DEAP (Dark Matter Experiment using Argon Pulse-shape discrimination) is a direct dark matter search experiment using liquid argon as target material. DEAP utilizes background discrimination based on the characteristic scintillation pulse shape in argon. A first-generation detector (DEAP-1) with a 7 kg target mass operated at Queen's University to test the available pulse-shape discrimination at low recoil energies in liquid argon, and was moved to SNOLAB in October 2007 and ran into 2011. Discrimination of beta and gamma events from nuclear recoils in the energy region of interest (near 20 keV of electron energy) is required to be better than 1 in 108 to sufficiently suppress backgrounds in the DEAP-1 detector. A larger detector with a 3600 kg active mass is planned for operation beginning in 2014, with sensitivity to WIMP-nucleon scattering cross-sections as low as 10−46 cm². A later ton-scale detector is planned.
Scintillation properties and background suppression
In order to uncover the faint signature of an interacting dark matter particle, the detector is sensitive to low energy interactions, and as such, the signal from the WIMP interaction is hidden within a large collection of events. These events are known as background events. The WIMP signal must be uniquely filtered from these events. In DEAP, as the light output is characteristic to the type of event, be it either a nuclear recoil event or a gamma event, the interaction of each event can be uniquely determined. This is done by finding the ratio of prompt light to late light in any given pulse. This ratio is known as fPrompt. Nuclear recoil events have a high fPrompt while gamma events have a low fPrompt. Since the interaction of WIMPs is expected to cause a nuclear recoil event, low fPrompt events such as gamma interactions can be considered as noise and cut from the data.
The first stage of the DEAP project, DEAP-1, was designed in order to characterize several properties of liquid argon, demonstrate the pulse shape discrimination of liquid argon and refine engineering. DEAP-1 utilizes 7 kg of liquid argon as a target for WIMP interactions. Two photomultiplier tubes are used to detect the scintillation light produced by a particle interacting with the argon. As the scintillation light produced is of short wavelength (120 nm) a wavelength shifting film is used to broaden the wavelength so that it falls within the visible spectrum (440 nm) enabling it to pass through ordinary windows without any losses and be detected by the PMTs.
The DEAP project is in commissioning. A large, well-experienced international collaboration from Canadian and US universities and laboratories are jointly working toward a large tonne-scale detector. The collaboration benefits largely from the experience many of the members and institutions gained on the SNO project, which detects neutrinos, another weakly interacting particle.
DEAP-1, demonstrated good pulse shape discrimination of backgrounds on the surface and began operation in SNOLAB. The deep underground location reduces unwanted cosmogenic signals ("background"). It ran 2007 to 2011, during which the 1 ton DEAP-3600 detector was constructed. DEAP-1 characterized the background, determining design improvments needed in DEAP-3600.
The DEAP-3600 detector has been installed underground, assembly i s near complete, and is expected to be filled with liquid argon in 2014. Projected sensitivity to WIMP cross-section is 10−46 cm2 after three years. DEAP-50T is the planned expansion with 50 tons of depleted argon, sensitivity of 10−48 cm2.
Collaborators are from Queen's University, Carleton University, Case Western Reserve University, Los Alamos National Laboratory, SNOLAB, TRIUMF, University of Alberta, University of New Mexico, University of North Carolina, and Yale University.
- DEAP-1 Project web site
- Lidgard, Jeffrey, J. M.Sc. thesis: Pulse shape discrimination studies in liquid argon for the DEAP-1 detector (Queen's University, April 2008)