Lead and bismuth are the heaviest stable elements. Circuitry of digital, analog-digital and digital-analog devices study

First, an article about what an "island of stability" is.

Island of stability: Russian nuclear scientists lead the race

The synthesis of superheavy elements that make up the so-called "island of stability" is an ambitious task of modern physics, in which Russian scientists are ahead of the rest of the 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 discovery priority 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 Livero Sea National Laboratory. Lawrence.

Academician of the Russian Academy of Sciences Yury Oganesyan, Head of the Laboratory of Nuclear Reactions at JINR

The new elements became the heaviest of those included in Mendeleev's periodic table, and received the temporary names ununquidium and unungexium, formed by the serial number in the table. Russian physicists proposed to name the elements "flerovium" in honor of Georgy Flerov, a Soviet nuclear physicist, a 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 at JINR, chemical elements with serial numbers 104, 113, 115, 117 and 118 were previously synthesized. And the 105th element of the table, in honor of the contribution of Dubna physicists to modern science, was given the name "dubnium".

Elements not found in nature

At present, 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), whose lifetime 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 had already decayed and did not survive to this day. Uranium, which has a half-life of about 4.5×10 8 years, will still decay and be radioactive. However, in the middle of the last century, researchers learned how to obtain elements that are not found 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 substantially shorter than that of uranium, but still long enough to suggest the possibility of heavier chemical elements. The concept of an atom, consisting of a nucleus that carries a positive charge and the main mass, and electron orbitals, suggests the possibility of the existence of elements with a serial 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 a different number of protons and neutrons) are found only up to lead and bismuth, followed by a small peninsula, including thorium and uranium found on Earth. But as soon as the ordinal number of an element exceeds the number of uranium, its lifetime decreases sharply. For example, the nucleus of the 100th element is 20 times less stable than the nucleus of uranium, and in the future this instability only increases due to spontaneous nuclear fission.

"Island of Stability"

The spontaneous fission effect was explained by Niels Bohr. According to his theory, the nucleus is a drop of charged liquid, that is, some 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 the Coulomb forces, under the influence of which the drop is deformed and divided into parts. Such a model predicts the possibility of the existence of elements up to the 104th - 106th serial numbers. However, in the 1960s, 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 charged liquid drop, but has an internal

structure. Moreover, the heavier the nucleus, the stronger the influence of this structure becomes, and the decay pattern will look completely different from that predicted by the liquid drop model. Thus, a 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 United States, 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. The half-lives of nuclei are represented by different colors (right scale). Black squares are isotopes of stable elements found in the earth's crust (half-life over 109 years). Dark blue is the "sea of ​​instability", where nuclei live for less than 10 −6 seconds. The "islands of stability" following the "peninsula" of thorium, uranium and transuranium elements are predictions of the microscopic theory of the nucleus. Two nuclei with atomic numbers 112 and 116, obtained in various nuclear reactions and their subsequent decay, show how close one can come to the "islands of stability" in 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 as possible into it, since neutrons are the "glue" that keeps nucleons in the nucleus. The first idea was to irradiate a source material with a neutron flux from a reactor. But with 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 (in fact, in a powerful pulsed neutron flux) were not successful either, the result of their experiments was the same fermium isotope. From that moment, another method of synthesis began to develop - to collide two heavy nuclei in the hope that the result of their collision would be the nucleus of the total mass. To conduct the experiment, one of the nuclei must be accelerated to a speed of approximately 0.1 of the speed of light using a heavy ion accelerator. All heavy nuclei obtained today have been synthesized in this way. As already noted, the island of stability is located in the region of neutron-rich superheavy nuclei; therefore, the target and beam nuclei must also contain an excess of neutrons. It is rather 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 the 114th element, the heaviest isotope of plutonium with an atomic mass of 244, produced in the reactor of the Livermore National Laboratory (USA), was used as a target and calcium-48 as a projectile nucleus. Calcium-48 is a stable isotope of calcium, which contains only 0.1% of ordinary calcium. The experimenters hoped that such a 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 exceeding all known accelerators by tens of times was required. Within five years, such an accelerator was created in Dubna, it made it possible to conduct an experiment several hundred times more accurate than experiments in other countries over the past 25 years.

Having received a beam of calcium of the required intensity, the experimenters irradiate the plutonium target. If, as a result of the fusion of two nuclei, atoms of a new element are formed, then they should 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) - plant for the separation of nuclei

The recoil nuclei emitted from the target layer stop in the graphite collector at a depth of several micrometers. Due to the high temperature of the collector, they diffuse into the chamber of the ion source, are drawn out of the plasma, accelerated by the electric field, and analyzed by mass by the magnetic fields as they move towards 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 place of its stop with high accuracy.

Separator operation scheme

To test the theory of the existence of an "island of stability," scientists observed the decay products of the nucleus of the 114th element. If the theory is correct, then the resulting nuclei of the 114th element 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 of the 114th to the 112th must be observed. Then the nuclei of the 112th also undergo alpha decay and pass into the nuclei of the 110th, and so on. Moreover, the lifetime of a new element should be several orders of magnitude longer than the lifetime of lighter nuclei. It is precisely such long-lived events, the existence of which was predicted theoretically, that the Dubna physicists saw. This is a direct indication that the 114th element is already experiencing the action of structural forces that form the island of stability of superheavy elements.

Examples of decay chains of the 114th and 116th elements

In the experiment on the synthesis of the 116th element, a unique substance, curium-248, obtained at a powerful reactor of the Research Institute of Atomic Reactors in the city of Dimitrovgrad, was used as a target. Otherwise, the experiment followed the same scheme as the search for the 114th element. The observation of the decay chain of element 116 was another proof of the existence of element 114, this time it was obtained as a result of the decay of a heavier parent. In the case of the 116th element, the experimental data also showed a significant increase in the lifetime with an increase in the number of neutrons in the nucleus. That is, the modern physics of the synthesis of heavy elements has come close to the border of the "island of stability". In addition, the 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 fundamental nature of Mendeleev's law regarding the periodicity of the chemical properties of elements in the table . With regard to heavy elements, it can be assumed that the 112th element has the properties of cadmium and mercury, and the 114th - of tin, lead, etc. Probably, at the top of the island of stability, there are superheavy elements with a lifetime of millions of years. This figure does not reach the age of the Earth, but the presence of superheavy elements in nature, in our solar system, or in cosmic rays, that is, in other systems of our galaxy, is still possible. 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 the world leader in the field of heavy ion physics and the synthesis of superheavy elements.

Anna Maksimchuk,
Researcher, JINR,
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 event to search for new almost stable elements?

It seems that when they find a way to produce these elements, then it will be seen.

But something is already being seen. For example, if someone 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. These new chemical elements are similar to uranium-235, but at the same time, the critical mass can be calculated in grams (in this case, 1 gram of this substance is equivalent to the explosion of 10 tons of TNT - such a good bomb the size of only a five-kopeck coin).

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

A. Levin

On the way to the island of stability

Scientists have been engaged in the latest version of alchemical craft for seven decades and have succeeded a lot in it: the list of officially recognized artificial elements, whose names are formally approved by the International Union of Pure and Applied Chemistry (IUPAC), includes 19 positions.

It opens with the 93rd element of the Periodic Table known since 1940 - neptunium and ends with the 111th - roentgenium, first made in 1994. In 1996 and 1998, elements with numbers 112 and 114 were received. They have not yet acquired the final names, and the temporary ones assigned to them until the decision of the IUPAC bureau sound terrible - ununbium and ununquadium. In 2004, there were reports of the synthesis of the 113th and 115th elements, so far endowed with equally unpronounceable names. However, they have their own logic, they are just serial numbers of elements encoded using the Latin names of single-digit numbers. For example, ununbium stands for "one-one-two".

Last fall, the world press was circulated by reports of the absolutely reliable receipt of another superheavy element, the 118th. The reliability of these results was by no means accidental. The fact is that for the first time such announcements appeared much earlier - in June 1999. Later, however, employees of the American Livermore Laboratory named after Lawrence, who made an application for this discovery, were forced to refuse it. It turned out that the data on which it was based were fabricated by one of the experimenters, Bulgarian Viktor Ninov. In 2002, this caused a considerable scandal. In the same year, scientists from Livermore, led by Kenton Moody, together with Russian colleagues from the Joint Institute for Nuclear Research in Dubna, headed by Yuri Oganesyan, resumed these attempts using a different chain of nuclear reactions. The experiments were completed only three years later, and now they have already led to the guaranteed synthesis of the 118th element - however, in the amount of only three nuclei. These results are presented in an article with twenty Russian and ten American signatures, which appeared on October 9, 2006 in the journal Physical Review by S.

We will talk about methods for obtaining superheavy artificial elements and about the joint work of the Oganesyan and Moody groups later. In the meantime, let's try to answer a not so naive question: why do nuclear physicists and chemists so persistently synthesize more and more new elements with three-digit numbers in the Periodic system? These works require complex and expensive equipment and many years of intensive research - and what is the result? Completely useless unstable exotic cores, which, moreover, can be counted on the fingers. Of course, specialists are interested in studying each such nucleus simply because of its uniqueness and novelty for science - for example, to study its radioactive decays, energy levels and geometric shape. For such discoveries, sometimes they give Nobel Prizes, but still - is the game worth the candle? What do these studies promise, if not technology, then at least fundamental science?

A LITTLE ELEMENTARY PHYSICS
First of all, we recall that the nuclei of all elements without exception, except for hydrogen, are composed of particles of two types - positively charged protons and neutrons that do not carry an electric charge (a hydrogen nucleus is a single proton). So all nuclei are positively charged, and the charge of a nucleus is determined by the number of its protons. The same number specifies the number of the element in the Periodic system. At first glance, this circumstance may seem strange. The creator of this system, D. I. Mendeleev, ordered the elements on the basis of their atomic weights and chemical properties, and then science did not suspect atomic nuclei at all (by the way, in 1869, when he discovered his periodic law, only 63 elements were known) . Now we know (but Dmitry Ivanovich did not have time to find out) that the chemical properties depend on the structure of the electron cloud surrounding the atomic nucleus. As you know, the charges of the proton and electron are equal in absolute value and opposite in sign. Since the atom as a whole is electrically neutral, the number of electrons is exactly equal to the number of protons - that's the desired connection and discovered. The periodicity of chemical properties is explained by the fact that the electron cloud consists of separate "layers" - shells. Chemical interactions between atoms are primarily provided by the electrons of the outer shells. As each new shell is filled, the chemical properties of the resulting elements form a smooth series, and then the capacity of the shell ends, and the next one begins to fill - hence the periodicity. But here we are entering the wilds of atomic physics, and it does not interest us today, we would have time to talk about nuclei.

Atomic nuclei are usually called "nuclides", from the Latin nucleus - nucleus. Hence the common name for protons and neutrons - "nucleons". Nuclei with the same number of protons, but different - neutrons differ in mass, but their electronic "clothes" are completely Marie Curie the same. This means that atoms that differ from each other only in the number of neutrons are chemically indistinguishable and must be considered varieties of the same element. Such elements are called isotopes (this name was proposed in 1910 by the English radiochemist Frederick Soddy, who derived it from the Greek words isos - equal, identical and topos - place). Isotopes are usually denoted by the name or chemical symbol of the element, followed by the designation of the total number of nuclear nucleons (this indicator is called the "mass number").

All naturally occurring elements have multiple isotopes. Let's say that hydrogen, in addition to the main one-proton version, has a heavy one - deuterium and a superheavy one - tritium (historically, hydrogen isotopes have their own names). The nucleus of deuterium consists of a proton and a neutron, tritium - of a proton and two neutrons. The second element in the Periodic Table, helium, has two natural isotopes: the very rare helium-3 (two protons, one neutron) and the much more common helium-4 (two protons and two neutrons). Elements of purely laboratory origin are also, as a rule, synthesized in different isotopic variants.

Not all atomic nuclei are stable. Some of them can spontaneously emit particles and turn into other nuclides. This phenomenon was discovered in 1896 by the French physicist Antoine Henri Becquerel, who discovered that uranium emits penetrating radiation unknown to science. Two years later, Frédéric Curie and his wife Marie detected a similar emission from thorium, and then discovered two unstable elements not yet included in the Periodic Table - radium and polonium. Marie Curie called the phenomenon, mysterious from the point of view of the then science, radioactivity. In 1899, Englishman Ernest Rutherford discovered that uranium emits two types of radiation, which he named alpha and beta rays. A year later, the Frenchman Paul Villard noticed radiation of the third type in uranium, which the same Rutherford designated by the third letter of the Greek alphabet - gamma. Later, scientists discovered other types of radioactivity.

Both alpha and gamma radiation arise as a result of internal rearrangements of the nucleus. Alpha rays are simply streams of nuclei of the main isotope of helium, helium-4. When a radioactive nuclide emits an alpha particle, its mass number decreases by four and its charge decreases by two. As a result, the element is shifted in the periodic table two cells to the left. Alpha decay is actually a special case of a whole family of decays, as a result of which the nucleus is rearranged and loses nucleons or groups of nucleons. There are decays in which the nucleus emits a single proton, or a single neutron, or even a more massive group of nucleons than an alpha particle (such groups are called "heavy clusters"). But gamma rays are immaterial - these are electromagnetic quanta of very high energy. So pure gamma decay is, strictly speaking, not radioactivity at all, since after it there remains a nucleus with the same number of protons and neutrons, only in a state with reduced energy.

Beta radioactivity is caused by nuclear transformations of a completely different kind. The particles that Rutherford called beta rays were simply electrons, which turned out very quickly. This circumstance puzzled scientists for a long time, since all attempts to find electrons inside nuclei led to nothing. It was only in 1934 that Enrico Fermi guessed that beta electrons were not the result of intranuclear rearrangements, but of mutual transformations of nucleons. The beta radioactivity of the uranium nucleus is explained by the fact that one of its neutrons turns into a proton and an electron. There is a different kind of beta radioactivity: a proton is converted into a positron and a neutron (the reader will notice that in both transformations the total electric charge is conserved). During beta decay, ultralight and super-penetrating neutral particles, neutrinos, are also emitted (more precisely, positron beta decay leads to the birth of the neutrino itself, and electronic beta decay leads to the birth of an antineutrino). With electronic beta decay, the charge of the nucleus increases by one, with positron decay, of course, it decreases by the same amount.

For a more complete understanding of beta decay, you have to dig even deeper. Protons and neutrons were considered truly elementary particles only until the mid-1960s. Now we know for sure that both of them consist of triplets of quarks - much less massive particles that carry positive or negative charges. The charge of a negative quark is equal to one third of the charge of an electron, and that of a positive quark is equal to two thirds of the charge of a proton. Quarks are closely soldered to each other due to the exchange of special massless particles - gluons - and simply do not exist in a free state. So beta decays are actually transformations of quarks.

The nucleons inside the nucleus are again connected by exchange forces, the carriers of which are other particles, pions (formerly they were called pi-mesons). These bonds are nowhere near as strong as the gluon bonding of quarks, which is why nuclei can decay. Intranuclear forces do not depend on the presence or absence of a charge (hence, all nuclei react with each other in the same way) and have a very short range, approximately 1.4x10-15 meters. The sizes of atomic nuclei depend on the number of nucleons, but in general the same order. Let's say the radius of the heaviest naturally occurring nuclide, uranium-238, is 7.4x10-15 meters, for lighter nuclei it is smaller.

PHYSICS IS MORE SERIOUS
We have done away with nuclear educational program, let's move on to more interesting things. To begin with, here are a few facts, the explanation of which opens the way to understanding the various mechanisms of nuclide synthesis.

Fact 1.
The first 92 elements of the Periodic System were discovered on Earth - from hydrogen to uranium (although helium was first discovered from spectral lines on the Sun, and technetium, astatine, promethium and francium were obtained artificially, but later they were all discovered in terrestrial matter). All elements with large numbers were obtained artificially. They are usually called transuranium, standing in the Periodic system to the right of uranium.

Fact 3.
The ratio between the numbers of intranuclear protons and neutrons is by no means arbitrary. In stable light nuclei, their numbers are the same or almost the same - say, for lithium 3:3, for carbon 6:6, for calcium 20:20. But with increasing atomic number, the number of neutrons grows faster and in the heaviest nuclei exceeds the number of protons by about 1.5 times. For example, the nucleus of a stable isotope of bismuth is composed of 83 protons and 126 neutrons (there are 13 more unstable ones, in which the number of neutrons varies from 119 to 132). In uranium and trans-uraniums, the ratio between neutrons and protons approaches 1.6.

Fact 2.
All elements have unstable isotopes, either naturally occurring or artificial. For example, deuterium is stable, but tritium undergoes beta decay (By the way, about two thousand radioactive nuclides are now known, many of which are used in various technologies and therefore are produced on an industrial scale.) But only the first 83 elements have stable isotopes periodic tables - from hydrogen to bismuth. The nine heaviest natural elements: polonium, astatine, radon, francium, radium, actinium, thorium, protactinium and uranium are radioactive in all their isotopic variants. Without exception, all transurans are also unstable.

How to explain this pattern? Why are there no carbon nuclei, say, with 16 neutrons (this element has 13 isotopes with the number of neutrons from 2 to 14, however, in addition to the main isotope, carbon-12, only carbon-13 is stable)? Why are all nuclides with more than 83 protons unstable?

Nuclear stability map

Atomic mass increases from the top of the map to the bottom. The number of protons increases towards the lower right corner, the number of neutrons increases towards the lower left. The lowest red block is the 112th element.

In textbooks of nuclear physics, you can find a very visual diagram, which is called the isotope map or the valley of nuclear stability. The number of neutrons is plotted along its horizontal axis, and the number of protons along its vertical axis. Each isotope corresponds to a certain point, say, the main isotope of helium - a point with coordinates (2,2). This diagram clearly shows that all really existing isotopes are concentrated in a rather narrow band. At first, its inclination to the abscissa axis is approximately 45 degrees, then it decreases somewhat. Stable isotopes are concentrated in the center of the band, and on the sides - prone to one or another decay.

This is where the ambiguity arises. It is clear that the nuclei cannot consist of protons alone - they would be torn apart by the forces of electrical repulsion. But neutrons seem to increase the interproton distances and thereby weaken this repulsion. And the nuclear forces that unite the nucleons in the nucleus, as already mentioned, act in the same way on both protons and neutrons. It would seem that the more neutrons in the nucleus, the more stable it is. And if it's not, then why?

Here is the explanation "on the fingers". Nuclear matter obeys the laws of quantum mechanics. Nucleons of both types have a half-integer spin, and therefore, like all other such particles (fermions), they obey the Pauli principle, which prohibits identical fermions from occupying the same quantum state. This means that the number of fermions of a given type in a certain state can be expressed only by two numbers - 0 (the state is not occupied) and 1 (the state is full).

In quantum mechanics, in contrast to classical mechanics, all states are discrete. The nucleus does not fall apart because the nucleons in it are pulled together by nuclear forces. This can be visually represented by such a picture - the particles sit in the well and just can't jump out of there. Physicists also use this model, calling the well a potential well. Protons and neutrons are not the same, so they sit in two pits, not one. Both in the proton and in the neutron well there is a set of energy levels that can be occupied by particles that have fallen into it. The depth of each pit depends on the average force interaction between its captives.

Now remember that protons repel each other, but neutrons do not. Consequently, protons are soldered weaker than neutrons, so their potential well is not as deep. For light nuclei, this difference is small, but it increases as the nuclear charge increases. But the energies of the uppermost non-empty levels in both wells must match. If the upper filled neutron level were higher than the upper proton level, the nucleus could reduce its total energy, "forcing" the neutron occupying it to undergo beta decay and turn into a proton. And as soon as such a transformation would be energetically favorable, it would happen over time, the nucleus would turn out to be unstable. The same finale would take place if any proton dared to exceed its energy scale.

Here we have an explanation. If the proton and neutron wells have almost the same depth, which is typical for light nuclei, then the numbers of protons and neutrons also turn out to be approximately the same. As one moves along the periodic table, the number of protons increases, and the depth of their potential well lags more and more behind the depth of the neutron well. Therefore, heavy nuclei should contain more neutrons than protons. But if this difference is artificially made too large (for example, by bombarding the nucleus with slow neutrons that do not break it into fragments, but simply “stick”, the neutron level will rise significantly above the proton level, and the nucleus will decay. This scheme, of course, is extremely simplified, but in principle it is correct.

Let's go further. Since, as the atomic number increases, there is a progressive excess of neutrons over protons, which reduces the stability of nuclei, all heavy nuclides must be radioactive. This is indeed the case, we will not repeat our Fact 2. Moreover, we seem to be right to assume that the heavier nuclides will become less and less stable, in other words, their life expectancy will constantly decrease. This conclusion looks absolutely logical, but it is wrong.

TREASURED ISLAND
Let's start with the fact that the scheme described above does not take into account much. For example, there is the so-called nucleon pairing effect. It consists in the fact that two protons or two neutrons can enter into a close union, forming a semi-autotomous state inside the nucleus with zero angular momentum. Members of such pairs are more strongly attracted to each other, which increases the stability of the entire nucleus. That is why, other things being equal, nuclei with even numbers of protons and neutrons show the greatest stability, and the smallest - with odd ones. The stability of nuclei also depends on a number of other circumstances too special to be discussed here.

But the main thing is not even that. The nucleus is not just a homogeneous accumulation of nucleons, even if they are paired. Numerous experiments have long convinced physicists that the nucleus most likely has a layered structure. According to this model, proton and neutron shells exist inside the nuclei, which are somewhat similar to the electron shells of atoms. Nuclei with completely filled shells are especially resistant to spontaneous transformations. The numbers of neutrons and protons corresponding to completely filled shells are called magic numbers. Some of these numbers have been reliably determined experimentally, such as 2, 8, and 20.

And here the most interesting begins. Shell models make it possible to calculate the magic numbers of superheavy nuclei - though without a full guarantee. In any case, there is every reason to expect that the neutron number 184 will turn out to be magical. The proton numbers 114, 120 and 126 can correspond to it, and the latter, again, must be magic. Therefore, it can be assumed that the isotopes of the 114th, 120th and 126th elements, containing 184 neutrons each, will live much longer than their neighbors. Special hopes are pinned on the last isotope, since it turns out to be doubly magical. According to the naming convention discussed in the first section, it should be called unbihexium-310.

So, one can hope that there are still undiscovered superheavy nuclides that live a very long time, at least by the standards of their immediate environment. Physicists call this hypothetical family the "island of stability." The hypothesis of its existence was first expressed by the remarkable American nuclear physicist (or, if you like, nuclear chemist) Glenn Seaborg, Nobel laureate in 1951. He was the leader or key member of the teams that created all nine elements from 94 (plutonium) to 102 (nobelium), as well as element 106, named after him seaborgium.
Now you can answer the question that ends the first section. The synthesis of superheavy elements, among other things, brings nuclear physicists step by step closer to their holy grail - the island of nuclear stability. No one can say with certainty whether this goal is achievable, but the discovery of the coveted island would be a great success for science.

Element 114 has already been created - this is ununquadium. Now it has been synthesized in five isotopic versions with the number of neutrons from 171 to 175. As you can see, up to 184 neutrons is still far away. However, the most stable isotopes of ununquadium have a half-life of just under 3 seconds. For the 113th element, this figure is about half a second, for the 115th - less than one tenth. This is reassuring.

U-400 accelerator at the Joint Institute for Nuclear Research (Dubna),

on which the 118th element was obtained

SYNTHESIS OF 118
All artificial elements from 93rd to 100th were | first obtained [by irradiating nuclei | neutrons or deuterium nuclei] (deuterons). This is not 1 always happened in the lab. Elements 99 and 100 - einsteinium and fermium - were first identified during radiochemical analysis of samples of matter collected in the area of ​​the Pacific Atoll of Eniwetok, where on November 1, 1952, the Americans detonated the ten-megaton thermonuclear charge "Mike". Its shell was made of uranium-238. During the explosion, uranium nuclei had time to absorb up to fifteen neutrons, and then underwent chains of beta decays, which ultimately led to the formation of isotopes of these two elements. By the way, some of them live quite a long time - for example, the half-life of einsteinium-254 is 480 days.

Transfermium elements with numbers greater than 100 are synthesized by bombarding massive but not too rapidly decaying nuclides with heavy ions accelerated in special accelerators. Among the world's best machines of this kind are the U-400 and U-400M cyclotrons belonging to the Flerov Laboratory of Nuclear Reactions of the Joint Institute for Nuclear Research. It was on the U-400 accelerator that the 118th element, ununoctium, was synthesized. In the periodic table, it is located exactly under radon and, therefore, must be a noble gas.
However, it is too early to talk about the study of the chemical properties of ununoctium. In 2002, only one nucleus of its isotope with an atomic weight of 294 (118 protons, 176 neutrons) was obtained, in 2005 two more. They did not live long - about a millisecond. They were made by bombarding a californium-249 target with accelerated calcium-48 ions. The total number of calcium "bullets" was 2x1019! So the performance of the ununoctium generator is extremely low. However, this is a typical situation. But the announced results are considered quite reliable, the probability of error does not exceed a thousandth of a percent.

Ununoctium nuclei underwent a series of alpha decays, successively turning into isotopes of the 116th, 114th and 112th elements. The last, already mentioned ununbium, lives for a very short time and is divided into heavy fragments of approximately the same mass.

That's the whole story so far. In 2007, the same experimenters hope to produce nuclei of element 120 by bombarding a plutonium target with iron ions. The assault on the island of stability continues.

What's New in Science and Technology, No. 1, 2007

Peter Armbruster, Gottfried Münzerberg

Subtle quantum mechanical effects stabilize nuclei that are much heavier than the nuclei that exist in nature. Experimenters had to rethink how best to synthesize such superheavy elements.

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

Experimental measurements of the masses of nuclei and theoretical analysis show that the stability of these new elements is due primarily to the microstructure of their proton and neutron systems, and not to 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 the nuclear structure and dynamics of nuclear fusion reactions.

Nucleosynthesis has come a long way from the early days when elements that don't exist in nature were made in nuclear reactors. Physicists used ever heavier accelerated ions to bombard target atoms. The last step in this development was the method of "cold fusion" of nuclei, in which the masses of the particles and the bombardment energy must be carefully determined so that the excitation of the newly formed nuclei is minimal.

In the course of our work, almost all initial ideas about the synthesis of superheavy elements had to be revised: the nuclei of elements that can be synthesized are deformed, anespherical, as was postulated in 1966. For the fusion, we used stable, widespread in nature, spherical nuclei and accelerated ions medium masses instead of the heaviest artificial nuclei and correspondingly selected light accelerated ions, as previously assumed. The fusion should take place at the lowest possible bombardment energy - as "softly" as possible, without the use of "brute force" in the form of excess interaction energy, which, as was previously believed, contributes to the fusion process.

Idea of ​​synthesis transuranium elements (with atomic number over 92) arose in the 30s. In 1934, Enrico Fermi bombarded thallium with slow neutrons in order to obtain 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 obtained by neutron capture and beta decay, proposed by Fermi: none of its isotopes undergo beta decay. During the same period, elements from 95 to 98 and 101 were obtained by irradiation with alpha particles. 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 for 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 the 99th are available because they can be synthesized in weighty macroscopic quantities. In Berkeley (USA) and Dubna (USSR), accelerators were built to produce heavy ions with energy sufficient to overcome the electrostatic forces that prevent the fusion of nuclei. Between 1958 and 1974 these heavy ion accelerators made it possible to synthesize elements from 102 to 106. The priority of discovering these elements and, consequently, the right to name them, is still a matter of debate.

The methods so successfully applied at Berkeley and Dubna proved ineffective for obtaining elements heavier than 100th. 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 the nuclei are kept together or fall apart and how the balance of various forces. which determines their stability, changes with increasing mass. Effects that can be neglected for lighter nuclei determine the difference between complete instability and the relatively long lifetimes of superheavy nuclei.

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

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

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

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 on which the electrons are located around the nucleus. Nucleon configurations that form completely filled (closed) shells are especially stable. For lead, the shell structure contributes to an increase in the binding energy of the nucleus by 11 million electron volts (MeV) compared to a hypothetical nuclear drop, 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 comparatively insignificant. However, for the heaviest elements, which are on the border of stability, "shell stabilization" can lead to a difference between instantaneous decay and 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 the hope that nuclei with a mass 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, in contrast to elements in the uranium-thorium region, must be unstable as homogeneous drops of nuclear matter.

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

Then we 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 the fine line" between those methods of bombardment in which fusion does not occur, and those methods that lead to the fission of the product nucleus, instead of leaving it in a relatively stable state. The decrease 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 colleagues from the Joint Institute for Nuclear Research in Dubna).

For example, when lead-208 or bismuth-209 is fused with chromium-54 or iron-58, the excitation energy of a 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.

A nucleus formed using light ions and isactinide targets cools down by emitting four neutrons. In contrast, a nucleus formed from lead or bismuth and heavier ions cools down, emitting only one neutron. Since the probability that a nucleus will cool down 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 the conservation of a newly formed nucleus.

Unfortunately, cold fusion also has a disadvantage: in this case, the electrostatic repulsive forces between the two nuclei to a greater extent prevent their fusion. When two nuclei approach, some 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 likelihood of fusion. In the case of a cold fusion using heavier ions, more kinetic energy is converted in the process of approaching and passing through 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, about 5 MeB above the fusion barrier.

While The theory of obtaining 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 the 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 ​​producing superheavy elements captured the imagination of physicists and chemists at the end of the 1960s, no one in the FRG had any experience in nucleosynthesis. Many "doors" have been opened for beginners in this area. Much could be learned from the earlier experiments at Berkeley and Dubna, but it was clear that no further progress could be made by copying these studies. A heavy ion accelerator, rapid separation methods for isolating new elements, and an appropriate technique for their identification were needed. There was also no answer to the question of what kind of reactions should lead to success.

In 1969, the German government, together with the state government of Hesse, decided to fund the establishment of a new institute for heavy ion research (Heavy Ion Research Society, Gays) in Darmstadt. The Universal Linear Accelerator (UNILAC), on which gay experiments are being conducted, 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 setup was designed to produce the most intense ion beams possible. Special efforts were made to ensure that the ion energy could be changed smoothly and set at a given level with sufficiently good reproducibility. Initially, the accelerator project was developed by K. Schmelzer and his collaborators in Heidelberg. The experience gained by 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 a linear accelerator.

When UNILAC was built, the question was posed to many scientists: 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 proved successful was cold fusion combined with the transport of recoil nuclei (fusion products).

Since discovery in 1941 of plutonium, about 400 tons of this element were synthesized, which corresponds to 10 30 atoms. On the other hand, only a few atoms of the 109th element were obtained and identified. Why are the heaviest elements obtained in such vanishingly small quantities? The answer is this: to produce plutonium, tons of neutrons bombard blocks of uranium-238 a few centimeters or more thick, while at UNILAC only 100 micrograms of iron-58 is accelerated to bombard a lead-208 target several hundred nanometers thick. In addition, the cross section of the neutron capture reaction that produces plutonium-239 is about 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. Being synthesized, elements such as 109 decay so quickly that the synthesis "does not 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 should be detected and identified during their production.

The methods of obtaining and detecting elements up to 106 were based mainly on mechanical means of transporting the formed atoms from the reaction zone to the detectors. The transport time between the formation and detection of reaction products was determined by the rates of their transfer in the 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 an unacceptable trade-off between detection speed and accuracy, so that using faster methods it was not possible to reliably identify new isotopes.

To transport the resulting nuclei to the detectors, we chose a technique based on the use of the recoil rate that the 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 original movement of the ion 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 complicated in this case. From the reaction zone, not only individual nuclei formed in the fusion reaction, but also trillions of heavy ions, as well as thousands of atoms knocked out of the target, leave the reaction zone at a high speed. To separate superheavy nuclei from the residual beam, we built a special velocity filter - the Separator for Heavy-Ion Reaction Products (SHIP), 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 rate of recoil of the fusion products can be calculated in advance. Therefore, they can be isolated in a relatively direct 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 nucleus with a certain speed, the influence of the fields is mutually excluded, and it continues to move in the median plane of the setup. Such a tandem filter reduces the number of accelerated ions falling into the detection region by a factor of 100 billion, and the number of knocked-out target nuclei by a factor of 1000. Excluding almost all unwanted particles from the beam, the SHIP spectrometer passes more than 40,070 fusion products. Detectors located behind the spectrometer register decay chains of particles that have passed through the spectrometer, which makes it possible to uniquely identify the fusion products.

The first element of the detecting system is a time-of-flight device that makes it possible to measure the particle velocity for the third time (the first two measurements are based on the principle of the velocity filter). After passing through this device, the particle is implanted into position-sensitive silicon surface-barrier detectors, which register its energy and impact location. Since the combination of time of flight and energy makes it possible to approximately determine the mass of the particle, it is possible to distinguish fusion products from scattered ions and knocked-out target nuclei.

To reliably identify a nucleus, it is nevertheless necessary to establish a correlation between its decay and the decay of its radioactive daughter products. The decay acts due to 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 hits the same spot as the fusion product under study may decay and produce a spatially correlated signal, it is highly unlikely that its decay energy, half-life, and decay type will match those expected for the fusion product. We observed such decay chains up to the fourth generation; the probability that such series of correlated events are random ranges from 10–15 to 10–18. If correlated events due to the isotope under study are observed once a day, then the random appearance of events simulating four generations of decay events can be expected for a time period 100 times longer 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 for the synthesis and identification of 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 an isotope of the 107th element with a mass number of 262 by bombarding bismuth 209 with chromium-54 ions. For the odd-odd isotope of element 107 (having an odd number of both protons and neutrons), we have established five alpha particle energies, which gives 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 recorded at 4:10 pm on August 29, 1982, in a reaction between iron-58 and bismuth-209. The nucleus 266 109 existed for 5 ms before emitting an alpha particle with an energy of 11.1 MeV; the resulting nucleus of the 107th element decayed into the 105th element after 22 ms; The 105th element decayed into the 104th element followed after 12.9 with spontaneous fission of its nucleus. From this single event, it was possible, albeit with limited accuracy, to determine the decay energy, half-life, and reaction cross section. Two more decay chains were observed at the beginning of 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 for the isotope 265 108 in the reaction between iron-58 and lead-208. The two identified isotopes of elements 107 and 109 are odd-odd and the probability of their fission is greatly reduced, however, the isotope of element 108 has an even number of protons and an odd number of neutrons. Although even-odd isotopes are much more likely to fission, the 265108 isotope also undergoes alpha decay.

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

The approximately constant level of stability shows how stabilizing shell effects compete with the overall drop in stability as the mass of the nuclei increases.

Behind the 104th and 105th elements contain a small "island" of nuclei, which, when emitted by alpha particles, decay to form known isotopes of lighter elements. Such acts of alpha decay 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 alpha decay energy can be used to calculate the binding energy of the parent nucleus. If the binding energy of the final product is known, then the chain of alpha decay events can lead to 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 (according to several events) was registered, 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 alpha decay process; 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 drop, the stability of the 100th and 108th elements is completely due to the quantum mechanical structure of their many-particle fermionic systems. Recent theoretical work predicts fission barriers that are consistent with our measurements.

The lifetime of an element relative to division is determined primarily by the height and width of the division barrier. Shell corrections increase the lifetimes of the 106th and 108th elements by 15 orders of magnitude. On a logarithmic scale, the observed lifetimes are in the middle of the range between the nuclear proper 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 be stable on this scale, lifetimes must increase by 12 orders of magnitude. However, nuclear physics is not based on the human time scale.

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 1960s begins much earlier than expected and gradually increases. In a narrow region of instability behind the 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 increases slowly and monotonically.

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-drop 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 could only be observed 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 about 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 it will be possible to detect them using existing methods. Particularly stable regions are assumed near the isotopes 273 109 and 291 115. At a neutron number of 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 nucleus 298 114 - have been unsuccessful. These superheavy isotopes have not been detected either in fusion reactions or in any other reactions involving heavy ions. Nevertheless, the main idea about the possibility of the existence of shell-stabilized nucleon systems, apart from stable nuclear drops, has been confirmed by the experiments described above. Theoretically, there is 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 come from our intensive research into fusion reactions. A shell-stabilized nucleus, spherical in the ground state, can be destroyed even at such a low excitation energy as 15 MeV, this was experimentally demonstrated by K.-Kh. 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 most suitable reaction available), the excitation energy is about 30 MeV. It follows from this 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 with the lightest possible ions. Despite this advantage, this most asymmetric combination has the disadvantage of maximizing the heating of the product core, resulting in high fission losses during the de-excitation process. The less asymmetric the combination, the lower the losses during the cooling stage. The best compromise between low losses in the final stage and a high probability of formation in the initial stage are more symmetrical combinations with target nuclei near lead.

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

Now we are beginning to understand why it will be very difficult to obtain even heavier elements. Only a combination of shell corrections for closed-shell fusion partners, 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 an already existing order into the merging process, avoiding unnecessary clutter.

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

How many elements are in the chemical periodic table? Do they all occupy a stable, stable and unconditional place? On the boundaries of the existence of elements in nature, neutron matter and the synthesis of superheavy elements - Corresponding Member of the Russian Academy of Sciences Yuri Oganesyan and Doctor of Physical and Mathematical Sciences Mikhail Itkis.

Theses for discussion:

What do we know and what do we want to understand about the problem of synthesis of superheavy elements?

Are there limits to the existence of elements in nature?

How did the nucleosynthesis of elements take place in the universe?

What determines the possible stability of superheavy elements?

How fundamental is this problem and does it have a political aspect?

Possibilities of modern experimental technique for its solution.

What is neutron matter? Is it possible to study it in the laboratory, and not only in the process of studying astrophysical objects, such as neutron stars, etc.? Trends in world science.

Does society need to study the above fundamental problems of science? Does it lead to new ideas in the form of new technologies, energy sources, medical devices, etc.

Topic Overview

It is known that all elements from the lightest (hydrogen) to the heaviest (uranium) make up the world around us. They exist on earth. This means that their lifetime is longer than the age of the Earth itself. All elements after uranium are heavier than it. They were formed sometime in the process of nucleosynthesis, but did not survive to this day. Today they can only be obtained artificially.

The concept of an atom is well known: the nucleus, which contains the entire mass of the atom and its positive charge, and the electron orbitals. Hypothetically, it can exist up to atomic numbers: 160 and, perhaps, 170. However, the boundary of the existence of elements is outlined much earlier, and the reason lies in the instability of the nucleus itself. Therefore, the question of the limits of the existence of elements should be addressed to nuclear physics. If you look at nuclei that contain different numbers of protons and neutrons, then stable elements are found only up to lead and bismuth. Then (Fig. 1) there is a "small peninsula", in which only thorium and uranium are found in the Earth. It follows from this that the question of the limits of the existence of elements depends on the stability of nuclei, and should be addressed to nuclear physics.

Rice. 1. Map of isotopes with atomic numbers 70 Zі. The stability of the atoms is shown by the color density according to the right scale. For the 112 Zі and 165 Zі regions, theoretical predictions of the half-lives of hypothetical superheavy atoms are given.

As soon as we move beyond uranium, the lifetime of nuclei drops sharply. Isotopes of transuranium elements are radioactive, they undergo alpha decay. The lifetime of nuclei decreases on a logarithmic scale. This logarithmic scale shows that from uranium (element 92) to element 100, the stability of nuclei decreases by more than 20 orders of magnitude.

In fact, the situation turned out to be even more complicated. Spontaneous fission - the fourth type of radioactivity - overtakes alpha decay in the region of the 100th element, and in the future, the lifetime of nuclei decreases much faster.

Spontaneous fission was discovered by K. A. Petrzhak and G. N. Flerov 60 years ago as a rare type of uranium decay. It becomes the main one when it comes to heavier elements.

The explanation of the phenomenon of spontaneous fission was given by Niels Bohr in 1939. According to N. Bohr, a similar process can occur if we assume that nuclear matter has the properties of structureless matter such as a drop of charged liquid. If a drop experiences deformation under the action of electric forces, then its potential energy grows up to a certain limit, and then it irreversibly decreases with increasing deformation until the drop is divided into two parts. Thus, a certain barrier will appear at the uranium nucleus, which keeps this nucleus from fission for 10 16 years.

If we move from uranium to a heavier element, in whose nucleus the Coulomb forces are much greater, the barrier is lowered, and the probability of fission is greatly increased. Finally, with a further increase in the charge of the nucleus, we will come to the limit when there is no longer any barrier, i.e., when even the spherical shape of the drop turns out to be unstable to separation into two parts.

This is the limit of kernel stability. According to the calculations of Bohr and Wheeler, this limit was expected for elements with atomic numbers 104–106.

Quite unexpected was the discovery in 1962 at the Dubnin Laboratory of Nuclear Reactions of yet another half-life for heavy nuclei, including uranium. That is, one and the same nucleus can have two decays of the same type with different probabilities, or two lifetimes. For uranium, one time is 10 16 years, which was discovered by Flerov and Petrzhak, and the second is very short, only 0.3 microseconds. With two half-lives, one must assume that the nucleus has two states from which fission occurs. This does not fit into the idea of ​​a drop in any way.

Two states can only exist if the body is not amorphous, but has an internal structure.

So, nuclear matter is not a complete analog of a charged liquid drop

A drop is a kind of approximation to the description of nuclear matter; the nucleus has an internal structure.

Questions of nuclear structure have been taken seriously by nuclear theorists; in our country - V. M. Strutinsky, S. T. Belyaev, V. V. Pashkevich and others. They solved a rather difficult problem - how to explain that the uranium barrier is two-humped and how the structure of the nucleus changes during its deformation.

And it has been explained. But if the explanation found by theorists correctly reflects the properties of nuclei, then when we come to superheavy elements, the picture will be completely different from what was predicted for a drop of liquid. In heavy elements, this structure will manifest itself in full measure where the drop is inconsistent, and the so-called structural barrier will appear. And this means that the nucleus can live for a very long time.

This non-trivial conclusion of the theory led, in essence, to the prediction of a hypothetical region of stability for superheavy elements located far from those elements that are known and with which we are accustomed to work.

As soon as this was predicted, all the largest laboratories in the world literally rushed to experimentally test this hypothesis. This was done in the United States, in France, in Germany. However, all experiments yielded negative results.

For the past two years, experiments have been carried out in the Dubna laboratory on the synthesis of new, heaviest elements with atomic numbers 114 and 116. The task was to obtain atoms of new elements, the nuclei of which have a large excess of neutrons. Only in this case would we be able to approach the boundaries of the hypothetical "island of stability" and observe an increase in the lifetime of superheavy nuclei.

The results of the experiments led to the conclusion that the "island of stability" really exists.

What are the ways to obtain (synthesis) superheavy nuclei? At first, the neutron synthesis method was used, when a lot of neutrons are driven into the nucleus. In this case, it would be natural to irradiate the initially starting substance with a powerful neutron flux. More and more powerful reactors were used for this. However, the reactor fusion method has exhausted itself on fermium (an element with atomic number 100), because the fermium isotope with a mass of 258, which should be obtained as a result of neutron capture, lives only 0.3 milliseconds. The entire chain of successive neutron captures broke at the capture stage of the 20th neutron. Here you need to go through more than 60 steps. The neutron method did not work.

An attempt by American researchers to use another method - to obtain superheavy elements in nuclear explosions, that is, in a powerful pulsed neutron flux, ultimately led to the formation of the same isotope of element 100 with a mass of 257.

The hopelessness of the neutron method led to the idea of ​​using a fundamentally different method for the synthesis of superheavy elements, which began to develop in the mid-1950s - "heavy nuclear". It consists in the fact that two heavy nuclei collide with each other in the hope that they will merge and, as a result, a nucleus of total mass will be obtained. In order for such a reaction to occur, one of the nuclei must be accelerated to a speed of about 0.1 of the speed of light. This function is performed by accelerators. What we know today about the properties of heavy elements in the second hundred was obtained using heavy ion accelerators in reactions of this type.

What are the properties of transuranium elements?

If the 92-element - uranium lives for a billion years, then the heavy nucleus of the 112-element lives only 0.1 milliseconds. Indeed, an increase in the atomic number by 20 units leads to a decrease in the lifetime of the nucleus by more than 1020 times. However, the "island of stability" is located where the nuclei contain significantly more neutrons. Therefore, it is necessary to move towards more neutron-rich nuclei. This is difficult to achieve, since the ratio of the number of protons to the number of neutrons in stable nuclides is strictly defined. It was decided to use reactions in which a large neutron excess was initially specified both in the core of the target material, which is produced in a nuclear reactor, and in the projectile core, which in this case was chosen as the calcium-48 core.

Calcium-48 is a stable isotope of calcium, an element with atomic number 20. There is a lot of calcium in nature. But the calcium isotope with a mass of 48 is extremely rare. Its content in ordinary calcium is only 0.18%. To isolate it from calcium is an incredibly difficult task. Nevertheless, if we were able to accelerate calcium-48 ions, then by irradiating uranium, plutonium, or curium, we could get into the treasured region where stability is expected to rise, and there we should feel the effect of a sharp increase in the lifetime of superheavy elements.

In a specific experiment, a reaction was chosen where plutonium (Z = 94), its heaviest isotope with a mass of 244, was used as the starting material, and calcium-48 isotope as the bombarding ion. We expected that the fusion reaction of these nuclei would lead to the formation of the 114-element, which should be more stable than the previously studied elements.

In order to set up such an experiment, it was necessary to create an accelerator with a calcium-48 beam power that is dozens of times superior to all known accelerators. At the same time, he had to give a high intensity of accelerated ions and spend as little as possible expensive calcium-48. This required a long and intense search for a solution to the problem. In the end, a solution was found and within 5 years such an accelerator was created in Dubna. At a very low consumption of the substance (0.3 mg/h), a beam intensity of several units per 10 12 ions per second was obtained. Now it was possible to set up an experiment a hundred and a thousand times more sensitive than was done previously by the Dubninists and their colleagues in other countries over the past 25 years.

The essence of the experiment itself was as follows. Having received a calcium beam, a plutonium target is irradiated. The heavy isotope plutonium-244 was provided by the Livermore National Laboratory (USA). If, as a result of the fusion of two nuclei, atoms of a new element are formed, then they should fly out of the target and, together with the beam, continue to move forward. Here they must be separated from calcium-48 ions and other reaction products. This function is performed by a separator (Fig. 2), in which there is a transverse electric field. Since the speeds of the nuclei are different, the beam hits the stopper, while the heavy recoil nuclei of the 114-element make a curvilinear trajectory and eventually reach the detector. The detector recognizes a heavy nucleus and records its decay.

What, in fact, can be expected next? If the hypothesis is true that there is an "island of stability" in the region of superheavy elements and these nuclei are very stable against spontaneous fission, they must experience another type of decay - alpha decay.

In other words, nuclei at and near the top of this island that are resistant to spontaneous fission must be alpha radioactive. An alpha radioactive nucleus, as is known, spontaneously ejects an alpha particle (helium nucleus), consisting of two protons and two neutrons, passing into a daughter nucleus. For the selected reaction, this is the transition of the 114th to the 112th element. The nuclei of element 112 must also undergo alpha decay and pass into the nuclei of element 110, etc. But as successive alpha decays, we move further and further away from the pinnacle of stability and eventually fall into a sea of ​​instability, where the predominant type of decay will be spontaneous fission. For the experimenter, this is a very vivid picture: as a result of successive alpha decays, each of which leaves an energy of about 10 MeV in the detector, fission occurs, in which an energy of about 200 MeV is immediately released. This breaks the chain of decay.

Such a chain can be observed if the theoretical hypothesis is valid. Indeed, during the experiment, which lasted continuously for three months, the scientists observed for the first time what they had been waiting for.

Rice. 3a. Chains of successive decays of superheavy atoms with Z = 114 and 116 registered in nuclear reactions with 48 Ca ions. For each decay, the values ​​of energy, signal arrival time, and its positional coordinates on the detector surface with an area of ​​50 cm² are indicated.

After the recoil nucleus came to the detector, which measures its energy, velocity and coordinates of its stopping place with high accuracy, an alpha particle with an energy of 9.87 MeV was registered a second after it stopped. Interestingly, in the heaviest nucleus synthesized earlier, this time took only one ten-thousandth of a second. Here is a second.

Then, after 10.3 seconds (also a long time), a second alpha particle with an energy of 9.21 MeV flew out and then, after 14.5 seconds, spontaneous fission occurred. The whole chain of decays took about 0.5 minutes.

The second event was the same as the first. Both of these events coincide with each other in 13 parameters. Therefore, the probability of random coincidences of signals in the detector simulating such a decay is only 10 −16 .

In the same experiment, another event, much longer lived, was also observed. Here, the decay is calculated in minutes and tens of minutes.

If we deviate into the region of nuclei with a deficit of neutrons, then spontaneous fission becomes more and more likely, which was discovered (when the lighter isotope, plutonium-242, was used instead of the plutonium-244 target). This exactly reproduces the scenario that was predicted by the theory that the island is on the right, among the nuclei enriched with neutrons.

Thus, the synthesized nuclei-isotopes of element 114 and their daughter products of alpha decay, new isotopes of elements 112 and 110 are already experiencing the action of these structural forces that form the "island of stability" of superheavy elements. And despite the fact that they are at a considerable distance from the top of the island, nevertheless, their times are minutes and tens of minutes (Fig. 4). This increases their stability by about 5 orders of magnitude compared to isotopes of the same elements located far from the island boundary.

A unique substance - curium-248 was obtained at a powerful reactor of the Research Institute of Atomic Reactors in Dimitrovgrad. Observation of the chain of decays of the 116-element would be another proof of the receipt of the 114-element - in the first case, it was obtained directly by irradiating a plutonium target; in the same reaction as a result of the decay of the heavier parent.

Rice. 4. A map of nuclides indicating the chains of radioactive decay of atoms synthesized in nuclear reactions under the action of accelerated 48 Ca ions. The topographic background demonstrates the strength of structural effects in the nucleus of an atom.

Such an experiment was carried out recently - and here the scientists took some risks.

If a 116-element is formed in the reaction, then after its alpha decay, the nucleus of the 114-element should be obtained; in other words, in this experiment the scientists had to observe once again (for the third time) the entire decay chain of the 114-element, in addition to the 116-element.

After the release of the alpha particle from the decay of the 116-element, the accelerator was turned off, and all the power equipment in the laboratory was turned off in order to create absolutely background-free conditions. Indeed, after the heavy recoil nucleus hit the detector, after 47 milliseconds, an alpha particle with an energy of 10.56 MeV flew out, which turned off all powerful equipment. After that, under completely calm conditions, another alpha particle was emitted, then another, and then spontaneous fission.

If we compare the decay chain after the accelerator was turned off with what was observed for the 114-element, then we can see a complete match in all parameters (Fig. 3b). It really was the decay of the 114th element, and, therefore, the previous alpha particle belongs to the 116th. It happened on July 19, 2000. In 2001, the experiment was continued and, as a result, 2 more nuclei of 116 elements were synthesized.

Now we can compare the prediction of the theory and the results obtained in the experiment. For the 116th element, according to the theory, with an increase in the number of neutrons in the nucleus from 166 to 176, the lifetime of the nucleus should have increased by 5 orders of magnitude. The experiment gave a value of about 6 orders of magnitude. For the 114th element, the picture looks the same way. With an increase in the number of neutrons in this nucleus from 164 to 174, the half-life period increases by more than 6 orders of magnitude. For the 112-element, an excess of 10 neutrons also increases the stability of the nucleus by 5–6 orders of magnitude. The same picture is typical for isotopes of element 110.

This is good agreement with the theoretical hypothesis. In addition, the experiment shows that the superheavy nuclides in this region are longer lived than it follows from the theory.

Attention should be paid to the top of the "island of stability". This peak can be millions of years old. It does not reach the age of the Earth, which is 4.5 billion years. However, if we take into account that in the experiment we have an excess of stability over the calculated values ​​on the spurs of the "island of stability", then the presence of superheavy elements in nature, in our system, or in cosmic rays, i.e., in other systems, is not excluded. Superheavy elements may exist there, the lifetime of which will be calculated in millions of years.

Another circumstance is important: now the table of elements has been replenished with new 114 and 116 elements. The experiments gave a new sound to the previously known elements 112, 110, 108, since the increase in neutrons led to a significant increase in their lifetime. This makes it possible to study the chemical properties of these elements. Elements 112, 110 and 108, which live for minutes, have become quite accessible for the study of their chemical properties using modern radiochemistry methods. You can set up experiments to verify the fundamental Mendeleev's Law regarding the unification of properties in columns. With regard to superheavy elements, we must assume that the 112th element is a homologue of cadmium, mercury; The 114th element is an analogue of tin, lead, etc. So far, this is just an extrapolation of our ideas to previously unknown elements. The fundamental law of the periodicity of the chemical properties of elements can now be verified experimentally.

Stable elements end in lead and bismuth. The nuclei of these atoms are magical, which determines the increased binding energy of nucleons in the nucleus. Then comes the area of ​​radioactive elements, among which thorium and uranium are the most stable. Their half-life is comparable to the age of our planet. As we move towards heavier elements, the lifetime of nuclei sharply decreases. The peninsula of radioactive elements has pronounced boundaries. The theory predicted that the "peninsula" would be followed by "islands of stability". They will be located in the region of very heavy elements, the nuclei of which are enriched with neutrons.

Attempts to obtain these nuclei in powerful neutron fluxes were unsuccessful. On the other hand, in reactions with heavy ions, starting from the 50s, it was possible to synthesize 12 artificial elements with atomic numbers over 100. But it was not possible to obtain an excess of neutrons in the nuclei of these elements, which would allow answering the question: the world ends with a » radioactive nuclei or it will be followed by an «island of stability» of even heavier - superheavy elements.

Using beams of accelerated ions of the calcium-48 isotope and choosing artificial elements as a target - heavy isotopes of plutonium and curium obtained in powerful reactors, scientists managed to approach only the boundaries of this hypothetical "island of stability" and already here to detect a significant increase in the stability of superheavy elements. The experiments continue, the 118th element is next in line.

What's next? The success achieved gave rise to new ideas for the development of open terra incognita. First of all, we would like to obtain nuclei of superheavy elements (SHE) in large quantities. Of course, the very fact of discovering a new element from just two observed atoms is impressive, but a much larger number is required for a more complete study. It is necessary to create fundamentally new, more efficient experimental facilities. The design work took half a year, and at present the Laboratory is implementing a project to create a Mass Analyzer of Superheavy Atoms (MASHA). There are no analogues of such an experimental setup in the world. With its commissioning, scientists expect to receive dozens of SHE atoms and study their properties more widely. The DRIBs project is also being implemented, in which two powerful accelerators are combined into a single complex, which will allow accelerating atoms of radioactive isotopes, in particular tin-132. This will provide fundamentally new possibilities for the synthesis of SHEs.

Minatom connected its organizations to the program and allocated the necessary finances (15 million rubles annually for 4 years). The Ministry of Science allocated a special grant in the amount of 1 million rubles. RAO ES received an exclusive right to allocate electricity to power the accelerators during experiments. The Americans from Livermore sent plutonium-244 free of charge. The Governor of the Moscow Region, B. V. Gromov, allocated funds from his reserve to the Joint Institute for Nuclear Research to finance research on superheavy elements (10 million rubles in 2001 and 15 million rubles in 2002). There is no doubt that the intellectual and technical resources accumulated in Dubna and other similar centers of Russia must be used for the development of modern high-tech and knowledge-intensive processes, which alone can ensure the competitiveness of Russian products on the world market in the future.

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