The department had planned a worldwide network using the new detectors, which were supposed to spot plutonium or uranium in shipping containers. The government wanted 1,300 to 1,400 machines, which cost $800,000 each, for use in ports around the world to thwart terrorists who might deliver a nuclear bomb to a big city by stashing it in one of the millions of containers that enters the United States every year.

The ingredient is helium 3, an unusual form of helium, that is formed when tritium, an ingredient of hydrogen bombs, decays. But the government stopped making tritium in 1989, and the number of h-bombs has been decreasing.

“I have not heard any explanation of why this was not entirely foreseeable,” said Representative Brad Miller of North Carolina, chairman of a House subcommittee that is investigating the problem. An official from the Homeland Security department testified last week before his subcommittee, the investigations and oversight subcommittee of the House Science Committee, that demand for helium 3 appeared to exceed supply by tenfold.

In fact, said Miller, some government agencies did anticipate a crisis, but Homeland Security appears not to have gotten the message.