Muon-assured defence

A CERTAIN sort of madness once seemed enough to defend the United States from nuclear attack. “MAD”—Mutually Assured Destruction—operated on the premise that the best defence is a good offence. But the American government worries that MAD-style deterrence will no longer work. Terrorists, for example, might not care about the threat of destruction. And to make matters worse, any nuclear weapons used by terrorists or rogue states may not arrive by ballistic missile. Instead, they are likely to come in a cargo container—either by ship or on the back of a lorry driven across one of the country's long land borders. So the best defence might rely on intercepting a nuclear weapon at the border, rather than in space.

But to intercept something, one must first find it. A nuclear weapon can be a tiny thing, and searching all incoming freight on the off chance of finding one is out of the question. Christopher Morris and Rick Chartrand, of America's Los Alamos National Laboratory in New Mexico, think they have found a way round the problem. As they explained to the AAAS, it would enable a lorry to be screened for the presence of fissile material (the uranium or plutonium that provides an atomic bomb's explosive power) in about a minute. And it is within the laboratory's traditional remit of doing clever things with radioactivity, for it relies on naturally occurring elementary particles called muons.

A muon is similar to an electron, but heavier and unstable. Muons are created in the atmosphere by cosmic rays (high-energy particles from outer space) colliding with air molecules. They arrive at the Earth's surface at a rate of one per square centimetre per minute. Crucially for Dr Morris's and Dr Chartrand's idea, they are good at penetrating metallic objects such as lorries, and yet also tend to be deflected from their paths by heavy atomic nuclei such as those of uranium and plutonium.

Fast-moving muons wreak havoc on air molecules as they pass by—stripping away electrons to create positively charged ions. That makes their passage easy to detect using banks of so-called drift tubes, which pick up the electrical signal generated by the ions. Dr Morris and Dr Chartrand's idea is to build boxes big enough for lorries to be driven into. Each box would have two layers of drift tubes above the lorry, and two below. This arrangement would allow muons to be tracked as they went into the lorry and as they came out. Any deviation from a straight line would mean that a muon had run into one or more atomic nuclei on its way through. By tracking enough muons and applying enough computing power, the two researchers think that they would be able to spot large concentrations of heavy nuclei—in other words, nuclear explosives.

So far, they have the computing power, but not the boxes. Simulations, though, suggest that the idea would work. A sphere of uranium weighing 20kg (about what is needed to make a bomb) shows up clearly. And even if the material were broken down into smaller batches alarm bells would go off in the detection software—although in this case it would not be able to locate the uranium precisely.

The next step is to build a real detector and then, if that works, to convince the authorities to support widespread deployment. A system that scanned every lorry coming into the United States would, the researchers estimate, cost about $1 billion. That is a lot of money in most contexts, but not all that much in the context of nuclear defence. The existing missile-defence programme has already cost over $130 billion, and is scheduled to spend at least another $50 billion in the next five years. And unlike missile interceptors, which on February 13th spectacularly failed their second test in two months, detectors at America's borders should actually work. Madness, it seems, may give way to science.