Radiographic imaging with cosmic-ray muons

Natural background particles could be exploited to detect concealed nuclear materials. Despite its enormous success, X-ray radiography1 has its limitations: an inability to penetrate dense objects, the need for multiple projections to resolve three-dimensional structure, and health risks from radiation. Here we show that natural background muons, which are generated by cosmic rays and are highly penetrating, can be used for radiographic imaging of medium-to-large, dense objects, without these limitations and with a reasonably short exposure time. This inexpensive and harmless technique may offer a useful alternative for detecting dense materials — for example, a block of uranium concealed inside a truck full of sheep.