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MOSCOW, October 28, Vladislav Strekopytov. Physicists from the Joint Institute for Nuclear Research (JINR) in Dubna have synthesized a previously unknown isotope of the 116th element of the periodic table — livermorium-288. This is an important step towards the discovery of the theoretically predicted element with atomic number 120 — unbinilium.
New Element Factory
There are no stable elements in nature with an atomic number (which corresponds to the number of protons in the nucleus) greater than 92, that is, heavier than uranium. All transuranium elements have a short life span. Physicists observe them in nuclear reactors, and superheavy elements (SHE) from 100th and above can only be obtained at accelerators by bombarding target nuclei with heavy ions.
The heaviest element known to date—the 118th—was synthesized at the G.N. Flerov Laboratory of Nuclear Reactions (FLNR) of JINR. He was given the name Oganesson — in honor of the head of the laboratory, Academician Yuri Oganesyan. In total, during the existence of the institute, founded in 1956, ten of the 18 new elements that have since been added to the periodic table were discovered here, and they also managed to repeat the synthesis of all transuranium elements obtained in other countries.
Since 2019, the DC-280 cyclotron has been operating at FLNR JINR — the basic installation of the Superheavy Element Factory (SHE) of the complex for the production of radioactive nuclear beams DRIBs (Dubna Radioactive Ion Beam accelerator complex). The intensity of the accelerated beams of calcium-48 produced at it is 60 trillion ions per second, which is an order of magnitude higher than the indicators achieved at other operating accelerators of this type in the world.
At the SHE Factory, scientists plan to study the chemical properties of previously discovered transuranium elements, and also synthesize new ones — 119th and 120th. For now, they were given the temporary names of ununennium (Uue) and unbinilium (Ubn) — from the Latin combinations “one-one-nine” and “one-two-zero”.
In search of the 120th element
Previously, all superheavy elements from 114th flerovium to 118th oganesson were obtained at FLNR JINR by fusion of calcium-48 nuclei with actinide targets — from plutonium to californium — the heaviest substance that can be produced in quantities sufficient to make a target.
But californium (98th element of the periodic table) when fused with calcium (20th element) forms the already known oganesson. To obtain heavier elements, researchers from Dubna tried to use projectile nuclei with an atomic number higher than that of calcium-48, and attempted to synthesize element 120 by bombarding a plutonium-244 target with iron-58 ions, as well as californium-249 — titanium-50, but all to no avail.
In September, a new experiment started in Dubna. This time, a beam of chromium-54 nuclei was fired at targets made of uranium-238. The result was somewhat unexpected. Instead of unbinilium, physicists obtained a previously unknown isotope of the 116th element of the periodic table — livermorium-288. Its lifetime was just under one millisecond. Despite the short duration of the event and the fact that the synthesis of a new isotope was not the immediate goal of the experiment, scientists consider this a major achievement.
“The observation of the formation of livermorium, even if it remains the only one, is a very good result,” says Scientific Secretary of FLNR JINR, Doctor of Physical and Mathematical Sciences Alexander Karpov. “After its analysis there will be less uncertainty: it will be possible to estimate the cross section and go on an experiment on the 120th element already with open eyes.»
Overcoming obstacles
The difficulty in synthesizing superheavy elements is that when moving from calcium-48 to a heavier bundle of titanium, chromium or iron, the likelihood of nuclear fusion decreases. Obtaining a high-intensity beam of heavy nuclei is also not an easy task. It was partially solved by increasing the power of the equipment. In terms of the efficiency of conducting experiments, the DC-280 is many times superior to the U-400 heavy ion cyclotron previously used at FLNR JINR.
In the current experiment, which will last another month, scientists plan to gain experience working with a beam of chromium nuclei and experimentally determine the cross section for the synthesis of superheavy elements in a reaction with them — that is, to assess the probability of their fusion. Alexander Karpov added that theoretical predictions for the synthesis cross section in reactions with chromium vary greatly, so it is extremely important to determine the cross section experimentally.
«The 116th element was synthesized by the fusion of calcium and curium nuclei. The cross section of this reaction is known. And now it is underway experiment on the synthesis of isotopes of the 116th element, but in the reaction chromium — uranium. We changed the target and the projectile nucleus, but the element is the same. By doing this experiment, we can compare the cross sections, and this is extremely important in preparation for the synthesis of the 120th element,» the scientist explained.
Physicists from FLNR JINR also consider the reaction of interaction between chromium and a curium target to be promising, which they plan to test in the future.
September 4, 08:00
Experiments continue
Over the three years of operation of the SHE Factory, JINR scientists discovered five more previously unknown isotopes of superheavy elements. At the very beginning of the installation, lawrencium-264 was obtained in the reaction of calcium-48 with americium-243. The lifetime of the new nuclide was about five hours, which is very long for such a heavy element. Following this, the shorter-lived (20 milliseconds) moscovium-286 was synthesized. And in 2022, using in practice the still unstudied reaction of calcium-48 with a target of thorium-232, they obtained three new isotopes at once — darmstadtium-276, hassium-272 and seaborgium-268. It turned out that darmstadtium-276, in a fraction of a millisecond, undergoes alpha decay and turns into hassium-272, which, in turn, after a hundred milliseconds, turns into seaborgium-268. The latter is capable of living for ten to fifteen seconds.
The main task of the experiments at the STE Factory is preparation for the synthesis of the 120th element and its implementation. When this goal is achieved, we can talk about new horizons of research. Rutherford once assumed that there are atoms with the number of protons in the nucleus equal to 170 or even more.