East German communist security police - the notorious Stasi, the local equivalent of the KGB - had a way of getting glowing reports of suspected dissidents and spies.

They clandestinely labelled their victims with a radioactive marker, and tracked them by geiger counter, according to officials of the Berlin-based Gauck commission, a government agency investigating the archives of former secret police.

According to today's New Scientist, while the plotters mingled with the crowds, secret policemen followed them with vibrating radiation detectors concealed in their armpits.

"It's a remarkable story. It's the first well-documented case of such a thing," said Klaus Becker, a radiation protection expert. According to the Stasi files, during the 1970s and 1980s, the security police would spray people, their documents or their money with a solution of radioactive scandium-46. They could also spray whole meeting rooms, thus identifying any conspirators or dissidents who turned up. They also developed an airgun that could fire radio-labelled silver wire into a car tyre from 25 yards. There was care not to expose the police themselves to much danger - but dissidents could be exposed to quite high doses every time they were labelled.

Geiger counters clicked intensely as they got nearer to any radiation source. So 30 years ago the secret police developed the "silent ringers" now used in today's cell phones and pagers, in order not to alarm the suspects they were trailing. They may also have created problems for thousands of unsuspecting - and unsuspected - citizens.

"The Stasi marked West German deutschmarks with large amounts to see how they circulated, to whom and for what purpose," said Dr Becker. "While they expected to retrieve them, they didn't, and the notes disappeared without trace."

The security police later calculated that while a single note was probably harmless, two or more together were not. The effect of more than one on male fertility could have had the same effect as castration

"It really is the stuff of James Bond movies," one London radiologist told New Scientist. "It's an unpleasant thing to do. The risk is not limited to the person being tagged. You'd be exposing other people, such as a spouse."