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 has been operated at Queen's University to test the available pulse-shape discrimination at low recoil energies in liquid argon, and has been moved to SNOLAB in October 2007. 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 construction beginning in 2008, and will have sensitivity to WIMP-nucleon scattering cross-sections as low as 10−46 cm².
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 currently in research and development. 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.
As of November 2007[update], the first generation detector, DEAP-1, after demonstrating good pulse shape discrimination of backgrounds on the surface, has begun operation in SNOLAB. The deep underground location reduces unwanted cosmogenic signals ("background") and allows a more sensitive dark matter search. A run of 18 months is planned, during which the 1 ton DEAP-3 detector is being constructed.
As of April 2013[update], the second-generation DEAP-3600 detector has had its major components installed underground, assembly is in progress, and it is expected to be filled with liquid argon in January 2014.
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.
- Walding, Joseph (2013-04-10), "The DEAP-3600 Dark Matter Experiment", IOP 2013: High Energy and Astrop Particle Physics, Institute of Physics, p. 14, retrieved 2013-05-19
- 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)