Subatomic particles that naturally bombard Earth could be used to make 3D images of industrial equipment akin to medical CT scans made with x-rays, a new study suggests. The technique could reveal the corrosion of pipes or the degradation within thick layers of concrete. It could also enable routine inspections of pipes and valves that are buried, wrapped in insulation, or otherwise inaccessible, even while the equipment is in use—and even if it lies deep within a heavily shielded nuclear reactor, scientists say.
The particles that make such probes possible are muons, heavier short-lived cousins of electrons. On Earth, most muons are formed when cosmic rays—high-energy subatomic particles that typically originate outside our solar system—crash into the atmosphere, triggering a cascade of lower energy particles. A muon carries the same negative charge as an electron but is 207 times as massive and lasts only a few microseconds before decaying into an electron and particles called neutrinos. On average, about one muon passes through each square centimeter of Earth’s surface each minute.
Because muons are massive but don’t interact too strongly with other materials, they can penetrate hundreds of meters of rock and soil, says Matt Durham, a nuclear physicist at Los Alamos National Laboratory in New Mexico and lead author of the study. In comparison, lighter electrons stop in material almost immediately, where heavier protons and atomic nuclei interact with them so strongly that they disintegrate into showers of particles. Muons' ability to penetrate makes them ideal for peering into objects. The denser the material the muons pass through, the more they are scattered and deflected from their original path.
In the study, researchers placed muon detectors on each side of the object they wished to image. Then they tracked the paths of muons as they passed through one set of detectors, then the object, and finally the second set of detectors. By mapping the “before” and “after” trajectories of a muon, researchers can determine how much its path was deflected. And by analyzing the deflections of many muons passing through different parts of the object, researchers can mathematically deduce the 3D distribution of mass in the space between the detector arrays, Durham says. The technique is something of a hybrid between traditional medical x-ray, which uses a material’s ability to block x-rays to directly make a 2D image of the mass distribution, and x-ray diffraction, which uses angles alone to probe the 3D structures of crystals. Durham and his colleagues describe their imaging technique, called muon tomography, online today in AIP Advances.
“This is a slick technique,” says Cas Milner, a physicist at Southern Methodist University in Dallas, Texas, who was not involved in the research. Besides using background radiation, which doesn’t expose workers to additional sources of radiation, the technique is noninvasive: Researchers don’t even have to shut down equipment, strip insulation off of a pipe, or enter a possibly hazardous environment, he notes.
One possible downside to the technique, however, is that it takes a long time to create an image. The team’s tests show that ghostly, low-resolution images of a stainless steel pipe can be built in just 15 minutes, but to create a high-quality model of the object in question can take hours, if not days, Durham says. Thus, muon tomography is probably best suited for routine inspections or monitoring equipment on an ongoing basis rather than conducting quick assessments of a catastrophic failure, he notes.
The technique is a smaller-scale version of technology previously developed at Los Alamos in the wake of the terrorist attacks on 11 September 2001 to search for nuclear materials or other contraband in shipping containers or vehicles. That technology has been commercialized and is now in use at some ports, says Konstantin Borozdin, a physicist at Decision Sciences International Corporation in Poway, California.
Borozdin’s company is now working to scale up the muon tomography technology to look for nuclear material in a much larger arena—the nuclear reactors destroyed by the earthquake and tsunami one-two punch that slammed Fukushima, Japan, in March 2011. For that effort, each array of muon detectors will measure 7 meters by 7 meters, and they’ll be placed about 50 meters apart on opposite sides of the devastated reactor building, Borozdin says. Milner says “the technology definitely has promise, when you look at the problem of trying to determine the internal configuration of a well-shielded object like a nuclear reactor.”