The story of Element 117, the latest addition to the periodic table, just discovered by a team of Russian and U.S. scientists.
Five years of preparation, eight months collecting a few drops of precious radioactive material from a nuclear reactor in Tennessee, five trans-Atlantic flights, millions in research dollars and rubles, and six months of nearly 24-hour-a-day bombardment in a Russian particle accelerator had come to this: Element 117.
Six times in the last few months, it had flashed in a detector for a few fractions of a second and then disintegrated away, earning a permanent spot on the periodic table.
This new atom was discovered during a six-month long experiment that ended in late February, according to the team of scientists from Tennessee, California, Nevada, and Russia, who are reporting their discovery this week. Before August of last year, element 117 had never before existed on Earth — and probably never before in the history of the universe.
Though it has yet to be named, element 117 is the latest in a series of super-heavy atoms to be synthesized in the last few years at the Joint Institute for Nuclear Research in Dubna, Russia. For technical reasons, it was by far the most difficult to make, but its discovery promises to be an important stepping-stone to synthesizing even heavier elements. And it may open the door to better understanding of the mysteries of the atomic structure at the extreme end of the material world.
“[Element 117] is the exploration of new territory — like the exploration of Africa by Livingston,” said nuclear physicist Karl-Heinz Schmidt of the GSI Helmholtz Centre for Heavy Ion Research in Darmstadt, Germany, who was not involved in the research.
Reached by telephone yesterday at his home near the institute where Element 117 was discovered, one of its discoverers spoke glowingly of the new element.
“This significantly expands the boundaries of the existence of the nuclei, atoms, elements,” said nuclear physicist Yuri Oganessian. “In a word, the boundaries of the material world.”
WORK BEGAN IN TENNESSEE
Oganessian is the head of the Russian side of the collaboration and ran the experiments in Dubna where element 117 was ultimately discovered, but the work began in Tennessee about five years ago after discussions between Oganessian and his longstanding collaborator Joseph Hamilton, a physicist at Vanderbilt University in Nashville, TN.
At the time, Oganessian and his colleagues had been using a new technique they developed called “hot fusion” to synthesize a number of the super-heavy elements. This technique used a steady and energetic beam of a rare form of calcium to bombard a radioactive target like uranium. Though hard to pull off, the experiments proved successful because occasionally a calcium atom would come just close enough to one of the heavier target atoms to stick, fusing together to form a new, super-heavy atom. From 2000-05, Oganessian and his collaborators from Lawrence Livermore National Laboratory in Livermore, CA, managed to make elements 113, 114, 115, and 116 in this way.
In 2005, Oganessian was hotly pursuing the heaviest atom ever created — element 118 — which he would soon succeed in making by bombarding a target of californium atoms, a highly radioactive element named after the U.S. state in which it was discovered.
Even with that goal still on the table, Oganessian and Hamilton were discussing what would be next. It would have to be element 117 — lighter than 118, but much harder to make. They knew it was going to prove even more difficult, Hamilton said, because what they needed as starting material was another highly radioactive element called Berkelium, which is also named after the place where it was discovered.
Berkelium is a by-product of producing californium, however. Californium was hard enough to obtain. Getting enough californium to get enough berkelium by-product to do their experiments was going to be prohibitively expensive because it would have to be made in a nuclear reactor that had a high concentration of subatomic particles known as neutrons.
The very best nuclear reactor in the world for this purpose is at the Department of Energy’s Oak Ridge National Laboratory in Tennessee. It was built in the 1960s specifically to produce radioactive elements like berkelium. In late 2004, Oganessian wrote a letter to Alex Zucker, the former director of ORNL, proposing the experiment and asking about the feasibility of producing enough berkelium.
“We saw immediately that this was an interesting experiment to both sides and that working together we could accomplish something that was pretty exciting,” recalled James Roberto, who was then the deputy director for science and technology at Oak Ridge.
Officials at Oak Ridge suggested that that they might piggyback on other work going on at the laboratory. Oak Ridge occasionally produces californium for a variety of commercial and research applications.
So they waited until 2008, when the next californium campaign was scheduled. That same year, a symposium was held at Vanderbilt to celebrate Hamilton’s career, and he invited both Oganessian and Roberto to attend. Over lunch, they made their plans.
THE THIRD TIME’S THE CHARM
They began in the spring of 2008 of by loading 40 grams of the radioactive element curium into target rods and lowering them into the reactor. These were then bombarded with neutrons for 23 days until the nuclear fuel in the reactor was spent. The reactor was shut down, the fuel was replaced, and then it was bombarded for another 23 days. After that fuel was spent, they replaced it again. And again.
The process lasted a total of 250 days, and it took 11 refueling cycles of the Oak Ridge reactor to produce about 22 milligrams of nearly pure berkelium in the end — just a few drops in the bottom of a test tube, but more than enough to do the experiment. It still had to be cooled for three months and then carefully chemically purified, which took another three months.
In the summer of 2009, they packed the material into five separate lead safety canisters and put them on a commercial flight to Moscow. At that point, they were working against the clock. The half-life of berkelium is 330 days, and in six months, there might not have been enough left to make element 117.
In something of a comedy of errors, the material was flown back-and-forth across the Atlantic Ocean five times because of incomplete or missing paperwork. It was refused entry into Russia twice, and twice it went back on the return flight to New York, before finally clearing customs only on its third trip to Moscow.
Nobody expected the berkelium to rack up so many frequent flier miles, Roberto quipped, but he added that the overall process was really only delayed for a few days.
From Moscow, the berkelium was transported to the Russian Research Institute of Atomic Reactors in Dimitrovgrad, where it was made into a “target” disk and sent on to Oganessian at the Joint Institute for Nuclear Research.
Finally, on July 27 last year, Oganessian and his colleagues placed the disk in the particle accelerator where it was blasted by a highly energetic beam smashing billions of calcium atoms per second against it. Because the beams were so energetic, the berkelium would have quickly vaporized if left still. So the disk was sent spinning at 2,000 revolutions per minute, and the beam was wobbled and wiggled to keep it from falling on one spot for too long.
What the scientists were hoping for was the rarest of rare events — a precise collision between a calcium atom flying out of the accelerator and a berkelium atom spinning about on the target.
THE PURSUIT OF MAGIC NUMBERS
Collisions like these are well known to science. Physicists have shown for decades that you can synthesize heavier atoms in the laboratory by smashing together two lighter elements. And atoms smash into each other and create heavier atoms throughout the universe in the wake of massive star explosions known as supernovae. Most of the matter on Earth is the result of atoms smashing into other atoms.
The key to these collisions lies in the nucleus, the heavy and compact heart of an atom. Imagine squeezing nearly the entire mass of Yankee Stadium into a speck the size of a flea sitting on home plate. An atom is like this by analogy — empty space the size of Yankee Stadium surrounding a tiny flea of great mass.
This is true regardless of how heavy an atom is — heavier atoms are just like fat fleas sitting in the same huge, empty ballpark.
What makes up the mass of the nucleus are two basic particles packed together: positively charged protons and uncharged neutrons. Protons naturally repel each other, but neutrons act like glue holding them together — sort of like putting tape on two magnets to hold them together head-to-head or tail-to-tail. The more powerful the magnets are, the more they repel each other and the more tape is needed to hold them together. Likewise, the heavier an atom is, the more neutrons it needs to hold its nucleus together.
“The extra neutrons supply that extra glue to hold these protons and neutrons together,” said Hamilton.
But at a certain point, even extra neutrons are not enough. Heavier atoms tend to break apart, throwing off a small fragment of their nuclei over time — a process known as radioactive decay. Every element known to science that is heavier than bismuth is radioactive and throws off mass like this. And in general, the heavier the element, the more quickly it decays.
Decades ago, physicists predicted the existence of a hypothetical island of stability, in which certain super-heavy atoms that had a particular “magic” number of protons and neutrons, would be more stable. Long sought, this island of stability around neutron “magic” number 184 has proven elusive because of the difficulty involved in synthesizing super-heavy elements.
While scientists generally agree that they do exist, there has been some disagreement over exactly what these magic numbers are, said Schmidt. Different theories have predicted different values for them, which is why the experiments in Dubna were so crucial.
“Since the available theories have already exploited all information available from the accessible nuclei, we need [new] experimental information to settle these questions,” Schmidt said.
LESS THAN A BILLIONTH
Back in the United States, Roger Henderson at Lawrence Livermore National Laboratory was downloading three or four data files every day from the Dubna experiment, analyzing them on a computer nicknamed Yana and looking for signs of element 117. When all was said and done, there were billions and billions of events or possible collisions to sort through.
What Henderson (who was duplicating the effort of the team in Russia) was looking for was one particular signature — a radioactive fingerprint of sorts — that would be the telltale sign they found the elusive atom. If they truly made element 117, it would exist for a brief time — a few hundredths of a second — and then decay into a series of lighter atoms as it threw off chunks of mass from its nucleus.
Several weeks after the experiment began they found the first one on Aug. 20. And by the time the experiment ended six weeks ago, they had detected a total of six events indicating the creation and subsequent decay of element 117.
One of these events occurred when Henderson’s colleague Mark Stoyer of Lawrence Livermore was visiting the Russian laboratory for a few weeks and on a side trip to Tobolsk, Russia, at a conference honoring the 175th birthday of Dmitri Mendeleev, who made the first periodic table in the 19th century. “How appropriate that we could add a chemical element to the known elements during this time!” reflected Stoyer.
There is always a chance that what they observed was really due to random events or background noise, but according to Stoyer, this was not likely. He calculated that there would be less than a billionth of one percent chance that the signatures they detected were random events.
So what does finding element 117 mean?
For one thing, it fills the only remaining gap in the periodic table, which is now complete from the first element (hydrogen) all the way through element 118. Element 117 will eventually be given a name by the researchers, though the discovery first has to be officially recognized by the International Union of Pure and Applied Chemistry.
Even before it has an official name, what the discovery provides is evidence for the island of stability, helps physicists better understand nuclear structure in general, and should help theorists narrow the range of predictions on those magic numbers.
“These experiments are a real tour de force,” said Yale University physicist Richard Casten, who was not involved with the research, which is described this week in the journal Physical Review Letters (http://physics.aps.org). “It’s an important experimental step that helps pin down interactions that constrain the theories.”
One of the most important findings involved isotopes that were formed as the element 117 they made decayed. As 117 decayed, it turned into first 115, then 113 and finally 111. All these lighter elements had already been created previously in the laboratory, but what was new this time was that they were made with more neutrons than even before — making them far more stable and giving them longer lifetimes.
The newly created element 113, in particular, lasted for about 30 seconds before decaying — about 100 times longer than previous isotopes of the same element created in the same laboratory in Dubna.
“This does verify that as you get closer to this island of stability you have much longer half lives,” said Hamilton.
The added time may also be long enough to be able to do see how these super-heavy elements react with other elements, which would allow scientists to garner information about their chemistry — — something that has never been done with super heavy atoms above element 112 before. Those experiments are ongoing now.
“The ultimate goal is a really comprehensive theory of nuclei — starting from the lightest and going to the heaviest,” said Casten.
The work also points the way forward for synthesizing even heavier elements, like element 120, but Oganessian said they will have to shut down and modify their facilities first. In order to achieve an even higher atomic number, they cannot rely on the calcium beams any longer and that they will have upgrade to more intense beams of titanium, which is slightly heavier than calcium.
Work towards that goal is set to begin this year, said Oganessian when a reporter asked him yesterday what is next for the laboratory, though he demurred in his response.
“Do not ask after a substantial dinner what we would like to have for supper,” he said.
-Jason Socrates Bardi
Inside Science News Service