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This creates an interim particle known as a compound nucleus, which churns and seethes with excess energy, literally warping its own nuclear architecture. (It also happens to be moving at an incredible rate of speed.) That excess energy can be expended in one of two ways: Either the compound nucleus spits out a neutron and settles down into a relatively stable state, or it splits in two in a process known as fission. In the lab's early glory years, Ghiorso and Seaborg's team struggled to infuse a light element with just enough energy to fuse the two nuclei together, but not so much that it would immediately split apart.
"You have to understand that the nucleus is very small compared to the size of the atom, and it's positively charged," says Rollie Otto, the director of LBNL's Center for Science and Engineering Education. "So the energy it takes to push one nucleus that close to another gets larger and larger the closer you get. Because strong nuclear forces are only short-range, you don't feel any attraction initially, you just keep pushing them down, and it takes millions of electron volts to get them that close. When they finally get close enough, the nuclear forces take over, and they amalgamate just like two liquid drops. In fact, a liquid drop is a good way to think about the nucleus. If you had two liquid drops, and you got them to touch, the surface tension would cause them to become one big drop.
"But now that compound nucleus is very excited, it's got a lot of excess energy in it. Now the question becomes, how can that nucleus survive? The lab spent its time trying to get it to survive by spitting off a neutron [rather than splitting in the process of fission]. So what [Ghiorso] and his team did over the years is find that delicate balance of getting them just close enough to fuse, but not give them so much energy that they end up falling apart. The energy level is very critical. Too little, and they don't get together. Too much, and they break in two."
As Seaborg and Ghiorso's team produced ever-heavier elements, they began to run up against a brick wall. The more protons they crammed into a nucleus, the more their electric repulsion tore at the nuclear structure. Strong nuclear force could only compensate so much, especially as the nucleus swelled in size, and nucleons on the edge of the nucleus edged out of the range of their sisters. Seaborg and Ghiorso tried to compensate by creating heavier isotopes, in hopes that additional neutrons would dilute the power of electric repulsion. But by 1974, when LBNL researchers discovered seaborgium, their last confirmed element, that particle had a half-life of just twenty seconds before decaying. They were finally confronted with what seemed to be the natural end of the spectrum of elements in the universe, and the lab's illustrious work appeared to be over.
Still, throughout the '70s and '80s, Swiatecki's theories continued to tantalize high-energy physicists around the world. If what he postulated was correct, the seemingly impermeable barrier of ever-dwindling half-lives was merely a rough patch that researchers had to slog through in order to find regions of nuclear stability unglimpsed by human beings -- a roster of new, superheavy elements that could last for millennia. All they had to do was find the right combination of isotopes to collide.
A useful analogy can be found in chemistry, in examining the so-called "noble" gases. According to atomic theory, protons and neutrons inhabit the nucleus, which is surrounded by electrons in what are variously called orbital shells or probability clouds. Each shell has a specific energy level associated with it and wants to have a certain number of electrons within it; the first orbital shell, for example, wants to have two electrons, and if that shell is unfilled, the hydrogen or helium ion will seek out an electron and absorb it. This is the essence of what is called "covalent bonding," and it's the reason why oxygen gas consists of two atoms of oxygen bonded together. Each oxygen atom hungers for two additional electrons, and when two atoms are close enough, each will latch onto an electron in the other's outermost shell and treat it as its own, thereby locking it into a symbiotic relationship with its twin. But the so-called "noble gases" such as neon, argon, and krypton are elements whose electron shells are completely filled; they have no such desire to seek out another electron and are dubbed "noble" because their behavior is aloof. They have achieved stability -- they don't want to change.
In a similar fashion, nucleons are organized into shells within the nucleus -- and want to fill up their own shells. There are separate, concentric shells for protons and neutrons, and once they are filled up, the nucleus achieves a certain nonreactive quality. The proton shells of helium, oxygen, calcium, and tin, for example, are all completely filled; they have accumulated what is known as a "magic number" of protons and have achieved a certain crystalline equilibrium; protons are arranged within each shell in such a way as to set the electric repulsion of each particle against one another, and the protons are held in an abeyance that lends a particular stability to the nucleus. When the shells of both protons and neutrons are filled, the nucleus is said to be "doubly magic," and particularly stable. Creating these doubly magic isotopes outside the known limits of the Periodic Table is at the heart of the quest for superheavy elements that continues to this day.
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