In 1964, workers at the Joint Nuclear Research Institute at Dubna (U.S.S.R.) bombarded plutonium with accelerated 113 to 115 MeV neon ions. By measuring fission tracks in a special glass with a microscope, they detected an isotope that decays by spontaneous fission. They suggested that this isotope, which had a half-life of 0.3 +/- 0.1 s might be 260-104, produced by the following reaction: 242Pu + 22Ne --> 260Rf +4n.
Element 104, the first transactinide element, is expected to have chemical properties similar to those of hafnium. It would, for example, form a relatively volatile compound with chlorine (a tetrachloride).
The Soviet scientists have performed experiments aimed at chemical identification, and have attempted to show that the 0.3-s activity is more volatile than that of the relatively nonvolatile actinide trichlorides. This experiment does not fulfill the test of chemically separating the new element from all others, but it provides important evidence for evaluation. Data issued by Soviet scientists reduced the half-life of the isotope they worked with from 0.3 to 0.15 s.
In 1969 Ghiorso, Nurmia, Harris, K.A.Y. Eskola, and P.L. Eskola of the University of California at Berkeley reported that they had positively identified two, and possibly three isotopes of Element 104. The group indicated that, after repeated attempts, they produced isotope 260104 reported by the Dubna groups in 1964.
The discoveries at Berkeley were made by bombarding a target of 249Cf with 12C nuclei of 71 MeV, and 13C nuclei of 69 MeV. The combination of 12C with 249Cf followed by instant emission of four neutrons produced Element 257104. This isotope has a half-life of 4 to 5 s, decaying by emitting an alpha particle into 253No, with a half-life of 105 s.
The same reaction, except with the emission of three neutrons, was thought to have produced 258104 with a half-life of about 1/100 s.
Element 259104 is formed by the merging of a 13C nuclei with 249Cf, followed by emission of three neutrons. This isotope has a half-life of 3 to 4 s, and decays by emitting an alpha particle into 255No, which has a half-life of 185 s.
Thousands of atoms of 257104 and 259104 have been detected. The Berkeley group believes their identification of 258-104 is correct, but attaches less confidence to this work than to their work on 257104 and 259104.