Geneseo faculty and students placed detectors at Letchworth State Park to show a subatomic particle interacts with us, every second.

By Kris Dreessen

Cosmic rays from outer space collide with atoms in the upper atmosphere, creating subatomic particles called muons that fall to earth. Muons pass through everything on their journey, including us.

If you hold your arm out right now, two or three will pass through your outstretched hand every second. “It’s impossible to see muons. You need special detectors to measure them,” says Kurt Fletcher, Distinguished Teaching Professor of Physics. “A lot of people have never heard of muons, but they are one step in the process of how energy is moved around in the universe.”

Like the better-known protons and neutrons, subatomic muons are one of the building blocks for everything in the universe. Muons have been getting more attention lately, as they have been used by scientists to “see” inside otherwise impenetrable objects, like Egyptian pyramids and magma chambers in volcanoes (see sidebar).

Fletcher and Associate Professor of Physics George Marcus were awarded a $10,000 Public Outreach and Informing the Public Grant from the American Physical Society for a public engagement project last summer to introduce people to muons.

“When people think about nature, they usually think of leaves or rocks or animals,” says Fletcher. “They don’t think about other parts of nature, like subatomic particles. But our view of the natural world is enriched when we realize that these subatomic particles connect us to outer space.”

Student researcher Kevin Seitz ’20 and Clint Cross, senior instructional support specialist, built 10 muon detectors, which were placed at heavily trafficked areas in Letchworth State Park. Matthew VanAllen ’21 and Lydia Fillhart ’20 created a project website and detector signage designed to make muons both interesting and accessible to visitors.

Each detector measures the number of muons passing through it, and the total number detected and overall muon detection rate are displayed on a digital screen. Visitors view the numbers in real time — and are often surprised by what they learn, says VanAllen. “I change the detector batteries and replace the memory cards, but my real role is talking to people who ask questions,” he says. “It’s easy to see nature and think of biology and geology, but how do we get people thinking about the subatomic world, which is around them all the time? Being able to put that idea into people’s minds to think of the invisible world while experiencing all that natural beauty is really exciting.”

As aspiring physics teachers, VanAllen and Fillhart say the project helped them to consider how people learn and how to make the information interactive and compelling for all visitors.

“Often, there’s this initial feeling that physics is so complex you won’t get it,” says Fillhart. “But you absolutely can.”

In addition to measuring muons on the earth’s surface, the team also measured the muon detection rate below and above the earth. Because muons are created in the upper atmosphere and decay into other particles as they fall to earth, the number detected is greater at higher elevations.

The team first measured the number of muons per second as they flew over Geneseo in a hot air balloon. The flight was inspired by a 1912 experiment by Austrian scientist Victor Hess, who tried to determine if radiation came from the earth or space by measuring levels in midair. The team also measured the muon detection rate deep within the American Rock Salt mine near the Geneseo campus. They demonstrated that more muons exist in the atmosphere a mile above the earth’s surface, fewer on the surface, and far fewer 1,200 feet underground.

Fletcher says the team hopes to install detectors at various locations on the Geneseo campus to remind students how physics affects us daily.

Watch a video about the project:

The 411 On Muons

MUONS ARE SO SMALL THEY are impossible to see, but they have been the big stars in mapping the insides of Egyptian pyramids, exploring magma chambers in volcanoes and even detecting uranium in nuclear waste.

MUONS — SUBATOMIC PARTICLES produced when cosmic rays collide with atoms in the upper atmosphere — are more than 200 times heavier than electrons. As they fall to earth, muons pass through objects and eventually decay into other particles. Few muons penetrate materials such as rock or lead; more pass through less dense material. Scientists can measure the number of muons passing through an object to detect, say, where a wall of rock may open up. The muons can be used to form an image — a process called muonography — like the radiograph produced by x-rays.

ACCORDING TO ARTICLES IN Nature, researchers used muon detectors to discover a 30-meter-long space in a 4,500-year-old Egyptian pyramid, giving clues to how it was built. Muons have mapped lava channels and the solidified lava plug in Mt. Vesuvius, which could one day help predict eruptions. Muonography may reveal hidden aqueducts in Roman ruins. Scientists have also shown how tracking muons’ directional changes can detect uranium in a barrel of nuclear waste.


Kurt Fletcher, center, Lydia Fillhart ’21, left, and Matthew VanAllen ’21 conduct their muon experiment in the American Rock Salt mine. /Photo by Keith Walters ’11

hot air balloon

The team takes off in the hot air balloon. /Photo courtesy of Tom Quartley