East German dissidents probably didn't spot the plain-clothes agent with the vibrating armpit. But agents could track suspected political opponents without even seeing them. They just followed a trail of radioactivity shed by their unwitting quarry.rnIn the 1970s and 1980s, the German Democratic Republic's secret police—the Stasi—frequently labelled suspected dissidents with highly radioactive chemicals so that agents wearing concealed Geiger counters could keep tabs on them, according to a paper by Klaus Becker, a leading radiation protection expert.
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