Curium - History

History

Although curium had likely been produced in previous nuclear experiments, it was first intentionally synthesized, isolated and identified in 1944, at the University of California, Berkeley by Glenn T. Seaborg, Ralph A. James, and Albert Ghiorso. In their experiments, they used a 60-inch (150 cm) cyclotron.

Curium was chemically identified at the Metallurgical Laboratory (now Argonne National Laboratory) at the University of Chicago. It was the third transuranium element to be discovered even though it is the fourth in the series – the lighter element americium was unknown at the time.

The sample was prepared as follows: first plutonium nitrate solution was coated on a platinum foil of about 0.5 cm2 area, the solution was evaporated and the residue was converted into plutonium dioxide (PuO₂) by annealing. Following cyclotron irradiation of the oxide, the coating was dissolved with nitric acid and then precipitated as the hydroxide using concentrated aqueous ammonia solution. The residue was dissolved in perchloric acid, and further separation was carried out by ion exchange to yield a certain isotope of curium. The separation of curium and americium was so painstaking that the Berkeley group initially called those elements pandemonium (from Greek for all demons or hell) and delirium (from Latin for madness).

The curium-242 isotope was produced in July–August 1944 by bombarding 239Pu with α-particles to produce curium with the release of a neutron:

Curium-242 was unambiguously identified by the characteristic energy of the α-particles emitted during the decay:

The half-life of this alpha decay was first measured as 150 days and then corrected to 162.8 days.

Another isotope 240Cm was produced in a similar reaction in March 1945:

The half-life of the 240Cm α-decay was correctly determined as 26.7 days.

The discovery of curium, as well as americium, in 1944 was closely related to the Manhattan Project, the results were confidential and declassified only in 1945. Seaborg leaked the synthesis of the elements 95 and 96 on the U.S. radio show for children, the Quiz Kids, five days before the official presentation at an American Chemical Society meeting on November 11, 1945, when one of the listeners asked whether any new transuranium element beside plutonium and neptunium had been discovered during the war. The discovery of curium (242Cm and 240Cm), their production and compounds were later patented listing only Seaborg as the inventor.

The new element was named after Marie Skłodowska-Curie and her husband Pierre Curie who are noted for discovering radium and for their work in radioactivity. It followed the example of gadolinium, a lanthanide element above curium in the periodic table, which was named after the explorer of the rare earth elements Johan Gadolin:

"As the name for the element of atomic number 96 we should like to propose "curium", with symbol Cm. The evidence indicates that element 96 contains seven 5f electrons and is thus analogous to the element gadolinium with its seven 4f electrons in the regular rare earth series. On this base element 96 is named after the Curies in a manner analogous to the naming of gadolinium, in which the chemist Gadolin was honored."

The first curium samples were barely visible, and were identified by their radioactivity. Louis Werner and Isadore Perlman created the first substantial sample of 30 µg curium-242 hydroxide at the University of California in 1947 by bombarding americium-241 with neutrons. Macroscopic amounts of curium fluoride were obtained in 1950 by W. W. T. Crane, J. C. Wallmann and B. B. Cunningham. Its magnetic susceptibility was very close to that of GdF₃ providing the first experimental evidence for the +3 valence of curium in its compounds. Curium metal was produced only in 1951 by reduction of curium fluoride with barium.