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"Magic numbers provide the same kind of stability as closed electron shells," says Lee Schroeder, the director of LBNL's Nuclear Science Division. "With that abeyance [of protons], you have a unique situation where the nucleus doesn't want to do anything or have things done to it, you have a nice closure on both the protons and the neutrons. In nuclear physics, the letter Z refers the number of protons in the nucleus, and as you push Z further [into the heavy elements], you have this peninsula running out, and then you go into deep water, and then there's this island that magically rises out there, which according to theoretical musings has a kind of stability. It was just a question of developing the tools that allow you to push out into that regime where you might see it."
In the mid-'90s, a convergence of events sparked a little-known but dramatic contest to finally reach Swiatecki's islands of stability. Lab engineers were perfecting a new generation of equipment to detect the signature "alpha decay chains" of superheavy elements; theoreticians developed new models of isotopic collision that might make such elements possible; and an incoming class of young researchers breathed new life into the endeavor. By 1998, three research teams -- the GSI lab in Germany, the LBNL team, and a consortium of researchers from Lawrence Livermore and Dubna, Russia -- were racing to put together experiments that would result in the discovery of an isotope of element 114, one of the "doubly magic" isotopes that could last as long as centuries. Since the Nixon years, GSI scientists have been the undisputed masters of heavy element work, discovering five of the last six known elements. For LBNL researchers, the race presented a chance to finally regain dominance in the field of heavy element research, dominance it had ceded more than twenty years ago. "Every island has a shore, and discovering elements 112 and 114 would be kind of like landing on the shore of this thing," Schroeder says.
In January 1999, just months after Seaborg suffered the stroke that ended his life, researchers with the Dubna/Livermore consortium announced that after bombarding plutonium with a calcium isotope for forty days, they had observed a single decay chain that fit the pattern predicted by the creation of element 114. After more than a month of bombardment, the Russians had seen just one atom of the element -- but it looked, nonetheless, like the Russian researchers had finally taken the first step onto the magic island, and, with this discovery, had put the world on notice that they were now serious players in the discovery of heavy elements.
Berkeley scientists immediately began preparing to repeat the experiment in order to confirm the discovery, but soon realized that they didn't have enough raw material to proceed. Fortunately, Polish theoretician Robert Smolanczuk was working at the lab on a Fulbright scholarship and proposed a different project to researcher Darleane Hoffman: Now that Ghiorso's Berkeley Gas-filled Separator was up and running, the lab could test Smolanczuk's prediction that firing high-energy particles of krypton gas at a lead target would produce not element 114, but element 118. Far beyond stepping onto the island's shore, Smolanczuk suggested, if his theory was correct the lab would have hiked deep into the island's brush.
Two things about this experiment were particularly appealing: Neither the particle nor the target was radioactive, which reduced the health and safety precautions; and Smolanczuk's model predicted results in just a few days, which promised not to monopolize use of the 88-inch cyclotron for very long (so many research teams compete for "beam time" on the cyclotron that for every project approved, two are rejected). In addition, Berkeley researchers feared that unless they ran Smolanczuk's experiment immediately, the Germans or the Russians would beat them to the punch yet again. "Smolanczuk had had some success with this model, and that gives you a little bit of a backbone," Schroeder says. "Well, here's a possible pathway, let's go off and try it." On April 8, 1999, researchers Ken Gregorich, Darleane Hoffman, and Victor Ninov fired up the cyclotron and went to work.
The key factor in any particle accelerator is a simple tenet of physics: A charged particle moving in a magnetic field will always bend in a direction that can be predicted. Line up enough magnets in just the right sequence, and you can move a charged particle any direction you want. So the first step is to take a particle and apply a charge to it -- in this case, by stripping electrons from particles of krypton to create positively charged ions. How do you strip electrons from krypton? You cook it.
"Krypton starts as a gas," Schroeder says. "You put it into an oven and heat it up, giving it random thermal motion, which eventually starts spitting some electrons off. You then apply a voltage by injecting the gas into a device called an 'electron cyclotron resonance ion source.' There are enormous radio frequency fields running around, which further create a plasma. What you're trying to do is get as many electrons off there as possible. Then you spit it out of the ion source. You then have krypton with a particular charge, which you then guide into the cyclotron."
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