In June 1953, I began postgraduate research at the Radiation Laboratory of the University of California, Berkeley, with Glenn T. Seaborg. Seaborg, with Albert Ghiorso, Stanley Thompson, and Bernard Harvey, were the members of the group on new-element research.
The group had initiated a program in 1952 to irradiate plutonium in a reactor in Idaho with neutrons to transmute plutonium into heavier actinides. For synthesis of Z = 101, -particles (He2+) from the cyclotron would be used to increase the atomic number of the target einsteinium by two units. We calculated that one-year irradiation of the Pu would yield about 1 billion atoms of Es. Since 253Es has a half-life of three weeks, we had a week after separation of the Es atoms from the irradiated Pu to do our experiments for making Z = 101.
We also estimated that to make one atom of Es per hour of cyclotron irradiation, we needed a beam of a-particles 100 times more intense than available. Seaborg applied for funds to upgrade the cyclotron to obtain the needed a-intensity, 1014 per second. Harvey worked on preparation of the 109 atoms of Es into a target by electrolytic precipitation on a thin gold foil, while Thompson and I concentrated on the chemical isolation of the few atoms of Z = 101 we hoped to make. I proposed using -hydroxyisobutyric acid to separate the few atoms of Z = 101 from the more numerous lower Z actinides. This did give us the improved separation needed and is still in common use for separation of trivalent actinides.
Ghiorso proposed we pass the cyclotron beam through the back of the target foil to give the atoms of element 101 enough momentum to be knocked out of the target and captured on another gold foil. We could reuse this target in successive bombardments by this recoil arrangement. We would dissolve the catcher foil and chemically isolate the atoms of element 101.
In the initial experiments (September 1954), we failed to observe -particles from decay of Z = 101. Ghiorso suggested that they had decayed by electron capture to atoms of 258Fm and that we should repeat the experiments with a new 253Es target to look for spontaneous fission decay of 256Fm atoms from the reactions (shown).
In February 1955, we had a new Es sample from the Pu irradiation, and the experiment was repeated. Since the cyclotron was on the campus of the University of California and the Radiation Laboratory was on an adjacent hill, Ghiorso would remove the catcher foil from the cyclotron and give it to Harvey, who would dissolve the foil in aqua regia and pass that solution through a small column of anion-exchange resin, where the gold and some of the other radioactive products would remain on the resin while the transuranium elements passed through.
NOVOSTI/SCIENCE PHOTO LIBRARY
The night of the discovery, the target was irradiated for a total of nine hours in three-hour intervals. The catcher foils from the three targets were processed individually. The counters were connected to the recorder so that the radioactive decay would be recorded as a large deflection on a chart, which allowed us to see how many radioactive decays would occur as well as the time of the decay event (from the distance between the deflections on the chart). By 4 AM, we had recorded five decay events from the third disk, which had the Z = 101 atoms. We left Seaborg a note on the successful identification of Z =101 and went home to sleep on our success.
We decided to name element 101, which marked the beginning of the second hundred elements of the periodic table, after the man most responsible for the periodic table, Dmitry Mendeleyev. This was during the Cold War era, but Seaborg convinced the U.S. government to allow our proposal to name the element for a Russian scientist, and the International Union of Pure & Applied Chemistry approved the name mendelevium.
Gregory R. Choppin received his Ph.D. from the University of Texas, Austin, in 1953, and then started with the new-element research group at the Lawrence Radiation Laboratory. He was one of the four codiscoverers of mendelevium. In 1956, he moved to Florida State University's chemistry department, where he initiated a research program in f-element chemistry and is studying the environmental behavior of the actinide elements.
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