Isotopes of selenium
Selenium (Se) has six natural isotopes that occur in significant quantities, along with the trace isotope Se, which occurs in minute quantities in uranium ores. Five of these isotopes are stable: Se, Se, Se, Se, and Se. The last three also occur as fission products, along with Se, which has a half-life of 327,000 years, and Se, which has a very long half-life (~10 years, decaying via double beta decay to Kr) and for practical purposes can be considered to be stable. There are 23 other unstable isotopes that have been characterized, the longest-lived  being Se with a half-life 327,000 years, Se with a half-life of 120 days, and Se with a half-life of 8.40 days. Of the other isotopes, Se has the longest half-life, 7.15 hours; most others have half-lives not exceeding 38 seconds. Selenium-75. The isotope selenium-75 has radiopharmaceutical uses.