If you want to breathe some of the most radioactive free, or “radio-pure,”
air on earth, go 4,850 feet underground to the site of the LZ (LUX-ZEPLIN)
experiment at the Sanford Underground
Research Facility (SURF).
A research team at South Dakota School of
Mines & Technology has built an air purifier that has reduced the radon
in the air to about 50 times lower than typical outdoor air. The team is
helping to ensure success for one of the world’s most sensitive dark matter
experiments — LZ.
Dark matter has never been directly observed. But it is believed to make up 85%
of all the matter in the universe. The mystery of dark matter is considered to
be one of the most pressing questions in particle physics. The LZ experiment is
run deep underground where it will be protected from high-energy particles,
called cosmic radiation, which can create unwanted background signals. But
underground environments pose other challenges. They are often higher in radon,
which can also impede sensitive experiments.
“Usually the concentration of radon
underground is quite high, but the equipment that has been installed in SURF
reduces radon background by a factor of a thousand,” says Richard Schnee,
Ph.D., the physics department head at South Dakota Mines. “Radioactive
particles are a real problem for these super sensitive dark matter detectors.”
Even miniscule amounts of radon could contaminate and throw off the experiment.
“Without this, the scientific community has no reason to trust our results,”
says Eric Morrison, a Ph.D. graduate student at South Dakota Mines who is
working on the project.
While radon gas in the air at SURF
may be enough to disrupt a sensitive experiment, it is not enough to be
dangerous to humans who work underground. Other air handling systems at SURF
keep the air in the rest of the underground lab at safe levels.
Schnee’s team also checked many of the components used to build LZ for
background radiation as the LZ detector was being assembled. This understanding
of the total level of background radiation in the equipment itself helps
researchers predict the number of false positive events that look like dark
matter interactions. The LZ
recently moved underground at SURF and is set to start the hunt for dark
matter in the coming year. South Dakota Mines is one of 37 institutions
worldwide working on LZ.
The South Dakota Mines team has also designed and installed a radon
reduction system for the SuperCDMS
SNOLAB experiment. This experiment is in the hunt for suspected dark matter
particles alongside LZ. SuperCDMS SNOLAB will be assembled and operated at the
Canadian laboratory SNOLAB, located 6,800
feet underground inside a nickel mine near the city of Sudbury, Ontario. It’s
the deepest underground laboratory in North America. SD Mines is one of 26
institutions worldwide working on SuperCDMS SNOLAB.
These two experiments, SuperCDMS SNOLAB and
LZ, are powerful new tools that will try to solve one of the biggest mysteries
of modern physics - dark matter.