Washington ? Federal inspectors are checking all travelers arriving in the United States for radiation as part of an expanded effort to screen for terrorist activity, a Customs official said Saturday.

Dean Boyd, a spokesman for the new Bureau of Customs and Border Protection, said inspectors began using small, pager-like detectors Saturday at U.S. ports of entry to check passengers for radiation. He said the inspectors, who ask incoming travelers for their passports, carried the detectors on their belts.

“If a source of radiation passes close by or within a certain distance, the pager will begin beeping or alerting, and you can look down at the pager and see the amount (of radiation) that the pager is picking up,” Boyd said.

Passengers may not notice the devices because inspectors do not have to sweep passengers with them to detect radiation. The pagers simply beep or vibrate to let inspectors know when something radiological is nearby.

The goal is to screen all the more than 500,000 people entering the United States every day. Inspectors who check passports at the arrival gates will carry them.

The government was using just 4,000 of the pagers before the Sept. 11 attacks but officials distributed more of the devices this weekend, bringing the total to 7,000, as Customs and 18 other federal agencies formally joined the Homeland Security Department.

The pagers cost about $2,500 each. Boyd said officials hoped to distribute more of them so that all 9,000 Customs inspectors would carry them.

Government auditors warned lawmakers in October that the pager’s range was too limited to be effective. Boyd says officials know all technology has its limits.

“The pagers — they are not a silver bullet. They are one piece of technology that we use, and we use a broad range of technologies to detect radiation,” he said.

The detectors can be set off by low levels, such as by patients undergoing chemotherapy or someone who recently had an X-ray. Potassium in bananas could also set it off.

As a backup, Boyd said a few hundred of the inspectors had hand-held devices called “isotope identifiers,” which signal what the radioactive material is.

“It allows us to determine whether it’s a nuclear bomb or components for a dirty bomb,” Boyd said.

The government has other equipment to check for radiological weapons, such as special monitors that screen trucks, plus 200 vans equipped with gamma ray machines for checking cargo containers.