Selenium (34Se) has six natural isotopes that occur in significant quantities, along with the trace isotope 79Se, which occurs in minute quantities in uranium ores.
The last three also occur as fission products, along with 79Se, which has a half-life of 327,000 years,[4][5] and 82Se, which has a very long half-life (~1020 years, decaying via double beta decay to 82Kr) and for practical purposes can be considered to be stable.
Of the other isotopes, 73Se has the longest half-life, 7.15 hours; most others have half-lives not exceeding 38 seconds.
For example, it is used in high-dose-rate endorectal brachytherapy, as an alternative to iridium-192.
[8] In paleobiogeochemistry, the ratio in amount of selenium-82 to selenium-76 (i.e, the value of δ82/76Se) can be used to track down the redox conditions on Earth during the Neoproterozoic era in order to gain a deeper understanding of the rapid oxygenation that trigger the emergence of complex organisms.