TRANSURANIUM ELEMENTS—SEABORG 255 
cium for target material were prepared by the intense neutron irradi- 
ation of plutonium for a long time; this process builds up the heavier 
elements by a series of neutron captures. Curium for target material 
was prepared in microgram amounts by the neutron irradiation of 
some of the americium. Both of these neutron bombardments were 
carried out in high-flux reactors. Berkelium, as the isotope Bk***, 
was discovered by S. G. Thompson, A. Ghiorso, and the author in 
December 1949, as a result of the bombardment of americium with 
helium ions. Californium was first synthesized and identified by 
S. G. Thompson, K. Street, Jr., A. Ghiorso, and the author in 
February 1950, the isotope Cf**> being produced by the bombardment 
of microgram amounts of curium with helium ions. The identifica- 
tion of californium was accomplished with a total of about 5,000 
atoms. 
EINSTEINIUM AND FERMIUM 
The first test thermonuclear explosion, which took place in the 
Pacific in November 1952, led to the discovery of elements 99 and 
100, einsteinium and fermium. These two elements were found in 
debris, collected first on filter papers attached to aeroplanes which 
flew through the explosion area and later in more substantial quanti- 
ties by gathering up surface materials from a neighboring atoll. The 
uranium in the fission-fusion device was subjected to an instantaneous 
intense neutron flux which gave rise to very heavy uranium isotopes. 
These rapidly decayed into heavy isotopes of plutonium, amer- 
iclum, curium, berkelium, californium, and elements 99 and 100. 
These reaction products were investigated by groups at the Uni- 
versity of California Radiation Laboratory, Argonne National Lab- 
oratory, and Los Alamos Scientific Laboratory. The two elements 
were discovered by Ghiorso and coworkers at the three laboratories. 
Einsteinium and fermium can be synthesized by a number of meth- 
ods. Chief among these is the irradiation of plutonium for several 
years with an extremely high neutron flux in such a reactor as the 
Materials Testing Reactor at Arco, Idaho. The einsteinium used to 
produce element 101 was prepared in the Materials Testing Reactor. 
MENDELEVIUM 
The synthesis of element 101 was planned and accomplished not only 
with an amount of target einsteinium (K**) so small as to be un- 
weighable, but also with the expectation that no more than one atom 
of element 101 per experiment would be produced. Only about 1,000 
million atoms of K*** were available for target material. In addition, 
the separation of the one atom of element 101 from the 10° atoms of 
target einsteinium and its ultimate complete chemical identification 
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