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 or radiation, bur 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.

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 soon to be released report.

The report raises the questions whether the new machines, at about $822. 000 each, are worth the cost if they're not that much better than current ones that costs about $308.000.

The department believes the benefits outweigh the additional cost. Department spokeswoman, Sara Kuban, said the department would not comment specifically on a report that has yet to be made public. She said the new machines are still being tested.

For years, Congress and the Government Accountability Office have been skeptical about the cost and testing of the new machines. The new report did not criticize the department's latest tests, but found the results disappointing.