High Quality Content by WIKIPEDIA articles! Naturally occurring xenon (Xe) is made of nine stable isotopes. (124Xe, 126Xe, 134Xe and 136Xe are predicted to undergo double beta decay, but this has never been observed, so they are considered to be stable.) Xenon has the second highest number of stable isotopes. Only tin, with 10 stable isotopes, has more. Beyond these stable forms, there are over 40 unstable isotopes and isomers that have been studied, the longest-lived of which is 127Xe with a half-life of 36.345 days. Of known isomers, the longest-lived is 131mXe with a half-life of 11.934 days. 129Xe is produced by beta decay of 129I (half-life: 16 million years); 131mXe, 133Xe, 133mXe, and 135Xe are some of the fission products of both 235U and 239Pu, and therefore used as indicators of nuclear explosions.