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The same place that gave the world the atomic bomb has now found a way to ferret out illicit nuclear material. Los Alamos National Laboratory has developed a method to search for heavy elements such as uranium via subatomic particles from space called muons. By 2008, “muon tomography” might be guarding U.S. borders.
About 10,000 muons reach every square meter of the earth’s surface a minute; these charged particles form as by-products of cosmic rays colliding with molecules in the upper atmosphere. Traveling at relativistic speeds, muons can penetrate tens of meters into rocks and other matter before attenuating as a result of absorption or deflection by other atoms. The scattering is most pronounced in dense substances such as uranium and plutonium—elements with high Z (the number of protons in an atom’s nucleus). “We use the fact that the scattering is sensitive to Z and particularly sensitive to the materials that you build nuclear bombs from or that you shield nuclear bombs with,” explains Los Alamos’s Christopher Morris, chief creator of the technology. “We measure the scattering angle for every muon, we measure the angle on the way in and the angle on the way out, and the change in the angle tells you how much material you’ve gone through.”