 Naturally occurring xenon consists of eight stable isotopes . There are also over 40 unstable isotopes that undergo radioactive decay . The isotope ratios of xenon are an important tool for studying the early history of the Solar System . <unk> xenon @-@ 135 is produced by beta decay from iodine @-@ 135 ( which is a product of nuclear fission ) , and it acts as the most significant neutron <unk> in nuclear reactors . 
