Such a system could blanket the nation with millions of cell phones equipped with radiation sensors able to detect even light residues of radioactive material. Because cell phones already contain GPS locators, the network of phones would serve as a tracking system, said physics professor Ephraim Fischbach.

"It's the ubiquitous nature of cell phones and other portable electronic devices that give this system its power," Fischbach said. "It's meant to be small, cheap and eventually built into laptops, personal digital assistants, and cell phones."

The system was developed by consulting scientist Andrew Longman, who developed the software and worked with Purdue researchers to integrate the software with radiation detectors and cell phones.

"The likely targets of a potential terrorist attack would be big cities with concentrated populations, and a system like this would make it very difficult for someone to go undetected with a radiological dirty bomb in such an area," said Longman, who also is Purdue alumnus. "The more people are walking around with cell phones and PDAs, the easier it would be to detect and catch the perpetrator." Tiny solid-state radiation sensors are commercially available.

Long before the sensors would detect significant radiation, the system would send data to a receiving center. "Say a car is transporting radioactive material for a bomb, and that car is driving down Fifth Avenue in New York," Fischbach said. "As the car passes people, their cell phones individually would send signals to a command center, allowing authorities to track the source."

The researchers tested the system in November, demonstrating that it is capable of detecting a weak radiation source 15 feet from the sensors.