0 a chemical element that is a soft, silver-white metal, and is radioactive: --
The remaining 7 transient elements (technetium, promethium, neptunium, americium, curium, berkelium, and californium) occur only rarely, as products of rare nuclear reaction processes involving uranium or other heavy elements.
Similar to the nearly simultaneous discovery of americium (element 95) and curium (element 96) in 1944, the new elements berkelium and californium (element 98) were both produced in 19491950.
The radioactive decay of uranium produces transient amounts of actinium and protactinium, and atoms of neptunium, americium, curium, berkelium and californium are occasionally produced from transmutation reactions in uranium ores.
Other elements are named after modern states or cities, with berkelium and californium being named after the city and the state where these elements were discovered.
All of the elements with higher atomic numbers, however, have been first discovered in the laboratory, with neptunium, plutonium, americium, curium, berkelium and californium later also discovered in nature.
The most difficult steps in the synthesis of berkelium were its separation from the final products and the production of sufficient quantities of americium for the target material.
Specifically, berkelium-249 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.
Berkelium is a soft, silvery-white, radioactive metal.
