Physicists and astrophysicists at the Los Alamos National Laboratory, US, believe that cosmic-ray muons could join the armoury of cross-border surveillance techniques, because of their ability to create images of dense objects. The technique would be relatively inexpensive and harmless as it involves only natural background radiation, and the team says that it could be used to detect, for example, "a block of uranium concealed inside a truck full of sheep".
The process relies on the slight deflection in the paths of cosmic-ray muons as they pass through an object. By measuring the amount of deflection, the object's density can be reconstructed, rather as X-rays reveal varying density through differing amounts of absorption.
To investigate the potential of muon radiography, the team at Los Alamos placed pairs of drift chambers with an active area of 60 x 60 cm2 above and below a test object of tungsten on steel supports. The reconstructed image clearly shows the tungsten, with some evidence for the steel. While one disadvantage of the technique is the relatively low flux of cosmic-ray muons, the team's simulations show that within only a minute they could detect a 10 cm cube of uranium in a large metal container full of sheep. The researchers conclude that the technique offers promise for the surveillance of commercial vehicles, cars and sea containers.
Further reading
K N Borozdin et al. 2003 Nature 422 277.