ELEMENT 117 FROM AN "ISLAND OF STABILITY"

by Nataliya TERYAEVA, Cand. Sc. (Phys. & Math.), journalist

On February 28, 2010, the Flerov Laboratory of Nuclear Reactions of the Joint Institute for Nuclear Research (JINR) completed a long-term experiment on synthesizing a new chemical element, the 117th, in the Mendeleyev Periodic Table. The new element was synthesized in a reaction of calcium-48 accelerated ions with the use of a unique target representing the isotope of the artificial 97th element, berkelium-249, obtained in the HIFR at the world's most powerful nuclear reactor at the Oak Ridge National Laboratory (USA). Nataliya Teryaeva has interviewed the manager of the experiment, Academician Yuri Oganesian.

—Now, here is the most exciting public issue: how will mankind benefit from this discovery of nuclear physicists?

—Since time immemorial people have been asking: Where are the limits of the world? Originally, they thought it to be a disk on the backs of elephants, then in a shape of a dome... Each model complied with the physical and philosophical ideas of a particular epoch. In 1911, when the English physicist Ernest Rutherford proposed a planetary model of the atom*, scientists predicted: the number of chemical elements would be limited to 137. This number resulted from a structural model of the atom, in particular, from the hypothesis that the atomic

* The atomic model proposed by Rutherford reminds the solar system: the positively charged nucleus is in the center, with negatively charged electrons moving around. The charge of the nucleus, numerically equal to the ordinal number in the Mendeleyev Periodic Table, is equilibrated by a sum of electron charges. -Ed.

nucleus is a point-like particle. But when this hypothesis was found not to be true, the ultimate limit of the world of chemical elements started shifting.

By the end of the last century, scientists had discovered 17 artificial elements to see every time that while moving towards heavier elements, their lifetime went down ever faster—the nuclei decayed ever more quickly. Between the 92nd (uranium) to 102nd (nobelium) elements, the half-life of uranium was 16 orders down: from 4.5 bln years to a few seconds. Proceeding from this fact, scientists concluded: even insignificant movements towards heavier elements would lead to the limit of their existence, which will, in fact, be the limit of the material world. This limit was theoretically calculated by way of drawing parallels with a drop of charged liquid (hence the name of the model—a drop model of the nucleus). As we all know, the drop is a small finite size object. An increase in the charge of the drop means that it will most likely break into two parts, which means its destruction thus prohibiting any movement toward heavier elements.

— That is to say the 117th element expanded the limits beyond which mankind could not see before?

—As early as the 1960s physicists of different countries introduced a hypothesis on the existence of a so-called "island of stability"* on the map of atomic nuclei-where no elements presumably exist. According to some theoreticians, this island is "inhabited" by superheavy elements with atomic numbers from 110 to 120, and, possibly, even heavier elements. Scientists calculated that their lifetime should dramatically increase with an increase in the number of neutrons in the nuclei. The most long-living elements have 184 neutrons. Compare: the nucleus of uranium, the heaviest element in our world, has 146 neutrons.

—Does it mean that the experiment on synthesis of the 117th element is a control check on this theory?

-Yes, it does. One more check, but not the last. We have already synthesized superheavy nuclei of new ele-

* See: Ye. Molchanov, "Searching for 'Islands of Stability'", Science in Russia, No. 3, 1999. -Ed.

Map of isotopes with atomic numbers Z≥70; theoretical forecasts of the half-life of hypothetic superheavy atoms are listed for elements with atomic numbers Z≥112 and number of neutrons N≥165.

merits with the atomic numbers from 112 to 116 and element 118 with its nucleus having many excessive neutrons*.

—Did anybody try to check on this hypothesis before you ?

—Yes, sure! Between 1970 to 1985, many well-known laboratories of the world tried to synthesize superheavy nuclei with ever higher lifetime. But all attempts failed—the problem was too hard to solve at one stroke. In 1975-1996, we together with fellow scientists from the particle acceleration centers in Darmstadt (GSI, Germany), Tokyo (RIKEN, Japan), and Berkeley (LBNL, USA), finally managed to synthesize 6 new elements. The heaviest elements—from the 109th to the 112th-were first obtained at GSI, then at RIKEN. Japanese physicists tried to nail down the success: for 240 days running they irradiated a bismuth target with zinc-70 ions and registered two events presumably related to the decay of element 113. But the half-life of the heaviest nuclei obtained in these reactions made up 0.10000 and 0.1000 seconds. For lack of neutrons, they were far away from an "island of stability".

The hypothesis on the existence of superheavy elements was first proved in our Laboratory. I'd like to explain in more detail: this theory could be verified only for nuclei of superheavy elements with many excessive neutrons. By our estimates, such nuclei could conceal a hypothetical "island of stability".

—How did you manage to succeed in what your fellow workers abroad failed?

—We overhauled the system of superheavy nuclei synthesis. Instead of lead and bismuth used as a target material in foreign laboratories, we used targets made of isotopes of artificial transuranium elements characterized by many excessive neutrons. As a "missile" we chose rare and very expensive isotope of element 20— calcium with a mass of 48 and 8 additional neutrons in the nucleus, relative to the main isotope of this element—calcium-40. Physicists working at the Dubna ion accelerator U-400 obtained an intensive beam of this element. Target material—isotopes of plutonium, curium and californium (elements 94, 96 and 98)— was obtained at powerful reactors of the Oak Ridge National Laboratory (USA) and Dimitrovgrad Research and Scientific Institute of Atomic Reactors (Ulyanovsk Region).

The results of experiments conducted in 2000-2004 surpassed even the most optimistic expectations: for 5 years of experiments on the beam of calcium-48 ions we first synthesized superheavy elements with the ato-ic numbers 114, 116, 118 and proved: their lifetime is hundreds and thousands of times longer compared with that of their lighter predecessors. That is to say, we finally landed on the "island" and gained a secure foothold on it (by the way, 5 to 8 years later, the results obtained at the Dubna Laboratory (synthesis of elements 112 and 114) were repeated in other laboratories of the world as well.

We expected the most intriguing results from the synthesis of elements with odd atomic numbers, in particular, elements 113, 115 and 117. Theoreticians forecasted: the 117th element should undergo alpha decay (release a helion) and transform to the 115th element that, in turn, after similar decay should transform to the 113th element. The 111th element should occur only after this chain transformation. In other worlds, we should have observed nuclear transformations in a series of generations of chain decay of a new element until the emission of alpha particles terminates and the nucleus splits into two parts. This moment might mean that we had approached a "coastline" with the "sea of instability"

* See: Yu. Oganesian,  "Nuclear Physics: Alchemy  Materialized?", Science in Russia, No. 1, 2008. -Ed.

ahead. The experiment we are talking about took real shape only after successful completion of works on elements 115 and 113.

—How did you synthesize the 117th element?

—The point is that this element could be synthesized only with an isotope of the artificial 97th element— berkelium-249 with half-life of 320 days—as a target. The problem we faced was that because of a short lifetime like that we had to produce the target material (about 20-30 mg) at a setup with a very high density of a neutron beam. Only the Oak Ridge National Laboratory could cope with that job: by the way, it was there that in 1943 plutonium for the American nuclear bomb of the Manhattan Project was first produced. In addition to Oak Ridge specialists, the Dubna experiment was carried out together with American physicists from ∣the Livermore National Laboratory (California), Vander-bilt University (Tennessee) and our people from the Research and Scientific Institute of Nuclear Reactors. These five groups of Russian and US scientists teamed up fast enough. Because of the time shortage (from the moment of production berkelium dwindles by half in 320 days!) preliminary work had to be completed as soon as possible. Not only physical laboratories were involved: Russian and American governmental bodies in charge of certification of the rare target material, transportation of the highly radioactive product by air and by land, and safety procedures worked fast, too.

— What was the chronology of the experiment?

—Toward the end of December 2008, just 250 days before the experiment, the Oak Ridge Laboratory produced the required quantity of the target material. It "cooled" for three months. Thereupon in two steps 22.2 mg of the berkelium-249 isotope was isolated and cleared of impurities. In early June 2009, a container with it was delivered to Moscow. Then it was transported to Dimitrovgrad, where specialists of the Research and Scientific Institute of Nuclear Reactors manufactured a target— a titanium foil covered with a thin layer of berkelium (300 nm). In July 2009, as many as 6 such targets with a total area of 36 sq. cm. were fixed on a plate rotating at a speed of 1,700 revolutions per minute and taken to Dubna. By that moment all preliminary works had been completed in our Laboratory, and after short-term tests we started continuous irradiation of the target with a dense beam of calcium-48.

— What is the production pattern of the new element's nuclei?

—In the course of irradiation, the nuclei formed in the fusion of berkelium (97th element) and calcium (20th element) are separated from a multitude of reaction byproducts and within one microsecond are fed to a detector where the decay process is registered.

We got the first positive results after the first 70-day irradiation of berkelium-249 isotope target: electronic devices registered an identical picture of the formation and decay of nuclei of the 117th element on 5 occasions. As we had expected, the nuclei released an alpha particle and transformed to nuclei of the 115th element. After the second decay the nuclei transformed to the 113th element, then to the 111th element that underwent spontaneous fission with a half-life of 26 seconds. This is an enormous period of time on a nuclear scale!

—How did you find it was exactly what you expected?

—As each nucleus in the chain of decay consisting of four nuclei was measured by three parameters and registered 5 times during the experiment, any events imitating formation and decay of the 117th element were excluded.

— What new did you learn about the decay products ? —For example, the "grandson" nucleus with the 113^th atomic number turned out to be tenfold as stable as the neighboring isotope obtained earlier in the experiment

on the synthesis of the 115th element. The half-life of the 111th element-the "great-grandson" of the 117th element—is 6,000 times longer compared with its known isotope that has only 3 neutrons less! This difference could have been even greater, if the chain of decays had not been cut short by a spontaneous fission. Our results not only proved the fact that a new element was synthesized, but also demonstrated a significant extension of the lifetime of its decay products while moving towards the peak of a "stability island". Now we can study their chemical properties. I'd like to emphasize: approaching the coveted "stability island" from the right side, we were able to significantly extend our knowledge. It means that our ideas of the nuclear structure have become closer to reality. We still have no clear-cut and definitive theory of nuclear interrelations, as, for example, in the case of electromagnetic interactions. Although we keep expanding our knowledge, we are still far away from a full understanding of the nature of nuclear forces.

—It looks like nuclear physics has much in common with medicine—both learn about the natural laws by experiment...

—A research scientist is satisfied if he sees that a predicted experimental result is confirmed or else—springs a surprise. We have obtained the predicted results, which means we are on the right track.