When cosmic rays collide with molecules in the upper atmosphere, they produce particles called muons. About 10,000 of these strike every square metre of the earth’s surface each minute. These particles are able to penetrate several tens of metres through most materials, but are scattered to an unusual extent by atoms that include large numbers of protons in their nuclei. Since this includes uranium and plutonium, muons could have valuable security applications.
Muon tomography is a form of imaging that can be used to pick out fissile materials, even when they are embedded in dense masses. For instance, a tunnel sized scanner could examine entire semi trucks or shipping containers in a short time. Such tunnels would be lined with gas-filled tubes, each containing a thin wire capable of detecting muons on the basis of a characteristic ionization trail. It is estimated that scans would take 20-60 seconds, and less time for vehicles and objects of a known configuration.