Washington ? Federal investigators say the government’s next generation radiation detectors are only marginally better at detecting hidden nuclear material than monitors already at U.S. ports, but would cost more than twice as much.

The machines are intended to prevent terrorists or criminals from smuggling into the U.S. a nuclear bomb or its explosive components hidden in a cargo container.

The monitors now in use can detect the presence of radiation, but they cannot distinguish between threatening and nonthreatening material. Radioactive material can be found naturally in ceramics and kitty litter, but would be of no use in making a bomb, for instance.

The Department of Homeland Security has said the new machines it is developing can distinguish between kitty litter and dangerous radioactive material and produce fewer false alarms than the current ones.

The new ones are also better at detecting lightly shielded material. But the machines perform at about the same level when detecting radiological and nuclear materials hidden in a lead box or casing, the most likely way a terrorist would try to sneak the materials into this country, the Government Accountability Office said in a report to be released today.

The report by Congress’ investigating agency was requested by Sen. Joseph Lieberman, the Connecticut independent who is chairman of the Senate Homeland Security and Governmental Affairs Committee.

The report raises the question whether the new machines, at about $822,000 each, are worth the cost if they’re not that much better than current ones that cost about $308,000.