Berkelium (pronounced /bərˈkiːli.əm/ bər-KEE-lee-əm, less commonly /ˈbɜrkli.əm/ BERK-lee-əm) is a synthetic element with the symbol Bk and atomic number 97. A radioactive metallic element in the actinide series, berkelium was first synthesized by bombarding americium with alpha particles (helium ions) and was named for the University of California at Berkeley. Berkelium was the fifth transuranic element to be synthesized.
Weighable amounts of 249Bk (half-life 330 days) make it possible to determine some of its properties using macroscopic quantities. It is a silvery metal that would easily oxidize in air at elevated temperatures and would be soluble in dilute mineral acids.
X-ray diffraction techniques have been used to identify various berkelium compounds such as berkelium dioxide (BkO2), berkelium fluoride (BkF3), berkelium oxychloride (BkOCl), and berkelium trioxide (BkO3). In 1962, visible amounts of berkelium chloride (BkCl3) were isolated that weighed 3 billionths of a gram. The first time visible amounts of a pure berkelium compound were produced was in 1958.
Like other actinides, berkelium bio-accumulates in skeletal tissue. This element has no known uses outside of basic research and plays no biological role.
Berkelium was first synthesized by Glenn T. Seaborg, Albert Ghiorso, Stanley G. Thompson, and Kenneth Street, Jr. at the University of California, Berkeley in December 1949. The team used a cyclotron to bombard a milligram-sized target of 241Am with alpha particles to produce 243Bk (half-life 4.5 hours) and two free neutrons.
24195Am + 42He → 24397Bk + 2 10n
One of the longest lived isotopes of the element, 249Bk (half-life 330 days), was later synthesized by subjecting a 244Cm target to an intense beam of neutrons.
19 radioisotopes of berkelium have been characterized, with the most stable being 247Bk with a half-life of 1380 years, 248Bk with a half-life of >9 years, and 249Bk with a half-life of 330 days. All of the remaining radioactive isotopes have half-lives that are less than 5 days, and the majority of these have half-lives that are less than 5 hours. This element also has 2 meta states, with the most stable being 248mBk (t½ 23.7 hours). The isotopes of berkelium range in atomic weight from 235.057 u (235Bk) to 254.091 u (254Bk).
In the nuclear fuel cycle, berkelium is produced by beta decay of curium. The first curium isotope to undergo beta decay is Cm-249 with a half-life of just over an hour, so Bk-249 is the only isotope of berkelium produced in significant quantities in nuclear reactors. Production of Bk-249 requires 11 successive neutron captures on uranium-238 without nuclear fission or alpha decay, so it is only produced in small amounts.
249Bk has a moderately large neutron capture cross section of 710 barns for thermal neutrons, 1200 barns resonance integral, but very low fission cross section for thermal neutrons. If still in a thermal reactor, much of it will therefore be converted to 250Bk which quickly decays to californium-250, but some alpha decays to americium-245.
Berkelium accumulates in the skeletal system. The radiation can cause damage to red blood cells. The maximum permissible body burden reported for the isotope Bk–249 in the human skeleton is 0.4 ng.
1. ^ "The Solution Absorption Spectrum of Bk3+ and the Crystallography of Berkelium Dioxide, Sesquioxide, Trichloride, Oxychloride, and Trifluoride", Ph.D. Thesis, Joseph Richard Peterson, October 1967, U. S. Atomic Energy Commission Document Number UCRL-17875(1967).