Fears that little known medical isotope could fall into the wrong hands.

• Position: SECURITY BEAT

Counterterrorism experts have long warned that unsecured radiological materials such as cesium-137--commonly used in the medical community--could fall into terrorist hands and be used to manufacture so-called dirty bombs.

But there is a less known medical isotope made from bomb grade uranium that could pose a far greater threat, a group of nonproliferation scholars said at a Capitol Hill briefing.

Christina Hansell, director of the Newly Independent States Non-Proliferation Program, said leftover material front the manufacture of Tc-99n--used for diagnosing ailments such as heart conditions--could be used by terrorists to make a nuclear bomb. Such a weapon poses a far greater risk than just a radiological dispersal device, the technical term for a dirty bomb.

"I'm talking about real nuclear terrorism with an improvised nuclear device," she said at the talk sponsored by the Center for Science, Technology and Security Policy.

Commercial facilities that store the uranium--all of them located overseas--are more vulnerable than military facilities that store nuclear material, she said.

Andrew Einstein, assistant professor of clinical medicine at Columbia University College of Physicians and Surgeons, said the United States has no capacity to domestically produce the Tc-99n isotope. Yet these suppliers are using...