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The synthesis of element 114 confirmed decades-old theoretical predictions 
of a little patch of nuclear stability in a sea of short-lived superheavy nuclei


by Yuri Ts. Oganessian, Vladimir K. Utyonkov and Kenton J. Moody
  Original: Scientific American


   
Yuri Tsolakovich Oganessian
Scientific Leader of the 
Flerov Laboratory of Nuclear Reactions
  Vladimir Klimentyevich Utyonkov
Flerov Laboratory of Nuclear Reactions
Head of the sector
 
  Kenton J. Moody
Lawrence Livermore National Laboratory

The creation of the element neptunium in the spring of 1940 launched chemists on a fascinating journey into uncharted terrain. In this transuranic world, atoms whose nuclei have more than the 92 protons of a uranium nucleus exhibit unusual or unique properties. With their large numbers of electrons, these heavy elements have given chemists invaluable insights into the arrangement of electrons in atoms and into chemical bonding. The elements have also found uses in technologies ranging from nuclear weapons to smoke detectors.
So far this research has produced 23 new elements with atomic nuclei that have more protons than uranium atoms do. Of those 23, only the "lightest" two —neptunium and plutonium — exist at all in nature.
Recently the creation of element 114 marked the end of a difficult leg of the grand transuranic voyage. The passage was like a perilous crossing of a sea of instability made up of elements with more than 106 protons in their nuclei. 

DUSAN PETRICIC
      DUSAN PETRICIC

By bombarding heavy nuclei with ion beams of lighter nuclei, scientists create superheavy nuclei that are so unstable that they split apart, oftentimes only a tiny fraction of a second after they are created. As they approached the "magic" number of 114, however, researchers found themselves coming to an island of stability where a collection of even heavier synthetic elements exhibit surprising stability and longevity.
Like the fabled city of El Dorado, the island of stability was long believed to exist but was considered impossible to reach; nuclear physicists had theorized about it as early as 1966. But unlike El Dorado, the whereabouts of the island of stability was no secret: its central, most stable point was predicted to be an isotope of element 114 with 184 neutrons, surrounded by neighboring but somewhat less stable elements between 109 and 115. Physicists knew exactly where they needed to go; the problems were how to get there and how to know when they had arrived.
The first attempts to synthesize an element occurred in 1934, when scientists began bombarding the nuclei of heavy elements with streams of neutrons. Each neutron captured by the target atom's nucleus underwent beta decay, changing into one proton and one electron, creating an element that had one more proton in its nucleus than the target nucleus had. In chemical terminology the created element had an atomic number that was greater, by one, than that of the target element. An element's atomic number is merely a tally of the protons in the nuclei of its atoms. The number defines an element and its place in the periodic table. Besides protons, atomic nuclei also contain neutrons, which carry no charge. All atoms of a single element must have the same number of protons, but different "isotopes" of the element have different numbers of neutrons and different degrees of stability.
By the mid-1950s researchers had produced elements 93, 94, 99 and 100 in this way. During the same period, they created elements 95, 96, 97, 98 and 101 by irradiating heavy nuclei with streams of alpha particles, which are helium nuclei, and boosted the atomic numbers two steps at a time.
The development of particle accelerators allowed scientists to direct high-intensity beams of ions of light elements such as boron (atomic number 5) at the nuclei of elements with atomic numbers between 94 and 98, to cause fusion of the two. For fusion to take place, the two nuclei must collide with enough energy to overcome the electrostatic force that causes the positively charged protons in each nucleus to repel each other. A great deal of energy is needed for this to occur, resulting in a new nucleus that is very hot. That heat, in turn, increases the likelihood that the new element will fission rather than "relax" into a stable state and remain intact. This technique yielded elements 102 through 106 between 1958 and 1974.

YURI GRIPAS Gamma Liaison
      YURI GRIPAS Gamma Liaison

RESEARCH TEAM at the Joint Institute for Nuclear Research in Dubna, Russia, posed near its experimental setup; 
the mass separator is in the top left corner of the photograph, and the protruding arms of the target apparatus 
are visible toward the right. The group includes authors Utyonkov (second from left) and Oganessian (fourth from left).

Above 106, the" tendency to fission made it impossible to synthesize new elements.
Then, in 1974, one of the authors (Oganessian) at the Joint Institute for Nuclear Research (JINR) in Dubna, Russia, discovered that using a beam of heavier ions to bombard lighter target nuclei could produce nuclei with lower excitation energies, allowing them to undergo fusion and remain intact. This is known as "cold fusion" but should not be confused with the discredited process of the same name that was widely publicized during the 1980s. Active research using this approach began after the 1975 opening of the UNILAC (Universal Linear Accelerator) at the Society for Heavy Ion Research in Darmstadt, Germany, which can accelerate even very heavy ions at varying energies.

Unfortunately, because so few nuclei of the desired new elements  are  produced during an experiment,  and  because the "daughter" nuclei resulting from the decay of the new element themselves decay so quickly that they must be detected while the synthesis process is still under way, existing methods could not detect elements produced by this new technique. Thus, no new elements were identified for several years.
In the early 1980s a research team at the Darmstadt facility developed a sophisticated and sensitive method for identifying the new fusion nuclei and was able to synthesize elements 107, 108 and 109. The barriers to synthesis and detection were enormous — the researchers had to operate the UNILAC for two weeks to produce a single atom of element 109. Nevertheless, by further adjusting the intensity of the ion beam and enhancing the sensitivity of the sensing devices, the same team was able to produce element 111 in 1994 and element 112 in 1996. Element 112 has a half-life of 240 microseconds, and only two atoms of it were produced in 25 days.
Since 1994, research groups in Germany, the U.S. and Russia have added six new elements to the periodic table, with atomic numbers as high as 118.
The most important synthesis has been the production of isotopes of element 114, which has conclusively demonstrated the existence of the island of stability. Reaching the island is so significant because it demonstrates the theoretical prediction that certain "magic" numbers of protons and neutrons result in especially tightly bound nuclei, stable closed shells similar to the configurations of electrons associated with filled atomic orbital shells that give the noble gases their inert chemical behavior and define the periodicity and reactivity of the chemical elements.
The known magic numbers of the nuclear shell model, which occur at the element lead (82 protons and 126 neutrons), among other places in the periodic table, were derived as early as 1948. The prediction of the next magic numbers at 114 protons and 184 neutrons in 1966 contradicted prevailing theory: at that time, it was expected that the decay half-lives of synthesized elements, along with their stability against fission, would decrease catastrophically as their nuclei grew heavier. The prediction of the magic numbers gave rise to the speculation that there was an island of unusually long-lived nuclear species well out in the uncharted sea of instability.
One reason the island is so difficult to reach is that these nuclei have more neutrons per proton than any known stable nuclei. We chose a reaction that introduced the most neutrons into the synthesized nuclei: irradiating Pu-244 (plutonium 244) — the exotic heaviest isotope of plutonium — with an intense stream of ions of Ca-48 (calcium 48) — a rare and expensive neutron-rich isotope of calcium. Our expectation was that the fusion would result in a compound nucleus with 114 protons and 178 neutrons. Such an isotope would be as close as possible to the doubly magic configuration of 114 protons and 184 neutrons. 
We knew that if the Ca-48 and Pu-244 nuclei collided with enough energy to overcome their mutual electrostatic repulsion, the excitation energy of the resulting compound nuclei would be low enough that at least some of them would not fission, because the evaporation of three neutrons — resulting in an isotope of element 114 with 175 neutrons — would cool the new nuclei below the fission barrier.

 OCEAN OF LARGE NUCLEI contains  many unstable specimens, or isotopes. An element's atomic nucleus has a set number of protons but typically has a variety of versions, called isotopes, each with a different number of neutrons and degree of stability. For nuclei with upward of 106 protons, especially, many of these isotopes are relatively unstable and are like a "sea of instability." In this metaphor, an unusually stable isotope of lead, with 82 protons and 126 neutrons, is a kind of magic mountain. And since 1966 researchers have theorized about an island of stability, at the center of which would be an isotope with 114 protons and 184 neutrons. Chemists recently reached the shores of the island by creating an isotope with 114 protons and 175 neutrons.

 
The decay properties of this isotope hint that the superheavy elements may be even more stable than predicted.
 
 

How We Did It


Previous searches for superheavy elements using similar reactions were unsuccessful because as few as one of these nuclei is produced over a period of a few weeks amid a background of trillions of other nuclear species. By increasing the sensitivity of our detection method by hundreds of times over those used in previous attempts, we were able to detect the newly synthesized elements before they decayed.
We performed our experiment at the heavy-ion cyclotron at JINR. Ions of Ca-48 were accelerated to approximately one tenth the speed of light and beamed at the target, which consisted of several milligrams of Pu-244 electroplated onto thin titanium foils.
To detect the new fusion nuclei we were trying to synthesize, we needed a means of separating the products in which we were interested from all others produced by the experiment. The expected signature of the decay of our superheavy nucleus would be a series of alpha decays as element 114 decays to element 112, which decays to element 110, which decays to element 108, until the island of stability is left behind and spontaneous fission occurs. Unfortunately, the alpha decay and fission rates from the decays of unwanted nuclei also generated by the experiment can produce sequences of random events that can mimic the decay sequence of element 114. Billions of these unwanted nuclei are produced per second, whereas the expected production rate for the element 114 isotope is far less than one atom per day. Consequently, it is vitally important to suppress these unwanted background reactions so as to recognize the element 114 reaction when it occurs.  To do this, the scientists at Dubna devised a gas-filled separator, which gave us effective transmission of the products we were seeking and very efficient detection of the radioactive decay sequences that would reveal their presence; it also effectively suppressed unwanted products. Heavy-ion fusion products (a mixture of the synthesized 114 nuclei and other fusion products) recoil from the target and enter a chamber filled with low-pressure hydrogen gas, which is confined between the pole faces of a dipole magnet. The recoiling heavy ions interact with the hydrogen gas atoms, and those whose electrons are bound to their nuclei with less energy than that supplied by the collision tend to be lost. The magnetic field is adjusted so that only the nuclei of interest will arrive at the detector array. Unreacted beam particles of Ca-48 pass into the hydrogen at high velocity and are so highly ionized that the magnetic field diverts them from the path of the particles we are seeking. The gas-filled separator also strongly suppresses other unwanted products of peripheral nuclear reactions.
 

 YURI GRIPAS Gamma Liaison
YURI GRIPAS Gamma Liaison

Only one atom of the new superheavy element was made in 40 days of irradiation.

Reaction products leaving the dipole magnets are focused with a set of magnetic quadrupoles, then pass through a time-of-flight (TOP) counter and bury themselves in a position-sensitive detector. The signal from the TOP counter enabled us to distinguish between the impact of products passing through the separator and the radioactive decay of products that are already implanted in the detector. The flight time through the TOP counter can be used to discriminate between low and high atomic numbers. In addition, the position-sensitive detector lowered the rate of background interference because it allowed us to identify and ignore unwanted reactions.

All of these capabilities permitted us to detect and measure the element 114 nuclei we sought.
We performed our first experiment over a period of 40 days in November and December of 1998. During that time, we observed the signals of a total of three spontaneous fission decays, indicating that three synthesized compound nuclei had been created and had passed through the separator before fissioning. Two of them lasted about one millisecond each and were unwanted reactions caused by the decay of the nucleus of Am-244 (americium 244). Only one of these events (one atom in 40 days of irradiation!) involved an implant in the detector followed by three alpha decays (the successive loss of two protons and two neutrons, each loss resulting in decay to a lesser element with a lower atomic number), all occurring at the same position in the detector array.
This is exactly the decay signature we had expected: the relations between the decay energies and the decay times were consistent with what had been theorized for the decay of an isotope of element 114 and its resulting daughter elements. The flight time of the initial recoil nucleus and its implantation energy in the detector were also consistent with predictions, and the random rates in the detector indicate less than a 1 percent chance that the event was caused by random correlations of background events.

The Decay Chain


The isotope of element 114 with 175 neutrons has a half-life of 30.4 seconds. It decays to element 112; 112, with a half-life of 15.4 minutes, decays to element 110; 110, with a half-life of 1.6 minutes, then decays to element 108. The element 108 isotope, with 169 neutrons, is off the edge of the island of stability and decays by spontaneous fission. A subsequent experiment performed at Dubna produced a lighter isotope of element 114 with 173 neutrons, which is closer to the edge of the island of stability. This lighter isotope has a half-life of about five seconds and then undergoes alpha decay into its daughter nucleus, element 112; the daughter nucleus decays by spontaneous fission in three minutes.

 LAURIE GRACE
LAURIE GRACE

DUBNA LABORATORY (left) creates beams that move from right to left in this photograph. To the left of this view was the gas-filled mass separator (above). Beams of reaction products from the plutonium target are bent by the dipole magnet; nuclei of interest are directed to an array of tiny silicon detectors. Researchers associate nuclear decays there with positions to determine which isotopes were created at the target.

We have confirmed the existence of the island of stability and have a measure of the magnitude of its effect. The lifetime of our element 114 isotope with 175 neutrons is more than 1,000 times longer than the lifetime of the 174-neutron isotope, which is produced as part of the decay chain from element 118 and was recently discovered at Lawrence Berkeley National Laboratory. Our isotope of element 112 with 173 neutrons is more than a million times longer-lived than the isotope with 165 neutrons, discovered in Darmstadt in 1996. These longer half-lives of our synthesized fusion products make it far easier to study them, and such studies may change the way we look at the fundamental properties of matter.

 Recently we also created the isotope of element 114 that has 174 neutrons (so far we have made a grand total of two atoms of it). The decay properties of this isotope suggest, tantalizingly, that the superheavy elements may be even more stable than predicted by theory.  Based on our experiment and the work of others, the future looks bright for the study of the limits of nuclear stability, with many prospects for new research and unexpected discoveries. With a concerted effort, we may even be able to solve for element 114 one of the traditional and most challenging difficulties of superheavy-element synthesis: finding a name for the new element that all interested parties can agree on!

The Authors

YURI TS. OGANESSIAN, VLADIMIR K. UTYONKOV and KENTON J. MOODY have been collaborating since 1989 on the creation of heavy elements. Oganessian, a physicist, is scientific director of the Flerov Laboratory of Nuclear Reactions at the Joint Institute for Nuclear Research (JINR) at Dubna, near Moscow. He graduated from the Moscow Physics and Engineering Institute in 1956 and since then has pursued research in the fields of nuclear physics and nuclear chemistry. Utyonkov graduated from the Moscow Engineering Physical Institute in 1978 and joined the scientific staff of Flerov Laboratory. Since 1997 he has been deputy head of the JINR research group, investigating the synthesis and properties of heavy nuclei. Moody received his Ph.D. in nuclear chemistry at the University of California, Berkeley, in 1983. Since 1985 he has worked in the Analytical and Nuclear Chemistry Division of Lawrence Livermore National Laboratory.

Further Information 

Search for the Missing Elements. Glenn T. Seaborg and Walter Loveland in New Scientist, Vol. 131, No. 1784, page 29; August 1991.
Transuranium Elements: A Half Century. Edited by Lester R. Morss and J. Fuger. American Chemical Society, 1992.
Making New Elements. Peter Armbruster and Fritz Peter Hessberger in Scientific American, Vol. 279, No. 3, pages 72-77; September 1998.
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