Plutonium-244
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Plutonium-244 has a halflife of 80 million years. This is longer than any of the other isotopes of plutonium. In fact, it is longer than any actinide except for the three naturally abundant ones U-235 (700my), U-238, and Thorium-232,longer than any other isotopes except samarium-146 (103my), potassium-40 (1.25by), and a number of nearly stable isotopes with halflives much longer than the age of the universe.
Some studies have claimed to detect a few atoms of primordial Pu-244. As the age of the Earth is about 50 half-lives, the amount of Pu-244 left would be very small. However, since Pu-244 cannot be easily produced by natural neutron capure in low neutron activity environment of uranium ores (see below), its presence is difficult to explain by any other means than creation by R process nucleosynthesis in supernovae.
Unlike Pu-238, Pu-239, Pu-240, Pu-241, and Pu-242, 244Pu is not produced in quantity by the nuclear fuel cycle, because further neutron capture on 242Pu produces 243Pu which has a short halflife (5 hours) and quickly beta decays to americium-243 before having much opportunity to further capture neutrons in any but very high neutron flux environments.
