it takes just a few pounds of plutonium or uranium to create a nuclear bomb. If a small amount of these elements were to land up in the wrong hands, then it would prove disastrous. "The threat is real,' says Graham Allison, director of the Belfer Centre for Science and International Affairs at Harvard University, the us.

Scientists at the us department of energy have found a way to detect concealed nuclear weapons-grade materials at border crossings around the world. Their method relies on particles generated by cosmic rays as they pass through the Earth's atmosphere. Cosmic rays are high energy particles that constantly bombard the Earth from outer space. As the rays pass through the Earth's atmosphere, nuclear interactions transform many of these particles into heavier particles called muons. Unlike the particles used in conventional x-ray machines to screen baggage at airports or search for broken bones in human skeletons, muons can penetrate dense objects like uranium and plutonium.

The scientists built an experimental set-up that traces the path of these heavy particles as they pass through dense materials and then, using a computer, generates an image of the object. "You put detectors above and below the thing you're trying to measure and look at the track of the muons and see which ones are bent,' says William Priedhorsky, one of the researchers. Muons make a straight line when they pass through organic objects such as human flesh, but bend by several degrees when they pass through dense objects such as uranium or plutonium. Processing where and how the muons bend, the computer generates an image.

The scientists suggest that a machine using muons could be an inexpensive way to screen medium or large objects such as cars or trucks. "This is plausible,' says Allison. But he would hold his final judgment until he has tested the technology. Till date, scientists have only worked out the initial science for the muon machine. Funding for the project is currently coming from various us agencies concerned with national security.