The chemical element californium, with an atomic number 98 and atomic symbol Cf, cannot be found in Nature. This is because it is a synthetic element, meaning that it is human-made and no form of it occurs naturally. Californium, in fact, was the sixth transuranium (“beyond” uranium or elements with atomic number greater than 92) element of the actinide series (radioactive, metallic elements found at the bottom of the periodic table) to be discovered.
The discovery of californium took place in 1950. Stanley Thompson, Kenneth Street Jr., Albert Ghiorso and Glenn Seaborg were American scientists plying their trade at the Lawrence Berkeley National Laboratory in the U.S.
Bombard curium with alpha particles
With the aid of a 60-inch cyclotron in the laboratory at Berkeley, California, the researchers were able to bombard curium-242 with alpha particles (helium atoms without electrons). Some of these collisions then caused the helium atom (atomic number 2) to be part of a curium atom (atomic number 96), resulting in the formation of a new element.
The new element formed, proved by the scientists after chemical analysis, was californium-245 with an atomic number 98. The scientists named it californium after the U.S. State of California and the University of California, the place where it was first produced, and announced the discovery on March 17, 1950.
Only trace quantities produced
This experiment produced only about 7,00,000 atoms of Cf-245, which has a radioactive half-life (time taken for half an initial amount to disintegrate) of about 45 minutes. Macro quantities of californium were isolated years later.
By employing prolonged (five years) neutron irradiation of plutonium-239, Thompson, along with fellow chemist Burris Cunningham, was able to get larger quantities of californium for the first time in 1958 at the Materials Testing Reactor in Idaho. The “large quantities” corresponded to the synthesis of approximately 1.2 micrograms of californium and 0.6 micrograms of berkelium.
20 known isotopes
One of the greatest stumbling blocks towards studying californium better has been the difficulty of obtaining a piece of it. Despite these hurdles, scientists have identified 20 isotopes of californium whose half-lives are known, with mass numbers ranging from 237 to 256. Of these, the longest lived isotope is californium-251, which has a half-life of 898 years.
Californium-252, which is produced in nuclear reactors and has a half-life of 2.645 years, has garnered particular interest owing to its unusual property of giving off neutrons when it breaks apart. As a result of this, Cf-252 is put to a variety of uses, including acting as a neutron emitter and providing neutrons for nuclear reactors’ start-up.
Even though only small quantities of californium have ever been produced, it is in fact among the heaviest elements that have been produced in weighable amounts. Our understanding of californium will only get better as we produce more and learn from it.
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More about the Lawrence Berkeley National Laboratory
The 60-inch cyclotron (pictured above), a type of particle accelerator, was housed in the Lawrence Berkeley National Laboratory and was among the world’s most powerful atom smashers at that time.
The Lawrence Berkeley National Laboratory was founded by Ernest Lawrence as the Radiation Laboratory of the University of California, Berkeley in 1931. It was centred around Lawrence’s cyclotron and has since been involved in major research in physics and chemistry.
The Lawrence Berkeley National Laboratory has almost been a treasure trove of sorts for the discovery of elements. The discovery of the chemical elements technetium (1936), astatine (1940), neptunium (1940), plutonium (1940), americium (1944), curium (1944), berkelium (1949), californium (1950), einsteinium (1952), fermium (1952), mendelevium (1955), nobelium (1958), lawrencium (1961), rutherfordium (1969), hahnium (1970) and seaborgium (1974) can all be traced down to this laboratory in one way or another.
