DEAP-3600

The international DEAP-3600 (Dark matter Experiment using Argon Pulse-shaped discrimination) collaboration is an experiment attempting the first direct detection of a dark matter particle. 

Based at SNOLAB, DEAP-3600 is one of five TRIUMF-SNOLAB collaborations involving some of the world’s most advanced experiments in dark matter detection (see also SuperCDMS) and neutrino science (SNO+, nEXO and HALO). SNOLAB, created with TRIUMF support, is the deepest underground laboratory in North America, based in a repurposed mine shaft two-kilometers under Sudbury, Ontario. 

The origin and nature of dark matter in our universe is one of the great mysteries in cosmology, astrophysics and particle physics.  

Through detailed observations of the gravitational behaviour of galaxies, scientists have inferred the existence of dark matter and that it makes-up about a quarter of cosmic mass. Dark energy is hypothesized to make-up about three quarters of the cosmic mass, with baryonic matter, the stuff of the Periodic Table–including people, planets and stars–making-up just four percent of the cosmos.  

Similarly, an extension of the Standard Model called Supersymmetry predicts the existence of dark matter particles dubbed WIMPS, for Weakly Interacting Massive Particles. As Earth orbits through the Milky Way galaxy, physicists believe we move through a sea of these dark matter particles. However, there’s never been a direct detection of one. 

The DEAP-3600 experiment is using a unique dark matter target made with 3600 kilograms of highly sensitive liquid-argon to try and make the first detection of a dark matter particle. DEAP-3600 is 20 times more sensitive to predicted large dark matter particles that other current experiments.  

The DEAP-3600 collaboration involves more than 75 researchers from ten institutions in the United Kingdom, Mexico, Germany, Italy, the United States, and Canada, including the University of Alberta, Carleton University, Queen’s University, SNOLAB and TRIUMF. 

Since dark matter doesn’t emit or scatter electromagnetic radiation (light), it can’t be seen using traditional particle detectors. However, physicists theorize that WIMPS have a small and measurable cross section with regular matter: if they get close enough to one another they’ll physically interact via the weak force. 

Thus, DEAP-3600 is designed to record a WIMP detection through its nuclear recoil reaction with argon. This is the equivalent of a WIMP billiard ball colliding with a stationary argon billiard ball energizing the argon, which would decay back to a resting state by emitting light.  

DEAP-3600 consists of three core components: the liquid-argon target; a system of sensors and related electronics for recording particle interactions; and an environment shielded from background particle noise both from the environment and the experimental apparatus itself.  

1. The liquid-argon target: At its core, DEAP-3600 houses its dark matter target: a circular acrylic vessel, about 85 cm in diameter, filled with 3600 kg of liquid argon (Ar). To increase the target’s detection efficiency, it’s cooled with liquid nitrogen to about -180 ºC. The liquid argon is a scintillator target: when an atom of argon is energized through a nuclear recoil, or electromagnetic interaction, it decays back to a rest state by emitting light, in Ar’s case ultraviolet (UV) light. The interior surface of the acrylic vessel is coated with tetraphenyl butadiene, a light wavelength shifter. The wavelength shifter converts the UV signal into visible light that’s recorded by DEAP-3600’s detectors. Because of its chemical and physical characteristics, argon is uniquely suited to detecting WIMPs, producing drastically different signals from collisions with different particles. This will enable DEAP-3600 scientists to easily differentiate between argon collisions with neutrons or electrons and a collision with a particle that fits the WIMP profile. The differentiating power of Ar is so strong that the likelihood of a mixed signal is less probable than one-in-ten-million.  
2. Signal detection: Covering the entire surface of the argon-acrylic vessel are 255 45 cm-long acrylic light guides, giving DEAP-3600 a porcupine-like appearance. The light guides funnel the light signals emit by the target to 255 photomultiplier tubes (PMT) detectors. Each of these is about 24 cm in diameter, analogous to 255 highly sensitive eyes staring unflinching into the sphere of Ar, recording every little flash of light, hoping for one that resembles that of a putative WIMP. The light signals are recorded and analyzed via a sophisticated TRIUMF-designed data acquisition system.  

3. Reducing background detections: DEAP-3600 is based at SNOLAB to shield the experiment from the detection of high-energy cosmic rays which would cause background noise and might mimic dark matter detections. Similarly, the entire DEAP-3600 experimental facility is housed in an 8-metre diameter tank of ultrapure water which provides additional particle shielding from the environment. Finally, DEAP-3600 is constructed from ultra-pure materials to minimize radiation from the structure itself.  

TRIUMF is providing three core hardware contributions to the DEAP-3600 experimental facility: 

1. Data Acquisition System and Electronics Optimized for Liquid Argon Scintillation Detection: With decades of expertise in developing Data Acquisition Systems (DAQ) for high-throughput, large data particle physics experiments, TRIUMF’s Electronics Group is equipping DEAP with a state-of-the-art TRIUMF-designed DAQ. This DAQ system is similar to the one TRIUMF is supplying to SNOLAB’s other dark matter search experiment, Super-CDMS. The TRIUMF-designed DAQ plays a key role in suppressing the background noise by providing electronic on-the-fly triggering. The liquid argon target contains traces of radioactive argon-39 which produce in total about 3000 decay events-per-second. As part of the DAQ, these all signals are analyzed electronically via a Digitizer and Trigger Module (DTM), which decides whether to trigger event readout, and thus filters-out the background signals. The DTM provides excellent photoelectric response and signal digitization in 16-microseconds, enabling particle identification, time-tagging and manageable data rates. The DTM hardware is based on a TRIUMF-designed 6U VME motherboard with an ALTERA Stratix IV GX field-programmable gate array. 
2. Novel Optical Calibration Hardware for PMTs: TRIUMF has developed lasers and laser drivers for characterizing different components of the DEAP-3600 photomultiplier (PMT) apparatus. Because the decay of excited Ar following a WIMP-Ar collision will produce a photon somewhere within the UV spectrum, DEAP scientists and technicians need to use lasers of variable pulse width, intensity, frequency, and wavelength, to mimic possible Ar decay signals and ensure every component of the PMT apparatus operates as designed.  
3. Pure Acrylic Light Guide Fabrication: The integrity of and precision of DEAP-3600’s 255 acrylic light guides is crucial to the experiment’s efficiency. The 45-cm long light guides join the cryogenic acrylic vessel holding the liquid argon and the room-temperature photo-multiplier tube detectors. TRIUMF’s Machine Shop manufactured the 255 acrylic light guides using specialized oil-free machining. Oil contamination would cause the degradation of the acrylic. The long-term integrity of the light guides is essential since they provide temperature isolation, insulating the photomultiplier tubes from the cryogenic liquid argon, and the acrylic is an effective neutron moderator, helping shield the experiment from environmental neutron interactions that might mimic a WIMP detection.