text: Radon has been produced commercially for use in radiation therapy, but for the most part has been replaced by radionuclides made in particle accelerators and nuclear reactors. Radon has been used in implantable seeds, made of gold or glass, primarily used to treat cancers.
  The gold seeds were produced by filling a long tube with radon pumped from a radium source, the tube being then divided into short sections by crimping and cutting. The gold layer keeps the radon within, and filters out the alpha and beta radiation, while allowing the gamma rays to escape (which kill the diseased tissue). The activities might range from 2 to 200 MBq/seed. The gamma rays are produced by radon and the first short-lived elements of its decay chain (Po, Pb, Bi, Po).
  Radon and its first decay products being very short-lived, the seed is left in place. After 11 half-lives (42 days), radon radioactivity is at 1/2 000 of its original level. At this stage, the predominant residual activity is due to the radon decay product Pb, whose half-life (22.3 years) is 2 000 times that of radon, and its descendants Bi and Po, totalling 0.03% of the initial seed activity.