An Island of Stability

By Robert Naeye|Monday, August 01, 1994
RELATED TAGS: ELEMENTS
After a brief hiatus, chemists are continuing their forced march up the periodic table. The latest addition is seaborgium: element 106.

Nearly all the elements from hydrogen to uranium occur naturally on Earth. Beyond that, doing chemistry gets exceptionally difficult. To study an element heavier than uranium--which has 92 protons in its nucleus, and thus an atomic number of 92--you have to make it first, usually by slamming a light element into a heavy one at high speed and hoping some of the nuclei stick together. The first nine transuranic elements were produced in a flurry between 1940 and 1955--eight of them, beginning with plutonium, by a group that included American chemist Glenn Seaborg. (The group found elements 99 and 100 in the fallout from the first hydrogen bomb explosion, at Eniwetok Atoll in 1952; element 100 was in a garbage can.) But the pace has been slower ever since. No new elements have been discovered since the early 1980s, when German researchers synthesized elements 107, 108, and 109. Even those discoveries have yet to be confirmed by a second laboratory.

Making the heaviest elements is so hard that only four labs in the world even attempt it. They do so for the challenge, for the glory of making additions to the periodic table, and because by creating artificial elements they can test their theories of how natural ones are put together. And recently, after a dry spell, they’ve had some notable success. Late last year a team at the University of California at Berkeley confirmed a 1974 discovery by Albert Ghiorso of the Lawrence Berkeley Laboratory and his colleagues, including Seaborg: they made element 106 a second time. Ghiorso promptly christened the element seaborgium, and it can now be officially added to the periodic table. Meanwhile, a group of Russian and American scientists has found that seaborgium is rather interesting: some forms of it survive for as long as 30 seconds before they decay into lighter elements. That result has raised hopes that somewhere in the upper reaches of the periodic table may lurk elements stable enough for us to get to know well.

The elements tend to get progressively less stable the further beyond uranium you go because their nuclei contain progressively more positively charged protons, and like charges repel one another. If the electrostatic force were the only force at work, however, every atom, transuranic or not, would fly apart immediately. Instead, as the protons and their electrically neutral cousins, the neutrons, zip around inside the nucleus, they are bound together by the strong nuclear force, one of nature’s four fundamental forces. That is, they are bound together until they get too close together: at close range the strong force turns repulsive. The balance of attractive and repulsive forces generally molds the nucleus into a sphere.

The interactions of dozens of particles are so complex, though, that theorists have a hard time predicting just how stable a particular element will be, and they can’t always agree--which is why making new elements is a useful test of competing theories. But one thing everyone agrees on, because quantum mechanics requires it, is that the protons and neutrons in the nucleus can only occupy discrete energy levels--chemists call them shells--just like the electrons that buzz around the nucleus. Although the shells represent energy levels, they also have a spatial significance. Particles with the most energy are more likely to be found in the outer shell of the nucleus.

The significance of the shell structure for the stability of the nucleus is this: each shell has room for only a certain number of particles, and an atomic nucleus is most stable when all its shells are filled to capacity. This gives it a tightly packed structure that makes it less susceptible to decay. As one moves up the periodic table, the shells fill from the inside out, from low energy to high. Protons and neutrons fill separate shells. At certain magic numbers, such as 2, 8, 20, 28, 50, and 82, the outermost shell is filled, such that the next particle would have to go into a new shell. Oxygen (8 protons and 8 neutrons) and lead (82 protons and 126 neutrons), for example, rank among the most stable elements because they have magic numbers of both protons and neutrons.

Theoretically, the shell structure could create accessible islands of stability among the transuranic nuclei. In particular, several theorists have predicted that 162 should be a peculiar type of neutron magic number. A nucleus containing 162 neutrons, they say, should have a filled outer shell, but one that is egg-shaped rather than spherical. The Russian and American team, led by Yuri Lazarev of the Joint Institute for Nuclear Research in Dubna, and Ron Lougheed of the Lawrence Livermore National Laboratory, set out to test this prediction by creating new isotopes of seaborgium as close to 162 neutrons as they could get. (Isotopes are atoms with the same number of protons but with different numbers of neutrons.)

Using an accelerator in Dubna, 60 miles north of Moscow, the researchers bombarded a two-square-inch target made of curium (atomic number 96) with a beam of neon (atomic number 10). They hurled 13 trillion neon atoms at the target every second for several weeks--16 million trillion neon atoms in all. Nearly all those atoms had too much energy or too little, or didn’t hit a curium atom at just the right angle and ended up either bouncing off the curium or passing right through it. But on a few hundred occasions, conditions were just right for a curium and a neon nucleus to meld into a nucleus of seaborgium. The researchers detected ten of those nuclei. Four had 159 neutrons, six had 160, and all were remarkably stable.

Actually the researchers never saw seaborgium directly. Instead they inferred its existence by observing daughter elements produced by the decay of the two different seaborgium isotopes, along with the alpha particles released by the isotopes as they decayed. From those observations the researchers could determine the isotopes’ half-lives: they ranged as long as 30 seconds. That doesn’t sound long, but for a nucleus with 106 protons, it is. The 157-neutron isotope of seaborgium that Ghiorso and Seaborg discovered in 1974 had a half-life of nine-tenths of a second.

The discovery at Dubna proves there is indeed nuclear stability to be had near the 162-neutron magic number. Presumably a nucleus with precisely 162 neutrons would be even longer-lived. If we got these two very long-lived isotopes, it means there is a new island of rather stable isotopes, says Lazarev. There is plenty of work for scientists to explore this island and make new, long-lived isotopes of elements 105, 106, and 107.

And perhaps beyond. By confirming the theoretical predictions for element 106, the Dubna-Livermore discovery increases our confidence in the predictions for heavier elements, says Adam Sobiczewski of the Institute for Nuclear Studies in Warsaw, who had predicted the long half- lives of the seaborgium isotopes. It confirms the essential role of the shell structure and gives hope for still heavier new elements. Indeed, the stability of an egg-shaped 162-neutron nucleus may be nothing compared with that of the next spherical proton shell, at magic number 114. Some theorists have predicted that elements 112, 113, and 114, especially their isotopes with the magic number of 184 neutrons, may boast half-lives in the range of billions of years.

Lazarev’s and Lougheed’s groups are working their way toward those elements step-by-step. This year they hope to make new and longer- lived isotopes of element 108 by bombarding uranium with sulfur (element 16), and the first definitive isotopes of element 110 by shooting a beam of sulfur at plutonium (element 94). Meanwhile, their success with long-lived 106 has spurred other labs. We’re talking about building a new type of detector that will enable us to do a far better job, says Ghiorso. This is only the beginning of exploring the upper region of the periodic table. The whole region is accessible.
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