The great majority of cosmic-ray particles reaching Earth’s surface are positive and negative muons. At sea level, the muon flux is about 104/m2 per minute. These muons are mostly decay products of short-lived π mesons created by cosmic-ray protons hitting the upper atmosphere. Because muons are 200 times heavier than electrons and impervious to the strong nuclear force, they are far more penetrating than electrons, neutrons, x rays, or even gamma rays. With a typical energy of 3 GeV, a cosmic-ray muon will pass through 15 m of water or 2.5 m of steel.
Christopher Morris and colleagues at Los Alamos National Laboratory are proposing to exploit this ubiquitous, penetrating flux for a new kind of radiographic surveillance. In a recent issue of Nature, 1 they reported experimental and computer simulations that suggest the feasibility of locating small quantities of fissile material concealed in ordinary...
