Polonium is a alpha source and it was the radioisotope that killed the russian Litvenenko recently. He had a meeting with an associate and detectives tracked down the existence of a hot teapot discovered by hotel staff. Here are more information about polonium: Because of its short half-life and rapid decay, a small quantity of polonium-210 is intensely active. Its "specific activity" is 4490 curies/g (166 TBq/g). Thus 4 millicuries (a minimal lethal dose for 80 kg person) is 0.89 micrograms, an almost invisible speck of matter.A detail story can be found here
