Uranium
In nature, uranium is found as uranium-238 (99.2739–99.2752%), uranium-235 (0.7198–0.7202%), and a very small amount of uranium-234 (0.0050–0.0059%). Uranium decays slowly by emitting an alpha particle. The half-life of uranium-238 is about 4.47 billion years and that of uranium-235 is 704 million years, making them useful in dating the age of the Earth.
Praseodymium
All other praseodymium isotopes have half-lives under a day (and most under a minute), with the single exception of Pr with a half-life of 13.6 days. Both Pr and Pr occur as fission products of uranium. The primary decay mode of isotopes lighter than Pr is inverse beta decay or electron capture to isotopes of cerium, while that of heavier isotopes is beta decay to isotopes of neodymium.