Californium - History

History

Californium was first synthesized at the University of California, Berkeley by the physics researchers Stanley G. Thompson, Kenneth Street, Jr., Albert Ghiorso, and Glenn T. Seaborg on or about February 9, 1950. It was the sixth transuranium element to be discovered; the team announced its discovery on March 17, 1950.

To produce californium, a microgram-sized target of curium-242 (242
96Cm) was bombarded with 35 MeV-alpha particles (4
2He) in the 60-inch-diameter (1,500 mm) cyclotron at Berkeley, California, which produced californium-245 (245
98Cf) plus one free neutron (n).

242
96Cm + 4
2He → 245
98Cf + 1
0n

Only about 5,000 atoms of californium were produced in this experiment, and these atoms had a half-life of 44 minutes.

The discoverers named the new element after California and the University of California. This was a break from the convention used for elements 95 to 97, which drew inspiration from how the elements directly above them in the periodic table were named. However, the element directly above element 98 in the periodic table, dysprosium, has a name that simply means "hard to get at" so the researchers decided to set aside the informal naming convention. They added that "the best we can do is to point out ... searchers a century ago found it difficult to get to California."

Weighable quantities of californium were first produced by the irradiation of plutonium targets at the Materials Testing Reactor at the Idaho National Laboratory; and these findings were reported in 1954. The high spontaneous fission rate of californium-252 was observed in these samples. The first experiment with californium in concentrated form occurred in 1958. The isotopes californium-249 to californium-252 were isolated that same year from a sample of plutonium-239 that had been irradiated with neutrons in a nuclear reactor for five years. Two years later, in 1960, Burris Cunningham and James Wallman of the Lawrence Radiation Laboratory of the University of California created the first californium compounds—californium trichloride, californium oxychloride, and californium oxide—by treating californium with steam and hydrochloric acid.

The High Flux Isotope Reactor (HFIR) at the Oak Ridge National Laboratory (ORNL) in Oak Ridge, Tennessee, started producing small batches of californium in the 1960s. By 1995, the HFIR nominally produced 500 milligrams of californium annually. Plutonium supplied by the United Kingdom to the United States under the 1958 US-UK Mutual Defence Agreement was used for californium production.

The Atomic Energy Commission sold californium-252 to industrial and academic customers in the early 1970s for $10 per microgram and an average of 150 mg of californium-252 were shipped each year from 1970 to 1990. Californium metal was first prepared in 1974 by Haire and Baybarz who reduced californium(III) oxide with lanthanum metal to obtain microgram amounts of sub-micrometer thick films.