IS THERE A LIMIT?
PERIODIC TABLE
D.I.MENDELEEV?

DISCOVERING NEW ELEMENTS

P The problem of systematization of chemical elements attracted close attention in the middle of the 19th century, when it became clear that the diversity of substances around us is the result of different combinations of a relatively small number of chemical elements.

In the chaos of elements and their compounds, the great Russian chemist D.I. Mendeleev was the first to restore order by creating his own periodic table of elements.

March 1, 1869 is considered the day the periodic law was discovered, when Mendeleev announced it to the scientific community. The scientist placed the 63 elements known at that time in his table in such a way that the main properties of these elements and their compounds changed periodically as their atomic mass increased. The observed changes in the properties of elements in the horizontal and vertical directions of the table followed strict rules. For example, the metallic (basic) character clearly expressed in group Ia elements decreased with increasing atomic mass along the horizontal table and increased vertically.

Based on the discovered law, Mendeleev predicted the properties of several as yet undiscovered elements and their place in the periodic table. Already in 1875, “ekaaluminium” (gallium) was discovered, four years later - “ekabor” (scandium), and in 1886 - “ekasilicon” (germanium). In subsequent years, the periodic table served and still serves as a guide in the search for new elements and anticipation of their properties.

However, neither Mendeleev himself nor his contemporaries could answer the question of what are the reasons for the periodicity of the properties of elements, whether and where the boundary of the periodic system exists.

Only many years after the creation of the periodic system of chemical elements, the complex structure of the atom was proven in the works of E. Rutherford, N. Bohr and other scientists. Subsequent achievements of atomic physics made it possible to solve many unclear problems of the periodic table of chemical elements. First of all, it turned out that the place of an element in the periodic table is determined not by atomic mass, but by the charge of the nucleus.

The nature of the periodicity of the chemical properties of elements and their compounds became clear.

The atom began to be viewed as a system in the center of which there is a positively charged nucleus, and negatively charged electrons revolve around it. In this case, electrons are grouped in the perinuclear space and move in certain orbits entering electron shells. All electrons of an atom are usually designated using numbers and letters. According to this notation, the main quantum numbers 1, 2, 3, 4, 5, 6, 7 refer to electron shells, and the letters, s, p, d, f g All electrons of an atom are usually designated using numbers and letters. According to this notation, the main quantum numbers 1, 2, 3, 4, 5, 6, 7 refer to electron shells, and the letters– to the subshells (orbits) of each shell. All electrons of an atom are usually designated using numbers and letters. According to this notation, the main quantum numbers 1, 2, 3, 4, 5, 6, 7 refer to electron shells, and the letters The first shell (counting from the core) has only s-electrons, the second may have All electrons of an atom are usually designated using numbers and letters. According to this notation, the main quantum numbers 1, 2, 3, 4, 5, 6, 7 refer to electron shells, and the letters-, s The first shell (counting from the core) has only p- And All electrons of an atom are usually designated using numbers and letters. According to this notation, the main quantum numbers 1, 2, 3, 4, 5, 6, 7 refer to electron shells, and the letters-,
s-, p The first shell (counting from the core) has only d- electrons, third –

-electrons, fourth –

- electrons, etc. Each shell can accommodate a very specific number of electrons: the first - 2, the second - 8, the third - 18, the fourth and fifth - 32 each. This determines the number of elements in the periods of the periodic table. The chemical properties of elements are determined by the structure of the outer and pre-outer electronic shells of atoms, i.e. by how many electrons they contain. The nucleus of an atom consists of positively charged particles - protons and electrically neutral particles - neutrons, often called in one word - nucleons. The atomic number of an element (its place in the periodic table) is determined by the number of protons in the nucleus of an atom of a given element. Mass number A atom of an element is equal to the sum of the numbers of protons Z and neutrons = Mass number + atom of an element is equal to the sum of the numbers of protons N

The chemical properties of different isotopes of the same element do not differ from each other, but the nuclear properties vary widely. This is manifested primarily in the stability (or instability) of isotopes, which significantly depends on the ratio of the number of protons and neutrons in the nucleus. Light stable isotopes of elements are usually characterized by an equal number of protons and neutrons. With increasing nuclear charge, i.e., the ordinal number of the element in the table, this ratio changes. Stable heavy nuclei have almost one and a half times more neutrons than protons.

Like atomic electrons, nucleons also form shells. As the number of particles in the nucleus increases, the proton and neutron shells are successively filled. Nuclei with completely filled shells are the most stable. For example, a very stable nuclear structure is characterized by the lead isotope Pb-208, which has filled proton shells ( Mass number= 82) and neutrons ( atom of an element is equal to the sum of the numbers of protons = 126).

Such filled nuclear shells are similar to the filled electron shells of noble gas atoms, which represent a separate group in the periodic table. Stable atomic nuclei with completely filled proton or neutron shells contain certain “magic” numbers of protons or neutrons: 2, 8, 20, 28, 50, 82, 114, 126, 184. Thus, atoms of elements in general, as well as in chemical properties, the periodicity of nuclear properties is also inherent. Among the different combinations of the number of protons and neutrons in the nuclei of isotopes (even-even; even-odd; odd-even; odd-odd), it is the nuclei containing an even number of protons and an even number of neutrons that are distinguished by the greatest stability.

The nature of the forces that hold protons and neutrons in the nucleus is not yet clear enough. It is believed that very strong gravitational forces of attraction act between nucleons, which contribute to increasing the stability of nuclei.

TO In the mid-thirties of the last century, the periodic table was developed so much that it showed the position of 92 elements. Serial number 92 was uranium - the last natural heavy element found on Earth back in 1789.

Of the 92 elements of the table, only elements with serial numbers 43, 61, 85 and 87 were not precisely identified in the thirties. They were discovered and studied later. The rare earth element with atomic number 61, promethium, was found in small quantities in ores as a product of the spontaneous decay of uranium. An analysis of the atomic nuclei of the missing elements showed that they are all radioactive, and due to their short half-lives, they cannot exist on Earth in noticeable concentrations.

Due to the fact that the last heavy element found on Earth was element with atomic number 92, one could assume that this is the natural limit of the periodic table. However, the achievements of atomic physics indicated the path along which it turned out to be possible to step over the boundary of the periodic table set by nature. Elements with b O

atomic numbers higher than those of uranium are called transuranium.

These elements are artificial (synthetic) in origin. They are obtained by nuclear transformation reactions of elements found in nature.

The first attempt, although not entirely successful, to discover the transuranium region of the periodic table was made by the Italian physicist Enrico Fermi in Rome shortly after the existence of neutrons was proven. But only in 1940–1941.

The opposite reaction is the transformation of a proton into a neutron with the emission of a positively charged + particle (positron). Such positron decay (+ decay) is observed when there is a lack of neutrons in the nuclei and leads to a decrease in the charge of the nucleus, i.e. to decrease the atomic number of an element by one. A similar effect is achieved when a proton is converted into a neutron by capturing a nearby orbital electron.

New transuranium elements were first obtained from uranium using the method of neutron fusion in nuclear reactors (as products of the explosion of nuclear bombs), and later synthesized using particle accelerators - cyclotrons.

The second type is the reaction between the nuclei of atoms of the initial element (“target”) and the nuclei of atoms of light elements (isotopes of hydrogen, helium, nitrogen, oxygen and others) used as bombarding particles. The protons in the nuclei of the “target” and “projectile” have a positive electrical charge and experience strong repulsion when approaching each other. To overcome repulsive forces and form a compound nucleus, it is necessary to provide the atoms of the “projectile” with very high kinetic energy.

Such enormous energy is stored in cyclotrons by bombarding particles. The resulting intermediate compound nucleus has quite a lot of excess energy, which must be released to stabilize the new nucleus. In the case of heavy transuranium elements, this excess energy, when nuclear fission does not occur, is dissipated by the emission of -rays (high-energy electromagnetic radiation) and the “evaporation” of neutrons from the excited nuclei. The nuclei of the new element's atoms are radioactive. They strive to achieve higher stability by changing the internal structure through radioactive electron decay or decay and spontaneous fission. Such nuclear reactions are characteristic of the heaviest atoms of elements with atomic numbers above 98.

In connection with this fact, the outstanding American scientist G.T. Seaborg, a Nobel Prize laureate who participated in the discovery of nine transuranium elements, believed that the discovery of new elements would probably end around the element with atomic number 110 (similar in properties to platinum). This idea about the boundary of the periodic table was expressed in the 60s of the last century with a caveat: unless new methods of synthesizing elements and the existence of as yet unknown regions of stability of the heaviest elements are discovered. Some of these opportunities have been identified.

The third type of nuclear reactions for the synthesis of new elements is the reaction between high-energy ions with an average atomic mass (calcium, titanium, chromium, nickel) as bombarding particles and atoms of stable elements (lead, bismuth) as a “target” instead of heavy radioactive isotopes. This way of obtaining heavier elements was proposed in 1973 by our scientist Yu.Ts. Oganesyan from JINR and was successfully used in other countries. The main advantage of the proposed synthesis method was the formation of less “hot” compound nuclei when the “projectile” and “target” nuclei merged. The release of excess energy of compound nuclei in this case occurred as a result of the “evaporation” of a significantly smaller number of neutrons (one or two instead of four or five).

An unusual nuclear reaction between ions of the rare isotope Ca-48, accelerated in a cyclotron
U-400, and atoms of the actinide element curium Cm-248 with the formation of element-114 (“eca-lead”) was discovered in Dubna in 1979. It was found that in this reaction a “cold” nucleus is formed that does not “evaporate” a single neutron , and all the excess energy is carried away by one particle. This means that for the synthesis of new elements it can also be implemented fourth type nuclear reactions between accelerated ions of atoms with average mass numbers and atoms of heavy transuranic elements.

IN In the development of the theory of the periodic system of chemical elements, a major role was played by the comparison of the chemical properties and structure of the electronic shells of lanthanides with serial numbers 58–71 and actinides with serial numbers 90–103. It was shown that the similarity of the chemical properties of lanthanides and actinides is due to the similarity of their electronic structures. Both groups of elements are an example of an internal transition row with sequential filling 4 d- or 5 d-electronic shells, respectively, after filling the outer All electrons of an atom are usually designated using numbers and letters. According to this notation, the main quantum numbers 1, 2, 3, 4, 5, 6, 7 refer to electron shells, and the letters The first shell (counting from the core) has only R-electronic orbitals.

Elements with periodic table numbers of 110 and higher were called superheavy. Progress towards the discovery of these elements becomes increasingly difficult and time-consuming, because... It is not enough to synthesize a new element; it is necessary to identify it and prove that the new element has properties unique to it alone. The difficulties are caused by the fact that a small number of atoms are available to study the properties of new elements. The time during which a new element can be studied before radioactive decay occurs is usually very short. In these cases, even when only one atom of a new element is obtained, the method of radioactive tracers is used to detect it and preliminary study of some characteristics.

Element 109, meitnerium, is the last element on the periodic table presented in most chemistry textbooks. Element-110, which belongs to the same group of the periodic table as platinum, was first synthesized in Darmstadt (Germany) in 1994 using a powerful heavy ion accelerator according to the reaction:

The half-life of the resulting isotope is extremely short. In August 2003, the 42nd IUPAC General Assembly and the IUPAC (International Union of Pure and Applied Chemistry) Council officially approved the name and symbol of element-110: darmstadtium, Ds.

There, in Darmstadt, in 1994, element-111 was first obtained by exposing a beam of 64 28 Ni isotope ions to 209 83 Bi atoms as a “target”. By its decision in 2004, IUPAC recognized the discovery and approved the proposal to name element-111 roentgenium, Rg, in honor of the outstanding German physicist W.K. Roentgen, who discovered X

-rays, to which he gave such a name because of the uncertainty of their nature.

According to information received from JINR, in the Laboratory of Nuclear Reactions named after. G.N. Flerov synthesized elements with serial numbers 110–118 (with the exception of element-117).

As a result of synthesis according to the reaction:

In February 2004, reports appeared in prestigious scientific journals about the discovery at JINR by our scientists together with American researchers from the Lawrence Berkeley National Laboratory (USA) of two new elements with numbers 115 and 113. This group of scientists in experiments carried out in July– In August 2003, on a U-400 cyclotron with a gas-filled separator, in the reaction between Am-243 atoms and Ca-48 isotope ions, 1 atom of the element-115 isotope with a mass number of 287 and 3 atoms with a mass number of 288 were synthesized. All four atoms of the element -115 decayed rapidly, releasing -particles and forming isotopes of element-113 with mass numbers 282 and 284. The most stable isotope 284113 had a half-life of about 0.48 s. It collapsed with the emission of -particles and turned into the roentgenium isotope 280 Rg.

In September 2004, a group of Japanese scientists from the Physicochemical Research Institute led by Kosuki Morita (Kosuke Morita) stated that they synthesized element-113 according to the reaction:

When it decays with the release of -particles, the roentgenium isotope 274 Rg is obtained. Since this is the first artificial element obtained by Japanese scientists, they considered that they had the right to propose calling it “Japan”.

The unusual synthesis of the isotope of element 114 with mass number 288 from curium has already been noted above. In 1999, a message appeared about the production of the same isotope of element-114 at JINR by bombarding plutonium atoms with a mass number of 244 with Ca-48 ions.

It was also announced that elements with serial numbers 118 and 116 were discovered as a result of long-term joint studies of nuclear reactions of the isotopes californium Cf-249 and curium isotope Cm-245 with a beam of heavy ions Ca-48, carried out by Russian and American scientists in the period 2002–2005. at JINR. Element-118 closes the 7th period of the periodic table; in its properties it is an analogue of the noble gas radon. Element-116 should have some properties in common with polonium.

Traditionally, the discovery of new chemical elements and their identification must be confirmed by a decision of the IUPAC, but the right to propose names for elements is left to the discoverers. Like a map of the Earth, the periodic table reflected the names of territories, countries, cities and scientific centers where elements and their compounds were discovered and studied, and immortalized the names of famous scientists who made a great contribution to the development of the periodic system of chemical elements. And it is no coincidence that element 101 is named after D.I. Mendeleev.

To answer the question of where the boundary of the periodic table may lie, at one time an assessment was made of the electrostatic forces of attraction of the inner electrons of atoms to a positively charged nucleus. The higher the atomic number of an element, the more the electron “coat” around the nucleus is compressed, the more strongly the internal electrons are attracted to the nucleus.

There must come a moment when electrons begin to be captured by the nucleus. As a result of this capture and reduction of the nuclear charge, the existence of very heavy elements becomes impossible.

A similar catastrophic situation should arise when the serial number of the element is 170–180.

This hypothesis was refuted and it was shown that there are no restrictions on the existence of very heavy elements from the point of view of ideas about the structure of electronic shells. Limitations arise as a result of the instability of the nuclei themselves.

However, it must be said that the lifetime of elements decreases irregularly with increasing atomic number. The next expected region of stability of superheavy elements, due to the appearance of closed neutron or proton shells of the nucleus, should lie in the vicinity of a doubly magic nucleus with 164 protons and 308 neutrons. The possibilities for discovering such elements are not yet clear.

Thus, the question of the boundary of the periodic table of elements still remains.

The synthesis of superheavy elements that make up the so-called “island of stability” is an ambitious task of modern physics, in solving which Russian scientists are ahead of the whole world.

On June 3, 2011, an expert commission, which included specialists from the International Unions of Pure and Applied Chemistry (IUPAC) and Physics (IUPAP), officially recognized the discovery of the 114th and 116th elements of the periodic table. The priority of the discovery was given to a group of physicists led by Academician of the Russian Academy of Sciences Yuri Oganesyan from the Joint Institute for Nuclear Research with the assistance of American colleagues from the Livermore National Laboratory. Lawrence.

RAS Academician Yuri Oganesyan, Head of the Laboratory of Nuclear Reactions at JINR

The new elements became the heaviest of those included in the periodic table, and received the temporary names ununquidium and unungexium, formed by the serial number in the table. Russian physicists proposed naming the elements “flerovium” in honor of Georgiy Flerov, a Soviet nuclear physicist, specialist in the field of nuclear fission and the synthesis of new elements, and “moscovium” in honor of the Moscow region. In addition to the 114th and 116th elements, chemical elements with serial numbers 104, 113, 115, 117 and 118 were previously synthesized at JINR. And the 105th element of the table in honor of the contribution of Dubna physicists to modern science was given the name “Dubnium”.

Elements that do not exist in nature

Currently, the entire world around us consists of 83 chemical elements, from hydrogen (Z=1, Z is the number of protons in the nucleus) to uranium (Z=92), the lifetime of which is longer than the lifetime of the solar system (4.5 billion years) . The heavier elements that appeared during nucleosynthesis shortly after the Big Bang have already decayed and have not survived to this day. Uranium, which has a half-life of about 4.5 x 10 8 years, is still decaying and radioactive. However, in the middle of the last century, researchers learned to obtain elements that do not exist in nature. An example of such an element is plutonium produced in nuclear reactors (Z=94), which is produced in hundreds of tons and is one of the most powerful sources of energy. The half-life of plutonium is significantly shorter than that of uranium, but is still long enough to suggest the possibility of the existence of heavier chemical elements. The concept of an atom consisting of a nucleus, which carries a positive charge and bulk, and electron orbitals, suggests the possibility of the existence of elements with an atomic number up to Z = 170. But in fact, due to the instability of the processes occurring in the core itself, the boundary of the existence of heavy elements is outlined much earlier. In nature, stable formations (nuclei of elements consisting of varying numbers of protons and neutrons) occur only up to lead and bismuth, followed by a small peninsula including thorium and uranium found on Earth. But as soon as the serial number of an element exceeds the number of uranium, its lifetime decreases sharply. For example, the nucleus of element 100 is 20 times less stable than the uranium nucleus, and in the future this instability only intensifies due to spontaneous fission of nuclei.

"Island of Stability"

The effect of spontaneous fission was explained by Niels Bohr. According to his theory, the core is a drop of charged liquid, that is, a kind of matter that does not have its own internal structure. The greater the number of protons in the nucleus, the stronger the influence of Coulomb forces, under the influence of which the drop is deformed and divided into parts. This model predicts the possibility of the existence of elements up to the 104th - 106th serial numbers. However, in the 60s, a number of experiments were carried out at the Laboratory of Nuclear Reactions of the Joint Institute for Nuclear Research to study the properties of fission of uranium nuclei, the results of which could not be explained using Bohr's theory. It turned out that the nucleus is not a complete analogue of a drop of charged liquid, but has an internal

structure. Moreover, the heavier the nucleus, the more pronounced the influence of this structure becomes, and the decay picture will look completely different from what the liquid drop model predicts. This is how the hypothesis arose about the existence of a certain region of stable superheavy nuclei, far from the elements known today. The area was called the "island of stability", and after predicting its existence, the largest laboratories in the USA, France and Germany began a series of experiments to confirm the theory. However, their attempts were unsuccessful. And only experiments at the Dubna cyclotron, which resulted in the discovery of the 114th and 116th elements, make it possible to assert that the region of stability of superheavy nuclei really exists.

The figure below shows a map of heavy nuclides. Nuclear half-lives are represented by different colors (right scale). Black squares are isotopes of stable elements found in the earth's crust (half-life more than 10 9 years). Dark blue color is the “sea of ​​instability”, where nuclei last less than 10 −6 seconds. "Islands of stability" following the "peninsula" of thorium, uranium and transuranium elements are predictions of microscopic nuclear theory. Two nuclei with atomic numbers 112 and 116, obtained in different nuclear reactions and their sequential decay, show how close one can get to the “islands of stability” during the artificial synthesis of superheavy elements.

Map of heavy nuclides

In order to synthesize a stable heavy nucleus, it is necessary to introduce as many neutrons into it as possible, since neutrons are the “glue” that holds nucleons in the nucleus. The first idea was to irradiate a certain starting material with a neutron flux from the reactor. But using this method, scientists were only able to synthesize fermium, an element with atomic number 100. Moreover, instead of the required 60 neutrons, only 20 were introduced into the nucleus. The attempts of American scientists to synthesize superheavy elements in the process of a nuclear explosion (essentially, in a powerful pulsed flow of neutrons) were also unsuccessful; the result of their experiments was the same fermium isotope. From that moment on, another method of synthesis began to develop - colliding two heavy nuclei in the hope that the result of their collision would be a nucleus of total mass. To carry out the experiment, one of the nuclei must be accelerated to a speed of approximately 0.1 the speed of light using a heavy ion accelerator. All heavy nuclei obtained today were synthesized in this way. As already noted, the island of stability is located in the region of neutron-excess superheavy nuclei, so the target and beam nuclei must also contain an excess of neutrons. It is quite difficult to select such elements, since almost all existing stable nuclides have a strictly defined ratio of the number of protons and neutrons.

In the experiment on the synthesis of element 114, the heaviest isotope of plutonium with an atomic mass of 244, produced in a reactor at the Livermore National Laboratory (USA), and calcium-48 as a projectile core were used as a target. Calcium-48 is a stable isotope of calcium, of which regular calcium contains only 0.1%. The experimenters hoped that this configuration would make it possible to feel the effect of increasing the lifetime of superheavy elements. To carry out the experiment, an accelerator with a calcium-48 beam power was required, tens of times greater than all known accelerators. Within five years, such an accelerator was created in Dubna; it made it possible to carry out an experiment several hundred times more accurate than experiments in other countries over the past 25 years.

Having received a calcium beam of the required intensity, experimenters irradiate the plutonium target. If, as a result of the fusion of two nuclei, atoms of a new element are formed, then they must fly out of the target and, together with the beam, continue to move forward. But they must be separated from calcium ions and other reaction products. This function is performed by the separator.

MASHA (Mass Analyzer of Super Heavy Atoms) - installation for nuclear separation

Recoil nuclei ejected from the target layer stop in a graphite collector at a depth of several micrometers. Due to the high temperature of the collector, they diffuse into the ion source chamber, are drawn out of the plasma, accelerated by the electric field, and are analyzed by mass by magnetic fields as they move toward the detector. In this design, the mass of an atom can be determined with an accuracy of 1/3000. The task of the detector is to determine that a heavy nucleus has hit it, to register its energy, speed and location of its stop with high accuracy.

Separator operation diagram

To test the theory of the existence of an “island of stability,” scientists observed the decay products of the nucleus of element 114. If the theory is correct, then the resulting nuclei of element 114 should be resistant to spontaneous fission, and be alpha radioactive, that is, emit an alpha particle consisting of two protons and two neutrons. For a reaction involving the 114th element, a transition from the 114th to the 112th should be observed. Then the nuclei of the 112th also undergo alpha decay and turn into the nuclei of the 110th, and so on. Moreover, the lifetime of the new element should be several orders of magnitude longer than the lifetime of lighter nuclei. It was precisely these long-lived events, the existence of which was predicted theoretically, that the Dubna physicists saw. This is a direct indication that element 114 is already experiencing the action of structural forces that form an island of stability for superheavy elements.

Examples of decay chains of elements 114 and 116

In the experiment on the synthesis of the 116th element, a unique substance was used as a target - curium-248, obtained at a powerful reactor at the Research Institute of Nuclear Reactors in Dimitrovgrad. Otherwise, the experiment followed the same pattern as the search for the 114th element. The observation of the decay chain of element 116 provided further evidence of the existence of element 114, this time resulting from the decay of a heavier “parent”. In the case of element 116, experimental data also showed a significant increase in lifetime with increasing number of neutrons in the nucleus. That is, modern physics of the synthesis of heavy elements has come close to the border of the “island of stability”. In addition, elements with atomic numbers 108, 109 and 110 formed as a result of the decay of the 116th element have a lifetime of minutes, which will make it possible to study the chemical properties of these substances using modern radiochemistry methods and experimentally verify the fundamentality of Mendeleev’s law regarding the periodicity of the chemical properties of elements in the table . In relation to heavy elements, it can be assumed that the 112th element has the properties of cadmium and mercury, and the 114th - tin, lead, etc. It is likely that at the top of the island of stability there are superheavy elements whose lifetime is millions of years. This figure does not reach the age of the Earth, but it is still possible that superheavy elements are present in nature, in our Solar system, or in cosmic rays, that is, in other systems of our Galaxy. But so far, experiments to search for “natural” superheavy elements have not been successful.

Currently, JINR is preparing an experiment to search for the 119th element of the periodic table, and the Laboratory of Nuclear Reactions is a world leader in the field of heavy ion physics and the synthesis of superheavy elements.

Anna Maksimchuk,
JINR Researcher,
especially for R&D.CNews.ru

Interesting, of course. It turns out that many more chemical elements and even almost stable ones can be discovered.

The question arises: what is the practical meaning of all this rather expensive undertaking to search for new almost stable elements?

It seems that when they find a way to produce these elements, then we will see.

But something is already visible now. For example, if anyone watched the movie "Predator", then the predator has a self-destruct device in a bracelet on his arm and the explosion is quite powerful. So here it is. These new chemical elements are similar to uranium-235, but the critical mass can be measured in grams (and 1 gram of this substance is equivalent to the explosion of 10 tons of TNT - a good bomb the size of only a five-kopeck coin).

So it makes a lot of sense for scientists to work hard, and for the state not to skimp on expenses.

By the end of the 60s, through the efforts of many theorists - O. Bohr and B. Motelson (Denmark), S. Nilsson (Sweden), V.M. Strutinsky and V.V. Pashkevich (USSR), H. Myers and V. Svyatetsky (USA), A. Sobichevsky and others (Poland), W. Greiner and others (Germany), R. Nix and P. Möller (USA), J. Berger (France) and many others created the microscopic theory of atomic nuclei. The new theory brought all the above contradictions into a harmonious system of physical laws.
Like any theory, it had a certain predictive power, in particular in predicting the properties of very heavy, still unknown nuclei. It turned out that the stabilizing effect of nuclear shells will work beyond those indicated by the droplet model of the nucleus (i.e. in the region Z > 106) forming the so-called. “islands of stability” around the magic numbers Z=108, N=162 and Z=114, N=184. As can be seen in Fig. 2, the lifetime of superheavy nuclei located in these “islands of stability” can increase significantly. This especially applies to the heaviest, superheavy elements, where the effect of closed shells Z=114 (possibly 120) and N=184 increases half-lives to tens, hundreds of thousands and, perhaps, millions of years, i.e. - 32-35 orders of magnitude more than in the absence of the effect of nuclear shells. This is how an intriguing hypothesis arose about the possible existence of superheavy elements, significantly expanding the boundaries of the material world. A direct test of theoretical predictions would be the synthesis of superheavy nuclides and the determination of their decay properties. Therefore, we will have to briefly consider the key issues associated with the artificial synthesis of elements.

2. Synthesis reactions of heavy elements

Elements heavier than Pm (Z=100) were synthesized in reactions with accelerated heavy ions, when a complex of protons and neutrons is introduced into the target nucleus. But this type of reaction is different from the previous case. When a neutron that does not have an electric charge is captured, the excitation energy of the new nucleus is only 6 - 8 MeV. In contrast, when target nuclei merge even with light ions such as helium (4 He) or carbon (12 C), heavy nuclei will be heated to an energy E x = 20 - 40 MeV. With a further increase in the atomic number of the projectile nucleus, it will need to impart more and more energy to overcome the electrical forces of repulsion of positively charged nuclei (the Coulomb reaction barrier). This circumstance leads to an increase in the excitation energy (heating) of the compound nucleus formed after the merger of two nuclei - the projectile and the target. Its cooling (transition to the ground state E x = 0) will occur through the emission of neutrons and gamma rays. And here the first obstacle arises.

A heated heavy nucleus will be able to emit a neutron only in 1/100th of cases; basically, it will split into two fragments because the energy of the nucleus is significantly higher than the height of its fission barrier. It is easy to understand that increasing the excitation energy of a compound nucleus is detrimental to it. The probability of survival of a heated nucleus drops sharply with increasing temperature (or energy E x) due to an increase in the number of evaporated neutrons, with which fission strongly competes. In order to cool a nucleus heated to an energy of about 40 MeV, it is necessary to evaporate 4 or 5 neutrons. Each time fission will compete with the emission of a neutron, as a result of which the probability of survival will be only (1/100) 4-5 = 10 -8 -10 -10. The situation is complicated by the fact that as the temperature of the core increases, the stabilizing effect of the shells decreases, therefore the height of the fission barrier decreases and the fission of the core increases sharply. Both of these factors lead to an extremely low probability of the formation of superheavy nuclides.

Advancement into the region of elements heavier than 106 became possible after the discovery in 1974 of the so-called. cold fusion reactions. In these reactions, “magic” nuclei of stable isotopes are used as target material - 208 Pb (Z = 82, N = 126) or 209 Bi (Z = 83, N = 126), which are bombarded by ions heavier than argon (Yu.Ts. Oganesyan , A.G. Demin, etc.). During the fusion process, the high binding energy of nucleons in the “magic” target nucleus leads to the absorption of energy during the rearrangement of two interacting nuclei
into a heavy core of total mass. This difference in the “packing” energies of nucleons in the interacting nuclei and in the final nucleus largely compensates for the energy required to overcome the high Coulomb barrier for the reaction. As a result, a heavy nucleus has an excitation energy of only 12-20 MeV. To some extent, such a reaction is similar to the process of “reverse fission”. Indeed, if the fission of a uranium nucleus into two fragments occurs with the release of energy (it is used in nuclear power plants), then in the reverse reaction, when the fragments merge, the resulting uranium nucleus will be almost cold. Therefore, when elements are synthesized in cold fusion reactions, a heavy nucleus only needs to emit one or two neutrons to go to the ground state.
Cold fusion reactions of massive nuclei were successfully used to synthesize 6 new elements, from 107 to 112 (P. Armbruster, Z. Hofmann, G. Münzenberg, etc.) at the GSI National Nuclear Physics Center in Darmstadt (Germany). Recently, K. Morita et al. at the RIKEN National Center (Tokyo) repeated the GSI experiments on the synthesis of 110-112 elements. Both groups intend to move on to elements 113 and 114 using heavier projectiles. However, attempts to synthesize increasingly heavier elements in cold fusion reactions are associated with great difficulties. With an increase in the atomic charge of ions, the probability of their fusion with target nuclei 208 Pb or 209 Bi decreases greatly due to an increase in Coulomb repulsive forces, which, as is known, are proportional to the product of the nuclear charges. From element 104, which can be obtained in the reaction 208 Pb + 50 Ti (Z 1 × Z 2 = 1804) to element 112 in the reaction 208 Pb + 70 Zn (Z 1 × Z 2 = 2460), the probability of merger decreases by more than 10 4 times.

Figure 3 Map of heavy nuclides. Nuclear half-lives are represented by different colors (right scale). Black squares are isotopes of stable elements found in the earth's crust (T 1/2 ≥ 10 9 years). Dark blue color is the “sea of ​​instability”, where nuclei live for less than 10 -6 seconds. Yellow lines correspond to closed shells indicating the magic numbers of protons and neutrons. “Islands of stability” following the “peninsula” of thorium, uranium and transuranium elements are predictions of the microscopic theory of the nucleus. Two nuclei with Z = 112 and 116, obtained in different nuclear reactions and their sequential decay, show how close one can get to the “islands of stability” during the artificial synthesis of superheavy elements.

There is another limitation. Compound nuclei obtained in cold fusion reactions have a relatively small number of neutrons. In the case of the formation of the 112th element considered above, the final nucleus with Z = 112 has only 165 neutrons, while an increase in stability is expected for the number of neutrons N > 170 (see Fig. 3).

Nuclei with a large excess of neutrons can, in principle, be obtained if artificial elements are used as targets: plutonium (Z=94), americium (Z=95) or curium (Z=96) produced in nuclear reactors, and rare elements as a projectile calcium isotope - 48 Ca. (see below).

The nucleus of the 48 Ca atom contains 20 protons and 28 neutrons - both values ​​​​correspond to closed shells. In fusion reactions with 48 Ca nuclei, their “magic” structure will also work (this role in cold fusion reactions was played by the magic nuclei of the target - 208 Pb), as a result of which the excitation energy of superheavy nuclei will be about 30 - 35 MeV. Their transition to the ground state will be accompanied by the emission of three neutrons and gamma rays. One could expect that at this excitation energy the effect of nuclear shells is still present in heated superheavy nuclei, this will increase their survival and allow us to synthesize them in our experiments. Note also that the asymmetry of the masses of interacting nuclei (Z 1 × Z 2 ≤ 2000) reduces their Coulomb repulsion and thereby increases the probability of merger.

Despite these seemingly obvious advantages, all previous attempts to synthesize superheavy elements in reactions with 48 Ca ions, undertaken in various laboratories in 1977 - 1985, failed. turned out to be ineffective. However, the development of experimental technology in recent years and, above all, the production in our laboratory of intense beams of 48 Ca ions on new generation accelerators, has made it possible to increase the sensitivity of the experiment by almost 1000 times. These achievements were used in a new attempt to synthesize superheavy elements.

3 Expected properties

What do we expect to see in the experiment if the synthesis is successful? If the theoretical hypothesis is true, then superheavy nuclei will be stable relative to spontaneous fission. Then they will experience another type of decay: alpha decay (emission of a helium nucleus consisting of 2 protons and 2 neutrons). As a result of this process, a daughter nucleus is formed that is 2 protons and 2 neutrons lighter than the parent nucleus. If the daughter nucleus has a low probability of spontaneous fission, then after the second alpha decay the grandchild nucleus will now be 4 protons and 4 neutrons lighter than the initial nucleus. Alpha decays will continue until spontaneous fission occurs (Fig. 4).

That. we expect to see not just one decay, but a “radioactive family”, a chain of successive alpha decays, quite long in time (on a nuclear scale), which compete with, but are ultimately interrupted by, spontaneous fission. In principle, such a decay scenario already indicates the formation of a superheavy nucleus.

To fully see the expected increase in stability, it is necessary to come as close as possible to the closed shells Z = 114 and N = 184. It is extremely difficult to synthesize such neutron-excess nuclei in nuclear reactions, since when merging nuclei of stable elements that already have a certain ratio of protons and neutrons, it is impossible to get to the doubly magic nucleus 298 114. Therefore, we need to try to use nuclei in the reaction that initially contain the maximum possible number of neutrons. This, to a large extent, also determined the choice of accelerated 48 Ca ions as a projectile. As you know, there is a lot of calcium in nature. It consists of 97% of the isotope 40 Ca, the nucleus of which contains 20 protons and 20 neutrons. But it contains 0.187% heavy isotope - 48 Ca (20 protons and 28 neutrons) which has 8 excess neutrons. The technology for its production is very labor-intensive and expensive; the cost of one gram of enriched 48 Ca is about $200,000. Therefore, we had to significantly change the design and operating modes of our accelerator in order to find a compromise solution - to obtain the maximum intensity of the ion beam with a minimum consumption of this exotic material.

Figure 4
Theoretical predictions about the decay types (shown in different colors in the figure) and half-lives of isotopes of superheavy elements with different numbers of protons and neutrons. As an example, it is shown that for the isotope of the 116th element with a mass of 293, formed in the fusion reaction of nuclei 248 St and 48 Ca, three successive alpha decays are expected, which end with the spontaneous fission of the great-grandson nucleus of the 110th element with a mass of 281. As can be seen in Fig. 8 is exactly such a decay scenario, in the form of a chain α - α - α - SF, observed for this nucleus in experiment. The decay of a lighter nucleus is the isotope of the 110th element with a mass of 271 obtained in the “cold fusion” reaction of nuclei 208 Pb + 64 Ni. Its half-life is 10 4 times less than that of the isotope 281 110.

Today we reached a record beam intensity - 8 × 10 12 / s, with a very low consumption of the 48 Ca isotope - about 0.5 milligrams / hour. As target material we use long-lived enriched isotopes of artificial elements: Pu, Am, Cm and Cf (Z = 94-96 and 98) also with a maximum neutron content. They are produced in powerful nuclear reactors (in Oak Ridge, USA and in Dimitrovgrad, Russia) and then enriched in special installations, mass separators at the All-Russian Research Institute of Experimental Physics (Sarov). Fusion reactions of 48 Ca nuclei with nuclei of these isotopes were chosen for the synthesis of elements with Z = 114 - 118.

Here I would like to make some digression.

Not every laboratory, even the leading nuclear centers in the world, has such unique materials and in such quantities that we use in our work. But the technologies for their production have been developed in our country and they are being developed by our industry. The Minister of Atomic Energy of Russia suggested that we develop a program of work on the synthesis of new elements for 5 years and allocated a special grant for carrying out this research. On the other hand, working at the Joint Institute for Nuclear Research, we widely collaborate (and compete) with leading laboratories in the world. In research on the synthesis of superheavy elements, we have been closely collaborating for many years with the Livermore National Laboratory (USA). This collaboration not only combines our efforts, but also creates conditions under which experimental results are processed and analyzed independently by two groups at all stages of the experiment.
Over 5 years of work, during long-term irradiation, a dose of about 2 × 10 20 ions (about 16 milligrams of 48 Ca, accelerated to ~ 1/10 the speed of light, passed through the target layers). In these experiments, the formation of isotopes of 112÷118 elements (with the exception of the 117th element) was observed and the first results were obtained on the decay properties of new superheavy nuclides. Presenting all the results would take too much space and, in order not to bore the reader, we will limit ourselves to describing only the last experiment on the synthesis of 113 and 115 elements - all other reactions were studied in a similar way. But before embarking on this task, it would be advisable to briefly outline the setup of the experiment and explain the basic principles of operation of our installation.

4. Setting up the experiment

Figure 5
Schematic view of the installation for the separation of recoil nuclei in experiments on the synthesis of heavy elements

Mentally, the entire surface of the detector can be represented as about 500 cells (pixels) in which decays are detected. The probability that two signals will randomly fall into the same place is 1/500, three signals - 1/250000, etc. This makes it possible to select, with great reliability, from a huge number of radioactive products very rare events of genetically related sequential decays of superheavy nuclei, even if they are formed in extremely small quantities (~1 atom/month).

5. Experimental results

In order to show the installation “in action”, we will describe, as an example, in more detail the experiments on the synthesis of element 115 formed in the fusion reaction of nuclei 243 Am(Z=95) + 48 Ca(Z=20) → 291 115.
The synthesis of a Z-odd nucleus is attractive because the presence of an odd proton or neutron significantly reduces the probability of spontaneous fission and the number of successive alpha transitions will be greater (long chains) than in the case of the decay of even-even nuclei. To overcome the Coulomb barrier, 48 Ca ions must have an energy E > 236 MeV. On the other hand, fulfilling this condition, if we limit the beam energy to E = 248 MeV, then the thermal energy of the compound nucleus 291 115 will be about 39 MeV; its cooling will occur through the emission of 3 neutrons and gamma rays. Then the reaction product will be the isotope 115 of the element with the number of neutrons N=173. Having flown out of the target layer, an atom of a new element will pass through a separator configured to transmit it and enter the detector. Further events develop as shown in Fig. 6. 80 microseconds after the recoil core stops in the frontal detector, the data acquisition system receives signals about its arrival time, energy and coordinates (strip number and position in it). Note that this information has the "TOF" (came from the separator) attribute. If within 10 seconds a second signal with an energy of more than 9.8 MeV follows from the same place on the detector surface, without the “TOF” sign (i.e. from the decay of the implanted atom), the beam is turned off and all further decay is recorded in conditions of almost complete absence of background. As can be seen in the top graph of Fig. 6, behind the first two signals - from the recoil nucleus and the first alpha particle - for a time of about 20 s. after the beam was turned off, 4 other signals followed, the positions of which, with an accuracy of ± 0.5 mm, coincided with the previous signals. Over the next 2.5 hours the detector was silent. Spontaneous fission in the same strip and in the same position was recorded only the next day, 28.7 hours later, in the form of two signals from fission fragments with a total energy of 206 MeV.
Such chains were registered three times. They all have the same appearance (6 generations of nuclei in the radioactive family) and are consistent with each other both in the energy of alpha particles and in the time of their appearance, taking into account the exponential law of nuclear decay. If the observed effect relates, as expected, to the decay of the isotope of element 115 with a mass of 288, formed after the evaporation of 3 neutrons by a compound nucleus, then with an increase in the energy of the 48 Ca ion beam by only 5 MeV, it should decrease by 5-6 times. Indeed, at E = 253 MeV there was no effect. But here another, shorter, chain of decays was observed, consisting of four alpha particles (we believe that there were also 5 of them, but the last alpha particle flew out of the open window) lasting only 0.4 s. The new chain of decays ended after 1.5 hours with spontaneous fission. Obviously, this is the decay of another nucleus, most likely the neighboring isotope of the 115th element with a mass of 287, formed in a fusion reaction with the emission of 4 neutrons. The chain of successive decays of the odd-odd isotope Z=115, N=173 is presented in the lower graph of Fig. 6, which shows the calculated half-lives of superheavy nuclides with different numbers of protons and neutrons in the form of a contour map. It also shows the decay of another, lighter odd-odd isotope of the 111th element with the number of neutrons N = 161 synthesized in the reaction 209 Bi+ 64 Ni in the German Laboratory - GSI (Darmstadt) and then in the Japanese - RIKEN (Tokyo).

Figure 6
Experiment on the synthesis of element 115 in the reaction 48 Ca + 243 At.
The top figure shows the times at which signals appear after implantation of a recoil nucleus (R) into the detector. Signals from the registration of alpha particles are marked in red, signals from spontaneous fission are marked in green. As an example, for one of the three events the positional coordinates (in mm) of all 7 signals from the R → decay chain are givenα 1 → α 2 → α 3 → α 4 →α 5 → SF recorded in strip No. 4. The lower figure shows the decay chains of nuclei with Z=111, N=161 and Z=115, N=173. Contour lines outlining regions of nuclei with different half-lives (different degrees of darkening) are predictions of microscopic theory.

First of all, it should be noted that the nuclear half-lives in both cases are in good agreement with theoretical predictions. Despite the fact that the isotope 288 115 is removed from the neutron shell N=184 by 11 neutrons, the isotopes 115 and 113 elements have a relatively long lifetime (T 1/2 ~ 0.1 s and 0.5 s, respectively).
After five alpha decays, isotope 105 of the element is formed - dubnium (Db) with N=163, the stability of which is determined by another closed shell N=162. The power of this shell is demonstrated by the huge difference in the half-lives of two Db isotopes differing from each other by only 8 neutrons. Let us note, once again, that in the absence of structure (nuclear shells), all isotopes of 105÷115 elements would have to undergo spontaneous fission in a time of ~ 10 -19 s.

(chemical experiment)

In the example described above, the properties of the long-lived isotope 268 Db, which completes the decay chain of element 115, are of independent interest.
According to the Periodic Law, element 105 is in row V. It is, as can be seen in Fig. 7, a chemical homologue of niobium (Nb) and tantalum (Ta) and differs in chemical properties from all lighter elements - actinides (Z = 90÷103) representing a separate group in the D.I. Table. Mendeleev. Due to its long half-life, this isotope of element 105 can be separated from all reaction products radiochemical method followed by measurement of its decay - spontaneous fission. This experiment provides an independent identification of the atomic number of the final nucleus (Z = 105) and of all nuclides produced in the successive alpha decays of element 115.
In a chemical experiment there is no need to use a recoil nuclei separator. The separation of reaction products by their atomic numbers is carried out by methods based on the difference in their chemical properties. Therefore, a more simplified technique was used here. The reaction products flying out of the target were driven into a copper collector located along the path of their movement to a depth of 3-4 microns. After 20-30 hours of irradiation, the collection dissolved. A fraction of transactinoids - elements Z > 104 - was isolated from the solution, and from this fraction, then the elements of the 5th series - Db, accompanied by their chemical homologues Nb and Ta. The latter were added as “markers” to the solution before chemical separation. A droplet of a solution containing Db was deposited on a thin substrate, dried, and then placed between two semiconductor detectors that recorded both fragments of spontaneous fission. The entire assembly was, in turn, placed in a neutron detector, which determined the number of neutrons emitted by fragments during the fission of Db nuclei.
In June 2004, 12 identical experiments were carried out (S.N. Dmitriev and others), in which 15 events of spontaneous division of Db were recorded. Spontaneous fission fragments Db have a kinetic energy of about 235 MeV, and an average of about 4 neutrons are emitted for each fission event. Such characteristics are inherent in the spontaneous fission of a fairly heavy nucleus. Let us recall that for 238 U these values ​​are approximately 170 MeV and 2 neutrons, respectively.
Chemical experiment confirms the results of physical experiment: the nuclei of the 115th element formed in the reaction 243 Am + 48 Ca as a result of successive five alpha decays: Z = 115 → 113 → 111 → 109 → 107 → 105 actually lead to the formation of a long-lived spontaneously fissile nucleus with atomic number 105. In these experiments, as a daughter product of the alpha decay of element 115, another, previously unknown element with atomic number 113 was also synthesized.

Figure 7
Physical and chemical experiments to study the radioactive properties of the 115th element.
In the reaction 48 Ca + 243 At, using a physical setup it was shown that five consecutive
alpha decays of the isotope 288 115 lead to the long-lived isotope of the 105th element - 268 Db, which
splits spontaneously into two fragments. In a chemical experiment, it was determined that a nucleus with atomic number 105 undergoes spontaneous fission.

6. The big picture and the future

Prospects

Now the task is to study in more detail the nuclear and atomic structure of new elements, which is very problematic, primarily due to the low yield of the desired reaction products. In order to increase the number of atoms of superheavy elements, it is necessary to increase the intensity of the 48 Ca ion beam and increase the efficiency of physical techniques. The modernization of the heavy ion accelerator, planned for the coming years, using all the latest achievements in accelerator technology, will allow us to increase the intensity of the ion beam by approximately 5 times. The solution to the second part requires a radical change in the experimental setup; it can be found in the creation of a new experimental technique based on the properties of superheavy elements.

Figure 8
Map of nuclides of heavy and superheavy elements.
For the nuclei inside the ovals corresponding to the various fusion reactions (shown in the figure), the half-lives and energies of the emitted alpha particles are given (yellow squares). The data is presented on a contour map of the separating region based on the contribution of the nuclear shell effect to the nuclear binding energy. In the absence of nuclear structure, the entire field would be white. As they darken, the effect of the shells increases. Two neighboring zones differ by only 1 MeV. This, however, is sufficient to significantly increase the stability of nuclei relative to spontaneous fission, as a result of which nuclides located near the “magic” numbers of protons and neutrons experience predominantly alpha decay. On the other hand, in the isotopes of the 110th and 112th elements, an increase in the number of neutrons by 8 atomic units leads to an increase in the periods of alpha decay of nuclei by more than 10 5 times.

The operating principle of the current installation - the kinematic separator of recoil nuclei (Fig. 5) is based on the difference in the kinematic characteristics of various types of reactions. The products of the reaction of fusion of target nuclei and 48 Ca that interest us fly out of the target in the forward direction, in a narrow angular cone ± 3 0 with a kinetic energy of about 40 MeV. By limiting the trajectories of recoil nuclei, taking these parameters into account, we almost completely tune out the ion beam, suppress the background of reaction by-products by a factor of 10 4 ÷ 10 6, and deliver atoms of new elements to the detector with an efficiency of approximately 40% in 1 microsecond. In other words, separation of reaction products occurs “on the fly”.

Figure 8 MASHA installation
The top figure shows a diagram of the separator and the principle of its operation. Recoil nuclei ejected from the target layer are stopped in a graphite collector at a depth of several micrometers. Due to the high temperature of the collector, they diffuse into the ion source chamber, are drawn out of the plasma, accelerated by the electric field, and are analyzed by mass by magnetic fields as they move toward the detector. In this design, the mass of an atom can be determined with an accuracy of 1/3000. The figure below shows a general view of the installation.

But in order to obtain high selectivity of the installation, it is important to preserve and not “smear” the kinematic parameters - departure angles and energies of recoil nuclei. Because of this, it is necessary to use target layers with a thickness of no more than 0.3 micrometers - approximately three times less than what is needed to obtain an effective yield of a superheavy nucleus with a given mass or 5-6 times less if we are talking about the synthesis of two isotopes of a given element neighboring in mass. In addition, in order to obtain data on the mass numbers of isotopes of a superheavy element, it is necessary to carry out a long and labor-intensive series of experiments - repeating measurements at different energies of the 48 Ca ion beam.
At the same time, as follows from our experiments, the synthesized atoms of superheavy elements have half-lives that significantly exceed the speed of the kinematic separator. Therefore, in many cases, there is no need to separate reaction products in such a short time. Then you can change the operating principle of the installation and separate the reaction products in several stages.
The diagram of the new installation is shown in Fig. 9. After implantation of recoil nuclei into a collector heated to a temperature of 2000 0 C, the atoms diffuse into the plasma of the ion source, are ionized in the plasma to a charge q = 1 +, are drawn out of the source by an electric field, are separated by mass in magnetic fields of a special profile and, finally, are registered (by decay type) by detectors located in the focal plane. The entire procedure can take, according to estimates, time from tenths of a second to several seconds, depending on the temperature conditions and physicochemical properties of the separated atoms. Inferior in speed to the kinematic separator, the new installation is MASHA (an abbreviation for the full name Mass Analyzer of Super Heavy Atoms) - will increase the operating efficiency by about 10 times and provide, along with the decay properties, a direct measurement of the mass of superheavy nuclei.
Thanks to a grant allocated by the Governor of the Moscow Region B.V. Gromov to create this installation, it was designed and manufactured in a short time - in 2 years, passed tests and is ready for operation. After reconstruction of the accelerator, with the installation of MASHA. We will significantly expand our research into the properties of new nuclides and try to go further into the region of heavier elements.

(search for superheavy elements in nature)

Another side of the problem of superheavy elements is related to the production of longer-lived nuclides. In the experiments described above, we only approached the edge of the “island”, discovered a steep rise, but are still far from its top, where nuclei can live for thousands and, perhaps, even millions of years. We do not have enough neutrons in the synthesized nuclei to get closer to the N=184 shell. Today this is unattainable - there are no reactions that would make it possible to obtain such neutron-rich nuclides. Perhaps in the distant future, physicists will be able to use intense beams of radioactive ions, with a number of neutrons greater than those of 48 Ca nuclei. Such projects are now being widely discussed, without yet touching on the costs required to create such accelerator giants.

However, you can try to approach this problem from a different angle.

If we assume that the longest-lived superheavy nuclei have a half-life of 10 5 ÷ 10 6 years (not much at odds with the predictions of the theory, which also makes its estimates with a certain accuracy), then it is possible that they can be detected in cosmic rays - witnesses of the formation elements on other, younger planets of the Universe. If we make an even stronger assumption that the half-lives of the "long-lived" could be tens of millions of years or more, then they could be present in the Earth, surviving in very small quantities from the formation of the elements in the solar system to the present day.
Among possible candidates, we give preference to isotopes of element 108 (Hs), whose nuclei contain about 180 neutrons. Chemical experiments carried out with the short-lived isotope 269 Hs (T 1/2 ~ 9 s) showed that element 108, as expected, according to the Periodic Law, is a chemical homologue of the 76th element - osmium (Os).

Figure 10
Installation for recording a burst of neutrons from spontaneous fission of nuclei during the decay of element 108. (Underground laboratory in Modan, France)

Then a sample of metallic osmium may contain the 108 element Eka(Os) in very small quantities. The presence of Eka(Os) in osmium can be determined by its radioactive decay. Perhaps the superheavy long-liver will experience spontaneous fission, or spontaneous fission will occur after previous alpha or beta decays (a type of radioactive transformation in which one of the neutrons of the nucleus turns into a proton) of a lighter and shorter-lived daughter or grandchild nucleus. Therefore, at the first stage, it is possible to carry out an experiment to register rare events of spontaneous fission of an osmium sample. Such an experiment is being prepared. Measurements will begin at the end of this year and will continue for 1-1.5 years. The decay of a superheavy nucleus will be detected by the neutron burst accompanying the spontaneous fission. In order to protect the installation from the neutron background generated by cosmic rays, measurements will be carried out in an underground laboratory located under the Alps in the middle of a tunnel connecting France with Italy at a depth corresponding to a 4000-meter layer of water equivalent.
If during a year of measurements at least one event of spontaneous fission of a superheavy nucleus is observed, then this will correspond to a concentration of element 108 in the Os sample of about 5 × 10 -15 g/g, assuming that its half-life is 10 9 years. Such a small value is only 10 -16 part of the concentration of uranium in the earth's crust.
Despite the ultra-high sensitivity of the experiment, the chances of detecting relict, superheavy nuclides are small. But any scientific search always has a small chance... The absence of an effect will give an upper limit on the half-life of a centenarian at the level of T 1/2 ≤ 3× 10 7 years. Not so impressive, but important for understanding the properties of nuclei in the new region of stability of superheavy elements.

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.

Peter Armbruster, Gottfried Münzerberg

Subtle quantum mechanical effects stabilize nuclei that are much heavier than those found in nature. Experimenters had to revise their ideas about how best to synthesize such superheavy elements

During Over the past 20 years, in many countries around the world, the attention of physicists has been attracted to the problem of obtaining superheavy elements. In Darmstadt at the Institute for Heavy Ion Research (HIR), we have achieved some success by synthesizing the nuclei of elements 107, 108 and 109. These nuclei are beyond the “threshold” of the 106th proton, which marks the limit for previously existing methods for obtaining and identifying heavy elements .

Experimental measurements of nuclear masses and theoretical analysis indicate that the stability of these new elements is determined primarily by the microstructure of their proton and neutron systems, rather than by the macroscopic properties that determine the stability of lighter nuclei. However, we have encountered problems that still make it difficult to achieve the goals set in the late 60s, when elements up to 114 seemed to be within reach. Overcoming these difficulties, we have advanced in the study of nuclear structure and the dynamics of nuclear fusion reactions.

Nucleosynthesis has come a long way from the early days when elements that do not exist in nature were produced in nuclear reactors. Physicists used increasingly heavier accelerated ions to bombard target atoms. The final step in this development was the method of "cold fusion" of nuclei, in which particle masses and bombardment energies must be carefully determined so that the excitation of newly formed nuclei is minimal.

In the course of our work, almost all the original ideas about the synthesis of superheavy elements had to be revised: the nuclei of elements that can be synthesized are deformed, aspherical, as postulated in 1966. For fusion, we used stable, widespread in nature, spherical nuclei and accelerated ions average masses instead of the artificial heaviest radioactive nuclei and appropriately selected light accelerated ions, as previously assumed. The fusion must occur at the lowest possible bombardment energy - as “softly” as possible, without the use of “brute force” in the form of excess interaction energy, which was previously believed to contribute to the fusion process.

The idea of ​​synthesis transuranium elements (with atomic number greater than 92) arose in the 30s. In 1934, Enrico Fermi bombarded thallium with slow neutrons to produce lead after beta decay (the decay of a neutron into a proton and an electron). As a result of neutron capture and subsequent beta decay, elements were formed with atomic numbers one higher than the original ones.

Between 1940 and the mid-1950s, elements 93, 94, 99, and 100 were produced by neutron irradiation. It is no coincidence that fermium, element 100, was the last in a series of elements that could be produced by neutron capture and beta decay, proposed by Fermi: none of its isotopes undergo beta decay. During the same period, irradiation with alpha particles produced elements 95 to 98 and 101. In this process, the heavy nucleus absorbs two protons and two neutrons; in this case, the atomic number increases by two units at once. Like all heavy elements, transuranium elements contain more neutrons than protons; for example, plutonium (element 94) contains 145 neutrons for a total mass of 239; the longest-lived fermium isotope has 157 neutrons with a total mass of 257.

The natural way to obtain elements above 100 was considered to be the fusion of the nuclei of the heaviest elements with the nuclei of light elements containing more protons and neutrons than helium. Elements up to 99 are available because they can be synthesized in macroscopic weight quantities. Accelerators were built in Berkeley (USA) and Dubna (USSR) to produce heavy ions with energy sufficient to overcome the electrostatic forces that prevent nuclear fusion. Between 1958 and 1974 these heavy ion accelerators made it possible to synthesize elements from 102 to 106. The priority of discovery of these elements and, therefore, the right to name them remains a subject of debate to this day.

The methods so successfully used in Berkeley and Dubna turned out to be ineffective for obtaining elements heavier than 100. To understand why it is so difficult to synthesize superheavy elements, and why some of them can be especially stable, it is necessary to understand how nuclei are held together as a whole or fall apart and how different forces balance. determining their stability, changes with increasing mass. Effects that can be neglected for lighter nuclei make the difference between complete instability and the relatively long lifetimes of superheavy nuclei.

Particularly important for all nuclei is the interplay between the strong nuclear forces that attract both protons and neutrons and the electrostatic forces that repel protons. The heavier the nuclei, the more neutrons they contain, which to some extent compensates for the influence of the repulsive forces between protons. However, the bond strength between nucleons peaks at iron (26 protons and 30 neutrons), less than a quarter of the way across the periodic table, and then decreases.

The fission of any nucleus heavier than iron must be accompanied by the release of energy, but the energy required to split nuclei less massive than lead is so great that such a reaction can only be carried out under special conditions. Since nuclei that are heavier than lead can transition to a more stable state, emitting even a small part of their nucleons, they are unstable. Naturally occurring isotopes of thorium and uranium decay primarily by emitting alpha particles. Only in uranium and heavier elements can unexcited nuclei undergo spontaneous fission.

Basically, as the atomic number (the number of protons in the nucleus) increases, the instability of atomic nuclei increases: their half-lives decrease from several thousand years to millionths of a second. However, from the theory of the structure of the nucleus it follows that elements that are only slightly heavier than those obtained to date will be more stable, not less.

Nuclei with certain combinations of neutrons and protons have particularly high binding energies; helium-4, oxygen-16, calcium-40, calcium-48 and lead-208 are very stable compared to neighboring elements. These large values ​​are due to the shell structure - the nuclear equivalent of the shells that hold the electrons around the nucleus. Configurations of nucleons that form completely filled (closed) shells are especially stable. For lead, the shell structure increases the binding energy of the nucleus by 11 million electron volts (MeV) compared to a hypothetical nuclear droplet, devoid of structure and having the same number of neutrons and protons. For most nuclei with binding energies up to 2 billion eV, such an increase is relatively insignificant. However, for the heaviest elements at the edge of stability, “shell stabilization” can make the difference between instantaneous decay and the relatively long existence of nuclei.

Nuclei with closed neutron and proton shells are especially stable; after lead, such shells appear at 114 protons and 184 neutrons. The success of shell theory in predicting binding energies for light nuclei gave rise to hope that nuclei with masses close to 298 could be so highly stabilized that, like uranium and thorium, they could form a region of relatively stable elements. Such shell-stabilized superheavy elements, unlike elements in the uranium-thorium region, should be unstable as homogeneous drops of nuclear matter.

The first of the shell-stabilized superheavy elements, 107, whose properties Fermi suggested would correspond to ecarenation, was identified in Darmstadt in 1981, 47 years after this prediction.

We then obtained and identified elements 108 and 109. Measurements of their binding energies show that we have already entered the region of superheavy elements. We are currently investigating the limitations that prevent the production of even heavier elements.

Synthesis of heavy elements in fusion reactions requires the experimenter to be able to “walk a fine line” between those bombardment methods in which fusion does not occur and those methods that lead to fission of the product nucleus rather than leaving it in a relatively stable state. The reduction in heating of the newly formed nucleus is the most important reason for the transition from the bombardment of heavy targets with relatively light ions to the bombardment of less massive targets with relatively heavier ions (a transition initiated by Yu. Ts. Oganesyan and his collaborators at the Joint Institute for Nuclear Research in Dubna).

For example, when lead-208 or bismuth-209 fuses with chromium-54 or iron-58, the excitation energy of the new nucleus is about 20 MeV. At the same time, the fusion of heavy actinide targets (californium-249, berkelium-249 or curium-248) with carbon-12, nitrogen-15 or oxygen-18 results in an excitation energy of about 45 MeV.

The nucleus, formed using light ions and isactinide targets, cools, emitting four neutrons. In contrast, a nucleus formed from lead or bismuth and heavier ions cools, emitting only one neutron. Since the probability that a nucleus will cool by emitting a neutron is only a few percent of the probability of its fission, the final yield of superheavy nuclei is significantly reduced at each stage of the neutron emission cascade. The single-neutron relaxation mechanism is much more suitable for preserving the newly formed nucleus.

Unfortunately, cold fusion also has a disadvantage: in this case, the electrostatic repulsive forces between the two nuclei prevent their fusion to a greater extent. When two nuclei come together, part of their kinetic energy is converted into excitation energy of the intermediate system of colliding nuclei and, therefore, cannot be used to overcome the fusion barrier, which in turn reduces the probability of fusion. In the case of cold fusion using heavier ions, more kinetic energy is converted in the process of approaching and passing the fusion barrier and the probability of overcoming this barrier is reduced compared to reactions between light ions and the heaviest targets.

If the initial energy is increased to compensate for these losses, the excitation energy will increase and the number of nuclei formed will decrease. As a result, only the 106th element shows the advantages of the cold fusion method.

We have shown that the maximum cross sections for the formation of heavy elements are in a narrow energy range - approximately 5 MeB above the fusion barrier.

While The theory of producing superheavy nuclei can be very interesting in itself, but in practice it is a much more difficult task. Theoretical calculations must be combined with the design of an accelerator and target, as well as the development of a detector system that can detect the existence of a superheavy nucleus as soon as it is synthesized. When the idea of ​​obtaining superheavy elements captured the imagination of physicists and chemists in the late 60s, no one in Germany had experience in carrying out nucleosynthesis. Many doors have been opened for beginners in this field. Much could be learned from the earlier experiments at Berkeley and Dubna, but it was clear that further progress could not be made by copying these studies. What was needed was a heavy ion accelerator, rapid separation methods for isolating new elements, and appropriate techniques for their identification. There was no answer to the question of what specific reactions should lead to success.

In 1969, the German government, together with the state government of Hesse, decided to finance the creation of a new institute for heavy ion research (Society for Heavy Ion Research, GE) in Darmstadt. The Universal Linear Accelerator (UNILAC), on which experiments are conducted in Gaia, began operating in 1975.

UNILAC can accelerate all ions up to and including uranium to energies exceeding the Coulomb barrier. From the very beginning, this installation was intended to produce the most intense ion beams possible. Particular efforts were made to ensure that the ion energy could be smoothly changed and set at a given level with fairly good reproducibility. The accelerator project was initially developed by K. Schmelzer and his collaborators in Heidelberg. In this case, the already accumulated experience of other scientific groups was taken into account: the ion sources were a modification of the sources used in Dubna to produce highly charged ions, and the Alvarez system developed at Berkeley was used in the high-frequency system of the linear accelerator.

When UNILAC was built, many scientists were faced with the question: what is the best way to use the accelerator? What reactions and what experimental methods should be used? In the initial period of its existence, UNILAC was used to test a wide variety of ideas, but the only strategy that was successful was cold fusion combined with the transport of recoil nuclei (fusion products).

Since After the discovery of plutonium in 1941, about 400 tons of this element were synthesized, which corresponds to 10 30 atoms. On the other hand, only a few atoms of element 109 were obtained and identified. Why are the heaviest elements obtained in such vanishingly small quantities? The answer is that to produce plutonium, tons of neutrons bombard blocks of uranium-238 several centimeters or more thick, and at UNILAC only 100 micrograms of iron-58 are accelerated to bombard a target of lead-208 several hundred nanometers thick. In addition, the cross-section of the neutron capture reaction that produces plutonium-239 is approximately 10 trillion times larger than the cross-section of the fusion reaction that produces element 109.

Difficulties in obtaining heavier elements are only part of the problem. Once synthesized, elements such as 109 decay so quickly that the synthesis cannot keep up with the decay. The heaviest elements are so short-lived that by the end of irradiation all the formed atoms have already decayed. Therefore, these atoms must be detected and identified during their production process.

Methods for obtaining and registering elements up to 106 were based mainly on mechanical means of transporting the resulting atoms from the reaction zone to detectors. The transport time between the formation and detection of reaction products was determined by the rates of their transfer in a gas flow, the time of their diffusion from solid surfaces, or the speed of rotating targets. These methods, however, were not good enough to detect elements heavier than 106, forcing unacceptable trade-offs between speed and detection accuracy, so that using faster methods made it impossible to reliably identify new isotopes.

To transport the resulting nuclei to detectors, we chose a technique based on the use of the recoil velocity that reaction products acquire from heavy ions. When a heavy ion collides with a target atom and fuses with it, the resulting nucleus moves in the direction of the ion's original motion at a speed of about a few percent of the speed of light. As a result, nuclei with half-lives up to 100 ns can be detected.

Although the technique of transporting recoil nuclei makes it possible to detect and identify very short-lived nuclei, the detection technique becomes more complex. Not only individual nuclei formed in the fusion reaction leave the reaction zone at high speed, but also trillions of heavy ions, as well as thousands of atoms knocked out of the target. To separate superheavy nuclei from the residual beam, we built a special velocity filter - a separator for reaction products with heavy ions SHIP (Separator for Heavy-Ion Reaction Products), developed jointly with specialists from the Second Physics Institute of the University of Giessen. Based on the kinematics of the collision and fusion of nuclei, the recoil velocity of the fusion products can be calculated in advance. Therefore, they can be isolated in a relatively straightforward way.

The velocity filter consists of two stages, each of which includes both electric and magnetic fields. These two fields deflect charged particles in opposite directions; Only for a core having a certain speed, the influence of the fields is mutually excluded, and it continues to move in the median plane of the installation. Such a tandem filter reduces the number of accelerated ions entering the detection region by 100 billion times and the number of knocked-out target nuclei by 1000 times. By eliminating almost all unwanted particles from the beam, the SHIP spectrometer allows more than 40,070 fusion products to pass through. Detectors located behind the spectrometer record the decay chains of particles passing through the spectrometer, which makes it possible to unambiguously identify fusion products.

The first element of the detection system is a time-of-flight device, which allows one to measure the particle’s velocity for the third time (the first two measurements are inherent in the principle of the velocity filter). After passing through this device, the particle is implanted into position-sensitive silicon surface barrier detectors, which record its energy and location of impact. Because the combination of time of flight and energy provides an approximate estimate of the particle's mass, fusion products can be distinguished from scattered ions and knocked-out target nuclei.

To reliably identify the nucleus, it is nevertheless necessary to establish a correlation between its decay and the decay of its radioactive daughter products. Decay events caused by the same nucleus must have the same spatial coordinates, and the type, energy and half-life of the daughter nuclei are known from previous measurements.

By establishing such correlated decay events, it is possible to uniquely identify each fusion product nucleus. Although a random nucleus that lands in the same location as the fusion product of interest may decay and produce a spatially correlated signal, it is highly unlikely that its decay energy, half-life, and decay mode will be those expected for the fusion product. We observed such decay chains up to the fourth generation; the probability that such a series of correlated events is random is from 10 –15 to 10 –18. If correlated events caused by the isotope under study are observed once a day, then the random occurrence of events simulating four generations of decay events can be expected for a period of time 100 times greater than the age of the Earth. As a result, even a single event can unambiguously indicate the existence of a given superheavy isotope.

Between 1981 and 1986 together with our colleagues P. Hessberger, Z. Hofmann, M. Leino, W. Reisdorf and K.-H. Schmidt, we used UNILAC, SHIP and its detection system to synthesize and identify elements 107-109. In these experiments, 14 isotopes of elements 104-109 were synthesized (five of which were previously known), as well as two more isotopes of elements 107 and 108 with mass numbers 261 and 264, respectively.

In 1981, we obtained the isotope of element 107 with a mass number of 262 by bombarding bismuth 209 with chromium-54 ions. For the odd-odd isotope of element 107 (which has an odd number of both protons and neutrons), we have established five alpha particle energies, which give an idea of ​​the nuclear energy levels; we can also report that this isotope has an isomer (long-lived excited state).

Element 109 was identified based on the observation of a single decay chain detected at 16:10 on August 29, 1982 in the reaction between iron-58 and bismuth-209. The 266 109 nucleus existed for 5 ms before emitting an alpha particle with an energy of 11.1 MeV; the resulting nucleus of the 107th element disintegrated into the 105th element after 22 ms; The 105th element decayed into the 104th element followed by 12.9 with the spontaneous fission of its nucleus. From this single event it was possible, although with limited accuracy, to determine the decay energy, half-life, and reaction cross-section. Two more decay chains were observed in early 1988, six years after the identification of the 100th element. They confirmed the interpretation of the event recorded in 1982.

In 1984 We have identified three decay chains of the 265108 isotope in the reaction between iron-58 and lead-208. The two identified isotopes of elements 107 and 109 are odd-odd and are highly unlikely to fission, but the isotope of element 108 has an even number of protons and an odd number of neutrons. Although odd-even isotopes are much more likely to fission, isotope 265108 also experiences alpha decay.

It is particularly interesting that none of the isotopes of elements 107–109 fission spontaneously, and the even-even isotopes 265104, 260106 and 264108 all have approximately the same stability relative to spontaneous fission.

The approximately constant level of stability shows how shell-stabilizing effects compete with a general drop in stability as nuclear mass increases.

Behind 104th and 105th elements there is a small “island” of nuclei, which, when emitting alpha particles, decay to form the known isotopes of lighter elements. Such alpha decay events make it possible to determine the binding energy of these superheavy elements. If the binding energy of the daughter nucleus is known, then at each stage the binding energy of the parent nucleus can be calculated from the alpha decay energy. If the binding energy of the final product is known, then through a chain of alpha decay acts one can arrive at the binding energies of the initial nucleus of the chain. Since the decay of the 108th and 100th elements (one event in each case) and the 106th element (over several events) was recorded, it is possible to reconstruct the chain 264 108 260 106 256 104 252 102. The binding energies of these nuclei are 120. 106 and 94 MeV, respectively.

The shell correction to the binding energy gradually increases for all isotopes from uranium-232 to 264,108, which are bound by the process of alpha decay; the corresponding values ​​increase from 1-2 to 6-7 MeV. In fact, all elements from uranium to element 108 have equally high fission barriers - about 6 MeV. Unlike uranium, which is still stable as a nuclear droplet, the stability of elements 100 and 108 is entirely due to the quantum mechanical structure of their many-particle fermion systems. Recent theoretical work predicts fission barriers that are consistent with our measurements.

The lifetime of an element relative to fission is determined mainly by the height and width of the fission barrier. Shell corrections increase the lifetimes of elements 106 and 108 by 15 orders of magnitude. On a logarithmic scale, the observed lifetimes are in the middle of the range between the intrinsic nuclear time (about 10 -21 s for the decay of an unbound nucleon system) and the age of the Universe (10 18 s). New elements are unstable only in comparison with the duration of human life (2·10 9 s). To match stability on this scale, lifetimes must increase by 12 orders of magnitude. However, nuclear physics is not based on human time scales.

Discovered by us The "island" of alpha radioactive isotopes is a direct consequence of their stabilization due to shell effects. Thus, the stabilization of spherical superheavy nuclei near element 114, predicted at the end of the 60s, begins much earlier than expected and gradually increases. In the narrow region of instability behind lead, between elements 83 and 90, the shell effects are weakened. However, in the interval between the 92nd and 114th elements, the value of the shell correction slowly and monotonically increases.

Even in the vicinity of the “island” of superheavy nuclei, stabilization occurs due to the quantum mechanical structure of fermion systems, while on the “mainland” the stabilization of nuclei is due to macroscopic liquid-droplet properties. The nuclei of elements 107 109 are located on the “dam” between the “island” and the “mainland”, so new isotopes can be attributed to both the “island” and the “mainland”. In any case - like superheavy elements - they were observed only due to the shell stabilization of their ground states.

From the latest theoretical predictions for shell corrections to binding energies, it follows that between elements 106 and 126 there should be a region of approximately 400 superheavy nuclei with fission barriers above 4 MeV. All of these isotopes must have half-lives greater than 1 μs; If they can be synthesized, then they can be detected using existing methods. Particularly stable regions are assumed to be near the isotopes 273109 and 291115. When the number of neutrons is about 166, the deformation of the ground state changes. Isotopes with fewer neutrons are deformed, while heavier isotopes are spherical.

During Over the past 20 years, all attempts to obtain isotopes near the expected center of stability - the 298,114 nucleus - have been unsuccessful. It was not possible to detect these superheavy isotopes either in fusion reactions or in any other reactions involving heavy ions. Nevertheless, the basic idea about the possibility of the existence of shell-stabilized nucleon systems, in addition to stable nuclear droplets, is confirmed by the experiments described above. Theoretically, there is still every reason to believe in extrapolation to even heavier elements.

Now an interesting question arises: what ultimately prevents the creation of these “fragile” objects? Some important clarifications have been obtained from our intensive studies of fusion reactions. A shell-stabilized nucleus, spherical in the ground state, can be destroyed even at an excitation energy as low as 15 MeV, this was experimentally demonstrated by K.-H. Schmidt back in 1979, while deformed nuclei can be preserved at excitation energies up to 40 MeV. Even in the reaction between calcium-48 and curium-248 (the best available reaction), the excitation energy is about 30 MeV. It follows that it is possible to obtain superheavy elements only with deformed nuclei. However, until now such attempts have been successful only for elements with atomic numbers less than 110.

As noted earlier, the fusion of two nuclei, leading to the formation of a superheavy nucleus, is complicated from the very beginning by the need to overcome the fusion barrier. For a given product nucleus, this barrier is minimal when the heaviest targets are bombarded by the lightest possible ions. Despite this advantage, this most asymmetrical combination has the disadvantage of maximizing the heating of the product core, which leads to large fission losses during the deexcitation process. The less asymmetrical the combination, the lower the losses during the cooling stage. The best compromise between low losses at the final stage and a high probability of formation at the initial stage are more symmetrical combinations with target nuclei close to the lead.

The use of lead and bismuth as targets provides the double benefit of the shell effect in these nuclei: the strong coupling in these nuclei with their doubly closed shells results in a reduction of more than 10 MeV in the energy transferred to the product nucleus and a corresponding reduction in losses due to fission. In addition, the probability of overcoming the fusion barrier increases if the reaction uses spherical, highly bound, and relatively rigid nuclei. Here again strong shell effects of lead appear, but this time in the dynamics of the process.

We are now beginning to understand why it will be very difficult to obtain even heavier elements. Only the combination of shell corrections for fusion reaction partners having closed shells, shell effects in dynamics and increased stability of excited deformed superheavy nuclei allowed us to synthesize several isotopes of the lightest superheavy elements. We had to extend the original question about the existence of shell-stabilized nuclei to the effect of shell corrections at all stages of the reaction. It is especially important when creating these complex and fragile objects to introduce pre-existing order into the merging process, avoiding unnecessary disorder.

How to get the following superheavy elements? For elements 110 and 111, it will be possible to apply the methods we developed in reactions between nickel-62 and lead-208 or bismuth-209. Once these elements are formed, detecting them will require not so much fundamentally new knowledge as meeting the needs for an enriched isotope and the patience to learn how to operate our equipment and conduct experiments over several months.