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 on 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.
On November 28, 2016, the International Union of Pure and Applied Chemistry (IUPAC) assigned names to four superheavy elements: nihonium (element 113 of the periodic table), moscovium (element 115), tennesine (element 117) and oganesson (element 118). Moscovium, tennessine and oganesson were first obtained in the Russian Federation in collaboration with American physicists. On the anniversary of this date N+1 together with Yandex Publishing House, we invite you to imagine yourself as an alchemist and try to synthesize one (or several, depending on your luck) superheavy elements at a particle accelerator.
Superheavy chemical elements with an atomic number greater than 100 can only be obtained through fusion reactions in charged particle accelerators. In them, a heavy target core is fired at with lighter projectile cores. The nuclei of new elements arise in the event of an accurate hit and fusion of the projectile and target nuclei. You have the opportunity to feel like an amateur alchemist and create a new element. You have projectile cores and target cores at your disposal. Select a pair and click the “Enable Accelerator” button. If you choose the right pair, you will get a superheavy element, see the products of its decay and find out who and when it was synthesized in reality.
We, together with Yandex Publishing House, have also prepared answers to questions about superheavy elements common on the Internet. Click on a question to see the answer.
Is it possible to predict how many superheavy elements may still be discovered? Is there a maximum number of protons that can be in the nucleus that would limit the mass of the element?
All such predictions are based on modern models of the stability of atomic nuclei. Based on the most naive considerations, it seems that any nucleus in which the Coulomb repulsion between positively charged protons is compensated by the bond strength between them due to the strong interaction can be stable. For this, in any case, there must be a certain number of uncharged neutrons in the nucleus, but the ratio between the number of neutrons and protons is not a sufficient condition for the stability of atomic nuclei. This is where the quantum nature of nucleons comes into play: they have half-integer spin and, like electrons, tend to cluster in pairs and form filled energy levels.
These effects lead to differences in the stability of proton-neutron systems relative to several decay paths - spontaneous fission (which occurs as a result of quantum mechanical effects and without external excitation leads to separation into lighter nuclei and neutrons), as well as α- and β-decay with emission of an alpha particle or electron (or positron), respectively. In relation to each of the decay channels, each nucleus has its own lifetime. Thus, with an increase in the atomic number of an element, the probability of spontaneous fission increases sharply, which imposes significant restrictions on the existence of stable nuclei of superheavy elements - all of them must be unstable with a rather short half-life. Therefore, for all elements heavier than lead there are no stable isotopes; they are all radioactive.
However, the theory predicts that even among superheavy elements there may be isotopes with relatively long lifetimes. They must exist for systems with a suitable ratio of protons and neutrons and completely filled proton and neutron levels. However, it has not yet been possible to synthesize such elements, and if it seems possible to get to the nearest “island of stability” (which is predicted for the flerovium nucleus with 184 neutrons) in the near future, then it will be possible to find heavier nuclei with the next filled shell among absolutely unstable systems much more difficult, if not impossible.
It is worth noting, however, that all these predictions are based on models that work well for relatively small nuclei, but for superheavy elements the shape of the nucleus, for example, begins to deviate quite noticeably from spherical, which requires corrections to be made to these models.
Do superheavy elements have any practical uses? Or perhaps it will appear in the future?
At the moment, superheavy elements have no practical use. This is due to several reasons. Firstly, their synthesis is an extremely complex technological process that takes quite a long time, as a result of which a very small number of nuclei are formed. Secondly, of all the elements with an atomic number greater than one hundred, only fermium (the 100th element) and mendelevium (the 101st) have relatively stable isotopes with half-lives of 100 and 50 days, respectively. For the remaining superheavy elements, even the most stable synthesized isotopes decay in a few tens of hours at best, and more often in seconds or even milliseconds.
Therefore, for now, the process of synthesis of superheavy nuclei is of only fundamental interest associated with the study of nucleon-nucleon interaction and interaction between quarks. The properties of synthesized isotopes help to build more accurate theoretical models that can be used not only to study the nuclei of atoms on Earth, but also, for example, when studying neutron stars, in the core of which the density of nucleons is significantly higher than the density in the nuclei of atoms.
Scientists expect that in the future, superheavy elements may have some practical applications, related, in particular, to the development of sensors or radiographic methods in medicine or industry. Perhaps these will be some new methods of use that cannot be predicted now, but they definitely should not be expected in the coming years, because for this the technologies for their production must radically change.
Is it possible to obtain stable isotopes of superheavy elements, or will they all only be radioactive?
The stable isotopes of the elements located in the periodic table after lead are currently unknown. The serial number of lead in the periodic table is 82nd. This means that all elements starting with bismuth will be radioactive in one way or another. The half-lives of these elements, however, can vary within very wide limits. Thus, the most stable isotope of bismuth, which was previously considered stable, has a half-life of 2 × 10 19 years, which is several orders of magnitude greater than the age of the Universe.
The currently synthesized isotopes of superheavy elements (with a serial number in the table of elements greater than one hundred) have a half-life that is significantly shorter than that of bismuth, and varies from one hundred days to fractions of a millisecond. All of them are also radioactive.
However, according to theoretical predictions, for some elements with a certain number of protons and neutrons in the nucleus, a significant increase in half-life is possible. The required number of neutrons and protons in the nucleus corresponds to completely filled neutron and proton shells and should presumably be equal to 114 for protons and 184 for neutrons. Theoretically, such a configuration should lead to an increase in half-life from hundreds of microseconds to 10 5 years. The relative stability of nuclei with numbers of protons and neutrons close to these values suggests the existence of an “island of stability” among superheavy elements. However, it has not yet been possible to confirm its existence experimentally. But even such a significant increase in the lifetime of nuclei will not make these isotopes stable - they will remain radioactive.
Is it possible, at least theoretically, to detect superheavy elements in nature? Or at least the products of their decay, which would prove that such elements existed?
None of the superheavy elements have been found in nature (which is not surprising, given that they all have very short half-lives). The element with the highest atomic number found in nature to date is uranium, with its 92 protons in the nucleus.
In the early 1970s, the presence of an element with serial number 108 (later synthesized under the name hassium) was reported in natural minerals; about ten years ago, traces of element 122 were discovered in thorium samples, but these facts were not confirmed.
On Earth, the conditions necessary for the synthesis of stable superheavy nuclei do not exist and never have existed, but it is believed that close to such conditions can be achieved during supernova explosions. In this case, the temperature rises to values sufficient to trigger the rapid absorption of neutrons by nuclei (the so-called r-process). So far, no reliable evidence of the natural formation of elements with a serial number greater than 100 in such processes has been recorded, but studies are being carried out on the composition of cosmic rays to determine the presence of traces of superheavy elements in them. In particular, the discovery of particles with atomic numbers greater than 100 in meteorite matter was discussed in 2011. These data, however, have also not been confirmed.
Where did the expression “transfermium wars” come from and why does the question of the primacy of one group or another in the synthesis of a new element so often arise?
This expression is usually used to refer to disputes between the United States and the USSR over priority in the discovery of elements with serial numbers 104, 105 and 106, which were discovered in the 60s and 70s of the 20th century. The term “transfermium wars” (all these elements are located in the periodic table just after fermium) was first proposed in 1994. In the Soviet Union, synthesis was carried out at the Joint Institute for Nuclear Research in Dubna, in the USA - at the Lawrence Berkeley and Livermore National Laboratories. The first successful attempts to synthesize element 104 now date back to 1964, element 105 to 1970, and element 106 to 1974.
The Soviet side believed that it was in Dubna that the 104th and 105th elements were first synthesized, and used the names “kurchatovy” and “nielsborium” for them, respectively. American scientists criticized the results of Soviet experiments and argued that they were the first to obtain these elements of physics in their laboratories and called them “rutherfordium” and “ganium” (in honor of Ernest Rutherford and Otto Hahn, respectively). However, due to the fact that a significant part of the data on synthesis was closed at that time, it was quite difficult to unambiguously determine the primacy of one or another group.
Because of this, the process of determining primacy lasted for 30 years and became one of the elements of the Cold War. Only in 1994 was an international commission assembled, which reviewed the known data and proposed its own variants of names for the elements. Initially, some of the decisions made were controversial, in particular the naming of elements in honor of a still living person (Glenn Seaborg), the transfer of the name from one element to another regarding the initial proposals (which involved a third party in the controversy - the German Society for the Study of Heavy Elements, whose scientists synthesized 107th, 108th and 109th elements).
As a result, a compromise solution was found, and in 1997 the priorities and names of the elements were finally approved. In particular, it was decided not to perpetuate the names of Igor Kurchatov and Otto Hahn, who were related to Soviet and Nazi nuclear projects. The 104th and 106th elements now use the names proposed by the American side (rutherfordium and seaborgium), the 105th element - in recognition of the merits of Soviet scientists, was called dubnium, for the 107th, 108th and 109th elements they use the names proposed by German scientists - bohrium, hassium and meitnerium (only the first of them differs from the proposed option - it was initially proposed to call it nilsborium). Now, thanks to the openness of data and the prescribed procedure for assigning names to elements, questions about priority are resolved much easier.
Miniature from the 16th century alchemical manuscript “The Splendor of the Sun”
Can superheavy elements be created in supernova explosions? And can we record this birth?
It is known that supernova explosions can produce nuclei of very heavy elements, such as uranium or thorium. These nuclei are formed by the mechanism of rapid neutron capture (the so-called r-process). It is believed that a supernova explosion produces enough temperature - about four billion degrees - to trigger this process. However, the frequency of formation of the heaviest nuclei even under such conditions is not very high. It is also believed that, in addition to uranium and thorium, during the explosion of supernovae, for example, the formation of californium (this is the 98th element) is possible.
For the formation of heavier nuclei as a result of the r-process, the launch of a thermonuclear reaction is necessary - thus, for example, on Earth it was possible to synthesize einsteinium (the 99th element) and fermium (the 100th) for the first time. It is assumed that several thermonuclear explosions can lead to the achievement of an island of stability as a result of the r-process. However, today it is generally accepted that during supernova explosions such conditions are not met and elements with atomic numbers greater than 100 are not formed. Nevertheless, traces of stable superheavy elements that could be formed during supernova explosions continue to be sought, for example, in cosmic rays and meteorites irradiated by them. Confirmation of the synthesis of lighter elements (for example, uranium or californium) is carried out by spectroscopic studies of the products of their spontaneous fission.
Why do synthesis reactions of superheavy elements so often fail when, according to theoretical calculations, they should work?
Superheavy nuclei are produced by the fusion reaction of lighter nuclei with each other. To do this, a target made of heavier elements is bombarded with nuclei of lighter ones. To obtain a nucleus with the required number of protons and neutrons, you need to correctly select those nuclei that are used as targets and projectiles. There may be several problems here that reduce the likelihood of the desired nucleus being formed and detected.
Firstly, to form the desired nucleus, it is necessary to overcome the electrostatic barrier - after all, both colliding nuclei have a fairly large positive charge (and before attractive forces begin to act at short distances between the protons, long-range electrostatic repulsion must be overcome). To do this, those nuclei with which the target is bombarded must initially be given a sufficiently high energy.
To reduce this barrier, it is more advantageous to use nuclei with a fairly large number of protons as incident particles. However, their choice is currently limited. Previously, to synthesize new nuclei, targets made of heavy elements, such as lead, plutonium or uranium, were bombarded with relatively light nuclei, such as neon-22 or oxygen-18. Later, various isotopes of heavier elements were used for these purposes: iron-58, nickel-62, nickel-64 or zinc-70. The reaction products of various targets with the calcium-48 isotope became extremely important.
Reactions in which a uranium target is bombarded with ions from superheavy elements - the same uranium, californium, einsteinium - are considered promising. To increase the probability of nucleus formation, it is necessary that the incident nucleus have a relatively small angular momentum, and the resulting “compound nucleus” has a shape close to spherical. Violation of these requirements leads to the fact that reactions do not occur. However, even with the correct selection of parameters, the synthesis process is very long - irradiation of the target for several months can lead to the synthesis of hundreds of the desired nuclei.
Thus, the limited choice of isotopes that can be used in fusion reactions, their complex, from a technical point of view, their implementation and the long reaction times significantly reduce the likelihood of the synthesis of the desired nuclei - even those that, according to theoretical predictions, should be stable.
Previously, it was believed that the center of the “island of stability” should be located in the region of element 114, but where is the “island of stability” located according to modern ideas? Maybe it doesn't exist at all?
The center of the “island of stability,” according to the shell model of the nucleus, corresponds to completely filled proton and neutron shells - an isotope with an order number of 114 and a mass number of 298, that is, a nucleus consisting of 114 protons and 184 neutrons.
Some scientists believe that the center of the "island of stability" may correspond to the next proton "magic number" and thus element number 120 (and maybe even number 126) should be more stable. In addition, due to the high probability of α-decay, the center of stability may be shifted from the number 114 to the 112th and 110th elements.
Since not only the number of protons in it, but also the number of neutrons is important for the formation of a relatively stable nucleus, so far it has not been possible to synthesize isotopes with the required number of nucleons due to the limited choice of isotopes in the experiment. Therefore, there is no necessary data to confirm the existence of an “island of stability”. However, those measurements that were carried out for less stable isotopes of superheavy elements are in fairly good agreement with the data of theoretical models.
However, it is worth noting that the position of the “island of stability” is determined within the framework of the concept of the shell model of the nucleus, which may not work entirely accurately with a large number of neutrons or protons. In particular, some effects associated with the interaction of quarks for neutron-excess nuclei cannot be explained using it.
What is the lifespan of the elements in the center of the “island of stability”?
According to theoretical predictions, the center of the “island of stability” corresponds to a nucleus consisting of 114 protons and 184 neutrons. It has not yet been possible to synthesize such a heavy isotope. However, according to theoretical models, exactly this number of nucleons in the nucleus corresponds to completely filled energy shells.
As for the half-lives of these elements, when nuclear fission occurs, three possible processes should be taken into account: spontaneous nuclear fission, as well as α- and β-decay. Thus, the half-life of 298 114, according to model predictions, should be approximately 10 16 years relative to spontaneous fission, 10 years relative to α-decay, and about 10 5 years relative to β-decay.
Taking into account all types of decay, the most stable nucleus turns out to be the 298 110 nucleus. According to theory, its half-life should be about 10 9 years. However, the region of stable nuclei is relatively broad, and almost all nuclei with an even number of protons from 110 to 114 and an even number of neutrons from 180 to 184 have a half-life greater than 1 year.
So far, these numbers are only the result of theoretical calculations. The heaviest and most stable isotope of element 114 (flerovium Fl) that has been obtained experimentally to date is 289 Fl. Its half-life is about 30 seconds. The period of the most stable isotope of element 110 (darmstadtium Ds) is about 10 seconds. Nevertheless, the experimentally obtained values agree quite well with the predictions of theoretical models, so if it is possible to synthesize the desired nuclei with a large number of neutrons, their lifetime can increase significantly.
Ten years ago, scientists said that there might be a second “island of stability.” Did you manage to find it?
In general, according to modern theoretical models, in the observable region of elements there may exist not two, but even more “islands of stability”, which will correspond to nuclei with completely filled neutron and proton shells, when the number of nucleons is equal to the so-called “magic number”. Currently, an element that may be an "island of stability" corresponds to an isotope consisting of 114 protons and 184 neutrons. According to modern shell models of the nucleus, the next “magic numbers” for protons are 126 and 164, and for neutrons - 196, 228 and 272.
The possible existence of relatively stable nuclei with 120 or 126 protons has been talked about for quite some time, and ten years ago they talked about the possible existence of an “island of stability” in the region of the 164th element. However, if a possible study of the 120th element in a relatively near future can still be expected, then there is no need to talk about the experimental study of the 126th, and especially the 164th element. For this, new accelerators of heavy nuclei are needed, which would allow working with low concentrations of short-lived isotopes. At the moment there are no such devices.
Currently, the heaviest element whose synthesis has been confirmed is oganesson with atomic number 118. In addition, it is worth noting that the applicability of the theoretical models used for such heavy nuclei has also not been proven.
Can neutron stars be viewed as a giant atomic nucleus? If not, what is the fundamental difference?
No, a neutron star, although it consists mainly of protons and neutrons, is not very similar to a giant atomic nucleus. In fact, the star has a rather complex structure - at least five layers with different properties, and heavy atomic nuclei are part of some of them as one of the important components. Moreover, in the outer layers of a neutron star there are, for example, electrons. And in the inner layers - closer to the center of the neutron star - there are a lot of free neutrons.
Despite the fact that the atomic nucleus is a quantum mechanical system with the maximum density of neutrons and protons on Earth, in neutron stars the nucleon density is much higher. The size of neutron stars is only a couple of tens of kilometers, and their mass often exceeds the mass of the Sun, so closer to the center of the star it has a very high density - several times more than in any atomic nucleus. The core of a neutron star contains only a few percent electrons and protons; the bulk is made up of neutrons, which are in the Fermi liquid state. In the very center of the star - in the inner core - the density of nucleons can be 10–15 times higher than the density in atomic nuclei, while the exact composition, state and mechanisms of interaction of particles in such dense systems are not reliably known.
Studies of neutron-rich nuclei provide important information about how neutrons and quarks can interact in the core of a neutron star, but the state of the nucleons at the center of a neutron star is in any case very different from what can be observed in the atomic nuclei of even the heaviest elements.
Alexander Dubov
Based on various theoretical models, the decay characteristics of superheavy nuclei were calculated. The results of one such calculation are shown in Fig. 4. The half-lives of even-even superheavy nuclei are given relative to spontaneous fission (a), α-decay (b), β-decay (c) and for all possible decay processes (d). The most stable nucleus with respect to spontaneous fission (Fig. 4a) is the nucleus with Z = 114 and N = 184. For it, the half-life with respect to spontaneous fission is ~10 16 years. For isotopes of element 114, which differ from the most stable one by 6-8 neutrons, half-lives decrease by 10-15 orders of magnitude. The half-lives relative to α-decay are shown in Fig. 4b. The most stable core is located in the Z region< 114 и N = 184 (T 1/2 = 10 15 лет). Для изотопа 298 114 период полураспада составляет около 10 лет.
Nuclei stable with respect to β-decay are shown in Fig. 4c with dark dots. In Fig. 4d shows the complete half-lives. For even-even nuclei located inside the central contour, they are ~10 5 years. Thus, after taking into account all types of decay, it turns out that nuclei in the vicinity of Z = 110 and N = 184 form an “island of stability.” The 294 110 nucleus has a half-life of about 10 9 years. The difference between the Z value and the magic number 114 predicted by the shell model is due to competition between fission (relative to which the nucleus with Z = 114 is most stable) and α-decay (relative to which nuclei with lower Z are stable). For odd-even and even-odd nuclei, the half-lives increase with respect to α-decay and spontaneous fission, and decrease with respect to β-decay. It should be noted that the above estimates strongly depend on the parameters used in the calculations and can only be considered as indications of the possibility of the existence of superheavy nuclei with lifetimes long enough for their experimental detection.
The results of another calculation of the equilibrium shape of superheavy nuclei and their half-lives are shown in Fig. 5, 11.11. In Fig. Figure 11.10 shows the dependence of the equilibrium deformation energy on the number of neutrons and protons for nuclei with Z = 104-120. The deformation energy is defined as the difference between the energies of nuclei in equilibrium and spherical form. From these data it is clear that in the region Z = 114 and N = 184 there should be nuclei that have a spherical shape in the ground state. All superheavy nuclei discovered to date (they are shown in Fig. 5 as dark diamonds) are deformed. Light diamonds show nuclei that are stable with respect to β-decay. These nuclei must decay by α decay or fission. The main decay channel should be α-decay.
The half-lives for even-even β-stable isotopes are shown in Fig. 6. According to these predictions, half-lives are expected for most nuclei much longer than those observed for already discovered superheavy nuclei (0.1-1 ms). For example, for the 292110 nucleus, a lifetime of ~51 years is predicted.
Thus, according to modern microscopic calculations, the stability of superheavy nuclei increases sharply as they approach the neutron magic number N = 184. Until recently, the only isotope of an element with Z = 112 was the isotope 277 112, which has a half-life of 0.24 ms. The heavier isotope 283112 was synthesized in the cold fusion reaction 48 Ca + 238 U. Irradiation time 25 days. The total number of 48 Ca ions on the target is 3.5·10 18. Two cases were recorded that were interpreted as spontaneous fission of the resulting isotope 283 112. The half-life of this new isotope was estimated at T 1/2 = 81 s. Thus, it is clear that an increase in the number of neutrons in the isotope 283112 compared to the isotope 277112 by 6 units increases the lifetime by 5 orders of magnitude.
In Fig. Figure 7 shows the measured lifetime of seaborgium isotopes Sg (Z = 106) in comparison with the predictions of various theoretical models. Noteworthy is the decrease in the lifetime of the isotope with N = 164 by almost an order of magnitude compared to the lifetime of the isotope with N = 162.
The closest approach to the island of stability can be achieved in the reaction 76 Ge + 208 Pb. A superheavy almost spherical nucleus can be formed in a fusion reaction followed by the emission of γ quanta or a single neutron. According to estimates, the resulting 284 114 nucleus should decay with the emission of α particles with a half-life of ~ 1 ms. Additional information about the occupancy of the shell in the region N = 162 can be obtained by studying the α-decays of nuclei 271 108 and 267 106. Half-lives of 1 min are predicted for these nuclei. and 1 hour. For nuclei 263 106, 262 107, 205 108, 271,273 110 isomerism is expected, the reason for which is the filling of subshells with j = 1/2 and j = 13/2 in the region N = 162 for nuclei deformed in the ground state.
In Fig. Figure 8 shows the experimentally measured excitation functions for the formation reaction of the elements Rf (Z = 104) and Hs (Z = 108) for the fusion reactions of incident ions 50 Ti and 56 Fe with a target nucleus 208 Pb.
The resulting compound nucleus is cooled by the emission of one or two neutrons. Information about the excitation functions of heavy ion fusion reactions is especially important for obtaining superheavy nuclei. In the fusion reaction of heavy ions, it is necessary to precisely balance the effects of Coulomb forces and surface tension forces. If the energy of the incident ion is not high enough, then the minimum approach distance will not be sufficient for the merger of the binary nuclear system. If the energy of the incident particle is too high, then the resulting system will have a high excitation energy and will most likely disintegrate into fragments. Effective fusion occurs in a rather narrow energy range of colliding particles.
Fusion reactions with the emission of a minimum number of neutrons (1-2) are of particular interest, because in synthesized superheavy nuclei, it is desirable to have the largest possible N/Z ratio. In Fig. Figure 9 shows the fusion potential for nuclei in the reaction
64 Ni + 208 Pb 272 110. The simplest estimates show that the probability of the tunneling effect for nuclear fusion is ~ 10 -21, which is significantly lower than the observed value of the cross section. This can be explained as follows. At a distance of 14 fm between the centers of the nuclei, the initial kinetic energy of 236.2 MeV is completely compensated by the Coulomb potential. At this distance, only nucleons located on the surface of the nucleus are in contact. The energy of these nucleons is low. Therefore, there is a high probability that nucleons or pairs of nucleons will leave the orbitals in one nucleus and move to the free states of the partner nucleus. The transfer of nucleons from an incident nucleus to a target nucleus is especially attractive in the case when the doubly magic lead isotope 208 Pb is used as a target. In 208 Pb the proton subshell h 11/2 and the neutron subshells h 9/2 and i 13/2 are filled. Initially, the transfer of protons is stimulated by proton-proton attractive forces, and after filling the h 9/2 subshell - by proton-neutron attractive forces. Similarly, neutrons move into the free subshell i 11/2, attracted by neutrons from the already filled subshell i 13/2. Due to the pairing energy and large orbital angular moments, the transfer of a pair of nucleons is more likely than the transfer of a single nucleon. After the transfer of two protons from 64 Ni 208 Pb, the Coulomb barrier decreases by 14 MeV, which promotes closer contact of interacting ions and the continuation of the nucleon transfer process.
In the works of [V.V. Volkov. Nuclear reactions of deep inelastic transfers. M. Energoizdat, 1982; V.V. Volkov. Izv. USSR Academy of Sciences, physical series, 1986, vol. 50 p. 1879] the mechanism of the fusion reaction was studied in detail. It is shown that already at the capture stage, a double nuclear system is formed after the complete dissipation of the kinetic energy of the incident particle and the nucleons of one of the nuclei are gradually transferred, shell by shell, to the other nucleus. That is, the shell structure of the nuclei plays a significant role in the formation of the compound core. Based on this model, it was possible to describe quite well the excitation energy of compound nuclei and the cross section for the formation of 102-112 elements in cold fusion reactions.
At the Laboratory of Nuclear Reactions named after. G.N. Flerov (Dubna) synthesized an element with Z = 114. The reaction was used
Identification of the 289 114 nucleus was carried out using a chain of α decays. Experimental assessment of the half-life of the isotope 289 114 ~30 s. The obtained result is in good agreement with previously performed calculations.
When synthesizing element 114 in the reaction 48 Cu + 244 Pu, the maximum yield is obtained by the channel with the evaporation of three neutrons. In this case, the excitation energy of the compound nucleus 289 114 was 35 MeV.
The theoretically predicted sequence of decays occurring with the 296 116 nucleus formed in the reaction is shown in Fig. 10.
 Rice. 10. Scheme of nuclear decay 296 116 |
The 296 116 nucleus is cooled by the emission of four neutrons and turns into the isotope 292 116, which then, with a 5% probability, as a result of two successive e-captures turns into the isotope 292 114. As a result of α-decay (T 1/2 = 85 days), the isotope 292 114 turns into the isotope 288 112. The formation of the isotope 288 112 also occurs through the channel
The final nucleus 288 112 resulting from both chains has a half-life of about 1 hour and decays by spontaneous fission. With approximately a 10% probability, as a result of the α-decay of the isotope 288 114, the isotope 284 112 can be formed. The above periods and decay channels were obtained by calculation.
When analyzing the various possibilities for the formation of superheavy elements in reactions with heavy ions, the following circumstances must be taken into account.
- It is necessary to create a nucleus with a sufficiently large ratio of the number of neutrons to the number of protons. Therefore, heavy ions with a large N/Z must be chosen as the incident particle.
- It is necessary that the resulting compound nucleus have a low excitation energy and a small angular momentum, since otherwise the effective height of the fission barrier will decrease.
- It is necessary that the resulting nucleus has a shape close to spherical, since even a slight deformation will lead to rapid fission of the superheavy nucleus.
A very promising method for producing superheavy nuclei are reactions such as 238 U + 238 U, 238 U + 248 Cm, 238 U + 249 Cf, 238 U + 254 Es. In Fig. Figure 11 shows the estimated cross sections for the formation of transuranium elements upon irradiation of targets consisting of 248 Cm, 249 Cf and 254 Es with accelerated 238 U ions. In these reactions, the first results on the cross sections for the formation of elements with Z > 100 have already been obtained. To increase the yields of the reactions under study, the target thicknesses were chosen in such a way that the reaction products remained in the target. After irradiation, individual chemical elements were separated from the target. α-decay products and fission fragments were recorded in the samples obtained over several months. Data obtained using accelerated uranium ions clearly indicate an increase in the yield of heavy transuranium elements compared to lighter bombarding ions. This fact is extremely important for solving the problem of fusion of superheavy nuclei. Despite the difficulties of working with appropriate targets, forecasts for progress towards high Z look quite optimistic.
Advances in the field of superheavy nuclei in recent years have been stunningly impressive. However, so far all attempts to discover the island of stability have not been successful. The search for him continues intensively.
A. Levin
On the way to the island of stability
Scientists have been engaged in the newest version of alchemical craft for seven decades and have succeeded a lot in it: the list of officially recognized artificial elements, the names of which 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 element, roentgenium, first produced in 1994. In 1996 and 1998, elements with numbers 112 and 114 were obtained. They have not yet received final names, and the temporary ones assigned to them until the decision of the IUPAC Bureau sound terrible - ununbium and ununquadium. In 2004, reports appeared about the synthesis of the 113th and 115th elements, so far endowed with equally unpronounceable names. However, they have their own logic; these are simply serial numbers of elements, encoded using the Latin names of single-digit numbers. For example, ununbium stands for “one-one-two.”
Last fall, reports circulated around the world press about the absolutely reliable receipt of another superheavy element, 118. It was not by chance that the reliability of these results was emphasized. The fact is that for the first time such announcements appeared much earlier - in June 1999. However, later, employees of the American Lawrence Livermore Laboratory, who made an application for this discovery, were forced to abandon it. It turned out that the data on which it was based was fabricated by one of the experimenters, Bulgarian Viktor Ninov. In 2002, this caused quite a scandal. That same year, Livermore scientists led by Kenton Moody, along with Russian colleagues at the Joint Institute for Nuclear Research in Dubna, led by Yuri Oganesyan, resumed these efforts using a different chain of nuclear reactions. The experiments were completed only three years later, and now they have led to the guaranteed synthesis of the 118th element - however, in the amount of only three nuclei. These results are presented in a paper with twenty Russian and ten American signatures, which appeared in the journal Physical Review on October 9, 2006.
We’ll talk about methods for producing superheavy artificial elements and the joint work of Oganesyan’s and Moody’s 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 Table? These works require complex and expensive equipment and many years of intensive research - but what is the end result? Completely useless unstable exotic cores, which can also be counted on one hand. Of course, it is interesting for specialists to study each such nucleus simply because of its uniqueness and novelty for science - for example, to study its radioactive decays, energy levels and geometric shape. Such discoveries are sometimes awarded Nobel Prizes, but still, is the game worth the candle? What do these studies promise, if not technology, then at least for fundamental science?
A LITTLE ELEMENTARY PHYSICS
First of all, let us recall that the nuclei of all elements, without exception, except hydrogen, are composed of particles of two types - positively charged protons and neutrons that do not carry an electrical charge (the 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 also determines the number of the element in the Periodic Table. At first glance, this circumstance may seem strange. The creator of this system, D.I. Mendeleev, ordered the elements based on their atomic weights and chemical properties, and science had no idea about atomic nuclei at that time (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 chemical properties depend on the structure of the electron cloud surrounding the atomic nucleus. As is known, the charges of a 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 - this is where the desired bond has been 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 electrons in 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 jungle of atomic physics, and it does not interest us today; we should 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 numbers of neutrons, differ in mass, but their electronic “clothes” are exactly the same, Marie Curie. 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 designated by the name or chemical symbol of the element, accompanied by a designation of the total number of nuclear nucleons (this indicator is called the “mass number”).
All naturally occurring elements have multiple isotopes. For example, hydrogen, in addition to the main one-proton version, has a heavy version - deuterium and a superheavy version - tritium (historically, hydrogen isotopes have their own names). The deuterium nucleus consists of a proton and a neutron, and the tritium nucleus consists of a proton and two neutrons. The second element of 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 transform 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, Frederic Curie and his wife Marie detected similar radiation 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 science of that time, radioactivity. In 1899, Englishman Ernest Rutherford discovered that uranium emits two types of radiation, which he called 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 from 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 shifts two cells to the left in the periodic table. Alpha decay is actually a special case of a whole family of decays, as a result of which the nucleus rearranges itself and loses nucleons or groups of nucleons. There are decays in which a 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 - they are electromagnetic quanta of very high energy. So pure gamma decay is, strictly speaking, not radioactivity at all, since after it a nucleus remains 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 became clear very quickly. This circumstance puzzled scientists for a long time, since all attempts to find electrons inside nuclei led nowhere. Only in 1934 did Enrico Fermi realize that beta electrons are the result not 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 beta radioactivity of a different kind: a proton turns into a positron and a neutron (the reader will notice that during both transformations the total electric charge is conserved). During beta decay, ultra-light and super-penetrating neutral particles - neutrinos - are also emitted (more precisely, positron beta decay leads to the birth of the neutrino itself, and electron - antineutrino). During electronic beta decay, the charge of the nucleus increases by one, and during positron decay, naturally, it decreases by the same amount.
To understand beta decay more fully, we have to dig even deeper. Protons and neutrons were considered truly elementary particles only until the mid-60s of the last century. Now we know for sure that both of them consist of triplets of quarks - much less massive particles carrying 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 welded 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 (previously they were called pi-mesons). These bonds are not nearly 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 charge (hence, all nucleons react with each other in the same way) and have a very short range of action, approximately 1.4x10-15 meters. The sizes of atomic nuclei depend on the number of nucleons, but in general they are of the same order. Let's say the radius of the heaviest naturally occurring nuclide, uranium-238, is 7.4 x 10-15 meters; for lighter nuclei it is smaller.
PHYSICS MORE SERIOUSLY
We're done with the nuclear education, let's move on to more interesting things. Here are a few facts to begin with, the explanation of which opens the way to understanding the various mechanisms of nuclide synthesis.
Fact 1.
The first 92 elements of the Periodic Table were discovered on Earth - from hydrogen to uranium (although helium was first discovered through spectral lines in the Sun, and technetium, astatine, promethium and francium were obtained artificially, but later they were all discovered in terrestrial matter). All elements with high numbers were obtained artificially. They are usually called transuranic elements, standing to the right of uranium in the Periodic Table.
Fact 3.
The relationship 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 as the atomic number increases, the number of neutrons grows faster and in the heaviest nuclei it exceeds the number of protons by about 1.5 times. For example, the nucleus of the 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). For uranium and trans-uranium, the ratio between neutrons and protons approaches 1.6.
Fact 2.
All elements have unstable isotopes, either naturally occurring or man-made. 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. All transurans, without exception, are also unstable.
How to explain this pattern? Why are there no carbon nuclei with, say, 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 - towards the lower left. The lowest red block is the 112th element.
In nuclear physics textbooks you can find a very visual diagram 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 the 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 actually existing isotopes are concentrated in a rather narrow band. At first, its inclination to the x-axis is approximately 45 degrees, then it decreases somewhat. Stable isotopes are concentrated in the center of the strip, and those prone to certain decays are concentrated on the sides.
This is where the confusion arises. It is clear that nuclei cannot consist of protons alone - they would be torn apart by electric repulsion forces. But neutrons seem to increase the interproton distances and thereby weaken this repulsion. And the nuclear forces that unite nucleons in the nucleus, as already mentioned, act equally on both protons and neutrons. It would seem that the more neutrons there are in the nucleus, the more stable it is. And if this is not the case, then why?
Here is an explanation “at your fingertips”. 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 are subject to the Pauli principle, which prohibits the same 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 in only two numbers - 0 (state not occupied) and 1 (state filled).
In quantum mechanics, unlike 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 with this picture - the particles sit in a well and cannot just jump out of there. Physicists also use this model, calling a well a potential well. Protons and neutrons are not the same, so they are seated in two pits, and not in one. In both the proton and neutron wells there is a set of energy levels that can be occupied by particles that fall into it. The depth of each hole depends on the average force interaction between its captives.
Now remember that protons repel each other, but neutrons do not. Consequently, protons are welded less tightly than neutrons, so their potential well is not so deep. For light nuclei this difference is small, but it increases as the nuclear charge increases. But the energies of the highest non-empty levels in both wells must coincide. 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 if such a transformation were energetically favorable, it would happen over time, and the nucleus would turn out to be unstable. The same ending would occur if any proton dared to exceed its energy scale.
So we found an explanation. If the proton and neutron wells have almost equal depths, which is typical for light nuclei, then the numbers of protons and neutrons also turn out to be approximately the same. As we move along the periodic table, the number of protons increases, and the depth of their potential well falls further and further behind the depth of the neutron well. Therefore, heavy nuclei must contain more neutrons than protons. But if you artificially make this difference too large (say, by bombarding the nucleus with slow neutrons, which do not break it into fragments, but simply “stick”), the neutron level will rise greatly above the proton level, and the nucleus will disintegrate. 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 the number of neutrons over protons, which reduces the stability of nuclei, all heavy nuclides must be radioactive. This is indeed true, we will not repeat our Fact 2. Moreover, we seem to have the right to assume that heavier nuclides will become less and less stable, in other words, their life expectancy will constantly decrease. This conclusion seems absolutely logical, but it is incorrect.
TREASURED ISLAND
Let's start with the fact that the scheme described above does not take into account a lot. 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 with zero angular momentum inside the nucleus. 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 exhibit the greatest stability, and those with odd numbers exhibit the least stability. The stability of nuclei also depends on a number of other circumstances, too special to be discussed here.
But that’s not even the main thing. A 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, inside nuclei there are proton and neutron shells, 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. Some of these numbers are reliably determined in experiments - these are, for example, 2, 8 and 20.
And this is where the fun begins. Shell models make it possible to calculate the magic numbers of superheavy nuclei - although without a complete guarantee. In any case, there is every reason to expect that the neutron number 184 will turn out to be magic. It can correspond to proton numbers 114, 120 and 126, and the latter, again, must be magical. Consequently, we can assume that the isotopes of elements 114, 120 and 126, containing 184 neutrons each, will live much longer than their neighbors. Particular hopes are placed 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, we can hope that there are still undiscovered superheavy nuclides that live for a very long time, at least by the standards of their immediate environment. Physicists call this hypothetical family the “island of stability.” The hypothesis about its existence was first expressed by the remarkable American nuclear physicist (or, if you prefer, nuclear chemist) Glenn Seaborg, Nobel laureate in 1951. He was a leader or key member of the teams that created all nine elements from 94 (plutonium) to 102 (nobelium), as well as element 106, named seaborgium in his honor.
Now we 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 - an island of nuclear stability. No one can say with certainty whether this goal is achievable, but the discovery of the treasured 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, 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 encouraging.
U-400 accelerator at the Joint Institute for Nuclear Research (Dubna),
on which the 118th element was obtained
SYNTHESIS OF 118TH
All artificial elements from 93rd to 100th were | first obtained [by irradiating nuclei | neutrons or deuterium nuclei] (deuterons). This did not 1 always happen in the laboratory. Elements 99 and 100 - einsteinium and fermium - were first identified during radiochemical analysis of samples of the substance collected in the area of the Pacific atoll of Enewetak, where on November 1, 1952, the Americans detonated a ten-megaton thermonuclear charge "Mike". Its shell was made of uranium-238. During the explosion, uranium nuclei managed 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 best machines of this kind in the world are the U-400 and U-400M cyclotrons, belonging to the G. M. Flerov Laboratory of Nuclear Reactions of the Joint Institute for Nuclear Research. The 118th element, ununoctium, was synthesized at the U-400 accelerator. In the periodic table it is located exactly below radon and, therefore, must be a noble gas.
However, it is too early to talk about studying 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, and two more in 2005. 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 productivity 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 transforming into isotopes of the 116th, 114th and 112th elements. The latter, already mentioned ununbium, lives very briefly and is divided into heavy fragments of approximately the same mass.
That's the whole story for now. 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
At the end of the second millennium, academician Vitaly Lazarevich Ginzburg compiled a list of thirty problems in physics and astrophysics that he considered the most important and interesting (see “Science and Life” No. 11, 1999). In this list, number 13 indicates the task of finding superheavy elements. Then, 12 years ago, the academician noted with disappointment that “the existence of long-lived (we are talking about millions of years) transuranium nuclei in cosmic rays has not yet been confirmed.” Today traces of such nuclei have been discovered. This gives hope to finally discover the Island of Stability of superheavy nuclei, the existence of which was once predicted by nuclear physicist Georgy Nikolaevich Flerov.
The question of whether there are elements heavier than uranium-92 (238 U is its stable isotope) remained open for a long time, since they were not observed in nature. It was believed that there were no stable elements with an atomic number greater than 180: the powerful positive charge of the nucleus would destroy the internal levels of the electrons of a heavy atom. However, it soon became clear that the stability of an element is determined by the stability of its core, and not the shell. Nuclei with an even number of protons Z and neutrons N are stable, among which nuclei with the so-called magic number of protons or neutrons - 2, 8, 20, 28, 50, 82, 126 - are especially prominent - for example, tin, lead. And the most stable are “doubly magic nuclei”, in which the number of both neutrons and protons is magic, say, helium and calcium. This is the lead isotope 208 Pb: it has Z = 82, N = 126. The stability of the element extremely depends on the ratio of the number of protons and neutrons in its nucleus. For example, lead with 126 neutrons is stable, but its other isotope, which has one more neutron in its nucleus, decays in more than three hours. But, noted V.L. Ginzburg, the theory predicts that a certain element X with the number of protons Z = 114 and neutrons N = 184, that is, with a mass atomic number A = Z + N = 298, should live approximately 100 million years.
Today, many elements have been artificially obtained up to and including the 118th - 254 Uuo. It is the heaviest non-metal, presumably an inert gas; its conventional names are ununoctium (it is formed from the roots of the Latin numerals - 1, 1, 8), eka-radon and moscovian Mw. All man-made elements once existed on Earth, but have decayed over time. For example, plutonium-94 has 16 isotopes, and only 244 Pu has a half-life T ½ = 7.6 10 7 years; neptunium-93 has 12 isotopes and 237 Np T ½ = 2.14 10 6 years. These longest half-lives among all isotopes of these elements are much less than the age of the Earth - (4.5–5.5) 10 9. Insignificant traces of neptunium, which are found in uranium ores, are products of nuclear reactions under the influence of neutrons from cosmic radiation and the spontaneous fission of uranium, and plutonium is a consequence of the beta decay of neptunium-239.
Elements that have disappeared during the existence of the Earth are obtained in two ways. Firstly, an extra neutron can be driven into the nucleus of a heavy element. There it undergoes beta decay, forming a proton, an electron and an electron antineutrino: n 0 → p + e – + v e. The nuclear charge will increase by one - a new element will appear. This is how artificial elements were obtained up to fermium-100 (its isotope 257 Fm has a half-life of 100 years).
Even heavier elements are created in accelerators, which accelerate and collide nuclei, for example gold (see “Science and Life” No. 6, 1997). This is exactly how the 117th and 118th elements were obtained in the laboratory of nuclear reactions of the Joint Institute for Nuclear Research (JINR, Dubna). Moreover, the theory predicts that stable superheavy nuclei should exist far beyond the currently known heavy radioactive elements. Russian physicist G. N. Flerov depicted the system of elements as a symbolic archipelago, where stable elements are surrounded by a sea of short-lived isotopes that may never be discovered. On the main island of the archipelago, there are peaks of the most stable elements - Calcium, Tin and Lead; beyond the Strait of Radioactivity lies the Island of Heavy Nuclei with peaks of Uranium, Neptunium and Plutonium. And even further away there should be a mysterious island of Stability of superheavy elements, similar to the already mentioned - X-298.
Despite all the successes of experimental and theoretical physics, the question remains open: do superheavy elements exist in nature, or are they purely artificial, man-made substances, similar to synthetic materials - nylon, nylon, lavsan - never created by nature?
There are conditions for the formation of such elements in nature. They are created in the depths of pulsars and during supernova explosions. The neutron fluxes in them reach a huge density - 10 38 n 0 / m 2 and are capable of generating superheavy nuclei. They scatter through space in a stream of intergalactic cosmic rays, but their share is extremely small - only a few particles per square meter per year. Therefore, the idea arose to use a natural detector-storage of cosmic radiation, in which superheavy nuclei should leave a specific, easily recognizable trace. Meteorites have successfully served as such detectors.
A meteorite - a piece of rock torn out of its mother planet by some cosmic catastrophe - travels through space for hundreds of millions of years. It is continuously “fired” by cosmic rays, which consist of 90% hydrogen nuclei (protons), 7% helium nuclei (two protons) and 1% electrons. The remaining 2% consists of other particles, which may include superheavy nuclei.
Researchers from the Physical Institute named after. P. N. Lebedev (FIAN) and the Institute of Geochemistry and Analytical Chemistry named after. V.I. Vernadsky (GEOKHI RAS) are studying two pallasites - iron-nickel meteorites interspersed with olivine (a group of translucent minerals in which Mg 2, (Mg, Fe) 2 and (Mn, Fe) 2 are added to silicon dioxide SiO 4 in different proportions ; transparent olivine is called chrysolite). The age of these meteorites is 185 and 300 million years.
Heavy nuclei, flying through an olivine crystal, damage its lattice, leaving their traces in it - tracks. They become visible after chemical treatment of the crystal - etching. And since olivine is translucent, these tracks can be observed and studied under a microscope. By the thickness of the track, its length and shape, one can judge the charge and atomic mass of the nucleus. Research is greatly complicated by the fact that olivine crystals have dimensions of the order of several millimeters, and the track of a heavy particle is much longer. Therefore, the magnitude of its charge must be judged by indirect data - the etching rate, a decrease in the track thickness, etc.
The work to find traces of superheavy particles from the island of stability was called “Project Olympia.” As part of this project, information was obtained on approximately six thousand nuclei with a charge of more than 55 and three ultra-heavy nuclei, the charges of which lie in the range from 105 to 130. All characteristics of the tracks of these nuclei were measured by a complex of high-precision equipment created at the Lebedev Physical Institute. The complex automatically recognizes tracks, determines their geometric parameters and, extrapolating measurement data, finds the estimated length of the track before it stops in the olivine massif (remember that the actual size of its crystal is several millimeters).
The experimental results obtained confirm the reality of the existence of stable superheavy elements in nature.
