Towards the “islands of stability” of superheavy elements

Radiation Physics and Chemistry 61 (2001) 259–269

Towards the ‘‘islands of stability’’ of superheavy elements Yu.Ts. Oganessian* Flerov Laboratory of Nuclear Reactions, Joint Institute for Nuclear Research, 141980 Dubna, Moscow region, Russia

Abstract For the 60 yr that have passed since the discovery of the first artificial elements Np and Pu investigations of the properties of new elements have become one of the most quickly developing fields of fundamental nuclear physics and nuclear chemistry. The transition from the traditional method of producing transuranium elements, where continuous and pulsed neutron fluxes have been used, to nuclear reactions induced by heavy ions has made it possible to synthesize 12 new elements heavier than fermium (Z ¼ 100). In the mid-1960s, the theoretical description of the masses and fission barriers of the new nuclei led to the prediction of ‘‘islands of stability’’ for the very heavy and superheavy nuclides in the vicinity of the closed proton and neutron shells. The experimental data that demonstrate enhanced stability of nuclei, relative to different decay modes, close to the deformed shells Z ¼ 108 and N ¼ 162; and also the reactions for their synthesis are discussed from the point of view of advancing into the unexplored region of heavier (superheavy) and significantly longer-lived nuclides, situated close to the spherical shells Z ¼ 114 and N ¼ 184: First results on the synthesis of superheavy nuclei in 48Ca-induced reactions are presented. The observed decay chains of individual atoms consisting of sequential a-decays and terminated by spontaneous fission, as well as the energies and half-lives, are in agreement with the predictions of theoretical models describing the structure of heavy nuclei. They are considered as a first evidence of the existence of the hypothetical region of stability of superheavy elements. The experiments were carried out at the FLNR heavy ion accelerator in the framework of a large collaboration with LLNL (Livermore), GSI (Darmstadt), RIKEN (Saitama), the Institute of Physics and Department of Physics of the Comenius University (Bratislava) and the Department of Physics of the University in Messina. r 2001 Elsevier Science Ltd. All rights reserved. Keywords: Superheavy elements;

48

Ca-induced reactions; Cross sections; Decay energies; Decay times

1. Introduction The well known conception of the atom consisting of a nucleus, carrying practically all the mass and positive charge of the system, and electron orbits at a large distance from the charge centre, according to quantum electrodynamics, is valid for very heavy atoms with atomic numbers up to Zp170: The limit of existence of stable atoms (elements) is significantly less than this, being due to the instability of the nucleus itself. Among the 2000 known nuclei, only 287 have survived in nature since nucleosynthesis. The changes

*Corresponding author. Fax: +7-09621-65083.. E-mail address: [email protected] (Y.T. Oganessian).

of the proton-to-neutron ratio in these nuclei generate b-decay. The neutron excess in a nucleus brings forth the decrease of the neutron binding energy; the limit of existence of neutron-rich nuclei is at En ¼ 0 (the neutron dripline). Similarly, a zero proton binding energy, Ep ¼ 0 (the proton dripline), determines the limit of existence of proton-rich nuclei. Another limitation is the maximum number of nucleons a nucleus can hold. Formally, the existence of the heaviest nuclei is determined by the probability for disintegration into parts of smaller mass. Such a process of nuclear transformationFthe spontaneous fission of heavy nucleiFwas observed for the first time for the isotope 238U (T1=2 ¼ 1016 y) in 1940 by Flerov and Petrzhak (Flerov and Petrzhak, 1940). At that time, Hahn and Strassman had already discovered

0969-806X/01/$ - see front matter r 2001 Elsevier Science Ltd. All rights reserved. PII: S 0 9 6 9 - 8 0 6 X ( 0 1 ) 0 0 4 1 2 - 1

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the induced fission of uranium. For the description of this phenomenon, Bohr and Wheeler offered the liquid drop model of the fissioning nucleus (Bohr and Wheeler, 1939). In the origin of this beautiful and essentially classical theory lies the assumption that nuclear matter is a macroscopically structureless (amorphous) body similar to a drop of charged liquid. The deformation of the drop induced by the Coulomb forces, which finally brings forth its fission into two parts of almost equal mass, is due to the overcoming of the potential barrier, which hinders the process of nuclear deformation. For the nucleus 238U, the fission barrier amounts to approximately Bf E6 MeV. The height of the fission barrier decreases rapidly with an increase in the atomic number Z; at a definite critical atomic number, the nucleus becomes absolutely unstable to spontaneous fission (TSF B1019 s). According to the estimations of Bohr and Wheeler, such a critical situation is reached for values of Z ¼ 1042106: It is interesting to note, that the properties of the first transuranium elements, which were synthesized considerably later with high-flux reactors, qualitatively confirmed the analogy between the liquid drop and nuclear matter: the probability of spontaneous fission for nuclei from 238U to 257Fm (Z ¼ 100) increased by more than a factor of 1013. Quite unexpectedly, and at the same time completely in contradiction to the liquid drop model of fission, came the discovery of spontaneously fissioning isomers (Polikanov et al., 1962). The shape isomerism in 33, at that time already known, nucleiFisotopes of U to BkFarises, as it is now well established, as a result of the complex structure of the potential energy surface of the nucleus, in particular, as a result of the two-humped shape of the fission barrier (the reader can find a rather detailed description of this phenomenon in the excellent review of Bjornholm and Lynn (Bjornholm and Linn, 1980). Another contradiction with the liquid drop model was found in the significant variations of the partial lifetimes of spontaneous fission, best shown for the isotopes of Cf, Fm and the transfermium elements, which have been synthesized in heavy-ion-induced reactions (Oganessian et al., 1975). A more detailed analysis of the theoretical and experimental values of the nuclear masses have shown that the deviations of the real values of the nuclear binding energies from the calculated ones has a regular behaviour: they are largest at definite ‘‘magic’’ numbers of protons and neutrons in the nucleus. For light and medium nuclei, the classical liquid drop model describes only the mean changes in the binding energies of the macroscopic system with the growth of its mass. For the nucleus, as an object of small size, quantum mechanics can bring forth strong local

variations of the energy levels occupied by the different nucleons. These states are far from homogeneous. In some sense, this reminds one of the closed electron orbits in the atomic structure, which determine the properties of the chemical stability of elements of group VIII of Mendeleev’s table of elements. In analogy with atomic physics, in a nuclear structure the so-called ‘‘magic numbers’’ of protons and neutrons, which determine the maximum binding energy of a nucleus, are called closed proton and neutron shells. Most strongly, the effect of nuclear shells is manifested in the ‘‘doubly magic’’ nuclei, such as 208Pb (Z ¼ 82; N ¼ 126), for which the value of the shell correction DEshell ¼ 14 MeV. In the works of Swiatecki, a phenomenological description was given for the shell anomalies in nuclear masses (Swiatecki, 1964; Myers and Swiatecki, 1966). Later, in 1966, Strutinsky suggested an original method for calculating the shell corrections to the liquid drop nuclear energy (Strutinsky, 1967, 1968). The calculations, carried out in the macro–microscopic model, have revealed regularities in the shell phenomena in deformed nuclei. This led to higher precision in defining their mass and shape in the ground state. In spite of the widely accepted opinion that shell effects are smeared out with the increase of the deformation of the nucleus, it was found that in highly deformed nuclei a substantial re-distribution of the nucleons takes place. With the increase of deformation shell effects do not vanish, but simply change, implying as usual a significant correction to the potential energy of the nucleus (Brack et al., 1972).

2. Nuclear shells and stability of heavy elements Similar to any other theory, the macro–microscopic model had a definite predictive power, which, in particular, concerned the masses and radioactive properties of very heavy, hitherto unknown nuclei. Such predictions were made in a series of works. Here, we shall present only the latest results obtained by Patyk, Smolanczuk and Sobiczewski (Patyk and Sobiczewski, 1991; Smolanczuk, 1997), who calculated masses and fission barriers of even–even nuclei with Z ¼ 1042120 and N ¼ 1402190: We shall first consider the probability for spontaneous fission of superheavy nuclei. As can be seen in Fig. 1b, the partial half-lives for spontaneous fission depend strongly on the amplitude of the shell correction. The significant rise in TSF ðNÞ when moving away from the N ¼ 152 shell, which manifests itself noticeably in the radioactive properties of the actinide nuclei, is due to the influence of another neutron shell at N ¼ 162: It is necessary to note that these two shells are related to deformed nuclei contrary to the case of the well-known doubly magic nuclei, e.g. 208Pb (Z ¼ 82; N ¼ 126),

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292

Fig. 1. (a) The map of the shell corrections to the nuclear liquid drop potential energy(in MeV). (b) Partial half-lives of even– even isotopes as a function of the neutron number. The element atomic numbers are indicated in the figure. The open circles denote calculations (Smolanczuk, 1997), the black pointsexperimental data. The solid lines connect the isotopes with Z ¼ 114:

which are spherical in their ground state. Maximum stability with respect to spontaneous fission is expected for the nucleus 270108 (N ¼ 162) for which the predicted TSF value can amount to several hours. When the neutron number increases, the deformation of the nucleus decreases due to it moving away from the deformed shell N ¼ 162; and the influence of another shell, viz. the closed spherical shell N ¼ 184: At N > 170; a significant rise of TSF is expected for nuclei up to

108 (N ¼ 184), whose partial half-life with respect to spontaneous fission is as long as TSF E3  10 yr. Here we come across a very interesting situation. If superheavy nuclei possess high stability with respect to spontaneous fission, they will undergo other modes of decayFa-decay and perhaps, b-decay. The probability for these modes of decay, hence the lifetimes, will be determined by the nuclear masses in the ground state. The latter can be calculated by different models, which are based on different assumptions for the fundamental properties of nuclear matter (of the nature of nuclear forces). In these circumstances, any experimental result becomes extremely informative as far as the verification of the theoretical models is concerned. Following the calculations of Smolanczuk and Sobiczewski, performed within the macro–microscopic model, the deformed nucleus 268106 (N ¼ 162) should undergo a-decay with a half-life of Ta E2 h. (Compare this with a different . . calculation by Moller and Nix (Moller and Nix, 1994), predicting a half-life of a few days). For the heavier spherical nucleus 294110 (N ¼ 184), Ta increases to several hundred, probably even thousand years (Fig. 1b). It is worthwhile noting that in the absence of a nuclear structure (in the liquid drop model) this nucleus should fission spontaneously with TSF o1019 s. The difference is, as we can see, about 30 orders of magnitude! Other calculations of the energy of the nucleus as a many-body system, carried out in the Hartry–Fock– Bogoliubov model choosing different interactions, as well as calculations in the relativistic mean field model, also indicate a significant increase in the nuclear binding energy when approaching the closed neutron shell N ¼ 184: However, the theoreticians have not yet come to a consensus for the magic proton number, when N ¼ 184; for which a maximum binding energy is realized for the doubly magic spherical nucleus. This, however, does not change the main conclusion that in the region of very heavy nuclei there may exist ‘‘islands of stability’’, which will significantly extend the limits of existence of superheavy elements.

3. Reactions of synthesis It is well known that the first artificial elements heavier than uranium were synthesized in reactions of sequential capture of neutrons during long exposures at high-flux reactors. The long lifetime of the new nuclides made their separation possible using radiochemical methods followed by the measurement of their radioactive decay properties. This pioneering work, which was performed in the Lawrence-Berkeley National Laboratory (USA) by Prof. Seaborg and colleagues in the period of 1940–1953 (Seaborg, 1963), led to the

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synthesis of eight artificial elements with Z ¼ 932100: The heaviest nucleus was 257Fm (T1=2 B100 d). The further advance into the region of heavier nuclei was blocked by the extremely short lifetime of 258Fm (TSF B0:3 ms). Attempts to overcome this barrier by means of pulsed neutron fluxes (underground nuclear explosions) were also limited by results leading to the observation of 257Fm. Elements with mass A > 257 were produced in heavy-ion-induced reactions. Unlike the method of sequential neutron capture, in the fusion process, the total mass of the projectile is imported into the targetnucleus. The fusion of complex nuclei leads to an excited compound nucleus. Its transition to the ground state is achieved by means of neutron- and g-ray emission. Fission strongly competes with this process; as a result, the survival probability of the nucleus strongly decreases. Compared to the cross-sections of ðn; gÞ reactions, leading to the formation of actinide elements with cross sections of tens and hundreds of barn, the EVRs crosssections in heavy-ion reactions are of the order of 106– 104 barn and exponentially fall with the advent into the region of heavier elements. However, in spite of such large losses in cross-section, heavy-ion-induced fusion reactions appear to be the only means of synthesis of transfermium elements (Z > 100). Another problem is connected with the nucleon composition of the synthesized nucleus. We should note that no combination of stable and even long-lived isotopes can bring us to nuclei in the peak of the ‘‘island of stability’’ with Z ¼ 114 and N ¼ 184: We can only hope to approach the boundaries of this unknown region so close as to come into the region of influence of the spherical shell N ¼ 184: However, this case also seems to be a problem. As follows from Fig. 1, in the transition region between deformed and spherical shells, the stability of nuclei rapidly diminishes. This is connected first of all with changes in the mass and shape of the nuclei in the ground state and with the changes in the structure of their fission barriers. The stable spherical configuration appears only when NX170; where according to the macro–microscopic calculations, the stabilizing effect of the spherical shell N ¼ 184 should already manifest itself. Such neutronrich nuclides can, in principle, be produced if, as target material and as projectiles, heavy isotopes of the actinides with Z ¼ 94298 and ions of the very rare isotope 48Ca are used, respectively. Due to the significant mass excess of the doubly magic nucleus 48Ca, the excitation energy of the compound nucleus at the Coulomb barrier amounts to about 30 MeV. Cooling of the nucleus will take place by 3neutron emission and by the emission of g-rays. It can be expected that at this excitation energy the shell effects still exist in the heated nucleus, which in turn increases

the survival probability of the EVRs in comparison with the case of hot fusion reactions (Ex X50 MeV). In spite of all these advantages, all previous attempts to synthesize new elements, undertaken during the years 1977–1985 in different laboratories (Hulet et al., 1977; Oganessian et al., 1978; Armbruster et al., 1985), have only provided upper limits for the production cross– sections of superheavy elements. However, the recent development of experimental techniques and the possibility of producing intense beams of 48Ca ions at the new accelerators have enabled an increase of the sensitivity of experimental studies by a factor of hundreds. We have chosen this way to advance into the region of stability of superheavy elements.

4. Experimental approach and set-up The planning of experiments for the synthesis of heaviest elements is determined to a great extent by their radioactive properties and above all, by the lifetime of the atoms to be synthesized. The lifetime, as mentioned earlier, can vary in a wide range depending on how justified predictions are concerning the influence of nuclear shells on the stability of heavy nuclides with different Z and A: Hence, the experimental set-up should be sufficiently fast. On the other hand, the evaporation residues, whose yield is extremely small, should be very quickly separated from the enormous background of incidental reaction products, which are formed with an 8–10 orders of magnitude higher probability. These conditions can be satisfied if the separation of the products is performed in-flight (within a period of 106– 105 s), taking into account the kinematic characteristics of the different reaction channels. It should be noted that in fusion reactions, leading to the production of compound nuclei, a total momentum transfer from the projectile to the composite system takes place and as a result, the momenta of the recoil atoms are well determined. The aim is now to separate the recoil atoms, emitted in a narrow angular interval (yL ¼ 0172:51) with respect to the beam direction, according to their velocities (or energies). Such an operation can be performed by Wien velocity selectors . (the separator SHIP in GSI (Munzenberg et al., 1979)) or the energy selector (the separator VASSILISSA in JINR (Yeremin et al., 1989)), where the reaction products are separated according to the electric rigidity in transverse electric fields (Fig. 2a). Essentially, such operations can also be performed by other techniques, such as gas-filled separators, where the separation of the recoil atoms is achieved by the magnetic rigidity in a gaseous hydrogen or helium atmosphere at a pressure of about 1 Torr (Lazarev et al., 1993) (Fig. 2b). The efficiency of the kinematic separators depends on the ratio of the masses of the interacting nuclei. For

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263

Fig. 2. The recoil separators used at FLNR in the experiments to synthesize new elements: (a) the separator VASSILISSA; (b) the Gas-filled separator; (c) the detector array used to register the recoil nuclei and their radioactive-decay products.

fusion reactions induced by relatively light projectiles (Ap p20) it amounts to only a few percent, but increases to 30–50% when going to ions with mass Ap X40: The experimental set-ups also have high selectivity. In the focal plane, practically all the background from the primary beam is eliminated and the products of incomplete fusion reactions are suppressed by a factor of 104–107, depending on the kinematical characteristics of the different channels leading to their formation. This is, however, not enough for the identification of the extremely rare events of the production of atoms of a new element. For this reason, the selection of the sought nuclei is further accomplished with a sophisticated registration device, which is schematically presented in Fig. 2c. The recoil atoms, which have reached the focal plane, are implanted into a position-sensitive strip silicon semiconductor detector, whose active area is determined by the size of the object image on the focal plane of the separator. Each strip has longitudinal position sensitivity. The front detector is surrounded by side detectors so that the entire array has the shape of a box with an open front wall. In this way, the detection efficiency for particles resulting from the decay of the implanted nucleus (a-particles or fission fragments) is increased

from 85% to 87%. For distinguishing between the signals of the recoil nucleus and those belonging to the particles from its decay, a time-of-flight (TOF) detector is situated before the front detector. The signals from the TOF detector are also used for determining the velocity of the implants. The selection of the events according to their generic decay significantly improves the background conditions. The parent nucleus, implanted into the detector, can be reliably identified if the decay chain of its sequential aand b-decays leads to nuclei with known properties. This method has been successfully used in the experiments on the synthesis of new elements with Z ¼ 1072112 whose isotopes have insignificant neutron excess (N2Zp53). Advancing into the region of spherical, more neutronrich nuclei this advantage is lost. Here, the decay of the parent nucleus results in the formation of hitherto unknown nuclei, whose properties can only be predicted with precision allowed by the theoretical calculations. At the same time, if the basic prediction of theory on the existence of the ‘‘island of stability’’ is justified, in any decay chain of sequential a- and b-decays, the daughter nuclei move more and more away from the closed spherical shells. This should result in a considerable increase of the probability of their spontaneous

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fission in comparison with other decay modes. Finally, the decay chains will be terminated by spontaneously fissioning nuclei. In principle, such a decay scheme appears to be a reliable sign of the formation of the heaviest of nuclei.

5. Production of intense

48

Ca-beams

The production of an intense ion beam of the rare and extremely expensive isotope 48Ca, as already pointed out, was the cornerstone in our attempts to synthesize superheavy elements. In order to achieve this, a modernization of the U400 accelerator was carried out. It included the development of a new external multi-charge ion source (ECR-4M) and an injection channel for a low-energy 48Ca5+-beam (Ep ¼ 60 keV) to the accelerator chamber. The neutral 48Ca atoms in the form of vapour were injected into the plasma of the ion source at the controlled temperature of the crucible. In order to avoid condensation of the vapour on the walls of the source chamber, hot screens with controlled temperature were applied. Metallic 48Ca, enriched to 70% was used in the experiments. The developed procedure of recuperation of the material from the ion source chamber, after a long-term operation, increased the efficiency of using the initial 48 Ca-material by up to 85%. Due to these improvements, an internal 48Ca-beam with an intensity of more than 1013 pps was obtained at a rate of material consumption equal to about 0.3 mg/h. The average intensity of the 48Ca-beam on the target was about 4  1012 pps.

6. Experiments on the synthesis of superheavy nuclei in the 48Ca+244Pu reaction In the fusion reaction of 48Ca and 244Pu, isotopes of element 114 most closely approaching the peak of the ‘‘island of stability’’ can be produced. At a bombarding energy close to the Coulomb barrier, we expect a maximum yield for the 3n- and 4n-evaporation channels. The experiments on the synthesis of these isotopes were carried out at the Gas-Filled Recoil Separator (Oganessian et al., 1999a). The target consisted of the enriched isotope 244Pu (98.6%) in the form of PuO2, deposited onto a 1.5 mm Ti-backing foil to a thickness of B0.4 mg/cm2. Each of the nine manufactured targets had the shape of a segment with an angular extension of 401 and an area of 3.5 cm2. They were mounted on a disc (R ¼ 60 mm), which was rotated at a speed of 2000 rpm in a direction perpendicular to the beam.

The projectile energy in the middle of the target was chosen to be equal to 236 MeV. Taking into account the energy losses in the target and the weak variations of the beam energy during the long-term irradiations, we have estimated that the excitation energy of the compound nuclei 292114 should be in the range 31.5–39 MeV. In these conditions, two experiments were carried out. In the first run (Oganessian et al., 1999a), in November and December 1998, with a beam dose of 5.2  1018 ions, a chain of sequential decays was observed, which was terminated by spontaneous fission with a total kinetic energy of the fragments equal to about 190 MeV. Considering the signals preceding the spontaneous fission, that could have been generated by a-particles with an energy Ea > 8:0 MeV in the same position intervals, we found out the chain of sequential decays, as shown in Fig. 3a. All five signals (recoil nucleus, a1 ; a2 ; a3 ; SF) appeared within a position interval of 1.5 mm, which strongly indicates that there is a strict correlation between the observed decays. The total time between the implantation of the recoil nucleus and the spontaneous fission amounts to 34 min. The probability of random coincidences imitating such a decay at any point of the working area of the detector was less than 0.6%. In such locations (the given position window in the considered strip), a probability of decay was even less (B104). For all steps of the decay sequence, the basic rule for a-decay giving the relation between the decay energy Qa and the decay probability (the half-life Ta ) was satisfied. Considering the conditions of performing the experiment and the decay characteristics, the origin of the decay chain is most probably found in the isotope 289 114, which has been produced in the 3n-evaporation channel. In the second experiment, carried out in June–October 1999, the total beam dose amounted to 1019 ions. Here, two more identical a-decay sequences, terminating in spontaneous fission, were observed (Fig. 3b). In the limits of the energy and position resolution of the detectors and the statistical uncertainty in the decay times, the two events coincide in all measured 11 parameters. The probability of random coincidences imitating recoil nuclei and their correlated decays is estimated to be less than 5  1013. It is noteworthy that in this long-term experiment only two spontaneous fission events were observed: they are characterized by a large energy release and both are preceded by the same a-decay sequences. The projectile energy, measured at the moment of registration of the given events, corresponds to an excitation energy of the compound 292114 nucleus, Ex ¼ 36237 MeV. At this energy, the most probable 4n-evaporation channel leads to the formation of the isotope 288114 in its ground state. The parent nucleus 288114 undergoes a-decay with an energy Qa ¼ 9:9870:05 MeV and half-life Ta ¼ 1:9þ3:3 0:8 s:

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265

Fig. 4. Experimental values of the decay energies and half-lives of sequential a-transitions observed in the experiments 48 Ca+242,244Pu (black symbols). The lines denote calculations obtained with the formula of Viola and Seaborg with parameters taken from ref. (Smolanczuk, 1997). Open symbolsFexperimental values for all known nuclides with ZX106:

280

110 undergoes spontaneous fission with a half-life TSF ¼ 7:5þ13:7 2:9 s: For the two events, the fission fragment energy release in the detectors (Etot ¼ 217 MeV) is about 40 MeV higher than the value obtained for the known spantaneous fission of the fissioning 252No nucleus (Z ¼ 102). Regardless of the relatively wide distributions of the fragment kinetic energy, this value also gives evidence that the progeny nuclei in the decay chain are also the result of the fission of a sufficiently heavy nucleus (Z > 106). The observed decay sequence also entirely reproduces the predicted decay scenario. The decay properties of the neighbouring odd isotope 289114 agree well with the above-mentioned properties of the even–even nucleus 288 114. For this nucleus, as it was expected, an increase is observed in the halflives Ta and TSF ; due to the additional odd neutron. Fig. 3. Sequential decay chains, obtained in the 48Ca+244Pu reaction. For the spontaneous fission fragments, the values of the energies deposited in the front and side detector are indicated. For all registered signals the position coordinates are also indicated.

We should recall that in the a-decay of an even–even nucleus it is possible to determine the atomic number of the initial nucleus with high precision. According to the relation Ta ðQa Þ; presented in Fig. 4, from the measured þ1:6 values it follows that a nucleus with Z ¼ 114:40:8 undergoes the observed decay. The progeny nucleus

7. Experiments with the

242

Pu target

If the identification of superheavy elements produced in the preceding experiments is correct, then it is not difficult to predict the properties of another isotope of element 114, viz. 287114 (N ¼ 173). It should predominantly undergo a-decay to the daughter nucleus 283112, which was formerly observed in the 48Ca+238U reaction. In the given case, we can expect a short decay sequence (a-SF), implying a few-seconds a-decay halflife, followed by spontaneous fission with considerably

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longer (several minutes) half-life. This isotope of element 114 can be synthesized via the 3n-evaporation channel in the 48Ca+242Pu reaction. The experiments were performed at the VASSILISSA separator. The rotating 0.2 mg/cm2 thick 242Pu target was bombarded by a 235 MeV 48Ca-beam with a total beam dose of 7.5  1018 ions. In the experiment, two short radioactive decay sequences were observed, connected with new nuclides which experience a-decay followed by a spontaneous fission of the daughter nuclei (Fig. 5). Although the energy of the a-particle was determined only in one decay sequence, the two events agree with each other in all measured parameters. The a-half-life determined from the two events amounts to about 5 s. The daughter nucleus undergoes spontaneous fission with a half-life of about 3 min. This value agrees well

with the properties of the nucleus 283112 (N ¼ 171), which was produced formerly in the 48Ca+238U reaction after the evaporation of 3 neutrons from the 286 112 compound nucleus. Now, in the 48Ca+242Pu reaction it is the daughter product of the a-decay of the mother nucleus 287114. The production cross section of the new isotope of element 114 amounts to about 2 pb. Its half-life and the decay sequence are shorter than the ones of the previously observed heavier isotope 289114, formed in the reaction 48Ca+244Pu (Fig. 3a). Such a trend is expected, according to theory, with the decrease of the neutron number of the superheavy nucleus, or in other words when moving away from the closed N ¼ 184 shell.

8. Comparison with theory

Fig. 5. Sequential decays observed in the 48Ca (235 MeV)+242Pu reaction (Oganessian et al., 1999b).

The data on the radioactive decay of nuclides, produced in the given series of investigations, can be compared with the predictions of different theoretical models. Most consistently and quantitatively, the properties of superheavy elements have been predicted in the framework of the macro–microscopic theory. First, we would like to point out that the heaviest isotopes with Z ¼ 110; 112 and 114; produced in the 48 Ca-induced reactions, undergo a-decay. Spontaneous fission in this region of nuclei is observed only for elements when ðN2ZÞp61: For the even–odd nuclei 277108 (N ¼ 169) and 283112 (N ¼ 171), the spontaneous fission half-lives turn out to be, respectively, 5 and 3 orders of magnitude higher than the predicted values for the neighbouring even–even nuclei. Such a difference can be explained by the presence of the extra neutron, which significantly diminishes the probability for spontaneous fission of the heavy nucleus. For the even–even nucleus 280110 (N ¼ 170), the experimental value (TSF B10 s) is also about 3 orders of magnitude larger than the calculated one, obtained in ref. Smolanczuk et al. (1996). Keeping in mind the uncertainty in calculating the probability for spontaneous fission, connected with tunneling through the potential barrier, this difference could be taken as evidence of the larger contribution of the shells to the deformation energy of the nucleus. Some conclusions can be drawn on the basis of the ground state properties of the observed nuclei. The experimental values of the a-decay energies of all known nuclides with ZX100 and NX148 are shown in Fig. 6, together with the calculated values of Qa ; obtained in the macro–microscopic theory, for all even–even isotopes of the same elements (Patyk and Sobiczewski, 1991; Smolanczuk, 1997). Certainly, the experimental results closely reflect the changes in Qa ðNÞ expected by

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267

Fig. 6. The a-decay energyFQa as a function of the number of neutrons for the isotopes with Z ¼ 100C114 (these are denoted in the Figure). The solid lines denote the calculated values obtained in ref. (Patyk and Sobiczewski, 1991; Smolanczuk, 1997), the black pointsFthe experimental values, the black squaresFthe results obtained in 48Ca-induced reactions, the open circlesFthe results of ref. (Ninov et al., 1999). The dashed lines are drawn through the experimental points to guide the eye.

theory for different N and Z; including the region of superheavy elements, where a transition from deformed to spherical shells is predicted. For the heaviest nuclides the measured decay energies Qa turn out to be less and the half-lives Ta longer than predicted by theory (Patyk and Sobiczewski, 1991; Smolanczuk, 1997). The calculations, carried out by . Moller et al. (1997), give deviations in the opposite direction: the value of Qa is about 0.7 MeV lower than the experimentally observed values and, as a result, the nuclear stability is 3 orders of magnitude higher than deduced from the experiment. Other calculations, carried out by Cwiok, Nazarewicz and Heenen using the Hartry–Fock–Bogoliubov method with selected Skyrme forces, concerned the heaviest nuclide 289114, produced in the 48Ca+244Pu reaction (Cwiok et al., 1999). Within this approach, which can definitely be applied to other nuclei also, quite good agreement is obtained between the calculation and the experiment (DQa p0:25 MeV) for the chain 289 114–285112–281110 (Fig. 3a). The recent calculations of the a-decay energies of the heavy nuclei, performed by Bender (2000) in the relativistic mean field theory, are presented in Fig. 7. In this model, where, according to the author, the spinorbit interactions of the particles are calculated more

Fig. 7. The a-decay energy of the isotopes of elements with Z ¼ 962116 and different number of neutrons. The upper panel represents the calculated values of Qa for the even–even nuclides, obtained in the relativistic self-consistent mean field theory (open circles and solid curve). The black diamonds, connected with the dashed lines denote the experimental values. The lower panel represents the calculated and experimental values for the decay sequences of the even–odd nuclides 277112 and 289114.

precisely, excellent agreement is achieved with experimental values for Qa for the sequence of even–even nuclei 288114–284112. For the decay sequence of the even–odd nucleus 289114–285112–281110 the calculated Qa values differ from the experimental results by DQa p0:3 MeV. Obviously, the uncertainty in the conclusions of the different models is explained by the fact that the heaviest isotopes produced in our experiments, with Z ¼ 1082114; are located on the slope of the ‘‘island of stability’’ and thus are rather far away from its centre. At the same time, the difference between theory and experiment, as it follows from the above, amounts to DQa p0:5 MeV when the total decay energy is greater than 10 MeV. This difference is less than 5%. However,

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all theoretical models predict a considerable decrease of the energy Qa with the increase of the neutron number in the superheavy nucleus when approaching the magic neutron number N ¼ 184: In fact, for the isotopes of element 112 when going from the isotope 277112 (Ta E2  104 s) to 285112 (Ta E10 min), which means a change in the neutron number by DN ¼ 8; there is a decrease in the a-decay energy DQa E2:8 MeV and, respectively, an increase of the half-life by a factor of 2.5  106 . A similar picture is observed for the isotopes of element 110. For them, the increase of the neutron number by almost the same amount DN ¼ 10 brings forth a decrease of the decay energy by DQa E2:2 MeV and an increase of the half-life by a factor greater than 5  105. For this reason, the observed properties of radioactive decay of the synthesized nucleiFthe isotopes of elements 114, 112, 110 and 108Fare direct experimental confirmation of theoretical predictions for the existence of a region of enhanced stability in the region of superheavy elements.

9. Consequences and perspectives The relatively long half-lives of isotopes with Z ¼ 1082114; obtained in 48Ca-induced reactions, open new opportunities for investigation of the chemical properties of these elements. A question arises as to whether these isotopes are homologues of the heavy metals Os to Pb, and to what extent their chemical properties depend on the relativistic effect of the heavy-atom electron structure. This question is a fundamental one for modern chemistry. Unfortunately, the advance into the region of heavier isotopes with ZX114 and N > 175 is highly limited. In principle, the isotopes of element 116 with N ¼ 176; 177 can be produced in the fusion reaction 48Ca+248Cm. A further increase in the neutron number can be achieved by using beams of neutron-rich radioactive nuclei. Such possibilities are at present discussed in connection with the projects of large accelerator complexesFfactories of radioactive ion beams. Another still open question is connected with the defining of the proton shell and, as a consequence, with the half-life of the most stable nucleus. If the half-lives of long-lived nuclei turn out to be T1=2 X108 years, it cannot be excluded that they exist in nature. While neglecting at the moment the question concerning the mechanism of the formation of superheavy elements in the nucleosynthesis, which is an interesting issue of its own, it is possible to consider different versions of such investigations. The experiments, which were undertaken by G.N. Flerov and his colleagues in the period 1978–1988 with the aim of searching for spontaneously-fissioning nuclei in natural samples,

potentially containing isotopes of Eka Pt–Eka Bi (ZX110), gave only the upper limits of their concentration at the level of 1014–1012 g/g (at T1=2 B108 years) (Flerov and Ter-Akopian, 1983). According to modern conceptions, it is more likely that the most stable nuclides may have atomic numbers Zp110: The experimental approach and especially the task of producing enriched samples are connected directly to the determination of the chemical properties of superheavy elements. The trends presented here for possible developments are far from limiting the scope of investigating the superheavy nuclides. These trends will manifest themselves even more definitely with the accumulation of new knowledge in this still scarcely studied region of physics.

Acknowledgements This paper presents the results obtained by a large group of physicists, many of them are co-authors of the original publications. I am taking the opportunity to express my warm gratitude to all of them. I consider it my duty to thank the JINR Directorate, in particular Profs. Ts. Vylov, A.N. Sissakian and V.G. Kadyshevsky for the help and support on all stages of performing the present work. I am indebted to Profs. M. Itkis, G. . Munzenberg, S. Hofmann, E.K. Hulet, N. Rowley, W. Greiner, V. Pashkevich, W. Nazarewicz, A. Sobiczewski, R. Smolanczuk, V. Volkov, A. Goverdovsky, B.I. Pustylnik and D.V. Shirkov for the interesting discussions. A significant part of this work was carried out with the support of the Ministry of Atomic Energy of Russia and the USA Department of energy, as well as of grants of the Russian Foundation for Basic Research and INTAS.

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