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Superheavy elements on the stability island

Theoretical and experimental study of the stability of the nucleus gave Soviet physicists a reason to revise the previously used methods for producing heavy transuraniums. In Dubna they decided to take new paths and target lead And bismuth.

The nucleus, like the atom as a whole, has shell structure. Particularly stable are atomic nuclei containing 2-8-20-28-50-82-114-126-164 protons (that is, atomic nuclei with the same atomic number) and 2-8-20-28-50-82-126- 184-196-228-272-318 neutrons, due to the complete structure of their shells. Only recently was it possible to confirm these views by computer calculations.

This unusual stability caught my eye, first of all, when studying the prevalence of certain elements in space. Isotopes, possessing these nuclear numbers are called magic. The bismuth isotope 209Bi, which has 126 neutrons, is such a magic nuclide. This also includes isotopes oxygen, calcium, tin. Twice magic are: for helium - the isotope 4 He (2 protons, 2 neutrons), for calcium - 48 Ca (20 protons, 28 neutrons), for lead - 208 Pb (82 protons, 126 neutrons). They are distinguished by a very special core strength.

Using ion sources of a new type and more powerful heavy ion accelerators - U-200 and U-300 units were paired in Dubna, the group of G. N. Flerov and Yu. Ts. Oganesyan soon began to have flow of heavy ions with extraordinary energy. To achieve nuclear fusion, Soviet physicists fired chromium ions with an energy of 280 MeV at targets made of lead and bismuth. What could have happened? At the beginning of 1974, nuclear scientists in Dubna recorded 50 cases of such bombing, indicating formation of element 106, which, however, decays after 10 -2 s. These 50 atomic nuclei were formed according to the scheme:

208 Pb + 51 Cr = 259 X

A little later, Ghiorso and Seaborg of the Lawrence Berkeley Laboratory reported that they had synthesized an isotope of a new 106 -th, element with mass number 263 by bombarding californium-249 with oxygen ions in the Super-HILAC apparatus.

By the end of 1976, the Dubna nuclear reaction laboratory completed a series of experiments on the synthesis of element 107; served as a starting substance for the Dubna “alchemists” magical"bismuth-209. When bombarded with chromium ions with an energy of 290 MeV, it turned into an isotope 107 -th element:

209 Bi + 54 Cr = 261 X + 2 n

Element 107 decays spontaneously with a half-life of 0.002 s and also emits alpha particles.

The half-lives of 0.01 and 0.002 s found for the 106th and 107th elements made us wary. After all, they turned out to be several orders of magnitude larger than predicted by computer calculations. Perhaps the 107th element was already noticeably influenced by the proximity of the subsequent magic number of protons and neutrons - 114, increasing stability?
If this is so, then there was hope to obtain long-lived isotopes of element 107, for example, by shelling Berkeley neon ions. Calculations showed that the neutron-rich isotope formed by this reaction would have a half-life exceeding 1 s. This would make it possible to study the chemical properties of element 107 - ecarenia.

The longest-lived isotope of the first transuranium, element 93, neptunium-237, has a half-life of 2,100,000 years; the most stable isotope of element 100, fermium-257, lasts only 97 days. Starting from element 104 half-lives are only fractions of a second. Therefore, there seemed to be absolutely no hope of discovering these elements. Why is further research needed?

Albert Ghiorso, a leading US specialist on transuraniums, once spoke in this regard: " The reason for continuing to search for further elements is simply to satisfy human curiosity - what is happening around the next corner of the street?“However, this, of course, is not just scientific curiosity. Ghiorso still made it clear how important it is to continue such fundamental research.

In the 60s, the theory of magic nuclear numbers became increasingly important. In the "sea of ​​instability" scientists desperately tried to find a life-saving " island of relative stability", on which the foot of an atomic explorer could firmly rest. Although this island has not yet been discovered, its "coordinates" are known: element 114, ekas lead, is considered the center of a large stability region. Isotope 298 of element 114 has long been a particular subject of scientific debate because, with 114 protons and 184 neutrons, it is one of those doubly magical atomic nuclei predicted to last for a long time. However, what does long-term existence mean?

Preliminary calculations show: the half-life with the release of alpha particles ranges from 1 to 1000 years, and in relation to spontaneous fission - from 10 8 to 10 16 years. Such fluctuations, as physicists point out, are explained by the approximation of “computer chemistry.” Very encouraging half-lives are predicted for the next island of stability - element 164, dvislead. The isotope of element 164 with mass number 482 is also doubly magical: its nucleus is formed by 164 protons and 318 neutrons.

Science is interesting and simple magical superheavy elements, such as isotope-294 of element 110 or isotope-310 of element 126, containing 184 neutrons. It’s amazing how researchers quite seriously juggle these imaginary elements, as if they already exist. More and more new data is being extracted from the computer and it is now definitely known what properties - nuclear, crystallographic and chemical - these superheavy elements must have. The specialized literature is accumulating precise data for elements that people will perhaps discover in 50 years.

Atomic scientists are currently navigating a sea of ​​instability, awaiting discoveries. Behind them was solid ground: a peninsula with natural radioactive elements, marked by hills of thorium and uranium, and a far-reaching solid ground with all the other elements and peaks lead, tin And calcium.
Brave sailors have been on the high seas for a long time. In an unexpected place, they found a sandbank: open elements 106 and 107 were more stable than expected.

In recent years, we have been sailing for a long time on a sea of ​​instability, argues G. N. Flerov, and suddenly, at the last moment, we felt the ground under our feet. Random underwater rock? Or a sandbank of a long-awaited island of stability? If the second is correct, then we have a real opportunity to create a new periodic system of stable superheavy elements with amazing properties.

After the hypothesis about stable elements near serial numbers 114, 126, 164 became known, researchers around the world pounced on these " super heavy"atoms. Some of them, with presumably long half-lives, were hoped to be found on Earth or in Space, at least in the form of traces. After all, when our Solar system arose, these elements existed just like all the others.

Traces of superheavy elements- what should be understood by this? As a result of their ability to spontaneously fission into two nuclear fragments with great mass and energy, these transurans would have to leave distinct traces of destruction in the surrounding matter.
Similar traces can be seen in minerals under a microscope after they have been etched. Using this method of destruction traces, it is now possible to trace the existence of long-dead elements. From the width of the traces left, one can also estimate the ordinal number of the element - the width of the track is proportional to the square of the nuclear charge.
They also hope to identify “living” superheavy elements based on the fact that they repeatedly emit neutrons. During the spontaneous fission process, these elements emit up to 10 neutrons.

Traces of superheavy elements were searched for in manganese nodules from the depths of the ocean, as well as in waters after the melting of glaciers in the polar seas. Still no results. G. N. Flerov and his colleagues examined the lead glass of an ancient showcase from the 14th century, a Leyden jar from the 19th century, and a lead crystal vase from the 18th century.
At first, several traces of spontaneous fission indicated ekas lead- 114th element. However, when the Dubna scientists repeated their measurements with a highly sensitive neutron detector in the deepest salt mine of the Soviet Union, they did not get a positive result. Cosmic radiation, which apparently caused the observed effect, could not penetrate to such a depth.

In 1977, Professor Flerov suggested that he had finally discovered " signals of new transuranium" while studying the deep thermal waters of the Cheleken Peninsula in the Caspian Sea.
However, the number of reported cases was too small for a clear classification. A year later, Flerov’s group registered 150 spontaneous divisions per month. These data were obtained while working with an ion exchanger filled with unknown transuranium from thermal waters. Flerov estimated the half-life of the element present, which he had not yet been able to isolate, to be billions of years.

Other researchers took different paths. Professor Fowler and his colleagues from the University of Bristol undertook experiments with balloons at high altitude. Using detectors of small amounts of nuclei, numerous areas with nuclear charges exceeding 92 were identified. English researchers believed that one of the traces even pointed to elements 102...108. Later they made an amendment: the unknown element has serial number 96 ( curium).

How do these superheavy particles get into the stratosphere of the globe? Several theories have been put forward so far. According to them, heavy atoms should appear during supernova explosions or other astrophysical processes and reach the Earth in the form of cosmic radiation or dust - but only after 1000 - 1,000,000 years. These cosmic deposits are currently being sought both in the atmosphere and in deep marine sediments.

So, superheavy elements can be found in cosmic radiation? True, according to the American scientists who undertook the Skylab experiment in 1975, this hypothesis was not confirmed. In a space laboratory that orbited the Earth, detectors were installed that absorb heavy particles from space; were only discovered tracks of known elements.
Lunar dust brought to Earth after the first lunar landing in 1969 was no less carefully examined for the presence of superheavy elements. When traces of “long-lived” particles up to 0.025 mm were found, some researchers believed that they could be attributed to elements 110 - 119.

Similar results were obtained from studies of the anomalous isotopic composition of the noble gas xenon contained in various meteorite samples. Physicists have expressed the opinion that this effect can only be explained by the existence of superheavy elements.
Soviet scientists in Dubna, who analyzed 20 kg of the Allende meteorite, which fell in Mexico in the fall of 1969, were able to detect several spontaneous fissions as a result of three months of observation.
However, after it was established that "natural" plutonium-244, which was once an integral part of our solar system, leaves completely similar traces, the interpretation began to be carried out more carefully.

At the end of the second millennium, academician Vitaly Lazarevich Ginzburg compiled a list of thirty problems in physics and astrophysics that he considered the most important and interesting (see “Science and Life” No. 11, 1999). In this list, number 13 indicates the task of finding superheavy elements. Then, 12 years ago, the academician noted with disappointment that “the existence of long-lived (we are talking about millions of years) transuranium nuclei in cosmic rays has not yet been confirmed.” Today traces of such nuclei have been discovered. This gives hope to finally discover the Island of Stability of superheavy nuclei, the existence of which was once predicted by nuclear physicist Georgy Nikolaevich Flerov.

The question of whether there are elements heavier than uranium-92 (238 U is its stable isotope) remained open for a long time, since they were not observed in nature. It was believed that there were no stable elements with an atomic number greater than 180: the powerful positive charge of the nucleus would destroy the internal levels of the electrons of a heavy atom. However, it soon became clear that the stability of an element is determined by the stability of its core, and not the shell. Nuclei with an even number of protons Z and neutrons N are stable, among which nuclei with the so-called magic number of protons or neutrons - 2, 8, 20, 28, 50, 82, 126 - are especially prominent - for example, tin, lead. And the most stable are “doubly magic nuclei”, in which the number of both neutrons and protons is magic, say, helium and calcium. This is the lead isotope 208 Pb: it has Z = 82, N = 126. The stability of the element extremely depends on the ratio of the number of protons and neutrons in its nucleus. For example, lead with 126 neutrons is stable, but its other isotope, which has one more neutron in its nucleus, decays in more than three hours. But, noted V.L. Ginzburg, the theory predicts that a certain element X with the number of protons Z = 114 and neutrons N = 184, that is, with a mass atomic number A = Z + N = 298, should live approximately 100 million years.

Today, many elements have been artificially obtained up to and including the 118th - 254 Uuo. It is the heaviest non-metal, presumably an inert gas; its conventional names are ununoctium (it is formed from the roots of the Latin numerals - 1, 1, 8), eka-radon and moscovian Mw. All man-made elements once existed on Earth, but have decayed over time. For example, plutonium-94 has 16 isotopes, and only 244 Pu has a half-life T ½ = 7.6 10 7 years; neptunium-93 has 12 isotopes and 237 Np T ½ = 2.14 10 6 years. These longest half-lives among all isotopes of these elements are much less than the age of the Earth - (4.5–5.5) 10 9. Insignificant traces of neptunium, which are found in uranium ores, are products of nuclear reactions under the influence of neutrons from cosmic radiation and the spontaneous fission of uranium, and plutonium is a consequence of the beta decay of neptunium-239.

Elements that have disappeared during the existence of the Earth are obtained in two ways. Firstly, an extra neutron can be driven into the nucleus of a heavy element. There it undergoes beta decay, forming a proton, an electron and an electron antineutrino: n 0 → p + e – + v e. The nuclear charge will increase by one - a new element will appear. This is how artificial elements were obtained up to fermium-100 (its isotope 257 Fm has a half-life of 100 years).

Even heavier elements are created in accelerators, which accelerate and collide nuclei, for example gold (see “Science and Life” No. 6, 1997). This is exactly how the 117th and 118th elements were obtained in the laboratory of nuclear reactions of the Joint Institute for Nuclear Research (JINR, Dubna). Moreover, the theory predicts that stable superheavy nuclei should exist far beyond the currently known heavy radioactive elements. Russian physicist G. N. Flerov depicted the system of elements as a symbolic archipelago, where stable elements are surrounded by a sea of ​​short-lived isotopes that may never be discovered. On the main island of the archipelago, there are peaks of the most stable elements - Calcium, Tin and Lead; beyond the Strait of Radioactivity lies the Island of Heavy Nuclei with peaks of Uranium, Neptunium and Plutonium. And even further away there should be a mysterious island of Stability of superheavy elements, similar to the already mentioned - X-298.

Despite all the successes of experimental and theoretical physics, the question remains open: do superheavy elements exist in nature, or are they purely artificial, man-made substances, similar to synthetic materials - nylon, nylon, lavsan - never created by nature?

There are conditions for the formation of such elements in nature. They are created in the depths of pulsars and during supernova explosions. The neutron fluxes in them reach a huge density - 10 38 n 0 / m 2 and are capable of generating superheavy nuclei. They scatter through space in a stream of intergalactic cosmic rays, but their share is extremely small - only a few particles per square meter per year. Therefore, the idea arose to use a natural detector-storage of cosmic radiation, in which superheavy nuclei should leave a specific, easily recognizable trace. Meteorites have successfully served as such detectors.

A meteorite - a piece of rock torn out of its mother planet by some cosmic catastrophe - travels through space for hundreds of millions of years. It is continuously “fired” by cosmic rays, which consist of 90% hydrogen nuclei (protons), 7% helium nuclei (two protons) and 1% electrons. The remaining 2% consists of other particles, which may include superheavy nuclei.

Researchers from the Physical Institute named after. P. N. Lebedev (FIAN) and the Institute of Geochemistry and Analytical Chemistry named after. V.I. Vernadsky (GEOKHI RAS) are studying two pallasites - iron-nickel meteorites interspersed with olivine (a group of translucent minerals in which Mg 2, (Mg, Fe) 2 and (Mn, Fe) 2 are added to silicon dioxide SiO 4 in different proportions ; transparent olivine is called chrysolite). The age of these meteorites is 185 and 300 million years.

Heavy nuclei, flying through an olivine crystal, damage its lattice, leaving their traces in it - tracks. They become visible after chemical treatment of the crystal - etching. And since olivine is translucent, these tracks can be observed and studied under a microscope. By the thickness of the track, its length and shape, one can judge the charge and atomic mass of the nucleus. Research is greatly complicated by the fact that olivine crystals have dimensions of the order of several millimeters, and the track of a heavy particle is much longer. Therefore, the magnitude of its charge must be judged by indirect data - the etching rate, a decrease in the track thickness, etc.

The work to find traces of superheavy particles from the island of stability was called “Project Olympia.” As part of this project, information was obtained on approximately six thousand nuclei with a charge of more than 55 and three ultra-heavy nuclei, the charges of which lie in the range from 105 to 130. All characteristics of the tracks of these nuclei were measured by a complex of high-precision equipment created at the Lebedev Physical Institute. The complex automatically recognizes tracks, determines their geometric parameters and, extrapolating measurement data, finds the estimated length of the track before it stops in the olivine massif (remember that the actual size of its crystal is several millimeters).

The experimental results obtained confirm the reality of the existence of stable superheavy elements in nature.

A century and a half ago, when Dmitry Ivanovich Mendeleev discovered the Periodic Law, only 63 elements were known. Arranged in a table, they were easily laid out into periods, each of which opens with active alkali metals and ends (as it turned out later) with inert noble gases. Since then, the periodic table has almost doubled in size, and with each expansion the Periodic Law has been confirmed again and again. Rubidium is also reminiscent of potassium and sodium, as xenon is of krypton and argon; below carbon is silicon, which is much similar to it... Today it is known that these properties are determined by the number of electrons rotating around the atomic nucleus.

They fill the “energy shells” of the atom one after another, like spectators occupying their seats in order in a theater: the one who is last will determine the chemical properties of the entire element. An atom with a completely filled last shell (like helium with its two electrons) will be inert; an element with one “extra” electron on it (like sodium) will actively form chemical bonds. The number of negatively charged electrons in orbits is related to the number of positive protons in the nucleus of an atom, and it is the number of protons that distinguishes different elements.

But there can be different numbers of neutrons in the nucleus of the same element; they have no charge, and they do not affect the chemical properties. But depending on the number of neutrons, hydrogen may turn out to be heavier than helium, and the mass of lithium may reach seven instead of the “classical” six atomic units. And if the list of known elements today is approaching 120, then the number of nuclei (nuclides) has exceeded 3000. Most of them are unstable and after some time decay, releasing “extra” particles during radioactive decay. Even more nuclides are unable to exist in principle, instantly falling apart into pieces. Thus, a continent of stable nuclei is surrounded by a whole sea of ​​unstable combinations of neutrons and protons.

Sea of ​​Instability

The fate of the nucleus depends on the number of neutrons and protons in it. According to the shell theory of the structure of the nucleus, put forward back in the 1950s, the particles in it are distributed among their energy levels in the same way as electrons that rotate around the nucleus. Some numbers of protons and neutrons give particularly stable configurations with completely filled proton or neutron shells - 2, 8, 20, 28, 50, 82, and for neutrons there are also 126 particles. These numbers are called “magic” numbers, and the most stable nuclei contain “twice-magic” numbers of particles—for example, lead has 82 protons and 126 neutrons, or two each in a normal atom of helium, the second most abundant element in the universe.

The successive "Chemical Continent" of elements found on Earth ends with lead. It is followed by a series of nuclei that exist much less than the age of our planet. In its depths they can be preserved only in small quantities, like uranium and thorium, or even in trace amounts, like plutonium. It is impossible to extract it from the rock, and plutonium is produced artificially, in reactors, bombarding a uranium target with neutrons. In general, modern physicists treat atomic nuclei as construction parts, forcing them to attach individual neutrons, protons or entire nuclei. This makes it possible to obtain heavier and heavier nuclides by crossing the strait of the “Sea of ​​Instability”.

The purpose of the journey is suggested by the same shell theory of the structure of the nucleus. This is the region of superheavy elements with a suitable (and very large) number of neutrons and protons, the legendary “Island of Stability”. Calculations say that some of the local “residents” may no longer exist for fractions of microseconds, but for many orders of magnitude longer. “To a certain approximation, they can be considered as droplets of water,” RAS Academician Yuri Oganesyan explained to us. — Up to lead, the nuclei are spherical and stable. Following them is a peninsula of moderately stable nuclei - such as thorium or uranium - which is stretched out by a shoal of highly deformed nuclei and breaks into an unstable sea... But even further, beyond the strait, there may be a new region of spherical nuclei, superheavy and stable elements with numbers 114, 116 and further." The lifetime of some elements on the “Island of Stability” can last for years, or even millions of years.

Island of Stability

Transuranic elements with their deformed nuclei can be created by bombarding targets made of uranium, thorium or plutonium with neutrons. By bombarding them with light ions accelerated in an accelerator, you can successively obtain a number of even heavier elements - but at some point the limit will come. “If we consider different reactions—the addition of neutrons, the addition of ions—as different “ships,” then all of them will not help us sail to the “Island of Stability,” continues Yuri Oganesyan. — This will require a larger “vessel” and a different design. The target would have to be neutron-rich heavy nuclei of artificial elements heavier than uranium, and they would have to be bombarded with large, heavy isotopes containing many neutrons, such as calcium-48.”

Only a large international team of scientists could work on such a “ship”. Engineers and physicists at the Elektrokhimpribor plant isolated from natural calcium the extremely rare 48th isotope, which is contained here in an amount of less than 0.2%. Targets from uranium, plutonium, americium, curium, californium were prepared at the Dimitrograd Research Institute of Atomic Reactors, at the Livermore National Laboratory and at the Oak Ridge National Laboratory in the USA. Well, key experiments on the synthesis of new elements were carried out by Academician Oganesyan at the Joint Institute of Nuclear Physics (JINR), at the Flerov Laboratory of Nuclear Reactions. “Our accelerator in Dubna worked 6-7 thousand hours a year, accelerating calcium-48 ions to approximately 0.1 the speed of light,” explains the scientist. “This energy is necessary so that some of them, hitting the target, overcome the forces of Coulomb repulsion and merge with the nuclei of its atoms. For example, element 92, uranium, will produce the nucleus of a new element numbered 112, plutonium 114, and californium 118.”

“The search for new superheavy elements allows us to answer one of the most important questions of science: where lies the border of our material world?”

“Such nuclei should already be quite stable and will not decay immediately, but will gradually emit alpha particles and helium nuclei. And we are very good at registering them,” continues Oganesyan. The superheavy nucleus will eject an alpha particle, transforming into an element two atomic numbers lighter. In turn, the daughter nucleus will lose an alpha particle and turn into a “grandchild” - four more lighter, and so on, until the process of sequential alpha decay ends with the random appearance and instantaneous spontaneous fission, the death of the unstable nucleus in the “Sea of ​​Instability”. Using this “genealogy” of alpha particles, Oganesyan and his colleagues traced the entire history of the transformation of nuclides obtained in the accelerator and outlined the near shore of the “Island of Stability.” After half a century of voyage, the first people landed on it.

New land

Already in the first decade of the 21st century, in the fusion reactions of actinides with accelerated calcium-48 ions, atoms of elements with numbers from 113 to 118, lying on the shore of the “Island of Stability” farthest from the “mainland”, were synthesized. Their lifetime is already orders of magnitude longer than that of their neighbors: for example, element 114 is stored not for milliseconds, like the 110th, but for tens and even hundreds of seconds. “Such substances are already available for chemistry,” says Academician Oganesyan. - This means that we are returning to the very beginning of the journey and now we can check whether Mendeleev’s Periodic Law is observed for them. Will element 112 be an analogue of mercury and cadmium, and element 114 an analogue of tin and lead? The first chemical experiments with the isotope of the 112th element (copernicium) showed that, apparently, they will. Copernicium nuclei ejected from the target during bombardment were directed by scientists into a long tube containing 36 paired detectors, partially coated with gold. Mercury easily forms stable intermetallic compounds with gold (this property is used in the ancient technique of gilding). Therefore, mercury and atoms close to it should settle on the gold surface of the very first detectors, and radon and atoms close to noble gases can reach the end of the tube. Obediently following the Periodic Law, Copernicium showed itself to be a relative of mercury. But if mercury was the first known liquid metal, then copernicium may be the first gaseous one: its boiling point is below room temperature. According to Yuri Oganesyan, this is only a faded beginning, and superheavy elements from the “Island of Stability” will open up a new, bright and unusual area of ​​chemistry for us.

But for now we lingered at the foot of the island of stable elements. It is expected that the 120th and subsequent nuclei may turn out to be truly stable and will exist for many years, or even millions of years, forming stable compounds. However, it is no longer possible to obtain them using the same calcium-48: there are no sufficiently long-lived elements that could combine with these ions to give nuclei of the required mass. Attempts to replace calcium-48 ions with something heavier have also not yielded results. Therefore, for new searches, marine scientists raised their heads and took a closer look at the skies.

Space and factory

The original composition of our world was not very diverse: in the Big Bang, only hydrogen appeared with small admixtures of helium - the lightest of atoms. All other respected participants in the periodic table appeared in nuclear fusion reactions, in the interior of stars and during supernova explosions. Unstable nuclides quickly decayed, while stable nuclides, like oxygen-16 or iron-54, accumulated. It is not surprising that heavy unstable elements such as americium or copernicium cannot be found in nature.

But if there really is an “Island of Stability” somewhere, then at least in small quantities superheavy elements should be found throughout the vastness of the Universe, and some scientists are searching for them among cosmic ray particles. According to Academician Oganesyan, this approach is still not as reliable as good old bombing. “The truly long-lived nuclei at the top of Stability Island contain unusually large amounts of neutrons,” says the scientist. “That’s why neutron-rich calcium-48 turned out to be such a successful nucleus for bombarding neutron-rich target elements.” However, isotopes heavier than calcium-48 are unstable, and the chances of them fusing to form ultra-stable nuclei under natural conditions are extremely low.”

Therefore, the laboratory in Dubna near Moscow turned to the use of heavier nuclei, albeit not as successful as calcium, for firing at artificial target elements. “We are now busy creating the so-called Factory of Superheavy Elements,” says Academician Oganesyan. — In it, the same targets will be bombarded with titanium or chromium nuclei. They contain two and four more protons than calcium, which means they can give us elements with masses of 120 or more. It will be interesting to see whether they will still be on the “island” or whether they will open a new strait beyond it.”

The work was carried out at the Laboratory of Nuclear Reactions (NLNR) named after. G.N. Flerov of the Dubna Joint Institute for Nuclear Research (JINR) successfully. The properties of the 117th and elements N 112-116 and 118 previously synthesized in Dubna are direct evidence of the existence of the so-called “island of stability” of superheavy elements, predicted by theorists back in the 60s of the last century and significantly expanding the limits of the periodic table. The editors of Izvestia were informed about the unique experiment back in March by the head of FLNR, Academician Yuri Oganesyan, but he only gave permission for publication now. The author of the discovery, Academician Yuri Oganesyan, told observer Pyotr Obraztsov about the essence of the experiment.

Izvestia: What caused the interest of scientists in the synthesis of superheavy elements, which exist for an insignificantly short time?

Yuri Oganesyan: After the discovery of the first artificial elements - neptunium and plutonium - in 1940-1941, the question of the limits of the existence of elements became extremely interesting for the fundamental science of the structure of matter. By the end of the last century, 17 artificial elements had been discovered and their nuclear stability was found to decrease sharply with increasing atomic number. When moving from the 92nd element - uranium - to the 102nd element - Nobelium, the half-life of the nucleus decreases by 16 orders of magnitude: from 4.5 billion years to several seconds. Therefore, it was believed that advancement into the region of even heavier elements would lead to the limit of their existence, essentially marking the boundary of the existence of the material world. However, in the mid-60s, theorists unexpectedly put forward a hypothesis about the possible existence of superheavy atomic nuclei. According to calculations, the lifetime of nuclei with atomic numbers 110-120 should have increased significantly as the number of neutrons in them increased. According to new ideas, they form a vast “island of stability” of superheavy elements, which significantly expands the boundaries of the table of elements.
and: Was it possible to confirm this experimentally?

Oganesyan: In 1975-1996, physicists from Dubna, Darmstadt (GSI, Germany), Tokyo (RIKEN) and Berkeley (LBNL, USA) managed to study these reactions and synthesize six new elements. The heaviest elements 109-112 were obtained for the first time at GSI and repeated at RIKEN. But the half-lives of the heaviest nuclei produced in these reactions were only ten-thousandths or even thousandths of a second. The hypothesis about the existence of superheavy elements was first experimentally confirmed in Dubna, in research conducted by our group in collaboration with scientists from the National Laboratory. Lawrence Livermore (USA). We managed to radically change the approach to the synthesis of superheavy nuclei, for example, by firing at a target made of the artificial element berkelium (N 97) with a projectile beam from an extremely rare and expensive calcium isotope (N 20) with a mass of 48. When the nuclei are fused, element N 117 (97 + 20 = 117). The results exceeded even the most optimistic expectations. In 2000-2004, almost within five years, it was in such reactions that superheavy elements with atomic numbers 114, 116 and 118 were synthesized for the first time.

and: What exactly was the scientific contribution made by American scientists?

Oganesyan: In a nuclear reaction with a calcium beam, element 117 can only be obtained using a target made of the artificial element berkelium. The half-life of this isotope is only 320 days. Due to the short lifetime, the production of berkelium in the required amount (20-30 milligrams) must be carried out in a reactor with a very high neutron flux density. Only the isotope reactor at the US National Laboratory in Oak Ridge can handle such a task. By the way, it was in this laboratory that plutonium for the American atomic bomb was first produced. Since from the moment of production of berkelium its quantity decreases by half after 320 days, it was necessary to carry out all work at a high pace. And not only in laboratories, but also in official structures in Russia and the United States related to the certification of unusual material, transportation of a highly radioactive product by land and air, safety precautions, and so on.

and: Worthy of an adventure story. What happened next?

Oganesyan: At the beginning of June 2009, the container arrived in Moscow. From this substance, a target was made at the Research Institute of Nuclear Reactors (Dimitrovgrad) in the form of a thin layer of berkelium (300 nanometers) deposited on thin titanium foil; in July the target was delivered to Dubna. By this time, all preparatory work at FLNR had been completed, and continuous irradiation of the target with an intense calcium beam began. Already in the first irradiation of the target for 70 days, we were lucky: the detectors recorded the formation and decay of nuclei of the 117th element five times. As expected, the nuclei of this element transformed into the nuclei of element 115, element 115 transformed into element 113, and then element 113 transformed into element 111. And element 111 decayed with a half-life of 26 seconds. On a nuclear scale, this is a huge time! Now the periodic table has been replenished with one more of the heaviest elements with atomic number 117.

and: Our readers will naturally be interested in what practical applications your discovery may have.

Oganesyan: Now, of course, none, because only a few atoms of element N 117 have been obtained. From a fundamental point of view, ideas about our world should now change greatly. Moreover, if elements with a huge half-life are synthesized, then it is possible that they exist in nature and could “survive” to our time since the formation of the Earth - 4.5 billion years. And we are conducting experiments to search for them; our installation is located in the depths of the Alpine mountains.

and: A question from a different plane. Why do you think the obvious successes in nuclear physics over the past 20 years have not been awarded Nobel Prizes?

Oganesyan: Physics is big. Apparently, the members of the Nobel Committee are more interested in other areas of this science. And there really are a lot of worthy scientists. By the way, I must name the participants in our experiment: Oak Ridge National Laboratory (Prof. James Roberto), University. Vanderbilt (Prof. Joseph Hamilton), National Laboratory. Lawrence Livermore (Dawn Shaughnessy), Research Institute of Nuclear Reactors, Dimitrovgrad (Mikhail Ryabinin) and the Laboratory of Nuclear Reactions of JINR (head Yuri Oganesyan).

From the editor. Temporarily, element N 117 will be called “one-one-seven” in Latin, that is, ununseptium. Academician Yuri Oganesyan's group - the authors of the discovery - has every right to give a real name to this element, as well as elements N 114-116 and 118 discovered by them. In the "Week" of March 26, we invited readers to submit their proposals for the name of "our" elements. For now, only “curly” for one of these elements seems reasonable. The competition continues.

Scientists from the University of New South Wales (Australia) and the University of Mainz (Germany) have suggested that one of the most unusual stars known to astronomers contains chemical elements from the island of stability. These are the elements at the very end of the periodic table; they are distinguished from their neighbors on the left by their longer lifetime. The study was published in the library of electronic preprints arXiv.org; its results and stable superheavy chemical elements are described.

The star HD 101065 was discovered in 1961 by Polish-Australian astronomer Antonin Przybylski. It is located about 400 light years from Earth in the constellation Centaurus. Most likely, HD 101065 is lighter than the Sun and is a main sequence star, a subgiant. A special feature of Przybylski's star is the extremely low content of iron and nickel in the atmosphere. At the same time, the star is rich in heavy elements, including strontium, cesium, thorium, ytterbium and uranium.

Przybylski's star is the only one in which short-lived radioactive elements, actinides, with an atomic number (the number of protons in the nucleus) from 89 to 103 are discovered: actinium, plutonium, americium and einsteinium. HD 101065 is similar to HD 25354, but the presence of americium and curium there is questionable.

The mechanism of formation of superheavy elements on Przybylski's star is still not entirely clear. It was assumed that HD 101065, together with a neutron star, forms a binary system - particles from the second fall onto the first, provoking fusion reactions of heavy elements. This hypothesis has not yet been confirmed, although it is possible that a dim satellite is located at a distance of about a thousand astronomical units from HD 101065.

Photo: N. Dautel / Globallookpress.com

HD 101065 is most similar to Ap stars, peculiar stars of spectral class A, in whose spectrum the lines of rare earth metals are enhanced. They have a strong magnetic field; heavy elements enter their atmosphere from the depths. HD 101065 differs from other Ap stars by short-term changes in the light curve, which made it possible to include it in a separate group of RoAp stars (Rapidly oscillating Ap stars).

It is likely that scientists’ attempts to fit HD 101065 into the existing classification of stars will someday be crowned with success. While Przybylski's star is considered one of the most unusual, this gives reason to suspect that it has a number of unusual properties. In particular, in the latest work devoted to HD 101065, Australian and German researchers assumed that chemical elements belonging to the island of stability are born in Przybylski's star.

Scientists proceeded from the shell model of the nucleus and its extensions. The model relates the stability of the atomic nucleus to the filling of the energy levels of the shells, which, by analogy with the electron shells of the atom, form the nucleus. Each neutron and proton are located in a certain shell (distance from the center of the atom or energy level) and move independently of each other in a certain self-consistent field.

It is believed that the more filled the energy levels of the nucleus, the more stable the isotope. The model explains well the stability of atomic nuclei, spins and magnetic moments, but is applicable only to unexcited or light and medium-sized nuclei.

In accordance with the shell model, nuclei with completely filled energy shells are characterized by high stability. Such elements form the “island of stability”. It starts with isotopes with serial numbers 114 and 126, corresponding to the magic and double magic numbers.

Nuclei with the magic number of nucleons (protons and neutrons) have the strongest binding energy. In the table of nuclides they are arranged as follows: horizontally from left to right in ascending order the number of protons is indicated, and vertically from top to bottom the number of neutrons. A doubly magic nucleus has a number of protons and neutrons equal to some magic number.

The half-life of flerovium isotopes (the 114th element) obtained in Dubna is up to 2.7 seconds. According to the theory, there should be an isotope of flerovium-298 with a magic number of neutrons N = 184 and a lifetime of about ten million years. It has not yet been possible to synthesize such a nucleus. For comparison, the half-life of neighboring elements with numbers of protons in the nucleus equal to 113 and 115 is up to 19.6 seconds (for nihonium-286) and 0.156 seconds (for moscovium-289), respectively.

The authors of the publication on arXiv.org believe that the presence of actinides in the atmosphere of HD 101065 suggests that there are also chemical elements from the island of stability there. Actinides in this case are a product of the decay of stable superheavy elements. The scientists propose searching the spectra of HD 101065 for traces of nobelium, lawrencium, nihonium, and flerovium and describe specific spectra that may produce stable isotopes.

Currently, new elements of the periodic table are being synthesized in Russia, the USA, Japan and Germany. Transuranium elements have not been found in the natural environment on Earth. The star HD 101065 may offer new opportunities to test nuclear physicists' theories that suggest the existence of an island of stability.