The University of Texas at Austin Maya Muon Group is building particle detectors that can be deployed underground to track cosmic-ray muons and reconstruct large-scale tomographic images of the overlying ground and structures. While the concept of cosmic-ray muon tomography has been known for a long time, the technology is rarely put to use. The Maya Muon Group has found that underground muon tomography is a practical tool for investigating large structures.
The first major experiment of the Maya Muon Group will bridge the disciplines of physics and archeology. The particle detectors and related systems are designed specifically to explore ruins of a Maya pyramid in collaboration with colleagues at the UT Mesoamerican Archaeological Laboratory. The Maya Muon Group will travel to La Milpa in northwest Belize to make discoveries about "Structure 1"—a jungle-covered mound covering an unexplored Maya ruin.
Muon tomography measures the rates of cosmic-ray muons detected underground at diverse angles and locations. Like an x-ray image, the differences in the numbers of muons detected along particular directions and at different locations indicate differences in the total mass of material between the detector and the open sky in the direction being studied. These differences can be assembled using methods of computer-assisted tomography (CT) into a 3D image of the overburden.
Cosmic ray muons are penetrating charged particles which arise from decays of other particles when high energy “primary” cosmic rays collide with atomic nuclei in the upper atmosphere. Csmic-ray muons form an appreciable fraction of the natural background of ionizing radiation at the earth’s surface, with the flux of muons approximately 1 min−1 cm−2 sr−1.
Cosmic ray muons strike the earth’s surface over a broad range of directions with a spectrum of energies. To be detected, those moving toward the detector must have energies at the surface sufficient to penetrate the intervening material. When muons pass through matter, atoms along the path becomes ionized, which results in a loss of energy for the muon proportional to the mass of material traversed. Thus, paths with more total material between the surface and the detector require higher-energy muons at the surface and, therefore, will have smaller yields for a given exposure time.
In the 1960s, L.W. Alvarez and co-workers invented muon tomography to study the internal structure of the Second Pyramid of Chephren. They used naturally occurring cosmic ray muons to “x-ray” the pyramid in the hopes of finding undiscovered chambers. The Alvarez group established the feasibility of muon tomography by observing the meter-scale limestone caps on the outside of the pyramid. Their principal scientific discovery was definitive, but negative: there are no additional chambers inside Chephren’s Second Pyramid.
Using improved technology and techniques standard in particle physics, it is possible to detect underground muons and to measure their trajectories with sufficient accuracy to point back to meter-scale structures as far as 50-80 meters away from the detector. To achieve muon detection rates large enough to be practical—exposure times measured in weeks to months—the detector system must have a sensitive area of several square meters. A system of moderate complexity—of order a thousand channels of information—is needed to track the muons with sufficient pointing accuracy. While large in comparison to traditional monitoring equipment, this scale detector is modest by contemporary standards in particle physics.
