DEAP-3600

First Results on Dark Matter Cross-Section with Liquid Argon

In July 2017, the DEAP-3600 collaboration published its first paper reporting on the search for Weakly Interacting Massive Particles (WIMPs) using four days of data collected during the commissioning phase. While the data showed no interactions between the liquid argon atoms and WIMPs (as expected based on the short exposure time), the result showed the true power of DEAP’s Pulse-Shape Discrimination (PSD), a methodology that differentiates electronic interactions (backgrounds) to nuclear interactions (signal). In Physics Review Letters, the collaboration showed that the experiment is performing to specifications and PSD can discriminate electronic recoils from nuclear recoils with an unprecedented measured power of 1.2x10^-7.

Completion of the DEAP-3600 Acrylic Vessel Inner Surface Sanding

To listen for interactions between DEAP’s argon detector and dark matter, the collaboration had to create one of the most radiogenically clean environments in the entire universe. To do this, the collaboration fabricated its inner detector, a sphere of radius 85 cm, from ultra-pure acrylic, and designed and implemented an 18 feet tall sanding robot (the Resurface) capable of removing the innermost layer of the acrylic sphere in a contained environment (with the aim of removing any contamination introducted during installation). This never-before-attempted, large-scale robot, a true experiment within the experiment, was deployed at the center of the DEAP vessel in October 2014 and after a month of operation, had successfully removed 500 microns uniformly from the detector surface, thus returning the purity levels of the acrylic back to production standards.

Commissioning of the Data Acquisition System and First Liquid Argon Signal

In August 2016, the DEAP-3600 experiment was finally completed with the filling of the acrylic vessel with 3.6 tonnes of liquid argon. This milestone marked the beginning of the physics operation for this experiment. The DAQ and the Science & Technology groups at TRIUMF worked extensively to ensure that the data acquisition system was ready for the data intake upon filling. One of the biggest challenges for the scientists at TRIUMF who drove this part of the project was to ensure that the experiment could keep up with the high rate of the intrinsic argon beta-emitter ³⁹Ar, of the order of ~3.6 kHz, while never missing any possible dark matter-argon interaction. The commissioning period was brief and smooth, and the data acquisition system operated to specification due primarily to the efforts of the TRIUMF team.