In this task you will be given a text passage from a trivia quiz. You should choose the category this question belongs to from these categories: History, Science, Social Science, Fine Arts, Literature. You can only choose one category, so if in doubt, please choose the closest match.
Q: This element was never present in the Canyon Diablo Troilite, which is why that body defines the origin for plotting the geochron. Ratios between isotopes of a daughter nuclide of this element were used by Clair Patterson to date the Earth. This element was the cause of reactions in the Oklo mine in Gabon. This element forms only in the r-process, and one of the main reasons the age of the galaxy is so imprecisely known is its uncertain production ratio relative to (*) Thorium in supernova emissions. Its decay to lead-206 and 207 is used in a radiometric dating scheme. Its 235 isotope is the only naturally-occurring fissile isotope. For 10 points, name this actinide, the heaviest naturally-occurring element.
A:
Science