False Alarms or No Alarms?

At more than $1 million each, thermal neutron analysis systems are designed to spot plastic explosives that can elude most other inspections. The FAA has installed TNA machines at two airports, New York City's Kennedy and Miami International, and plans to require U.S. airlines to purchase 150 of them, at a cost of $175 million. But the presidential commission contends that the machines are duds: if set to find a small bomb like the one that shattered Pan Am Flight 103 (apparently between 1 and 2 lbs.), they produce excessive false alarms.

The 12-ft.-long, 6-ft.-high machines generate a cloud of neutrons that penetrate the luggage. These combine with the nitrogen in plastic explosives to generate gamma rays; an array of detectors identifies the substance. But other items containing nitrogen, including wool sweaters and padded ski boots, can set off warnings. The manufacturer, Science Applications International Corp. of San Diego, says the false alarms can be reduced with further experience. At the moment, says FAA administrator James Busey, "we have no other system available."