A new bomb-detection machine touted by federal officials as an advancement against airport terrorism will be put into use this month at United Airlines' terminal at Dulles International Airport, officials said yesterday.

The $1 million machine, one of six in the world, will be operated at Dulles as part of a test and is considerably more sophisticated than the X-ray machines now used by U.S. carriers to examine checked baggage on international flights.

The new device showers each piece of luggage with neutrons, which when combined with nitrogen atoms in explosives give off a distinctive gamma ray, which is detected by the machine.

Although acknowledged by most aviation groups as a step forward in airline security, the thermal neutron analysis machines have been the focus of debate this year among bureaucrats, lawmakers and airline officials, who disagree about the new machines' effectiveness.

At Dulles, the machine will scan luggage on United's three daily international flights to Frankfurt, Mexico City and Cancun, Mexico.

Dulles is the fourth airport to get one of the machines in the test program, which is being conducted by the Federal Aviation Administration. Similar machines are being tested at New York's John F. Kennedy International Airport, Miami International Airport and Gatwick International Airport in London.

The FAA, which owns the machines and paid for a $15.4 million program to develop them, also hopes to put one of the devices in Frankfurt and is expected to decide soon on a sixth test site.

The FAA agrees that the devices have limitations but considers them to be the best way yet to detect bombs in airport luggage.

But last spring the machines were called unreliable by members of the President's Commission on Aviation Security and Terrorism, a panel created after the 1988 terrorist bombing of Pan Am Flight 103 over Lockerbie, Scotland. The panel said its tests revealed that the machines frequently failed to detect plastic explosives, supposedly one of the machine's strengths.

In tests at Kennedy last May, one of the machines sounded its alarm on thick wool sweaters, padded ski boots and some other items that apparently contained nitrogen. Officials said the machine's sensitivity may have been set too high.

The president's commission urged the FAA to suspend its plan to eventually put the machines in 150 airports used by U.S. carriers around the world, a plan that would cost more than $175 million.

But for now, FAA officials say they intend to go ahead with the machines, which they say have a 95 percent accuracy rate. Federal officials said they know of no instance in which one of the machines in New York, Miami or London has detected a live bomb that was not part of a test.

"Our view is that it's the best system we have at the moment," FAA spokesman Fred Farrar said. "Until something better comes along, we'll go with this. It's better than what we have now."

After the Pan Am 103 bombing, most airlines began using the same type of X-ray machines used to examine carry-on luggage to try to detect bombs in checked baggage on international flights. Scanning devices are rarely used on checked baggage on domestic flights, federal and airport officials said.

Although FAA officials say the machines are able to examine more than 500 bags an hour, Dulles manager Keith Meurlin said the additional security step inevitably will cause some delays for passengers on international flights.

KEY FACTS

How It Works: The machine showers luggage with neutrons. When combined with nitrogen atoms in explosives neutrons give off detectable gamma rays.

Test Sites: Dulles International, John F. Kennedy International, Miami International; and Gatwick International.