Occurrence
Very minute amounts of californium exist on Earth due to neutron capture reactions and beta decay in very highly-concentrated uranium-bearing deposits. Traces of californium can be found near facilities that use the element in mineral prospecting and in medical treatments. The element is fairly insoluble in water, but it adheres well to ordinary soil; and concentrations of it in the soil can be 500 times higher than in the water surrounding the soil particles.
Fallout from atmospheric nuclear testing prior to 1980 contributed a small amount of californium to the environment. Californium isotopes with mass numbers 249, 252, 253, and 254 have been observed in the radioactive dust collected from the air after a nuclear explosion. Californium is not a major radionuclide at United States Department of Energy legacy sites since it was not produced in large quantities.
Californium was once believed to be produced in supernovas, as their decay matches the 60 day half-life of 254Cf. However, subsequent studies failed to demonstrate any californium spectra, and supernova light curves are now thought to follow the decay of nickel-56.
Read more about this topic: Californium
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