Super-heavy element research

The largest neutron excess in the CN with 114 protons can be achieved in the ²⁴⁴Pu + ⁴⁸Ca complete-fusion reaction. The first super-heavy nucleus was discovered on June 25, 1999, in experiments performed at the DGFRS by Dubna (FLNR)-Livermore (LLNL) collaboration. Two identical decay chains were observed. Each consisted of two consecutive α decays terminated by SF of the third nucleus (see figure 6). In this experiment, carried out at the lowest projectile energy only, the parent nucleus was assigned to ²⁸⁸Fl [39]. Later on, the cross-section measurement in combination with decay properties of produced nuclei gave correct identification of the mass number of 289 for this isotope of element 114.

In 2003 the study of the ²⁴⁴Pu + ⁴⁸Ca reaction was continued at higher ⁴⁸Ca energies [42] (see figure 7). The same isotope was observed at two higher excitation energies. At increased energies, another isotope, ²⁸⁸Fl, with different decay properties was produced also. Its decay chain consisted of α decay of parent nucleus and SF of daughter nuclide which half-life is lower by factor of 300 than that for daughter isotope in the first decay chain. Finally, at the highest energy the third isotope, ²⁸⁷Fl, was observed. The consideration of decay properties of daughters in the three chains evidently indicated that the second chain should originate from even-N nucleus and neighbouring isotopes should have odd number of neutrons. This is explained by the hindrance factor increasing SF half-lives of odd-N and odd-Z nuclei by a few orders of magnitude that was established for many nuclei (see, e.g. [38]). Such assignment is in agreement with fine structure in α decays of synthesized nuclei, which will be discussed below (see figures 6 and 11).

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The measured production cross sections of all the three isotopes agree well with expectations for complete-fusion reactions with evaporation of three, four and five neutrons from CN ²⁹²Fl [56–59, 61, 62]. The decay properties of ²⁸⁷Fl were investigated in more detail and the new lighter isotope ²⁸⁶Fl was synthesized in the cross bombardment of ²⁴²Pu by ⁴⁸Ca [44] in 2003. The isotope ²⁸⁷Fl, product of the 5n-evaporation channel of the ²⁴⁴Pu + ⁴⁸Ca reaction, was observed also in the ²⁴²Pu(⁴⁸Ca,3n) reaction at the three lowest excitation energies of the CN ²⁹⁰Fl [44] (see figures 6 and 7). Further increase of projectile energy allowed synthesis of the new isotope ²⁸⁶Fl. Its decay chain was similar to that of its heavier even–even neighbour ²⁸⁸Fl, an α decay of ²⁸⁸Fl and SF of daughter nuclide ²⁸⁴Cn. In agreement with theoretical expectations, the stability of nuclei, especially against SF, decreases with receding from the neutron magic number N = 184: even–even isotope ²⁸⁶Fl with approximately equal probabilities undergoes α decay and SF.

In the ²³⁸U + ⁴⁸Ca reaction, investigated in 2003–2004 [44], one event of SF was attributed to the decay of ²⁸²Cn and seven α-decay chains of ²⁸³Cn were detected at three projectile energies (see figures 6 and 7). In one case ²⁷⁹Ds underwent α decay, instead of the more probable SF mode, then two more α decays of ²⁷⁵Hs and ²⁷¹Sg and SF of ²⁶⁷Rf were registered. A similar long chain was observed in the ²⁴²Pu(⁴⁸Ca, 3n)²⁸⁷Fl reaction also [44]. Once again this long decay chain has been registered in the third cross bombardment ²⁴⁵Cm(⁴⁸Ca, 2n)²⁹¹Lv [42, 45] (see below).

Experiments on the synthesis of Fl isotopes were repeated in other laboratories. A study of the ²⁴²Pu + ⁴⁸Ca fusion reaction at the BGS was published in 2009–2010. One and two decay chains of ²⁸⁷Fl and ²⁸⁶Fl, respectively, were observed [52, 53] with decay modes, half-lives and decay energies in agreement with results published by the DGFRS group [42–45]. Besides, one more decay chain of the lightest isotope ²⁸⁵Fl, product of the 5n-evaporation channel of the ²⁴²Pu + ⁴⁸Ca reaction, was found to consist of five consecutive α decays terminated by SF of ²⁶⁵Rf (see figures 6 and 7). In addition, the ²³⁸U + ⁴⁸Ca reaction was studied at one ⁴⁸Ca energy at the SHIP [50] in 2005–2007. Here two decay chains were measured, which fully confirm data that were previously assigned to the isotope ²⁸³Cn in experiments at the DGFRS. Two ER-SF chains were assigned to a 50% SF branch of this isotope; this, however, was not evident from the data where ²⁸³Cn was observed as daughter nucleus after α decay of ²⁸⁷Fl [42–45, 52] and the upper limit of 0.1 was set for a SF branch of ²⁸³Cn [44]. And finally in 2009, the decay properties of ^288,289Fl were also confirmed in experiments [32, 54] performed with use of the TASCA (see figures 6 and 7). In addition, in this work a rare α-decay branch for ²⁸¹Ds and SF of ²⁷⁷Hs were detected in one decay chain of ²⁸⁹Fl.

In 2011 a joint IUPAC/IUPAP Working Party (JWP) recommended that the Dubna–Livermore collaboration be credited with discovery of the new element 114 produced in the ²⁴²Pu(⁴⁸Ca, 3n)²⁸⁷114 reaction [63]. The element with atomic number 114 was named flerovium (Fl) to honour the Flerov Laboratory of Nuclear Reactions which was founded by Flerov and where super-heavy elements were synthesized [64].

The relatively high stability of ²⁸³Cn (T_1/2 ≈ 4 s) allowed researchers to investigate for the first time, the chemical properties of element 112. In 2006–2007, the IVO setup was applied for collecting the ²⁴²Pu + ⁴⁸Ca reaction products in the chamber filled by a He/Ar carrier gas which delivered volatile reaction products (including short-lived isotopes of Hg and Rn) to detection system COLD. Here, atoms were deposited according to their interaction with the detector surfaces. The COLD consisted of an array of 32 pairs silicon detectors, with the active surfaces facing each other. The surface of one detector in each of 32 pairs was covered by gold layer. The temperature gradient was established along detectors by a thermostat heating at the inlet and a liquid–nitrogen cryostat cooling near the outlet. The primary fusion-evaporation reaction product ²⁸⁷Fl has a half-life of about 0.5 s which was too short compared with the average transport time from the reaction chamber to the detector. Thus, only the daughter nucleus ²⁸³Cn could reach the detector. In these experiments, five decay chains of ²⁸³Cn were registered [46, 47]. By directly comparing the adsorption characteristics of ²⁸³Cn to that of mercury and the noble gas radon, it was found that the element Cn was more volatile than Hg and unlike radon, reveals a metallic interaction with the gold surface. These adsorption characteristics establish element Cn as a typical element of group 12.

The results of the study of the chemical properties of Fl are less definite. In the first experiment [49] performed in 2007, three decay chains of ^287,288Fl were observed in the reactions of ⁴⁸Ca with ^242,244Pu. Their deposition on the detectors with temperature −90, −88 and −4 C indicates that element Fl is at least as volatile as element 112 (noble-gas-like behaviour). In the second work [65] published in 2014, two decay chains were attributed to the ²⁴⁴Pu + ⁴⁸Ca reaction products ^288,289Fl. But both events were observed at room temperature (+21 C) indicating volatile-metalic behaviour of Fl.

The first nucleus of element 116 was discovered at the DGFRS on July 19, 2000 [40, 41]. Like in 1999 experiment ²⁴⁴Pu + ⁴⁸Ca, the first study of the ²⁴⁸Cm + ⁴⁸Ca reaction in 2000–2001 was performed at low-energy side of the excitation function [43, 44]. In this series of experiments, three similar decay chains of parent nucleus were observed which were followed by a sequence of two α decays and SF in each case. The decay properties of these descendants were in full agreement with those measured in 1999 in the ²⁴⁴Pu + ⁴⁸Ca reaction [39]. After measuring the excitation function of the ²⁴⁴Pu + ⁴⁸Ca reaction [42], ERs observed in the ²⁴⁸Cm + ⁴⁸Ca reaction were identified as the product of the 3n-evaporation channel, ²⁹³Lv.

Investigation of this reaction was continued in 2004 at higher ⁴⁸Ca energy [43, 44]. Here, like in the case of the ²⁴⁴Pu + ⁴⁸Ca reaction, increase of the excitation energy allowed us to observe simultaneously two isotopes: ²⁹³Lv and a new lighter isotope ²⁹²Lv. If the same type of nuclear reaction occurred in both cases, then after α decay of the parent nuclei in the reaction with ²⁴⁸Cm the descendant isotopes should be the same as those observed in primary reaction with ²⁴⁴Pu. Indeed, decay properties of the daughter nuclei in the ²⁴⁸Cm + ⁴⁸Ca reaction were identical to those registered directly in the reaction with ²⁴⁴Pu [39, 42] (see figures 6 and 7).

Experiments were continued with a ²⁴⁵Cm target. In 2003, at the first (lowest) bombarding energy two new isotopes of element 116 were synthesized [42]. The decay properties of daughter nuclei in one of these were in agreement with those observed in the ²⁴⁴Pu(⁴⁸Ca, 5n)²⁸⁷Fl and ²⁴²Pu(⁴⁸Ca, 3n)²⁸⁷Fl [42–44] as well as in the ²³⁸U(⁴⁸Ca, 3n)²⁸³Cn [44] reactions. The α decay of the second isotope lead to decay chains seen in the ²⁴²Pu(⁴⁸Ca, 4n)²⁸⁶Fl [44] and ²³⁸U(⁴⁸Ca,4n)²⁸²Cn [44] reactions. This indicates observation of the 2n- and 3n-evaporation channels of the ²⁴⁵Cm + ⁴⁸Ca reaction, i.e. ²⁹¹Lv and ²⁹⁰Lv, respectively (see figures 6 and 7). In 2005, further increase of ⁴⁸Ca energy resulted in a reduction in the yield of ²⁹¹Lv and increase of that of ²⁹⁰Lv [45]. Finally, at the highest energy, only 3n-evaporation channel was observed. Such variation of production cross sections of two isotopes is in agreement with what it should be expected for the behaviour of the xn channels of the complete-fusion reactions. The decrease of the neutron number in the target nuclei, e.g. from ²⁴⁴Pu to ²⁴²Pu or from ²⁴⁸Cm to ²⁴⁵Cm, results in rise of the neutron binding energy and somewhat lower excitation energy of CN at the fusion barrier (figure 7). This leads to a relative increase in the yields of lower xn channels. Note, the 2n-evaporation channel was also observed in two other reactions with somewhat lower-N target nuclei. A single decay chain was assigned to the 2n-reaction product in the experiment with ²⁴²Pu [44] (figure 7) and four chains were observed in the ²⁴³Am(⁴⁸Ca, 2n)²⁸⁹115 reaction [66] (see below).

Owing to strong correlations in three α-decay chains of ²⁹¹Lv with following ²⁸⁷Fl and lighter descendants (even up to ²⁶⁷Rf in one chain), the IUPAC/IUPAP JWP recommended that the Dubna–Livermore collaboration be credited with discovery of element 116 [63]. The element with atomic number 116 was named livermorium (Lv) in honour of the Lawrence Livermore National Laboratory because of experiments on the synthesis of super-heavy elements, including element 116, were performed in collaboration with group of researchers from this laboratory [64].

The independent confirmation of the results obtained at the DGFRS in 2000, 2001 and 2004 in the ²⁴⁸Cm + ⁴⁸Ca reaction [40, 41, 43, 44], where element 116 was observed for the first time, followed in 2007 in the SHIP experiments [51]. Four and one decay chains of ²⁹²Lv and ²⁹³Lv, respectively, were registered at the excitation energy of 40.9 MeV (see figures 6 and 7). The decay properties of the parent and all the descendant nuclei are in agreement with those measured at the DGFRS. One more chain was not definitely assigned. The energy and lifetime of the first α decay agree with data determined for ²⁹³Lv but energies of the next three α particles are larger than those measured for ²⁸⁹Fl, ²⁸⁵Cn [39–42] and ²⁸¹Ds [32, 54] as well as much longer decay time was observed for the terminating fission event than that seen at the TASCA experiment [32, 54]. The decay properties of detected nuclei are in good agreement with those observed in long decay chains originating from ²⁹¹Lv which could be produced in the 5n-evaporation channel of the reaction with ²⁴⁸Cm or in the reaction with lighter Cm isotope, e.g. ²⁴⁶Cm (percentage of 3.1%) (private communication by Hofmann). Indeed, α-particle energies and lifetimes of the second to fourth detected nuclei coincide well with data for ²⁸⁷Fl, ²⁷⁹Ds and ²⁷⁵Hs as well as decay time of SF nucleus is in agreement with half-life for ²⁷¹Sg (see figure 6). Somewhat lower α-particle energy for the parent even-odd ²⁹¹Lv and missing of one of five decays (²⁸³Cn) are quite possible.

For the first time, the decay chain of the element 118 was synthesized on March 19, 2002, in the reaction ²⁴⁹Cf(⁴⁸Ca, 3n)²⁹⁴118 studied at the DGFRS [45]. Two more decay chains of ²⁹⁴118 were observed in 2005 at higher projectile energy [45]. Finally, the fourth decay chain of the same isotope was registered during two experiments aimed at the synthesis of element 117 in the ²⁴⁹Bk + ⁴⁸Ca reaction performed in 2009–2010 and 2012 [36] ². Identification of the isotope ²⁹⁴118 was based on the results of the four cross reactions: ²⁴⁹Cf(⁴⁸Ca, 3n)²⁹⁴118 → ... → ²⁸²Cn [45], ²⁴⁵Cm(⁴⁸Ca, 3n)²⁹⁰Lv → ... → ²⁸²Cn [42, 45], ²⁴²Pu(⁴⁸Ca, 4n)²⁸⁶Fl → ²⁸²Cn [44] and ²³⁸U(⁴⁸Ca, 4n)²⁸²Cn [44] (see figure 6). Among the daughter nuclei in the decay chain of ²⁹⁴118, decay properties of two isotopes ²⁸⁶Fl and ²⁸²Cn were confirmed in independent experiments at the BGS [52, 53] (see subsection 4.1.1. above).

The discoveries of elements 113 and 115 as the products of the complete-fusion reaction ²⁴³Am + ⁴⁸Ca that led to the synthesis of isotopes of element 115 and their α-decay product, element 113 that was also unknown at that time, were reported in 2004 [67]. The first decay chain was observed at the DGFRS on 24 July, 2003. This involved two new elements at once, ²⁸⁸115 and ²⁸⁴113, followed by α decays of three new neutron-rich isotopes of the known elements ²⁸⁰Rg, ²⁷⁶Mt and ²⁷²Bh and SF of ²⁶⁸Db (see figure 8). The electron-capture (EC) of ²⁶⁸Db leading to presumably rapid SF of ²⁶⁸Rf (T_SF ~ 1 s [7]) could not be excluded as well. The run was performed at two projectile energies, which resulted in observation of two isotopes ²⁸⁸115 and ²⁸⁷115 as well as their descendant nuclides down to ²⁶⁸Db and ²⁶⁷Db (figure 8). In the ²⁴³Am + ⁴⁸Ca reaction, the energies of the bombarding particles and reaction cross sections were comparable with results of experiments where excitation functions for the reactions ^242,244Pu, ^245,248Cm + ⁴⁸Ca have been measured (see figures 7 and 9). Two neighbouring isotopes of the new element were detected at different ⁴⁸Ca energies, in agreement with expectations for the fusion-evaporation reactions. After observing decays of the three nuclei ²⁸⁸115 at the excitation energy E* ≈ 40 MeV, with the increase of energy to E* ≈ 45 MeV a new decay chain originating from different isotope was registered [26, 67] (see, e.g. the difference in half-lives of the neighbouring ^280,Rg and ²⁷⁹Rg, ²⁷⁶Mt – ²⁷⁵Mt and other descendant nuclei in figure 8). The decay properties of SHN will be discussed in the following section but here we emphasize that the decay properties of nuclei observed in the ²⁴³Am + ⁴⁸Ca reaction evidently differ from those produced in the reactions with even-Z target nuclei. One can compare figures 6 and 8 where isotopes of odd-Z elements show decay properties intermediate between those of neighboring even-Z nuclei. Their decay chains are longer which is caused by unpaired protons increasing stability of nuclei against SF by orders of magnitude, etc.

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In addition to the above-mentioned measurements, a chemical experiment was performed for identification of the long-living SF isotope ²⁶⁸Db that was observed after five sequential α decays of ²⁸⁸115 nuclei [71]. The ²⁴³Am target was bombarded by ⁴⁸Ca ions with an energy corresponding to 247 MeV in the middle of the target that means E* = 39 MeV for the CN ²⁹¹115 (compare with the DGFRS data in figure 9). On leaving the target, the recoiling nuclei passed through a collimator suppressing the yield of transfer-reaction products (capture angle of ±12.5°) and were stopped in a copper catcher. Each one–two days the front layer of catcher with a thickness exceeding range of ERs was mechanically cut from the surface. This part of the catcher could contain ²⁶⁸Db (T_SF ~ 1 d) atoms. Then elements of group 4 and 5 were isolated from actinides [71].

The test experiments [75] have shown that the factor of separation of transactinides from actinides was more than 10⁴. On the other hand, from two possible transactinides with Z = 104 and 105, the element of group 4 (Rf) preceded by five α decays could be produced in the ²⁴³Am + ⁴⁸Ca reaction only in the pxn channel or as a result of EC/ β⁺ decay of one of elements 115-Bh. However, the pxn channel for x = 1–5 or EC/ β⁺ decay of ²⁸⁸115 or ²⁸⁴113 would lead to the now known isotopes of Fl or Cn whose decay chains strongly differ from the observed ones (see figures 6 and 8). The EC/β⁺ decay of lighter descendants ²⁸⁰Rg–²⁷²Bh leads to even–even isotopes ²⁸⁰Ds–²⁷²Sg for which SF is expected with half-lives less than 1 s (see [7] and figure 13 below). Therefore, the ~1-d SF, if observed in transactinides fraction, could originate from the element of group 5 (Db) only (direct SF or SF with short half-life following EC). In 2004, in the chemical experiment [71] 15 SF events of the nuclei of a transactinide element were detected. These showed a half-life of 32$_{-7}^{+11}$  h, high total kinetic energy of SF fragments (~235 MeV) and average neutron multiplicity per fission act (4.2) and were produced with a cross section of about 4.2 pb. All the values, within experimental errors, are in agreement with those (T_1/2, TKE, σ_3n) measured for ²⁶⁸Db at the DGFRS in 2003 [67] and later experiments [66, 70] (figures 8 and 9). These results were later (in 2005 [72]) corroborated in another chemical experiment that attempted studying more delicate chemical properties of Db within group 5.

In chemistry experiments [71, 72] the SF activity was produced in the same²⁴³Am + ⁴⁸Ca reaction (I) at the same projectile energy (II) with the same decay properties (namely, decay mode (III), half-life (IV), total kinetic energy (V)) and the same cross section (VI) as it was observed in the experiment performed at the DGFRS [67]. All the factors allowed one to conclude that one and the same isotope has been observed in both the physical and chemical experiments [26, 67, 71, 72]. Simultaneously, all the precursors (Z = 107, 109, 111, 113 and 115) discovered in [67] were identified by the method of genetic relation between ancestor and descendant [37]. In our further investigation of the region of odd-Z SHN we studied neutron-deficient isotopes and continued, as well, the ²⁴³Am + ⁴⁸Ca experiment in a more extended range of projectile energies for observation of more nuclei and detailed measurement of the excitation function of this reaction. In 2006 in the ²³⁷Np(⁴⁸Ca, 3n) reaction, we observed two decay chains originating from the lighter odd–odd isotope ²⁸²113 [68] (figure 8). Decay properties of ²⁸²113 and its descendant nuclei ²⁷⁸Rg, ²⁷⁴Mt, ²⁷⁰Bh and ²⁶⁶Db were in full agreement with those following from decay properties of heavier isotopes ^283,284113 and other descendants previously produced in the ²⁴³Am + ⁴⁸Ca reaction.

In contrast to even-Z target nuclei, application of the cross-bombardment method for odd-Z nuclei is limited by the number of target nuclei available for experiments, that is ²³⁷Np, ^241,243Am and ²⁴⁹Bk. The ²⁴⁹Bk(⁴⁸Ca, 3-4n) reactions lead to heavier isotopes of element 115 and their descendants; therefore only the isotope ²⁸⁹115, the product of the 2n-evaporation channel of the reaction ²⁴³Am + ⁴⁸Ca, seems to be useful for cross-bombardment experiments. In 2010–2012, with aim of synthesizing this isotope as well as measuring excitation function in a wider energy range, we performed a new series of experiments with ²⁴³Am [66, 70]. The results of all these experiments are presented in figures 8 and 9. Indeed, at the two lowest ⁴⁸Ca energies, we detected the product of the 2n-reaction channel, ²⁸⁹115, undergoing two consecutive α decays and terminated mainly by SF of ²⁸¹Rg, as it was observed for the same nuclei synthesized in the reaction ²⁴⁹Bk + ⁴⁸Ca (see below). These chains were not detected at higher ⁴⁸Ca energies.

In sum with results of 2003, at four energies, 31 decay chains of ²⁸⁸115, product of the evaporation of three neutrons, with maximum yield at the excitation energy of about 36 MeV, were registered. At the energy E* ≈ 45 MeV, we detected two decay chains of ²⁸⁷115 that were not found at lower ⁴⁸Ca energies. Such a behaviour of σ_xn(E*) is expected for cooling process of exited CN ²⁹¹115 and is in full agreement with numerous similar observations for fusion-evaporation reactions³. The radioactive decay properties of ²⁸⁸115, ²⁸⁷115 and their daughter nuclei discovered in 2003 [26, 67] were completely confirmed by registration of 28 new decay chains in the new series of experiments [66, 70].

In four decay chains of ²⁸⁸115, α decay of ²⁶⁸Db was searched for within beam-off intervals from 2.7 h up to 1–3 d following decay of ²⁷²Bh (see discussion in [34]). No events were found with E_α = 7.7–8.2 MeV which could be expected for ²⁶⁸Db. This result is in agreement with chemistry experiments [71, 72] where SF activity was found in the fraction of transactinide elements. Thus, an upper limit of 7% can be set for α-decay branch b_α for ²⁶⁸Db.

In 2012, the same ²⁴³Am + ⁴⁸Ca reaction was studied at two close beam energies of 242.1 and 245.0 MeV (E* = 35.1 and 37.5 MeV) at the TASCA [73] applying a high-resolution α, x-ray and γ-ray coincidence spectroscopy technique. In total, thirty correlated α-decay chains were detected which contain five ER-α-α-SF and two ER-α-SF events 'compatible with decay chains proposed to originate from either ²⁸⁹115 or ²⁸⁸115 [73]'. The excitation function of the reaction was not measured in this experiment; short decay chains and reasons for doubts about their assignment are not given in [73]. One long chain of five α decays was clearly compatible with the characteristics of the decay chain attributed to start from the isotope ²⁸⁷115 [66, 67]. The remaining 22 chains are compatible with the 31 chains previously assigned to the decay of ²⁸⁸115 [26, 66, 67, 70]. Here α decay of ²⁶⁸Db was not observed as well. An upper limit b_α ≤ 4% follows from results of all these experiments. The cross-section values are not given in [73]. The summary decay properties of ^287,288115 are shown in figure 8.

In both experiments at the DGFRS [26, 66, 67, 70] and TASCA [73], complex spectra of the α-particle energy were measured (see figure 12 below). In decay of ²⁷⁶Mt, that shows the most complex α-particle spectrum, we could not exclude observation of two states with different lifetimes [66]. Despite the single half-life for ²⁷⁶Mt proposed in [73], its decay time in one chain was t = 8.95 s whereas the average lifetime for other 15 events was τ = 0.55 s (probability to decay with t ≥ 8.95 s for 16 events with τ = 0.55 s is only 1.5  ×  10⁻⁶). Thus, this new long decay time just could confirm the assumption of [66]. In figure 8 we present result of a two-exponential fit [76] of all the available data for ²⁷⁶Mt which suggests two half-lives. However, more statistics is still needed for definite conclusion.

In addition to studying the formation and radioactive properties of the products of the ²⁴³Am + ⁴⁸Ca reaction, the TASCA experiment [73] was aimed at the measuring x- and γ-rays in coincidence with α particles of nuclei starting from element 115. In one decay chain the escape event with E = 0.825 MeV was 'firmly attributed' to ²⁷⁶Mt (probability of its random origin is not given in [73]) and was coincident with two Ge-detector entries at 136 and 167 keV, which are consistent with Z = 107 K_α2 and K_β energies, respectively [77]. Because of 10–15% probability of random α-photon coincidences, authors noted that either one or both of events can also represent γ-rays or Compton scattered or background events. Besides, several α–γ coincidences were observed for ²⁸⁰Rg and ²⁷⁶Mt.

In spite of the fact that registration of x-rays in coincidence with α particles of ²⁷⁶Mt was not definitely established, the experiment demonstrated the prospects for investigation of nuclear structure of SHN. However, these studies call for considerable increase of production rate of nuclei which could be reached at the new experimental facilities which are planned to be put into operation in a few years (see section 5).

The synthesis of heavier element 117 became feasible when 22.2 mg of the ²⁴⁹Bk target material was produced at the High Flux Isotope Reactor (HFIR) at the Oak Ridge National Laboratory (ORNL) [27] and sent to Dubna. For the first time, element 117 was observed at the DGFRS on August 20, 2009, in the complete-fusion reaction ²⁴⁹Bk(⁴⁸Ca, 4n)²⁹³117 [69].

In 2009–2010, at two projectile energies corresponding to the excitation energies of the CN ²⁹⁷117 of 39 and 35 MeV, we synthesized two isotopes of element 117 [27, 69] (see figures 8 and 9). At the excitation energy of 39 MeV that corresponds to the expected maximum for production yield of the 4n-evaporation channel we have registered five decay chains of the odd-even isotope ²⁹³117. From the well established behavior of the excitation functions measured for numerous reactions, it followed that a reduction of the projectile energy should result in a decrease of the cross section for the 4n channel and increase of the cross section of a heavier odd–odd isotope with lower α-particle energy and longer lifetime. Indeed, at 35 MeV excitation energy, we produced one longer decay chain of the isotope ²⁹⁴117. As was expected for an odd–odd nucleus, fission of ²⁸²Rg and ²⁷⁸Mt is suppressed because of the unpaired neutrons compared to α emission and gives a longer α chain.

Identification of isotopes of the new element 117 was made similarly to that of the isotopes with Z = 115 produced in the ²⁴³Am(⁴⁸Ca, 2-4n)^287-289115 reaction. Neutron-rich ²⁹⁰115 and ²⁸⁹115 that are daughters of α decay of the isotopes of element 117, produced in the ²⁴⁹Bk(⁴⁸Ca,3-4n)^294,293117 reaction should have lower α-particle energies and respectively, longer lifetimes compared with the isotopes ²⁸⁸115 and ²⁸⁷115 produced in the reaction with ²⁴³Am. Indeed, α-decay energies of all the descendant nuclei ^289,290115, ^285,286113, ²⁸²Rg, ²⁷⁸Mt and ²⁷⁴Bh (products of the ²⁴⁹Bk + ⁴⁸Ca reaction) have smaller E_α and longer T_α when compared with neighboring isotopes ^287,288115, ^{282 − 284}113, ^{278 − 280}Rg, ^{274 − 276}Mt and ^{270 − 272}Bh observed in the reactions with ²⁴³Am and ²³⁷Np (see figures 8 and 12 below). Moreover, analogously to lighter isotopes of odd-Z nuclei with Z ≤ 111 and N ≤ 163, the α-decay energies of all the products of the reaction with ²⁴⁹Bk including parents ²⁹³117 and ²⁹⁴117 have intermediate values between those measured for neighboring even-Z nuclei (see figures 6, 8, 11 and 12 below).

The number of consecutive α decays originating from ²⁸⁹115 and ²⁸⁸115 differ. Decay chains of ²⁸⁹115 are terminated mainly by SF of ²⁸¹Rg but all 31 [26, 66, 67, 70] and 22 [73] observed decay chains of ²⁸⁸115 ends in SF of ²⁶⁸Db (figure 8). Furthermore, in spite of complex α-particle spectra of odd-Z nuclei, decay properties of ²⁸⁹115 and ²⁸⁸115 and descendant nuclei are different (compare T_α values for nuclei in figure 8 and shape of α-particle spectra in figure 12 below, especially for isotopes of element 113). At the same time, α-particle energies, decay times and decay modes of isotopes ²⁸⁹115, ²⁸⁵113, and ²⁸¹Rg observed in the reactions with ²⁴³Am and ²⁴⁹Bk agree. Therefore, one can conclude that isotope ²⁸⁹115 was produced in cross reactions with two target nuclei to provide cross bombardment evidence for the discovery of the new elements 113,115 and 117.

These conclusions were confirmed also by the following experiments aimed at the measurement of excitation functions for isotopes of element 117 [34, 36]. In 2012 the ²⁴⁹Bk + ⁴⁸Ca reaction was studied at five excitation energies within interval E* = 30.4–48.3 MeV. In two campaigns with ²⁴⁹Bk target four decay chains of ²⁹⁴117 were produced at two laboratory-frame beam energies E_lab = 244 and 247 MeV (E* = 32.6 and 35.1 MeV) and 16 chains of ²⁹³117 were observed at three higher energies E_lab = 252, 256 and 260 MeV (E* = 39.3, 42.6 and 46.0 MeV) (see figures 8 and 9) providing additional evidence of the identification of the nuclei of element 117.

The same reaction was studied in 2012 at the TASCA at ⁴⁸Ca energies of 252, 254 and 258 MeV [74]. In total, four decay chains, two long and two shorter ones (not published yet), all terminated by SF, were observed. The decay properties of the nuclei in the long chains registered at the two largest ⁴⁸Ca energies and assigned to ²⁹⁴117 are in good agreement with results reported from the DGFRS group. In addition, in both chains events with E_α = 7.89 and 7.90 MeV were found 1.3 h and 1.6 h after α decay of ²⁷⁴Bh and assigned to ²⁷⁰Db (T_α = 1 h) followed by SF of ²⁶⁶Lr (T_SF = 11 h). This was not observed in experiments at the DGFRS and needs a short comment.

Consecutive α decays of nuclei ²⁸⁸115–²⁷²Bh or ²⁹⁴117–²⁷⁴Bh in all the observed decay chains occurred within 2 and 7 min, respectively. The probability of random observation of any of daughter nuclei with E_α = 8.5–10.5 MeV (see figures 5 and 12 below) within beam-off interval of 10 min at the DGFRS was about 2.5  ×  10⁻³ (at E_lab = 243 MeV in [66, 70]) and 2.8  ×  10⁻⁴ (at E_lab = 247 MeV in [34, 36]). But the situation differs at the end of decay chains where the time interval between the last α particle and SF increases up to hours and tens of hours. The probability of the appearance of a random α particle within this interval increases accordingly.

However, the probability of the random origin P_err of events within some energy range (e.g. E_α = 7.7–8.2 MeV in [34]) and relatively long time interval, say 2 h, was not presented in [74]. The given total numbers of beam-off α particles with E_α = 6–12 MeV in the detector pixels where decay chains were observed [74] results in a rather large P_err value of 0.5. Furthermore, one could expect similar decay properties for ²⁷⁰Db (N = 165) and ²⁶⁸Db (N = 163) which follows from close predicted Q_α values (8.31 and 8.26 MeV, respectively [78]) as well as from comparable α-decay half-lives of other N = 163, 165 isotopes ^269,271Sg and ^270,272Bh (see figures 6 and 8 and table 1 below). But, as mentioned above, α decay of ²⁶⁸Db was not seen in the DGFRS [26, 66, 67, 70], TASCA [73] and chemistry experiments [71, 72]. Thus, the partial T_α value for ²⁶⁸Db exceeding that for ²⁷⁰Db, suggested in [74], by more than two orders of magnitude (≥300 h) seems to be very unlikely. That is why we do not include results for ²⁷⁰Db in figure 8 and below until finishing detailed analysis of the data ([36] in [74]).

Table 1. Decay properties of nuclei.

Z	N	A	No. observed a	Decay mode, branch b	Half-life c	Eα (MeV)	Qαexp (MeV)	Qαth (MeV)
118	176	294	4 (4/4)	α	0.69$_{-0.\text{22}}^{+0.64}$ms	11.66  ±  0.06	11.82  ±  0.06	12.11
117	177	294	5 (5/5)	α	51$_{-16}^{+38}$ms	10.81–11.07	11.18  ±  0.04	11.43
	176	293	15 (15/15)	α	22$_{-4}^{+8}$ms	10.60–11.20	11.32  ±  0.05	11.53
116	177	293	5 (5/5)	α	57$_{-17}^{+43}$ms	10.56  ±  0.02	10.71  ±  0.02	11.09
	176	292	9 (8/7)	α	13$_{-4}^{+7}$ms	10.63  ±  0.02	10.78  ±  0.02	11.06
	175	291	4 (4/4)	α	19$_{-6}^{+17}$ms	10.74  ±  0.07	10.89  ±  0.07	10.91
						10.50  ±  0.02
	174	290	11 (11/11)	α	8.3$_{-1.\text{9}}^{+3.\text{5}}$ms	10.85  ±  0.07	11.00  ±  0.07	11.08
115	175	290	6 (5/6)	α	650$_{-200}^{+490}$ms	9.78–10.31	10.41  ±  0.04	10.65
	174	289	16 (15/16)	α	330$_{-80}^{+120}$ms	10.15–10.54	10.49  ±  0.05	10.74
	173	288	46 (41/46)	α	164$_{-21}^{+30}$ms	10.29–10.58	10.63  ±  0.01	10.95
	172	287	3 (3/3)	α	37$_{-13}^{+44}$ms	10.61  ±  0.05	10.76  ±  0.05	11.21
114	175	289	16 (14/16)	α	1.9$_{-0.\text{4}}^{+0.\text{7}}$s	9.84  ±  0.02	9.98  ±  0.02	10.04
						9.48  ±  0.08
	174	288	35 (31/30)	α	0.66$_{-0.\text{10}}^{+0.\text{14}}$s	9.93  ±  0.03	10.07  ±  0.03	10.32
	173	287	19 (18/17)	α	0.48$_{-0.\text{09}}^{+0.\text{14}}$s	10.03  ±  0.02	10.17  ±  0.02	10.56
	172	286	27 (22/14)	α:0.6 SF:0.4	0.12$_{-0.\text{02}}^{+0.\text{04}}$s	10.21  ±  0.04	10.35  ±  0.04	10.86
	171	285	1 (1/0)	α	0.13$_{-0.\text{06}}^{+0.\text{60}}$s			11.11
113	173	286	6 (5/6)	α	9.5$_{-2.\text{7}}^{+6.\text{3}}$s	9.61–9.75	9.79  ±  0.05	9.98
	172	285	17 (17/17)	α	4.2$_{-0.\text{8}}^{+1.\text{4}}$s	9.47–10.18	10.01  ±  0.04	10.21
	171	284	47 (39/47)	α	0.91$_{-0.\text{13}}^{+0.\text{17}}$s	9.10–10.11	10.12  ±  0.01	10.68
	170	283	2 (2/2)	α	75$_{-30}^{+136}$ms	10.23  ±  0.01	10.38  ±  0.01	11.12
	169	282	2(2/2)	α	73$_{-29}^{+134}$ms	10.63  ±  0.08	10.78  ±  0.08	11.47
112	173	285	17 (16/16)	α	28$_{-6}^{+9}$s	9.19  ±  0.02	9.32  ±  0.02	9.49
	172	284	37 (34/-)	SF	98$_{-14}^{+20}$ms			9.76
	171	283	33 (23/31)	α:1. SF: ≤ 0.1	4.2$_{-0.\text{7}}^{+1.\text{1}}$s	9.53  ±  0.02	9.66  ±  0.02	10.16
						9.33  ±  0.06
						8.94  ±  0.07
	170	282	14 (14/-)	SF	0.91$_{-0.\text{19}}^{+0.\text{33}}$ms			10.68
	169	281	1 (1/1)	α	0.10$_{-0.\text{05}}^{+0.\text{46}}$s	10.31  ±  0.04	10.46  ±  0.04	11.21
111	171	282	6 (6/6)	α	100$_{-30}^{+70}$s	8.86–9.05	9.16  ±  0.03	9.85
	170	281	20 (17/2)	α:0.1 SF:0.9	17$_{-3}^{+6}$s	9.28  ±  0.05	9.41  ±  0.05	10.48
	169	280	45 (41/42)	α	4.6$_{-0.\text{7}}^{+0.\text{8}}$s	9.09–9.92	9.91  ±  0.01	10.77
	168	279	3 (2/2)	α	90$_{-40}^{+170}$ms	10.38  ±  0.16	10.53  ±  0.16	11.08
	167	278	2 (2/2)	α	4.2$_{-1.\text{7}}^{+7.\text{5}}$ms	10.69  ±  0.08	10.85  ±  0.08	11.30
110	171	281	17 (17/1)	α:0.07 SF:0.93	12.7$_{-2.\text{5}}^{+4.\text{0}}$s	8.73  ±  0.03	8.85  ±  0.03	9.30
	169	279	36 (31/4)	α:0.1 SF:0.9	0.21$_{-0.\text{04}}^{+0.\text{04}}$s	9.71  ±  0.02	9.85  ±  0.02	10.24
	167	277	1 (1/1)	α	0.006$_{-0.\text{003}}^{+0.\text{027}}$s	10.57  ±  0.04	10.72  ±  0.04	10.79
109	169	278	5 (5/5)	α	4.5$_{-1.\text{3}}^{+3.\text{5}}$s	9.38–9.55	9.58  ±  0.03	9.55
	168	277	2 (2/-)	SF	5$_{-2}^{+9}$ms			9.84
	167	276	43 (43/41)	α	0.45$_{-0.\text{09}}^{+0.\text{12}}$s	9.17–10.01	10.03  ±  0.01	10.09
					6$_{-3}^{+5}$s
	166	275	3 (3/3)	α	20$_{-7}^{+24}$ms	10.33  ±  0.01	10.48  ±  0.01	10.34
	165	274	2 (2/2)	α	440$_{-170}^{+810}$ms	10.0  ±  1.1	10.2  ±  1.1	10.63
						9.76  ±  0.10
108	169	277	1 (1/1)	SF	3$_{-1}^{+15}$ms			9.03
	167	275	4 (4/4)	α	0.20$_{-0.\text{06}}^{+0.\text{18}}$s	9.31  ±  0.02	9.45  ±  0.02	9.41
	165	273	1 (1/1)	α	0.2$_{-0.1}^{+1.\text{2}}$s	9.59  ±  0.04	9.73  ±  0.04	9.78
107	167	274	6 (5/6)	α	44$_{-13}^{+34}$s	8.73–8.84	8.94  ±  0.03	8.83
	165	272	44 (39/44)	α	10.9$_{-1.\text{5}}^{+2.\text{0}}$s	8.55–9.15	9.18  ±  0.01	9.08
	164	271	2 (2/2)	α	1.5$_{-0.\text{6}}^{+2.\text{8}}$s	9.28  ±  0.07	9.42  ±  0.07	9.07
	163	270	1 (1/1)	α	61$_{-28}^{+292}$s	8.93  ±  0.08	9.06  ±  0.08	8.63
106	165	271	4 (4/2)	α:0.6 SF:0.4	1.6$_{-0.\text{5}}^{+1.\text{5}}$min	8.54  ±  0.08	8.67  ±  0.08	8.71
	163	269	1 (1/1)	α	2$_{-1}^{+10}$min	8.57  ±  0.10	8.70  ±  0.10	8.32
105	165	270	6 (6/-)	SF e)	15$_{-4}^{+10}$h			8.11
	163	268	66 (66/-)	SF e)	26$_{-3}^{+4}$h d)			7.80
	162	267	3 (3/-)	SF e)	1.3$_{-0.\text{5}}^{+1.\text{6}}$h			7.41
	161	266	1 (1/-)	SF e)	22$_{-10}^{+105}$min			7.52
104	163	267	2 (2/-)	SF	1.3$_{-0.\text{5}}^{+2.\text{3}}$h
	161	265	1 (1/-)	SF	2$_{-1}^{+8}$min			7.27

^aNumber of observed decays and number of events used for calculations of half-lives / α-particle energies, respectively. ^bBranching ratio is not shown if only one decay mode was observed. ^cError bars correspond to 68%-confidence level. ^dThe value obtained combining the results of physical and chemical experiments. ^eThe SF mode was observed but EC/β⁺ or α decay is not excluded.

The energies of α particles for even-Z nuclei ²⁸¹Cn, ²⁹⁴118, ²⁹¹Lv, ²⁹²Lv, ²⁹³Lv and their descendants registered by the focal-plane detector only or together with the side one at the DGFRS [39–45] are shown in figure 11. Here we also show results of experiments carried out with the use of other facilities (see figure 6): chemical setup IVO + COLD [46–49] and separators SHIP [50, 51], BGS [52, 53] and TASCA [32, 54].

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In agreement with expectations, the α decays of the even–even nuclei ²⁹⁴118, ^290,292Lv and ^286,288Fl are terminated by SF of short-lived isotopes ^282,284Cn (see figures 6, 11). The isotope ²⁸⁶Fl has an SF branch of 40$_{-10}^{+11}$ % (68% confidence level). The α-particle energy spectra of these nuclei are characterized mainly by one α line, which corresponds to the ground-state to ground-state transitions as the most probable for even–even nuclei. In figure 11 the spectra of events registered solely by the focal-plane detectors with energy resolution better than 0.1 MeV are shown by green histograms.

The decay chains originated from the even–odd nuclei ^291,293Lv are terminated mainly at later stage, by SF of ^279,281Ds which have small α-decay branch of 11$_{-4}^{+6}$ % and 7$_{-5}^{+9}$ %, respectively. Two SF events of four observed in the ²³⁸U + ⁴⁸Ca reaction at the SHIP [50] were assigned to 50% SF branch of ²⁸³Cn that seemingly does not contradict results observed at the DGFRS in the same reaction [44]. Here, the three decay chains of seven were detected as ER-SF sequences with presumably missing α particles of ²⁸³Cn. However, when ²⁸³Cn is produced after α decay of the parent nucleus ²⁸⁷Fl [42–45, 52], only the upper limit of 7% for the SF branch of ²⁸³Cn can be derived. It seems that the population of isomeric and ground states, even with comparable lifetimes, in direct reaction and after α decay cannot be excluded (compare with ²⁶¹Rf ( [91] and references therein). Further α decay of the even–odd nuclei leads to SF of ^265,267Rf and ²⁷⁷Hs (SF branch for ²⁷¹Sg is not excluded with probability of 40%).

The α-particle energy spectra of the even–odd isotopes are broader. The complex spectra are clearly seen for ²⁸³Cn, ²⁸⁹Fl and ²⁹¹Lv. In addition to the main α line at E_α = 9.53 MeV, two lines with lower energies of 9.33 and 8.94 MeV were registered for ²⁸³Cn [42–47] whereas a spectrum of ²⁸⁷Fl is consistent with a single α transition. The hindrance factors (HF) of 1.5 $_{-0.3}^{+0.5}$ , 0.3 $_{-0.14}^{+1.5}$ and 2.9$_{-1.6}^{+1.7}$, respectively, for three energies of ²⁸³Cn can be estimated as a ratio of partial half-lives which are determined by numbers of observed events and half-lives expected for Z = 112 nucleus with measured α-particle energies from, e.g. semi-empirical systematics (Viola–Seaborg formula [92]). Such data could be decisive for predictions of the low-lying quasi-particle states of nuclei. Predominantly single-line spectrum is seen for ²⁹³Lv but two different energies were observed for ²⁸⁹Fl with E_α = 9.48 and 9.84 MeV. Both these α decays are unhindered (HF = 1.1 $_{-0.8}^{+0.6}$ and 1.1 $_{-0.2}^{+0.5}$, respectively).

The energy spectra of α particles for odd-Z nuclei ²⁸²113, ^287,288115, ^293,294117 and their descendants registered at the DGFRS [26, 27, 34, 36, 66–70] and TASCA [73, 74] are shown in figure 12. Even in cases with relatively low statistics, one can see wider energy distributions for these nuclei than those shown in figure 11. The α-decay chains of four parent nuclei end by SF of isotopes ^{266,267,268,270}Db. Only odd–even ²⁸¹Rg undergoes SF with small branch for α decay (b_α = 12 $_{-7}^{+9}$ %) which is followed by SF of ²⁷⁷Mt (see figure 8).

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The decay properties of nuclei produced in the fusion-evaporation reactions with ⁴⁸Ca are shown in figures 6 and 8 and table 1. The α-decay energies of nuclei Q_α^th calculated within macroscopic–microscopic model [88, 89] are given in column 9. For even–even nuclei with Z = 114–118 and N = 172–176 the values ΔQ_α = Q_α^exp–Q_α^th amount to from −0.5 to −0.1 MeV. Measured in experiments α-decay energies Q_α^exp are lower and correspondingly, the half-lives T_α^exp of super-heavy nuclei are larger than theoretical values. For even-Z isotopes of elements 106–116 with odd number of neutrons N = 165–177 the difference ΔQ_α varies within −0.8–0.4 MeV. Finally, for odd-Z nuclei of elements 107–117 the discrepancies are ΔQ_α = −1.1–0.4 MeV in assumption that the measured maximum α-particle energy corresponds to transitions through the ground states of odd-Z nuclei which is not always valid. From comparison of decay properties of 43 α-decaying nuclei produced in the Act + ⁴⁸Ca reactions, it follows that their agreement with theoretical predictions is not only qualitative but, to some extent, quantitative. Here one should bear in mind that accuracy of predictions of masses of the known nuclei reaches 0.3–0.4 MeV [89] and 0.4–0.7 MeV for some other approaches (see, e.g. review [22] and references therein). Note, results of Q_α^th calculations within other MM models and purely microscopic approaches (HFB, RMF) also agree with experimental data ([22] and references therein). Therefore, the hypothesis of existence of super-heavy nuclei received experimental confirmation.

For 16 of the 54 synthesized nuclei SF was observed. For seven even-Z nuclei SF is the predominant mode of decay (see figure 6 and table 1). In five nuclei, SF competes with α decay. As to the odd-Z nuclei, SF was registered for six nuclides (figure 8 and table 1). For the remaining nuclides SF was not observed. The partial SF half-lives of even–even isotopes with N ≥ 162, produced in fusion reactions with ⁴⁸Ca, together with the half-lives of SF nuclides with N < 162, are shown in figure 13. Two isotopes of element 112 with N = 170 and 172 are located in a region, where a steep rise of T_SF(N) is expected. Indeed, in the even–even isotopes ²⁸²Cn and ²⁸⁴Cn the difference in two neutrons increases the partial half-life T_SF by two orders of magnitude⁴. A similar effect is also observed for the even–even isotopes of element 114. Addition of two neutrons to the nucleus ²⁸⁶Fl (T_SF ≈ 0.3 s) leads to increase of the stability of ²⁸⁸Fl with respect to SF by at least a factor of 15. For even–even isotopes ²⁸⁸Fl, ²⁹⁰Lv, ²⁹²Lv and ²⁹⁴118 SF was not observed.

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It is significant that the rise of stability with respect to SF is observed for the heavy nuclei with Z ≥ 110, which are 10–12 neutrons farther from the closed neutron shell N = 184. On moving to the nuclei with Z < 110 and N < 170 the probability of SF decreases again when the closed deformed shell N = 162 is approached. The stabilizing effect of the N = 162 shell manifests itself in the properties of the even–even isotopes of Rf, Sg and Hs.

Because of the high hindrance of SF in the nuclei with odd number of protons (and neutrons) and relatively low T_α, the isotopes of elements 113 and 115 produced in the reactions ²³⁷Np + ⁴⁸Ca and ²⁴³Am + ⁴⁸Ca with N = 169–173 undergo α decay [26, 27, 34, 36, 66–70, 73]. Spontaneous fission is observed only at the end of chain for isotopes of element 105 (or their EC/α-decay products). For ²⁶⁸Db α decay seems to be less probable (see above).

In case of the reaction ²⁴⁹Bk(⁴⁸Ca,3-4n)^294,293117, the daughter nuclei have one or two extra neutrons compared to the reaction with ²⁴³Am. In analogy with the neighbouring even-Z isotopes all the nuclei in the decay chains of ²⁹³117 and ²⁹⁴117 with Z > 111 and N ≥ 172 will undergo α decay. The nucleus ²⁸¹Rg (N = 170) belonging to the 'critical' region between neutron shells N = 162 and 184 might avoid SF due to the hindrance resulting from an odd proton. The hindrance of the SF in ²⁸¹Rg with respect to its even–even neighbuor ²⁸²Cn [42–45, 52, 53] is ~ 2  ×  10⁴. Despite this, the isotope ²⁸¹Rg undergoes SF with a probability of about 90%. Accordingly, even the high hindrance governed by oddness does not 'save' the odd nucleus (²⁸¹Rg, ²⁷⁷Mt) from SF which is caused by the weakening of the stabilizing effect of the neutron shells at N = 162 and N = 184. However an extra neutron and the double effect of oddness favor the α decay in the neighboring isotopes ²⁸⁰Rg and ²⁸²Rg.

Practically for none of the odd-Z nuclei the branch for EC/β⁺ was observed. For example, isotope ²⁸⁸Fl, EC product of ²⁸⁸115 and its descendants were not seen in the ²⁴³Am + ⁴⁸Ca reaction, which indicates that for nuclei in the decay chain of ²⁸⁸115 the probability for EC/β⁺ is less than 2%. Meanwhile, nuclei at the end of decay chains, neutron-rich odd–odd isotopes ^266,268,270Db, could undergo EC decay leading to SF of even–even Rf isotopes for which T_SF = 23 s, 1.4 s and 20 ms are predicted [7]. For odd–even isotopes ²⁶⁷Db as well as ²⁸¹Rg values of the observed half-lives are somewhat lower than the bulk of data in the systematics of T_β versus Q_β for known isotopes of Np-Db (see, e.g. figure 7 in [27]). This could indicate a larger probability of undergoing SF than EC/_β⁺ decay leading to 1 h ²⁶⁷Rf and 13 s ²⁸¹Ds.

Spontaneously fissioning nuclei with Z = 108–112 and N = 168–170 located between N = 162 and N = 184 neutron shells have the lowest T_SF values. The α-decay chains of the heaviest nuclei both with even and odd Z, N numbers are terminated here by SF. The decay chains of nuclei outside this region end by SF of neutron-rich isotopes of Rf and Db located closer to the neutron shell at N = 162. Thus, the decay chains of the heaviest nuclei synthesized in the Act + ⁴⁸Ca reactions are not connected with a region of known nuclei. They form an isolated region of the heaviest nuclei—some 'island of super-heavy nuclei'. The existence of this island and relatively high stability of SHN are determined in full by the new closed shells at N = 184 and Z = 114 (possibly 120–126).

Investigation of SF of SHN is of independent interest. Extensive information about the SF process, obtained for nuclei in the region of actinides (especially for ²⁵²Cf), can be essentially extended for heavier and super-heavy nuclei whose fission barrier is entirely determined by the influence of the new nuclear shells.