serves to hold the cryogenic target and 92 optical cassettes.

MiniCLEAN (see Figs.1&2), a first-generation experiment with a target (fiducial) mass of 500 kg (150 kg), will be the first "single-phase" LAr detector capable of 3-D event reconstruction [2]. The ability to inject a radioactive spike of 39Ar into the MiniCLEAN detector provides a unique opportunity to study background discrimination at the unprecedented level of parts in 1010. The data from MiniCLEAN will inform the ultimate PSD capability of next generation experiments like DEAP-3600 and DarkSide-G2 and provide the bedrock for design of a massive detector capable of extending the reach for WIMP dark matter by several orders of magnitude. The MiniCLEAN detector is presently being assembled in the Cube-Hall at SNOLAB (see Fig.3) and includes collaborators from the U.S., the United Kingdom and Canada.

[1] M.G. Boulay and A. Hime, A Technique for Direct Detection of Weakly Interacting Massive Particles Using Scintillation Time Discrimination in Liquid Argon, Astroparticle Physics 25, 179-182 (2006).

[2] A. Hime, The MiniCLEAN Dark Matter Experiment, Proceedings of the DPF-2011 Conference, providence, RI (2011); arXiv:1110.1005.