If you’ve got physics on your brain, there’s a good chance the word “particle” immediately summons the subatomic realm. Maybe it calls to mind the protons, neutrons, quarks and electrons that make up our bodies and the world around us, or super-high-energy particles like neutrinos that zoom through space at nearly the speed of light.
But there’s a whole other class of particles. The kind that are kicked up when the wind blows, that collect on your countertops and windowsills, that visibly cloud the air when there’s nearby smoke and pollution. A major obstacle to many particle physics experiments is that these types of particles—gas, dust, soot, smoke—can cause pesky background noise and obscure experimental results. This is why many of the highly sensitive detectors used for these experiments are kept in cleanrooms.
“A lot of times when you talk about particle detectors in high-energy and nuclear physics, it’s the kind that detects particles like neutrinos,” says Peggy Norris, Education and Outreach Deputy Director at Sanford Underground Research Facility, or SURF, in South Dakota, the deepest underground lab in the US. “But instruments called particle counters, which measure the amount of dust and other particulates in the air, are crucial to maintaining the cleanliness of the air in the cleanrooms where many of these experiments are built or performed."
Escaping the dust
Deep underground, where many experiments are performed at SURF, the surrounding rocks are laced with radioactive elements such as thorium and uranium, which decay and produce radon gas. These radon gas particles can stick to plastic and contaminate materials.
“The whole reason you go a mile underground is to get away from the cosmic rays and cell phone signals,” says Mark Hanhardt, an experiment support scientist at SURF. “But something else that causes background is dust, and a large amount of dust down there contains some radioactive elements. If you can’t get rid of this dust, then what’s the point of going underground?”
SURF employs a collection of particle counters to keep track of the levels of these and other particles (such as microscopic flakes of human skin) that might compromise experiments. Although there are a few different types of particle counters, they all pull in surrounding air and use tricks of light, such as scattering or blocking, to count and measure the size of the particles in any given space. When the counts are high enough to endanger the data they’re collecting, the researchers know to take extra cleaning precautions to bring them back down.
Hanhardt often tests the instruments in his office to make sure they’re running correctly. On a typical day, his office—which he keeps quite tidy—has a particle count of about a million 0.5-micron-sized particles per cubic foot.
“Step down into the Common Corridor at the Davis Campus, a part of the lab that is kept as clean as possible, and that particle count drops to only a few hundred particles per cubic foot,” Hanhardt says. “Once you enter a cleanroom, that particle count will drop below 10, rarely going above 100.”
At first, to monitor the particle count Hanhardt and his colleagues would have to physically travel to each counter every three or four months to download its data onto a USB drive. But in July of 2017, Hanhardt worked with an undergraduate summer intern at SURF to hook the instruments up to tiny microcomputers called Raspberry Pis, which enabled them to track the particle count in real time.
“In the past, we wouldn’t know about spikes in the particle count until after they occurred,” he says. “With the new system, we have alarms built in that alert us when the counts start going up. This makes it easier to pinpoint what’s causing the increase.”
Counting the invisible
In addition to tracking the cleanliness of the air, these devices also provide a learning opportunity for young students, giving them the chance to take and analyze real data.
“Some years ago we hosted a group of seventh-grade girls at Sanford Lab,” Norris says. “I set up a particle counter in an empty room and sent them into the room one at a time so we could see how the particle count changed with each new person.”
The exercise illustrated why scientists cover themselves head-to-toe when entering a cleanroom. Each person sheds millions of skin particles per day and may also leave behind hair, clothing fibers, cosmetics particles, microbes and dust.
“[The students] were shocked to learn just how much invisible stuff is in the air,” Norris says. “Eventually we used the data to plot a graph of dust versus number of students.”
Stephen Gabriel, a physics teacher at a local high school, is involved in a project investigating ventilation at the lab. His students participate by analyzing the data, and he hopes that this will get them interested in STEM fields.
“Getting involved in real research with real data is what got me hooked on science,” Gabriel says. “But it’s hard to show students what science is really like when you’re tied into a typical high school schedule. My hope is that if I give students first-hand experience doing real research, they'll be inspired to pursue careers in science.”