US researchers are developing technology to power a network of mobile phones that could find and track radiation sources.

It is hoped that the system could prevent terrorist attacks using nuclear 'dirty bombs'.

The Purdue University scientists are working with the State of Indiana to "blanket the nation" with millions of mobile phones equipped with radiation sensors able to detect even light residues of radioactive material.

Purdue physics professor Ephraim Fischbach said that such a network of phones could serve as a tracking system because handsets already contain global positioning locators.

Professor Fischbach is working with Jere Jenkins, director of Purdue's radiation laboratories within the School of Nuclear Engineering.

"It is the ubiquitous nature of cell phones and other portable electronic devices that give this system its power," said Professor Fischbach.

"It is meant to be small, cheap and eventually built into laptops, PDAs and cell phones."

The system was developed by Andrew Longman, a consulting instrumentation scientist, who worked with Purdue researchers to integrate the software with radiation detectors and mobile phones. Cellular data air time was provided by AT&T.

"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.

"The more people are walking around with cell phones and PDAs, the easier it would be to detect and catch the perpetrator. We are asking the public to push for this."

Although suitable small solid-state radiation sensors are commercially available, the detection system would require additional circuitry. However, this would not add significant bulk to portable electronic products, Fischbach said.

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