Two kilometers underground, beneath the Canadian rock, sits a tank filled with three tonnes of liquid argon. The argon is chilled to minus 186 degrees Celsius and is as clear as water. This tank is a trap for dark matter particles, which scientists believe stream through us every second but rarely interact with ordinary matter.
To make the trap work accurately, it needs tuning. So researchers inject radioactive sodium-22 into the detector. It emits three gamma rays simultaneously, like three bells ringing at once. These triple signals make it easy to distinguish calibration events from random noise. The team compared the detector's response to the energy of these rays with predictions from a computer model. The match was good, which means the instrument can be trusted.
This calibration is like checking a scale before weighing something. If the scale lies, all measurements are useless. Now DEAP-3600 is ready for a long hunt: maybe one day, a faint flash in the argon will signal a collision with a WIMP — a hypothetical dark matter particle. And then, the tank in the mine will become our first window into the invisible world.
The DEAP-3600 detector is so sensitive it can spot even a single photon of light produced in the argon by a particle collision.