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.
Neutron
Outside the nucleus, free neutrons are unstable and have a mean lifetime of (about 14 minutes, 42 seconds); therefore the half-life for this process (which differs from the mean lifetime by a factor of ) is (about 10 minutes, 11 seconds). Beta decay of the neutron, described above, can be denoted by the radioactive decay: