Muons to detect shielding?

The BBC has a story on a group looking to use muon detectors to detect the shielding used to hide plutonium or uranium. The idea is fairly simple: we already have a muon flux of 10,000 muons/square meter/minute and we might as well use them as probes of trucks to see if they have anything really dense aboard.

The reporter Paul Rincon says "Muons are also harmless, unlike X-rays or gamma-rays," which is hardly accurate. The point is not that muons are harmless, but that they're already part of your everyday background radiation no matter what you do.

The idea seems fine at first glance. But think a bit. They say that "The scattering of muons is very sensitive to the density and atomic number of a material. It could therefore easily detect uranium, plutonium or the shielding material that would have to surround them to make these materials undetectable by other methods." The problem is that "easy" is a rather relative term. First note that the size of the hidden fissionable and lead box is going to be rather small: on the order of 400 square cm cross section or so. So now we're talking about only 400 muons/minute. Assume that they're making a muon telescope, with several layers of strips of scintillator making a sort of garage that you drive into. The presence of dense chunks of lead or iron will make a shadow that you can readily detect using those 400 muons you get in a minute. But figuring out where that shadow is takes a few more tracks, and trying to figure out from scattering whether this is a chunk of scrap iron or a lead box full of loose uranium foil is going to take some subtle analysis. You can model this, and work out how to program it to be done automatically, but I don't see how you can determine if even an isolated sample is loose lead or dense iron with only 400 muons, and if the load is complicated (lots of chunks of stuff), I guarantee you'll need more muon tracks to untangle the structure.

That means more time. How long can you have each truck sitting inside your muon garage? One minute? 10 minutes? 20?

Muons make very nice probes, but you have to be patient.