EINSTEINIUM AND FERMIUM

ALBERT GHIORSO, LAWRENCE BERKELEY NATIONAL LABORATORY

As a codiscoverer of all of the elements from 96 to 106 and of the earliest work on element 110, I can attest to the fact that all of that research was exciting: It was an exploration into the unknown that required the development of new techniques that eventually developed sensitivities for identifying one atom at a time. But the discovery of elements 99 and 100 was something quite different. It was completely unexpected, a most extraordinary event that mimicked the r-process by which the heaviest elements were put on our Earth in the first place.

It occurred in 1952, when the first thermonuclear device in history was exploded in the South Pacific by Los Alamos Scientific Laboratory. This was a highly secret event, and initially, the UC Radiation Laboratory was not involved in any way. In fact, our first news that something very unusual had happened came when Glenn T. Seaborg received from Washington, D.C., a telegram, which said that the heretofore unknown isotope of plutonium with mass 244 had been found in a recent test and therefore its existence was now classified secret even if it were to be produced by any other nonsecret means!

This was truly sensational news indeed! Somehow, six neutrons had been added in an instant to U-238, and this had then undergone -decay to Pu-244. We had an ongoing program to do the same thing over a time span of years by subjecting reactor-grade Pu to a high flux of neutrons, but we expected that it would take a long time to produce anything. I discussed this new information with Stan Thompson, and we both became very excited. What could we do? It had been some five weeks since their discovery of Pu-244. What else had been found by the weapons diagnostic groups at Los Alamos and Argonne laboratories in the filter papers that had been flown through the explosion cloud? We had heard rumors about an H-bomb explosion, but since we had no connections with the work, we knew nothing more. Surely they must have discovered other isotopes and maybe a new element or two. Was there some way by which we could join forces with them to help with the research? After all, we had pioneered the heavy-element field and felt that there was much that we could offer in the way of help.

	DEBRIS Both einsteinium and fermium were discovered in the debris of the "Mike" hydrogen bomb test.
	DOE PHOTO

That night, at home, I came up with the wildest idea of my entire career. The key to it was a simple estimate of the amount of Pu-244 that might have been produced, an estimate which turned out accidentally to be remarkably accurate. I assumed that the amount of Pu-244 was of the order of 0.1%, the nominal mass spectrometer detection limit at that time, and that it was produced by an enormous flux of neutrons impinging on U-238, which I assumed was probably a major component of the nuclear device. Momentarily, U-244 would be created and this would then undergo b-decay to Np-244 and then Pu-244. I assumed that the same process would occur at higher mass numbers with the yield decreasing by a factor of 1,000 for every five masses. Thus, I speculated that as many as 16 neutrons might be captured by U-238 with a yield of 10-9, creating the ultraheavy isotope, U-254; this would then -decay all the way up to mass 254 at element 100! A reasonable prediction based on what was known at that time was that the end result would be an

Ghiorso
PHOTO BY DENNIS GALLOWAY

This was an exciting moment. Would we find anything at all? Almost immediately, I saw 6.6-MeV -particles in the transcalifornium fraction, an energy that I had never seen before! And the intensity was about 1 c/m. This was eerie. Could this be the element 100 that my guesstimate had predicted?

Because of the imprecision of the first chemical separation, we did not know whether the 6.6-MeV -particles came from element 99 or from 100, so further experiments were performed, this time with the inclusion of a Cf-246 tracer. These showed conclusively that the new element that we were observing had the atomic number 99. After following its decay for a few weeks, we knew that its half-life was about a month.

These startling results were communicated to Los Alamos and Argonne, and now we were bona fide members of the team. The next order of business was to look for element 100. For that we would need more material from the test, since we knew that its yield would be much smaller. This was obtained from fallout that had settled on the coral of a neighboring island. Barrels of the highly radioactive material were flown to the U.S., and in short order, Thompson set up a small chemical processing plant and proceeded to isolate an element-100 fraction. Once again, we were surprised to find a small amount of a new activity, 7.1-MeV -particles with a half-life of only a day. It had been about two months since the explosion; they had been kept alive by a -emitting isotope of element 99 with a half-life of about a month.

By September 1953, the three laboratories--Berkeley, Argonne, and Los Alamos--had established with certainty the mass assignments of isotopes of elements 96-100 that had been generated by the Mike explosion, but there was no indication as to when this information might be declassified so that it could be published. We became concerned that other, lighter isotopes of elements 99 and 100 might be discovered by heavy-ion reactions and these would not be classified secret and could be published. Not being aware of our work, these discoverers, of course, would want to name these elements, since this signal honor traditionally goes to the people who first find them. It was not clear that our secret work would be recognized as having priority, so we decided to forestall this problem by investigating these heavy-ion reactions ourselves.

Bombardments of U-238 with N-14 ions produced by the Crocker Laboratory cyclotron did produce a short-lived light isotope of element 99 that we were able to isolate chemically, so we published the work as due to element 99. When we published this experiment in 1954, we indicated that there was prior research on this element that had not yet been declassified, so that our heavy-ion work did not constitute the discovery of element 99.

At about the same time, something similar happened in the case of element 100. When we examined the products of the long neutron bombardments of Pu-239, as expected, we found 99-253, the same isotope that we had discovered in the Mike research. When this material was bombarded with neutrons at the MTR high-flux reactor in Idaho, we found a short-lived new -emitting isotope, 99-254. This activity decayed to -emitting 100-254, which we were able to isolate chemically. Since this research was not secret, we were able to publish it immediately. Again, it was accompanied with a disclaimer that said that there was prior work on element 100.

With this incentive, the Mike work was declassified and the discovery of elements 99 and 100 was published by the large team of scientists from Berkeley, Argonne, and Los Alamos. At this time, I suggested that we should start a new paradigm and name new elements after famous scientists. A natural choice that was strongly supported by everyone was einsteinium for element 99 and fermium for element 100, and I had the honor of announcing this selection at the Geneva International Conference on the Peaceful Uses of Atomic Energy in 1955.

A more detailed description of this amazing episode in science can be found in Chapter 6 of "The Transuranium People: The Inside Story" by Darleane C. Hoffman, Albert Ghiorso, and Glenn T. Seaborg (Imperial College Press, 2000)

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