Yuri Oganessian discusses the search for superheavy elements

An image to illustrate the search for superheavy elements.

Yuri Oganessian describes the search for superheavy elements, their discovery, and subsequent placing in the Periodic Table – including one, ‘oganesson’, which was named after himself.

The periodic table of the elements published by D I Mendeleev in 1869 demonstrated the regularity in the chemical behaviour of the 63 elements known at that time and showed that the atom (element) must have an internal structure that would be the basis of this regularity. Indeed, 28 years later J J Thomson discovered the smallest negatively charged particles in the atom – the electrons (1897).

14 years later, Ernst Rutherford proposed the well-known planetary model of the atom (1911) in the form of a compact nucleus that comprises almost the entire mass of the atom and all the positive charge, and electrons that move around the nucleus at a large distance. In the earliest theoretical model (G A Gamov, 1928), the atomic nucleus was considered to be a spherical, uniformly charged drop of a specific liquid-like substance. The model proved to be very fruitful. On this basis, G Gamov himself developed a theory of alpha-decay (1928), the author of the well-known formula C F von Weizsäcker calculated the binding energy of protons and neutrons in nuclei (1936), and N Bohr and J A Wheeler developed their liquid-drop-based theory of nuclear fission (1939).

Structure and stability of the heaviest nuclei

According to the theory of Bohr and Wheeler, the heavy nucleus is protected from dividing into two fragments by a potential barrier. The height of this fission barrier for the nucleus of 238U is 6 MeV. For 238U the partial period of decay with respect to spontaneous fission is TSF=1016 years (G.N. Flerov and K.A. Petrzhak, 1940). For the nuclei heavier than uranium, increased charge of the nucleus results in the decrease of the fission barrier and strong decrease of TSF. With vanishing fission barrier (Bf≈0), the nucleus splits into two fragments in the shortest time TSF=10-19s. In the liquid drop model, this limit is already reached for the nuclei with Z≥100. As to the limit of existence of elements (atoms), this occurs somewhat earlier since at TSF≤10-14s the nucleus decays before the orbital electrons appear around it.

However, the liquid-drop fission model could not describe all of the data obtained in studies of this process in later years. Discovery of the spontaneously fissioning isomers (1962) in 33 isotopes from U to Cf clearly contradicted the liquid-drop model. The isomerism of nuclear forms, the existence of two (sometimes three) states from which spontaneous fission occurs, is not consistent with the vision of the nucleus as structureless amorphous matter. Theory that is more adequate was developed by many theorists around the world.
One of the fundamental outcomes of the new theory is the prediction of the possible existence of the hypothetical super heavy elements (SHEs). In a heavy nucleus on its way to fission, the motion of single nucleons is linked with the collective degrees of freedom of the whole system. The most striking effect of this link – the so-called effect of closed shells that occur at definite numbers of protons and neutrons: 2, 8, 20, 28, 50, 82, and 126. From this point of view, the synthesis and study of properties of super heavy nuclei (SHN), which can exist only due to their specific internal structure, is a direct way for checking the basic statements of the microscopic nuclear theory. On the map of the nuclides, the SHN outline the border of the heaviest nuclear masses. SHN set the limits of the periodic system of chemical elements.

The existence of the nuclei of the second hundred is expected, where new closed shells emerge. According to the macro-microscopic model, new shells in the transactinide region were expected to occur in spherical superheavy nuclei with Z=114 and N=184, similar to the nucleus 208Pb. With moving away from the magic numbers, the effect of shells rapidly decreases, which results in a sharp drop in the stability of the nuclei. Therefore, the domain of the superheavy elements looks like ‘an Island’, with steep slopes immersed in a deep ‘sea of instability’, where, in the absence of the nuclear shell effect, the nuclei cannot exist.

The search for hypothetical superheavy elements

After the first impressive publications of half-life estimates of nuclear heavyweights at the top of the island, that were, in some cases, comparable with the age of the Solar system, there began an experimental assault on the hypothetical SHE.

Superheavy elements were searched for in nature: in terrestrial and lunar samples, in cosmic rays, and meteorites. Many attempts have been made to synthesise them artificially, using high-flux nuclear reactors and even nuclear explosions, and powerful heavy ion accelerators. Unique setups and techniques were developed, with record sensitivity for the separation and registration of rare events of formation and decay of superheavy nuclides. Unfortunately, in all the extensive attempts undertaken for over 15 years, SHE have not been found. This caused certain pessimism; in articles and speeches, one could often read and hear judgments that a beautiful theoretical hypothesis about SHE might have the right to live, but it is almost impossible to prove it.

Here, we must make an important excursus. In 1974, with a spread of several months, first results on the synthesis of element 106 were obtained in reactions of different type: 249Cf+18O→263Sg+4n in Berkeley and 208Pb+54Cr→261,260Sg+1n, 2n in Dubna. The cross sections for producing Sg isotopes in these reactions are nearly the same. However, there is a significant difference in methods of production. In fusion with magic nucleus 208Pb, the excitation energy of the compound nucleus 262Sg* on the Coulomb barrier is about three times lower than in the reaction of Cf+O. In the reaction with lead target, a kind of ‘cold fusion’ occurs; this facilitates the survival of the compound nucleus in the process of cooling. In cold fusion reactions with targets of 208Pb or 209Bi, it is possible to increase the mass and charge of the bombarding ion and to always obtain a slightly heated compound nucleus. This scenario was then followed for the next 38 years.

The major work on the synthesis of the sixth heaviest elements at that time (Z=107-112) was carried out in GSI (Darmstadt) with the participation of physicists and chemists from Europe and other continents. Finally, much later, in the new century, having shown courage and patience, researchers of RIKEN (Tokyo) have worked for over 10 years to synthesise three atoms of the element 113 in the cold fusion reaction 209Bi+70Zn.
The existence of the nuclei with Z= 106-112 and the characteristics of their successive alpha decays appeared to be very close to the predictions based on the macro-microscopic nuclear model. Therefore, predictions of the existence of an ‘island’ of heavier (let us call them ‘superheavy’) and more stable nuclei formed due to the effect of spherical shells Z=114 and N=184, now looked more reasonable. However, all the problems of production and study of SHE have been associated with the synthesis reactions, since none of the feasible target-projectile combinations could produce such a massive nucleus that contains more than 60% of neutrons.

Reactions of synthesis

In such a situation, the only option is to increase the number of neutrons in nuclei that are to fuse. With an aim of synthesising element 114, a hot fusion reaction 244Pu+48Ca→292Fl* was chosen; long-lived isotope 244Pu (8∙107) is a target nucleus, and stable but rare and very expensive isotope 48Ca is used as a projectile.
The experiments were performed employing the Dubna Gas-Filled Recoil Separator (DGFRS) and the heavy-ion cyclotron U-400 at the Flerov Laboratory for Nuclear Reactions at JINR.

Compound nucleus 292Fl* that has 114 protons and 178 neutrons is still six neutrons away from the doubly magic 292Fl (Z=114, N=184). Yet here theory already predicted a noticeable growth of the fission barrier of the superheavy compound nucleus; this should significantly increase their survivability in the process of cooling. Expectations were met: the cross section of 288Fl production in the reaction 244Pu+48Ca appeared to be some 500 times above the cross section of production of a lighter nucleus 278Nh (Z=113).
Results of the very first experiments with reactions 244Pu, 248Cm+48Ca were obtained in 2000, and decay scenarios and properties of nuclei in the chains of successive alpha-decays were in good agreement with the above-mentioned calculations made in macroscopic-microscopic models. With respect to the heaviest isotopes of elements 110 and 112 produced in cold fusion reactions, these nuclei have eight extra neutrons. As a result, their half-lives increase by about 105 times! Such an effect shows that neutron-rich isotopes of these elements have already entered the field of reach of the neutron shell N=184. Note that heavy isotopes of elements 114 and 116 are still away from the shell N=184 by nine and seven neutrons, respectively.

Further events developed rather quickly. With the targets of available and relatively long-lived isotopes of Ra, U, Np, Pu, Am, Cm, Bk and Cf, numerous isotopes of the elements from 108 to 118 have been synthesised in nuclear reactions with 48Ca. Practically all the synthesised nuclei undergo one or a sequence of alpha decays that end in spontaneous fission. The energies of alpha-transitions of the nuclides originating from the mother nuclei with even proton number agree with the predictions of different models within 5-7%.

Superheavy elements in the chart of nuclides

Synthesis of the new elements in the reactions of cold and hot fusion 48Ca projectiles has considerably enriched the “north-eastern” area of the map of nuclides (see Fig. 1). In these reactions, 12 new chemical elements with atomic numbers 107-118 were synthesised, which completed the seventh row of the D I Mendeleev‘s Periodic Table. In the elliptic contour of Figure 1, there are 52 of the most neutron-rich nuclides with Z=104-118, synthesised in reactions induced by 48Ca.

Now, the heaviest nucleus produced in the laboratory has a mass of 294. We observed two A=294 isobars in two reactions: in reaction with the target of 249Cf as an even-even isotope with Z=118, N=176 that undergoes α-decay with T1/2≈0.5ms and in reaction with target of 249Bk – as an odd-odd isotope with Z=117, N=177 and T1/2≈50 ms.
The principal conclusion, that follows from experimental studies carried out over the past 40 years, is that when moving from Pb/Bi that are the last stable elements to the region of still heavier nuclides, we observe the remarkable survivability of the atomic nuclei. Fundamental predictions of the microscopic nuclear theory concerning the possible existence of superheavy elements have now got direct experimental evidence.

Along with this, the discovery of superheavy elements gave rise to many questions:

  • Can there exist elements heavier than the already synthesised superheavy elements, new nuclear shells and more remote islands?
  • Can superheavy elements be formed in the Universe, in various astrophysical scenarios of nucleosynthesis?
  • What is the location of superheavy elements in the Periodic Table?
  • How similar are they to their light homologues?

Many other questions arise, naturally.

Naming the new superheavy elements

Priority of claims for the discovery of the elements of atomic numbers 113-118 was determined by a Joint Working Party of independent experts drawn from the International Union of Pure and Applied Chemistry (IUPAC) and the International Union of Pure and Applied Physics (IUPAP).

For the element with atomic number 113, the discoverers at RIKEN Nishina Center (Japan) proposed the name ‘nihonium’ and the symbol Nh. Nihon is one of the two ways to say ‘Japan’ in Japanese, and literally means ‘the Land of Rising Sun’.

For the element with atomic number 114, the name ‘flerovium’ with the symbol Fl was proposed, and for element with atomic number 116 the name ‘livermorium’ with the symbol Lv are proposed jointly by the discoverers at the Joint Institute for Nuclear Research, Dubna (Russia) and Lawrence Livermore National Laboratory (USA).
Flerovium will honor the Flerov Laboratory of Nuclear Reactions. G Flerov was a pioneer in heavy ion physics and founder of the laboratory which, since 1991, has borne his name and where numerous new elements have been synthesised.Livermorium honours the Lawrence Livermore National Laboratory. A group of researchers at this Laboratory was involved in the work carried out in Dubna on the synthesis of superheavy elements including element 116.

For the element with atomic number 115, the name proposed is ‘moscovium’, with the symbol Mc, and for element with atomic number 117, the name proposed is ‘tennessine’, with the symbol Ts. These are in line with the tradition of honouring a place or geographical region and are proposed jointly by the discoverers at the Joint Institute for Nuclear Research, Dubna (Russia), Oak Ridge National Laboratory (USA), Vanderbilt University (USA) and Lawrence Livermore National Laboratory (USA).

Moscovium is in recognition of the Moscow region and honours the ancient Russian land that is the home of the Joint Institute for Nuclear Research, where the discovery experiments were conducted at the Flerov Laboratory of Nuclear Reactions.
Tennessine is in recognition of the contribution of the Tennessee region, including Oak Ridge National Laboratory, Vanderbilt University, and the University of Tennessee at Knoxville, to superheavy elements research.

The element with atomic number 118 was named after the author of this article, ‘oganesson’. The author is deeply grateful for the appreciation of his contribution to the synthesis and study of new elements.

The new arrivals in the Table of Elements

According to the QED calculations in the non-relativistic approximation, the Periodic law works for all the elements from Z=1 to Z=172. However, when moving to still heavier elements, with increasing nuclear charge, the speed of electrons increases and, according to the theory of relativity, this leads to an increase in the total energy of the electron (the effect of growth of mass) thus reducing the size of the heavy atom.

‘Relativistic contraction’ leads to a change in the binding energy and quantum characteristics of the outer electrons that are responsible for the chemical behaviour of the element. Therefore, going along the seventh row of the Table from Z=104 to Z=118, we should see the influence of the ‘relativistic effect’ as progressively increasing difference in the chemical behavior of SHE compared to their light homologues.
Figure 2 shows the heaviest elements of the sixth and seventh rows of the Periodic Table of the elements.

In first experiments, the behaviour of elements in Cn/Hg pair was compared regarding their adsorption on gold surface at various temperatures. Estimates show that boiling point of element 112 (84±110°C) appeared to be considerably less than that of Hg (356.8°C).
High volatility is observed, as well, for element 113 in atomic state, or in the form of hydroxide. However, the situation changed a lot when going to the pair Fl/Pb of group 14. According to recent data by FLNR (Dubna) and GSI (Darmstadt), the difference in this pair increases significantly compared to the previous cases. Element 114 is most likely a gaseous noble metal (new substance!) while Pb is known to boil at 1,750°C.
Now, attention is drawn to the heaviest element with Z=118. In non-relativistic approach, it belongs to group 18, a homologue of Rn (Z=86). Now, more than 100 years after the discovery of noble gases by Sir William Ramsay (Nobel Prize of 1904), we are looking for an answer to the question of whether element 118 is a noble gas. Judging by the latest theoretical publications, more often the most likely answer is no, it is not! Again, extraordinary theoretical predictions are to be verified experimentally.

The present and the future

Evidently, we are at the very beginning of a long journey in the study of superheavy atoms and the chemical properties of SHE. The road we are travelling, however, is wide and is illuminated by strict theory (QED), meaning that our progress should be exciting and productive. Yet, the very small production cross section of SHE
being measured by few picobarns (that is, a part of 10-12 of the total reaction cross section) presents something of a challenge, and a significant increase in the sensitivity of studies is required. This is exacerbated by the fact that, in recent years, experimental activity has somewhat declined as the emphasis is now on the development of new accelerators and more advanced experimental facilities.

Of course, every laboratory has its own approach to solving this problem. For instance, in FLNR (Dubna) a special laboratory, the ‘SHE-Factory’, will be put into operation in 2019. The laboratory is equipped with the universal heavy-ion accelerator – DC-280 cyclotron (K=280) (see Fig. 3) which is capable of accelerating ions from carbon up to uranium up to energy 4–8 MeV/A with stepwise and smooth energy variation. For ions with masses A<70, the beam intensity should be not below 6·1013 1/s (10 pµA).

On 26 December 2018, the first beam of accelerated ions was produced at the DC-280 cyclotron. This accomplishes one of the most important stages of the launch of the factory. Experimental set-ups will be installed in three radiation-isolated caves with the total area of 1,500m2. In combination with the new gas-filled recoil separator DGFRS-II (see Fig. 4), the luminosity of the experiments at the ‘SHE-Factory’ will increase by some 50-100 times. Synthesis of new elements 119 and 120, which are the first elements of the eighth period of the Periodic Table, will be one of the key objectives of the Factory.

In conclusion, I would like to congratulate all my colleagues working in the field of SHE and all those interested in this issue on occasion of the coming International Year of the D I Mendeleev’s Periodic Table of the Elements (IYPTE), which, for the last 150 years, has been the subject of exceptional scientific interest in various fields of knowledge.

Yuri Oganessian
Scientific Leader
Flerov Laboratory of
Nuclear Reactions
Joint Institute for Nuclear Research
oganessian@jinr.ru
http://flerovlab.jinr.ru/flnr/people/oganessian.html

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