 Naturally occurring xenon is made of eight stable isotopes , the most of any element with the exception of tin , which has ten . Xenon and tin are the only elements to have more than seven stable isotopes . The isotopes <unk> and <unk> are predicted to undergo double beta decay , but this has never been observed so they are considered to be stable . Besides these stable forms , there are over 40 unstable isotopes that have been studied . The longest lived of these isotopes is <unk> , which has been observed to undergo double beta decay with a half @-@ life of 2 @.@ 11 × <unk> <unk> . 129Xe is produced by beta decay of 129I , which has a half @-@ life of 16 million years , while <unk> , 133Xe , <unk> , and 135Xe are some of the fission products of both <unk> and <unk> , and therefore used as indicators of nuclear explosions . 
