The cosmic rays that constantly bombard the earth could be just the thing to detect concealed nuclear materials, say Los Alamos National Laboratory physicists who have designed a detection system based on them. Security officials currently scan vehicles and containers with x-rays and gamma rays, but they are harmful to people and penetrate lead and steel poorly. Cosmic rays end up producing particles called muons, which scatter off of heavy nuclei such as those of uranium and plutonium atoms. Muons can burrow through thick steel and lead; detectors placed above and below a sample can measure the flight paths of the particles and reconstruct the shape of dense materials in-between, shielded or not. A midsize prototype for scanning sections of cars should be ready this summer, the Los Alamos team reported at the February meeting of the American Association for the Advancement of Science.