NOBELIUM

MATTI NURMIA, JYVÄSKYLÄ UNIVERSITY, FINLAN

NOBELIUM AT A GLANCE Name: Named for scientist Alfred Nobel. Atomic mass: (259). History: Discovery was first claimed by scientists at the Nobel Institute in Stockholm in 1957, but their claim could not be verified. In 1958, Albert Ghiorso and colleagues at the Lawrence Berkeley National Laboratory identified a shorter lived isotope. Occurrence: Artificially produced. Appearance: Solid of unknown color. Behavior: Radioactive. Uses: No commercial uses.

Albert Ghiorso, the legendary element hunter of Lawrence Berkeley National Laboratory, had already established his reputation before nobelium's time. He had participated in the discovery of berkelium and californium, produced from plutonium with the Berkeley 60-inch cyclotron, and found einsteinium and fermium in the debris of the "Mike" hydrogen device that exploded in 1952. His team then produced the next element, mendelevium, from einsteinium and helium ions at the 60-inch cyclotron and reported its discovery in 1955.

After mendelevium, a pause to upgrade the equipment was needed in Berkeley. A new accelerator, the HILAC, was built. Because of the ever-shortening half-lives of each new element, a new detector system was needed to capture individual short-lived nuclei and measure their properties in a hostile, radiation-filled environment.

While Ghiorso's team was busy with these preparations, surprising news arrived from Stockholm. A group working at the Nobel Institute in Sweden had bombarded a curium target with a beam of ¹³C and observed a few high-energy -particles with an apparent half-life of several minutes. On this basis, they reported in 1957 the discovery of element 102 and proposed the name "nobelium" for it.

Attempts to reproduce the Stockholm results in Berkeley failed, and it became clear that the Swedish claim was premature. By that time, a group working at Dubna, Russia, also reported the observation of the new element. Both of these claims were based on inaccurate techniques that enabled only a very approximate determination of the energy and half-life of the -particles observed.

After painstaking work, the Berkeley group reported in 1958 their results on element 102 that were at variance with these early announcements and claimed the right to name the element. They recommended that the old name "nobelium" be retained. The group then used their new equipment to discover lawrencium in 1961.

	DISCOVERERS Nurmia (left), James Harris, Kari Eskola, Seaborg, Pirkko Eskola, and Ghiorso.
	COURTESY OF MATTI INURMIA

There was plenty to do. Besides working on the 104 problem, we gathered more data about the various isotopes of nobelium and lawrencium. Hundreds of bombardments were made using a detector system invented by Ghiorso, in which the nuclear reaction products were knocked off the target foil by the bombarding beam and stopped in helium gas and then deposited onto the rim of a wheel with a tiny jet of helium. A stepping motor moved the wheel periodically to place the spots of deposits in front of a series of detectors. A different helium jet system was used for chemical experiments, and in 1967 we were able to report that, in fulfillment of a prediction made by Glenn T. Seaborg in 1949, nobelium exhibited a divalent state in aqueous solutions that was more stable than that of its lanthanide homolog, ytterbium.

Two of my Finnish students, Pirkko and Kari Eskola, joined us in 1968, and our work settled into an efficient pattern. Kari and Pirkko painstakingly analyzed the reams of data produced in those days when computers were still in their infancy, and I applied Ghiorso's ideas and built and tested equipment. We carried our project forward, balancing Ghiorso's interest in pioneering new techniques with the Eskolas' desire for more scientific data. I remember how Ghiorso, a warm, intense Italian-American genius, and Pirkko, a statuesque, blond Finnish lady, had rapid-fire discussions about the use of the next allotment of precious accelerator time. Many times Pirkko won, and more data were taken while Al and I waited for our turn. It was teamwork at its best!

A fascinating pattern emerged as the properties of the various isotopes of nobelium were elucidated. The heavier even-even isotopes of No became unstable against spontaneous fission: While the half-life of ²⁵⁶No was about three seconds, that of ²⁵⁸No was only 1.2 milliseconds. We had encountered the edge of the "continent" of relatively long-lived nuclides! In front of us was the "sea" where fission dramatically intervened. It had been known that the mutual repulsion of protons would eventually limit the extent of the continent of nuclides, but it was still quite exciting to actually find the edge.

But all was not well at Berkeley. The noble and exciting field of scientific discovery became tainted by the Cold War-inspired tactics of the Soviet group. The funding needed to maintain the excellence of our work was not forthcoming, and the leadership of new-element work passed to a well-organized group in Germany. It was time for me to move, and I returned to my native Finland to work on the challenge posed by the greenhouse effect.

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