Journal of Nuclear and Radiochemical Sciences,Vol. 3, No. 1, pp. 5–8, 2002

5

Synthesis and Properties of Even-even Isotopes with Z = 110–116 in 48 Ca Induced Reactions
Yu. Ts. Oganessian∗Flerov Laboratory of Nuclear Reactions, Joint Institute for Nuclear Research, 141980 Dubna, Moscow
Region, Russian federation
Received: November 13, 2001; In Final Form: November 13, 2001
The paper presents results on the synthesis of superheavy nuclides with Z = 114 and 116 in the fusion reactions with48
Ca ions. In the irradiation of targets made from enriched 244 Pu and 248 Cm isotopes with beam doses of 1.5 ×1019
and 2.3 ×1019 , respectively, the detector array situated in the focal plane of the gas-filled separator registered heavy
atoms of new elements undergoing sequential α decays terminated by spontaneous fission. The time of the decay
chains is approximately one minute. Decay properties of the synthesized nuclei are consistent the consecutive α
decays originating from the parent nuclides 288 114 and 296 116 produced in the 4n-evaporation channels with thecross section of about a picobarn. Comparison of TSF and Tα values for the nuclei with Z = 110 and 112 with thoseobtained earlier for lighter isotopes of these elements points to an enhanced stability of heavy nuclei with an increasein the neutron number. The α-decay energies Qα measured experimentally in the chains 116-α1 –114-α2 –112-α3 –110
are compared with theoretical predictions of different nuclear models. This comparison shows that the difference
between the experiment and theory is in the range of ±0.5 MeV. From this it follows that the theoretical models
predicting the decisive influence of the nuclear structure on the stability of superheavy elements are well-founded
not only qualitatively but in some sense also quantitatively. The prospects of further investigations in the field ofsuperheavy nuclei are discussed briefly.

1. Introduction
It is known that one of the fundamental consequences of nuclear theory is the prediction of the “island of stability” of superheavy elements in the region of hypothetical superheavy elements. This intriguing hypothesis suggested 35 years ago has
been developed lately and now seems to find its experimental
confirmation in the currently conducted experiments.
A considerable increase in the nuclear stability at approaching closed spherical shells Z = 114 (and possibly 120–126) andN = 184 which follow the doubly magic nucleus 208 Pb (Z = 82,
N = 126) is expected for the isotopes of superheavy elementswith a high neutron excess (Figure 1). That is why for the synthesis of nuclei with Z = 114 and 116 we chose the fusion reactions 244 Pu, 248 Cm + 48 Ca in which the reaction products after
the evaporation of neutrons have the maximal neutron excess.1
In the fusion reactions with a doubly magic nucleus 48 Ca thecompound nuclei 292 114 and 296 116 formed at the Coulomb barrier have the excitation energies of 33 and 31 MeV, respectively.
It can be assumed that at these energies shell effects are stillpresent in the excited nucleus which enhances the survival probability of evaporation residues as compared with typical “hotfusion” reactions (Ex ≥ 45 MeV) used by us earlier for the synthesis of heavy isotopes with Z = 106, 108, and 110.2
Despite of these advantages, all previous attempts to synthesize new elements in reactions with 48 Ca ions and actinide targets only yielded the upper limit of their production cross section.3 It was vital to increase the sensitivity of the experimentby three orders of magnitude to go down to the level of 0.5 pb
where formation of superheavy nuclides was expected in the 3n
and 4n evaporation channels. An increase in the sensitivity ofthe experiments could be achieved first of all due to increasing
the intensity of the 48 Ca ion beam. For this purpose a new ion
source ECR-4M operating on metallic Ca vapors was created.
At a consumption of about 0.3 mg/h, a 48 Ca ion beam with theenergy EL = 6 MeV/A and intensity of about 0.5–0.8 pµA wasproduced.4
2. Experiment
Enriched 242,244 Pu and 248 Cm isotopes were used as a target
matter. Rotating targets (about 0.3 mg/cm2 in thickness) de∗E-mail: oganessian@flnr.jinr.ru.

posited on a Ti (1.5 µm) backing having a total area of about
30 cm2 were used. Recoil nuclei knocked out of the target layer
were separated in-flight from 48 Ca ions and other products of incomplete fusion reactions by kinematic separators. Two typesof recoil separators were used in the experiments: VASSILISSA(energy selector) and a gas-filled separator DGFRS (Dubna GasFilled Recoil Separator).
Briefly about the conditions of the experiment.
Separated heavy atoms are implanted into a strip positionsensitive detector (∼50 cm2 in area) situated in the focal plane.
The front detector is surrounded with side detectors so that thewhole assembling looks like a box with an open front wall. It
increases the efficiency of detection of α particles from the decay of implanted nuclei up to 87% of 4π. For every implanted
atom velocity is measured (by TOF detectors) as well as energy
and location of implants on a sensitive surface of the front detector. In the case the nuclei of implanted atoms emit α particles
or fission fragments, the last-mentioned will be registered in astrict correlation with the implant.
Experiments have been carried out in such setting since late1998 and up to date. Without mentioning the results of the first

Figure 1. The map of nuclides in the region of heavy elements. The
intensity of the color reflects the half-lives of the nuclei (the right-handscale). The crosses indicate the location of the doubly magic nuclei withclosed spherical shells Z = 82, N = 126 and Z = 114, N = 184 and withclosed deformed shells Z = 108, N = 162. The white squares indicate
the compound nuclei with Z = 112 and 114 formed in the reactions ofcold fusion 70 Zn + 208 Pb and hot fusion 48 Ca + 244 Pu, respectively.

c 2002 The Japan Society of Nuclear and Radiochemical Sciences
Published on Web 6/30/2002

6 J.Nucl.Radiochem.Sci.,Vol. 3, No. 1, 2002

Oganessian

Figure 2. Chains of radioactive decay of nuclei synthesized in the reactions (a) 48 Ca + 244 Pu and (b) 48 Ca + 248 Cm. The shaded area corresponds to theswitched-off beam.

experiments with 238 U, 242 Pu, and 244 Pu targets which were published in Reference 5, we will only present the data obtained
within the past year.
In May–November 2000, the second irradiation of the 244 Pu
target was performed with 48 Ca ions, the beam dose was 1 ×1019
ions.6The highly enriched (98.5%) target matter was provided byour colleagues and collaborators from the Livermore National
Laboratory. In that experiment two more identical decay chainswere observed. Each of them consisted of two sequential α decays and was terminated with spontaneous fission accompanied
by a high energy release in the detector array (Figure 2a). Only
the above mentioned events of spontaneous fission were registered in this experiment. The lifetime of the new nuclei wasabout 0.5 min. The probability that the observed decays were
the result of incidental coincidences of the signals in the front
detector was less than 5 ×10−13 . Note that both events were observed at an energy of the 48 Ca ion beam corresponding to theexcitation energy of the compound nucleus Ex = 36 ±2 MeV. At
this energy the most probable channel of the 292 114 nucleus deexcitation corresponds to the emission of 4 neutrons and γ rays.
Proceeding from this, new decay chains may be attributed to the
decay of the even-even isotope of element 114 with mass 288.
The experiment that followed was devoted to the study of thereaction 48 Ca + 248 Cm and was aimed at testing this conclusion.
The target matter — enriched isotope of 248 Cm (Z = 96) —was produced at the high-flux reactor in Dimitrovgrad (Russia)
in the quantity of 10 mg. The other target matter of enriched isotope 248 Cm was provided by the Livermore National Laboratory(USA).
Changing the 244 Pu target to the 248 Cm one, all other conditions of the experiment being kept, should lead to the formationof the new heavy nucleus with Z = 116 and mass 292 in the 4n
evaporation channel of the fusion reaction 48 Ca + 248 Cm. As aresult of the α decay of this nucleus, expected with a high probability, we should obtain the daughter nucleus-isotope 288 114
earlier produced in the reaction 48 Ca + 244 Pu. That is why after
the decay of the 292 116 nucleus the whole chain of the daughter nucleus decay 288 114 → 284 112 → 280 110 (SF) should also be
observed in the experiment.
Usually the separator operates with a continuous 48 Ca beam.
In the experiment on the synthesis of element 116 this regime
was changed.
After the implantation into the focal plane detector of theheavy nucleus with the expected parameters (energy and velocity) and its decay with emission of an α particle with

Eα ≥ 10 MeV (two signals are strictly position-correlated) the
beam was switched off.
Measurements made straight after switching off the beam
showed that the rate of the α particles (Eα ≥ 9 MeV) and spontaneous fission fragments in any strip within ∆x = 0.8 mm,
which is determined by the position resolution of the detector,
is 0.45/year and 0.1/year, correspondingly. Incidental coincidences of the signals simulating the 3-step 1-min chain of thenucleus decay 288 114 (α-α-SF) are practically excluded even fora single event.7
In these conditions, at a beam dose of 2.25 ×1019 ions three
decay chains of element 116 were registered (Figure 2b). After the emission of the first α particle (Eα = 10.53 ± 0.06 MeV)
the following decay occurred in the absence of the beam (seethe gray area). As is seen from Figure 2, two decay chainsof the 288 114 nucleus produced in the reaction 48 Ca + 244 Pu andthree new decay chains observed in the reaction 48 Ca + 248 Cm

Figure 3. (a) Position and (b) energy correlations in the decay chainsshown in Figure 2.

Synthesis and Properties of Even-even Isotopes
are strictly correlated: five signals arising in the front detector,
i.e., the recoil nucleus, three α particles and fission fragments(in the case of the 288 114 nucleus decay: R-α-α-SF), differ intheir position by no more than 0.6 mm (Figure 3a).
Energies of α particles and nuclear half-lives obtained in thereaction 48 Ca + 248 Cm coincide with each other within the limitsof the detector energy resolution (∆Eα ∼ 60 keV) and statistical fluctuations determined by the nuclear decay in the chains.
Figure 3b shows α-particle energy spectra for the three events
corresponding to the 292 116 α-decay and the five events corresponding to the 288 114 and 284 112 decay, as well as the spectrumof combined energies of fission fragments from the five eventsof the 280 110 spontaneous fission obtained in the experimentswith 244 Pu and 248 Cm targets.
Isotopes of elements 114 and 116 produced in the reactions
Pu, 248 Cm + 48 Ca are most likely formed in the 4n evaporation
channels. This conclusion follows from the excitation energy
values for compound nuclei which after the emission of neutronsand γ rays led to the observed evaporation products.

244

As it had been expected, in the case of even-even nuclei
the experimentally observed α radiation is characterized by thestrictly definite decay energy which corresponds to the difference between the masses of mother and daughter nuclei in the
ground states (Figure 3b). For the allowed α transitions (eveneven nuclei) the decay energy Qα and the decay probability (orthe half-life Tα ) are connected by the well-known relation ofGeiger-Nuttall. This relation is strictly fulfilled for all the known
by now 60 even-even nuclei heavier than Pb, for which Qα andTα have already been measured. Figure 4 shows experimental
and calculation data for the region of heavy nuclei with Z ≥ 100.
The results obtained for the new even-even superheavy nuclei
with Z = 112, 114, and 116 are also presented there. On theother hand from the quantities Qα and Tα it is possible to determine the atomic numbers of the α-emitters. With a probabilityof ≥96% the observed α-decays can be attributed to the chain
with Z = 116–114–112–110.

J.Nucl.Radiochem.Sci.,Vol. 3, No. 1, 2002

7

TABLE 1: Radioactive properties of even-even isotopes of the superheavy elements with Z = 110, 112, 114, and 116.

Z
116
114
112
110

Decay mode
α
α
α
SF

Qα / MeV
10.68 ± 0.06
9.96 ± 0.06
9.28 ± 0.06
TKE ∼ 230

Tα
53+62
−19 ms
2.6+2.0−0.8 s0.75+0.57−0.23 min7.6+5.8−2.3 s3. Properties of Superheavy Nuclides
Radioactive properties of the new nuclides are presented inTable 1. Comparison of the partial half lives TSF and Tα of thesynthesized nuclei with Z = 110 and 112 with the data obtained
earlier for lighter isotopes of these elements9 point to a considerable increase in the stability of heavy nuclei with increasing theneutron number. Addition of 10 neutrons to the 270 110 nucleus(Tα ∼ 100 µs) leads to an increase in the half-life almost by 105
times. For the nuclei of element 112 with A = 277 and the isotopes with A = 284 and 285 formed in the synthesis of element
114 nuclei in the reaction 244 Pu(48 Ca; 3n,4n) this difference is
2 ×105 and about 106 , respectively.
Comparison of the experimental Qα values for even-even nuclei with the calculation carried out using different theoreticalmodels8, 10–13 shows that the difference between the experimentand theory is in the range of ±0.5 MeV (Figure 5).
Without making so far a more detailed analysis, one can conclude that theoretical predictions about the decisive influence ofthe nuclear structure on the stability of superheavy nuclides has
been confirmed not only qualitatively but also to some extent
quantitatively.
An increase in the half-lives of the most heavy nuclei up totens of seconds and minutes substantially extends the area for
the investigation of superheaviers, including the study of theirchemical properties, measuring atomic masses, etc. On the other
hand, development of acceleration and experimental techniquewill allow us to make an advent into the region if even heavier
nuclei in the future.

Finally, in spontaneous fission of 280 110 nuclei the total energy release from the fission fragments in the detectors is about
E = 206 MeV. This value with a correction to the energy losses inthe dead layers of the focal plane pentane gas, and side detectors
corresponds to the mean total kinetic energy of the fission fragments TKE ∼ 230 MeV. Such a high value of the fission fragment energy points to the spontaneous fission of a rather heavy
nucleus. Note that in the fission of 235 U induced by thermal neutrons TKE = 168 MeV.

(1) Experimental and calculated cross sections of evaporation
products of the fusion reactions with 48 Ca ions obtained in thesynthesis of superheavy nuclides with Z = 112–116 show thatthese reactions can also be in principle used for the synthesisof nuclides with Z > 116. In particular, the reaction 249 Cf + 48 Caleading to the formation of the Z = 118 element nuclei in the
xn-evaporation channels can be regarded as a next step. In thisFigure 4. Calculated8 and the experimental Tα vs. Qα relationship foreven-even isotopes of transfermium elements.

Figure 5. The difference between calculated and experimental valuesof Qα for even-even isotopes with Z = 106–116 and N = 154–176.

4. Prospects

8 J.Nucl.Radiochem.Sci.,Vol. 3, No. 1, 2002
reaction the excitation energy of the 297 118 compound nucleus at
the Coulomb barrier is about 26 MeV. At this energy, the largest
formation cross section of the evaporation products is expected
for the 3n evaporation channel with the formation of the eveneven isotope 294 118 (N = 176).
For the registration of sequential decays of this nuclide, thesame setting of the experiment as earlier used in the synthesis of
nuclei with Z = 116 can be employed. The energy of the allowed
α transitions in the chains of the 294 118 nucleus decay together
with earlier obtained data for Z = 114 and 116 can yield more
information on the location and strength of the closed protonshell in the region of superheavy nuclei.
(2) Relatively long half-lives of the new nuclides amounting
to several seconds in the case of Z = 114 and nearly one minutein the case of Z = 112 open up possibilities for the investigation of chemical properties of superheavy elements. In our opinion, the most suitable candidates are the isotopes of Eka Hg due
to their relatively long half-lives and expected unique chemical
properties of element 112. One of possible approaches to thistask is to determine the probability of formation of intermetallic
bonds between Eka Hg and Pd, Au, or Pt which are well-known
for Hg. On-line experiments of this type will soon be launchedusing 48 Ca ion beams.
(3) An enhanced stability of superheavy nuclei changes theentire approach to their synthesis in the fusion reactions withheavy ions. The necessity of separating in-flight the reaction
products for a speedy delivery of superheavy atoms to detector array is not a decisive factor in the synthesis of superheavy
nuclei due to their long half-lives. The use of classical on-line
separators is more preferable since the latter alongside their highefficiency, connected with a possibility of using thick targets anda higher selectivity, allow us to measure the superheavy nucleus
mass.
This idea should be implemented in the design of the setup
MASHA (Mass Analyzer of Superheavy Atoms), which is now
being developed at the Flerov Laboratory.
(4) The use of fusion reactions with 48 Ca ions presently employed for the synthesis of superheavy elements is restricted by
the availability of the target material. The isotope 249 Cf which isplanned to be used as a target in the experiments on the synthesis of nuclei with Z = 118 is apparently the last one in the row ofactinides. At the same time, according to predictions of microscopic models of the type of Hartree-Fock-Bogoliubov or Relativistic Mean Field, theory a change in the nuclear density (theso call “semi-bubble nuclei”) also leading to the stability of hyperheavy nuclei is expected in the case of nuclei with Z ≥ 120.13
For the synthesis of these nuclei it will be necessary to use fusion reactions with ions heavier than 48 Ca, i.e. of the 58 Fe, 64 Ni,etc., type. Two fusion reactions, 248 Cm + 54 Cr and 238 U + 64 Ni,
lead to the formation of the weakly excited compound nucleus
302
120 with N = 182. It is planned to study the fusion and fission
processes involving such massive nuclei.
Acknowledgements. Experiments described in the present
talk were carried out at the heavy ion accelerators of the JINR
in collaboration with LLNL (Livermore).
Taking this opportunity, I would like to express gratitude to
my colleagues who took part in the experiments and to Profs.
W. Greiner, S. Hofmann, G. Münzenberg, A. Sobiczewski, M.
Itkis, V. Zagrebaev, M. Ohta, Y. Abe, S. N. Dmitriev, and P.
Heenen for their interesting discussions and valuable remarks.
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