John W. Poston, Sr. is head of the nuclear engineering department at Texas A&M University and is a Fellow of the American Nuclear Society. He offers the following explanation.
Conventional wisdom tells us that plutonium (Pu) does not exist in nature. Plutonium and other so-called transuranic elements are considered by most to be man-made elements. Thus, they assume that when plutonium is found in the environment, human technology has put it there.
This element has usually been considered synthetic because it is produced most efficiently in nuclear reactors. But in the strictest sense, the answer to the question is yes, plutonium does occur naturally. Plutonium appears at very low concentrations in nature, on the order of one part in 1011 in pitchblende, the ore of uranium (U).
Image: Seaborg Center
The element plutonium was discovered by Nobel Laureate Glenn T. Seaborg and his colleagues in February 1941. It was the second transuranic element to be discovered; neptunium (Np) was identified in 1940. The 60-inch cyclotron at University of California at Berkeley produced the first isotope of plutonium, Pu-238. It was made by bombarding a U-238 target with deuterons, producing Np-238. This material had a brief radioactive half-life--the time required for half of the atoms in a sample to decay or transform--of 2.12 days. The radionuclide Np-238 decayed (by emitting beta-radiation) to Pu-238, which has a half-life of 87.7 years.
The isotope Pu-239 was produced on March 28, 1941 by bombarding a U-238 target with neutrons to produce U-239 (half-life of 23.5 minutes). This radionuclide decayed by beta emission to Np-239 (half-life of 2.12 days), which subsequently decayed by beta emission to Pu-239 (which has a very long half-life of 24,600 years).
Plutonium is produced in nature through the reasonably well-understood process discussed above. Uranium is a naturally occurring element that is ubiquitous in the Earth's crust. The isotopes of uranium decay primarily by alpha-particle emission, but there is also a process called "spontaneous fission" that occasionally competes with alpha decay.
In spontaneous fission, the nucleus splits ("fissions") and additional neutrons are released. There is a possibility that these released neutrons are absorbed (captured) by another U-238 nucleus. If this occurs, it triggers a process that produces Pu-239 in a manner similar to that discussed above. Thus, we have plutonium produced naturally in the environment (admittedly in trace quantities). This reaction has been going on since the creation of the Earth.
In 1971, Darlene Hoffman of Los Alamos National Laboratory discovered trace quantities of another isotope of plutonium in the environment. Pu-244 was found in Precambrian Age phosphate from southern California. This isotope of plutonium had a radioactive half-life of 80 million years. Scientists have postulated that, because of its long radioactive half-life, this isotope has existed since the creation of Earth about 4.5 billion years ago.
Some scientists also suspect that there was another isotope of plutonium (Pu-241) in the Earth's crust at its formation. The reasons are that the radioactive progeny (daughter radionuclides) of Pu-241 exhibit some similarities properties to those of U-235, U-238 and Thorium-232. But whereas the three other radionuclides have very long half-lives on the order of the age of the Earth, Pu-241 has a short half-life of about 13 years. Thus, it would have decayed away long before scientists could have discovered it.