SNOLAB officially becomes one of the coolest places on Earth

SNOLAB staff escort the dilution fridge 1.2 kilometers through the mine drift to the lab entrance. (Mike Whitehouse/SNOLAB)

Deep underground in Sudbury’s SNOLAB, a major new experiment has just reached a critical milestone.

The very centre of a series of large, nested copper vessels that make up the heart of the newest and one of the most sensitive detectors in the world has now reached a temperature a hundred times colder than outer space.

After several years of fabricating and installing, a team of international scientists and SNOLAB staff working on the Super Cryogenic Dark Matter Search (SuperCDMS) experiment have successfully cooled the experiment to its base temperature — just tens of millikelvin, or a hundredth of a degree above absolute zero.

Reaching base temperature marks a major transition for SuperCDMS, from construction and installation to commissioning and science operations.

The SuperCDMS experiment is poised to join the international search for dark matter, the elusive sub-atomic particle thought to comprise up to 85 per cent of the mass of the universe.

Quiet and sensitive

We know from astrophysical observations that our solar system and Milky Way galaxy sit inside a halo of dark matter, and that dark matter is passing through us all the time.

The challenge is to build a detector quiet enough and sensitive enough to observe the very rare interactions of dark matter particles.

At the heart of SuperCDMS are 24 cryogenically cooled detectors made from ultra-pure silicon and germanium crystals, each just larger than a hockey puck. When a dark matter particle strikes one of these crystals, it produces a tiny vibration called a phonon, along with a small electrical signal.

To detect those minuscule signals, the crystals are outfitted with superconducting sensors that only work when they are extremely cold.

SuperCDMS is optimized to explore a region of dark matter models that has remained largely uncharted: light dark matter, a hypothesized form of matter that interacts so weakly with ordinary matter that it has so far escaped direct detection, says Miriam Diamond, assistant professor in the Department of Physics and the Department of Astronomy & Astrophysics, University of Toronto and a SuperCDMS collaborator.

“What sets SuperCDMS apart from other dark matter searches is its low threshold for detecting tiny energy depositions,” Diamond says.

Worked with cryogenic experts

“This gives it exquisite sensitivity to low-mass dark matter candidates, including WIMP-like particles, axion-like particles, dark photons, and lightly-ionizing particles.”

Reaching base temperature is the culmination of years of preparation and months of detailed planning. Over the last year, the team developed a step-by-step cooldown plan, working closely with cryogenics experts responsible for different parts of the system.

“The detectors simply don’t function unless they’re cold enough to enter the superconducting transition,” said SLAC scientist Richard Partridge, who manages the experiment’s installation.

“For us, that means roughly 15 to 30 millikelvin.”

Cooling the experiment reduces thermal noise, the random motion of atoms that can mask faint signals.

“When everything is that cold, the crystals are basically quiet,” Partridge said. “Even very small energy deposits become detectable.”

Building an experiment underground is always a unique experience, especially one that is on the scale of SuperCDMS, says SNOLAB research scientist Matt Stukel.

There have been countless checks: mechanical, thermal, vacuum integrity; and for the most part things have gone remarkably smoothly.

“Plans that work on surface don’t always translate when you get to the underground facility. Luckily SuperCDMS is an incredibly talented collaboration, filled with innovative thinkers who can rapidly come up with solutions,” said Stukel.

“I’m excited that we reached base temperature, but more excited for the future science that we will do.”

Will collect data for a year

With base temperature achieved, the collaboration will move into detector commissioning, a months-long process of turning on, calibrating and optimizing each detector channel.

Once commissioning is complete, SuperCDMS will begin its first science run, expected to last about a year.

Even the first few months of data could be enough to set world-leading limits on light dark matter or reveal something entirely new.

Beyond dark matter, SuperCDMS will allow scientists to probe previously inaccessible energy scales thanks to its unprecedented sensitivity, and maybe even uncover entirely new kinds of particle interactions.

The SuperCDMS SNOLAB experiment is a joint project of the U.S. Department of Energy Office of Science, the U.S. National Science Foundation, the Canada Foundation for Innovation, the Natural Sciences and Engineering Research Council of Canada, and the Arthur B. McDonald Institute

SNOLAB is Canada’s deep-underground science laboratory, located at a depth of two kilometres in Vale’s Creighton Mine near Sudbury.

Using the Canadian Shield to protect experiments from the cosmic rays that constantly bombard the Earth’s surface, SNOLAB supports world-class astroparticle physics research and has attracted innovative life science and quantum technology programs.

By hosting and enabling the world’s most advanced and sensitive underground experiments, SNOLAB bolsters Canada’s scientific reputation, attracts new talent to our country and northern Ontario, trains more highly skilled people, provides more opportunities for Canadian researchers to lead international projects, and generates economic benefits for Ontarians and Canadians.

Mike Whitehouse is SNOLAB’s senior communications officer