How physicists and archaeologists "see" inside ancient monuments
The prototype muon detector can identify stacks of bricks through the lab's concrete roof. A plot of the data (far right) shows that fewer muons make it through the densest material. Similarly, buildings around the detector cast muon "shadows" (below). The narrow green band below the center corresponds to a corridor within the building. A similar signal might indicate a tomb in a Maya pyramid. (Photos Courtesy Roy Schwitters)
Some ancient Maya looked to our galaxy, the Milky Way—a hazy streak across the night sky—and saw the "cosmic monster," a fluid, two-headed serpent sometimes associated with clouds, water, and rain. To modern skywatchers, the Milky Way is less monster than menagerie, a swirling disc of billions of stars of a dozen species, from brown dwarves to red giants. But it does produce a kind of rain, a constant shower of high-energy cosmic rays. Teams of physicists and archaeologists are collecting this cosmic rain for an inside look at the ancient Maya and other past cultures. The Maya carefully observed the night sky. Now the sky seems to be looking back.
In what was once a low-energy nuclear physics lab on the campus of the University of Texas at Austin, particle physicist Roy Schwitters stands next to a gaping hole in the floor of a cavernous, garagelike workshop. In the hole, which is surrounded by day-glo orange safety netting, is Schwitters's prototype, an almost featureless aluminum cylinder 5 feet in diameter that extends 14 feet to a sub-floor. It looks like a missile in a silo, but it is a particle detector, silently counting cosmic flotsam called muons (pronounced mew-ons). Based on the number and direction of muons the detector senses, it "sees" a pile of bricks on top of the building's 3-foot-thick concrete roof. Schwitters and UT–Austin archaeologist Fred Valdez plan to use this technology to take a picture of the inside of a Maya pyramid at the jungle-covered site of La Milpa in Belize next year.
Physicist Roy Schwitters examines the brittle plastic strips used to sense the ionization trails left by muons. Light created by the particles travels to the end of fiberoptic strips for amplification and processing. (Photo: Samir S. Patel, Fred Valdez, Courtesy of the PfBAP)
Cosmic rays—mostly made up of protons accelerated to near light speed by exploding stars, or supernovae—collide with the atoms of our atmosphere and break apart into a cascade of rapidly decaying particles. By the time the rays reach earth's surface, they are primarily composed of muons, which are much like heavy electrons and barely interact with the nuclei of other atoms. They're ghost particles that zip through matter. Muons can survive up to 2.2 microseconds, which makes them the Methuselahs of the cosmic particle parade, and they live long enough to pass through hundreds of miles of atmosphere and into the earth. "We're bathed, all of us, in a sea of muons," Schwitters says. As they pass through matter, including us, they knock electrons off other atoms, losing a little energy and leaving a charged trail, like the vapor behind a high-flying jet. The denser the material muons travel through, the more energy they lose, until they eventually just fade away.
Schwitters's detector works on the principle that dense material stops more muons. A detector next to a Maya pyramid, for example, will see fewer particles coming from the direction of the structure than from other angles: a muon "shadow." And if a part of that pyramid is less dense than expected—containing an open space for, say, a royal burial—it will have less of a shadow. Count enough muons that have passed through the pyramid over the course of several months, and they will form an image of its internal structure, just like light makes an image on film. Then combine the images from three or four devices and a 3-D reconstruction of the pyramid's guts will take shape.
Samir S. Patel is an associate editor at ARCHAEOLOGY. Stay tuned for updates on findings from muon detection projects.