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HEAVIEST NUCLEI FROM
48
Ca-INDUCED
REACTIONS
YURI OGANESSIAN
Flerov Laboratory of Nuclear Reactions,
Joint Institute for Nuclear Research,
141980 Dubna, Russia
Relatively long half-lives of the new nuclides open possibilities for the
study of the structure of superheavy atoms, in particular chemical prop-
erties of the new elements. Experiments on the observation of the so-
called “relativistic effect†in the electron structure of elements 112 and
114 using express radiochemical methods are now underway at the
48
Ca
ion beam. The search for the most long-lived superheavy elements in
nature is aimed at the registration of decay of element 108 isotopes.
Preliminary results of measurements of spontaneous ï¬ssion rare events
in the underground laboratory at Modane (France) are discussed in con-
text of the maximal nuclear half-lives at the top of the island of stability
and in context of setting up new experiments.
Keywords
: Superheavy nuclei; long-lived isotopes.
A fundamental outcome of modern nuclear microscopic theory is the predic-
tion of the “islands of stability†in the region of hypothetical superheavy
elements. A signiï¬cant enhancement in nuclear stability when approach-
ing the closed spherical shells with
Z
= 114 (possibly 120 and 122) and
N
= 184, which follow the doubly magic
208
Pb nucleus is expected for the
nuclei with large neutron excess.
Because of this, for the synthesis of nuclei with
Z
= 112–116 and 118
we chose the reactions
238
U,
242
,
244
Pu,
243
Am,
245
,
248
Cm and
249
Cf +
48
Ca,
which are characterized by evaporation residues with a maximal number of
neutrons.
The formation and decay of the nuclei with
Z
= 112–116 and 118 were
registered with the use of the Gas Filled Recoil Separator (Fig. 1), installed
at the beam of the heavy ion accelerator.
1
The mechanism of production of the heaviest nuclei induced by
48
Ca
ions was studied separately. From the yield of the evaporation residues —
products of the above-mentioned reactions (excitation functions), measured
at different ion-beam energies — it follows that they are formed in the
3
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Yu. Oganessian
Fig. 1.
Dubna Gas Filed Recoil Separator.
process involving the emission of two to ï¬ve neutrons depending on the
excitation energy (or temperature) of the compound nucleus.
2
The maximal formation cross-section for the heaviest nuclei substan-
tially depends on the neutron number in the compound nucleus and its
position relative to the closed neutron shell
N
= 184. Figure 2 shows the
formation cross-section of the evaporation products as a function of the
ï¬ssion barrier height of compound nuclei produced in the cold fusion (1n
channel) and hot fusion (4n channel) reactions. In the cold fusion reac-
tions (with a
208
Pb or
209
Bi target) a decrease in the cross-section with
increasing
Z
CN
is connected with dynamical prohibitions for the fusion of
massive nuclei,
3
whereas in the reactions of hot fusion (
48
Ca + Act.) the
major losses are determined by the strong ï¬ssility of the heated nucleus.
The survivability of compound nuclei grows with an increase in their ï¬s-
sion barrier height in the vicinity of closed neutron shells
N
= 152
,
162.
4
,
5
An increase in the cross-section of the evaporation products of the heav-
iest nuclei, observed in the experiment, with the growing atomic number
in the region of
Z
CN
= 112–116 is connected with an increase in the neu-
tron number and nuclear ï¬ssion barrier as the neutron shell
N
= 184 is
approached.
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Heaviest Nuclei from
48
Ca-Induced Reactions
5
Fig. 2.
The upper panel shows cross-sections of the 1n and 4n evaporation channels
of the cold and hot fusion reactions, correspondingly, as a function of the compound
nucleus neutron number. The lower panel shows the fission barrier heights as a function
of
N
CN
.
The new nuclides undergo mainly sequential
α
-decays, which are ter-
minated by spontaneous ï¬ssion (SF). The total time of the decays ranges
from 0.5 ms to
∼
1 d, depending on the proton and neutron numbers in the
synthesized nuclei.
The experimental method used can be demonstrated with the example
of the synthesis of elements 113 and 115 in the reaction
243
Am +
48
Ca.
6
,
7
The evaporation of three neutrons and the emission of
γ
-rays by the com-
pound nuclei of element 115, produced in the fusion reaction, lead to the
formation in the ground state of the odd–odd nuclide with 115 protons and
173 neutrons. This nuclide is the parent of a radioactive “family†consisting
of the
Z
= 115(
α
)
→
113(
α
)
→
111(
α
)
→
109(
α
)
→
107(
α
)
→
105(SF)
nuclei, formed as a result of the ï¬ve consecutive emissions of
α
-particles,
and terminated by spontaneous ï¬ssion of the Db isotope (
Z
= 105).
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Yu. Oganessian
Fig. 3.
Synthesis of elements 113 and 115.
The properties of the long-lived Db isotope are themselves of special interest
(Fig. 3).
Due to the long lifetime, the atoms of element 105 can be separated by
a classical off-line radiochemical method of ion-exchange chromatography,
with the consequent measurement of their decay by spontaneous ï¬ssion. In
eight identical experiments
243
Am +
48
Ca, after the chemical separation,
15 spontaneous ï¬ssion events were detected with
T
1
/
2
∼
1 d.
8–10
It was
shown that the spontaneous ï¬ssion observed in the decay of the
288
115
nucleus comes from an element that is a chemical homologue of Nb and
Ta, which are representatives of the ï¬fth group of Mendeleev’s Periodic
Table of the Elements. The chemistry experiment gives an independent
and unambiguous identiï¬cation of the atomic number of the ï¬nal nucleus
(
Z
= 105) and at the same time the atomic numbers of all nuclides in the
fully correlated chain of the decay of the parent nucleus
288
115.
Another example is the study of a chemical behavior of the isotope
283
112 (
T
1
/
2
∼
3.8 s). To what extent element 112 is a homologue of Hg
depends on the so-called “relativistic effect†in the electronic structure of
the superheavy atom. According to some relativistic calculations, the chem-
ical behavior of element 112 (as well as of the other atoms with a higher
atomic number) will somewhat differ from that of its light homologue. Both
characteristics — the high volatility and the ability to form inter-metallic
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48
Ca-Induced Reactions
7
compounds — of Hg/Au and (112)/Au independently conï¬rm the identi-
ï¬cation of the atomic numbers of element 112.
11
And ï¬nally, after a few
attempts failed to synthesize the isotope
283
112 in the reaction
238
U +
48
Ca
with different set-ups,
12
,
13
its decay properties were successfully reproduced
in the recent experiments with the set-up SHIP (GSI).
14
The majority of our experiments have been dedicated to the synthesis
of even-
Z
elements. In the
238
U,
242
,
244
Pu,
245
,
248
Cm +
48
Ca reactions 15
new nuclides were synthesized for the ï¬rst time: these are the heaviest
isotopes of elements 110, 112, 114 and 116. For the
291
116 isotope and
its daughter nuclei
287
114 and
283
112, a rare branch of 6–4 consecutive
α
-
decays was observed: 116(
α
)
→
114(
α
)
→
112(
α
)
→
110(
α,
SF)
→
108(
α
)
→
106(
α,
SF)
→
104(SF). It was terminated by spontaneous ï¬ssion of the
neutron-rich isotope
267
Rf (
T
1
/
2
∼
1
.
3 h). Simultaneously, ï¬ve new odd-
Z
isotopes of elements 111, 113 and 115 were obtained in reactions
237
Np,
243
Am +
48
Ca.
For the ï¬rst time, results are reported on the synthesis of the heaviest
element with
Z
= 118. It has been shown that in the fusion of two massive
nuclei
249
Cf +
48
Ca, after the emission of three neutrons, the even–even iso-
tope
284
118, which undergoes
α
-decay (
E
α
= 11
.
65 + 0
.
06 MeV) with a half-
life
T
1
/
2
= 0
.
9
+1
.
1
−
0
.
3
ms, is formed. Its further decay takes place within about
0.2 s via the chain
284
118(
α
)
→
280
116(
α
)
→
276
114(
α,
SF)
→
272
112(SF).
The properties of the daughter nuclei of
284
118 — the isotopes with
Z
= 116, 114 and 112 — were studied in an independent way in the
238
U,
242
Pu and
245
Cm +
48
Ca reactions (Fig. 4). A self-consistent picture of the
decay of the isotope of element 118 was obtained.
15
In the series of experiments using the
48
Ca-beam, performed during the
past six years, in total 84 events, corresponding to the formation and decay
of 34 new nuclides with
Z
= 104–118 and
N
= 161–177, were observed
(Fig. 5).
A comparison of the half-lives of the known nuclei with
Z
= 110–112
with those of the newly observed neutron-rich isotopes of the same elements
shows that the increase of their mass by adding 6–8 neutrons brings forth
an increase in nuclear stability by a factor of 104–105 (Fig. 6).
The decay properties of the isotopes of the heaviest elements are now
being compared with the predictions of microscopic nuclear models. This
comparison gives evidence of the decisive influence of the nuclear structure
of superheavy elements on their stability with respect to different modes
of radioactive decay. A more detailed analysis shows that experiments do
not solely reproduce the theoretically expected decay scenarios, but are
also consistent (within
∼
5% accuracy) with the decay energies of all the
synthesized 26
α
-radioactive nuclei with
Z
= 106–118.
17
From this point
of view, the obtained results can be considered as the ï¬rst experimental
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Yu. Oganessian
Fig. 4.
(a) Decay chains of the isotopes of element 116 synthesized in 2n- and
3n-evaporation channels of the reaction
245
Cm +
48
Ca (bottom). The daughter prod-
ucts of
α
-decay of these nuclides — the isotopes of elements 114 and 112 — were also
produced in the reactions
242
Pu,
238
U +
48
Ca. (b) Decay of the nuclei of element 118
synthesized in the reaction
249
Cf +
48
Ca.
15
evidence of the existence of “islands of stability†in the region of the heaviest
elements, considerably extending the boundaries of the material world.
16
Keeping in mind that the yield of the new nuclides in heavy ion reactions
is extremely low, any further progress in the ï¬eld has to be connected
primarily with the sensitivity of the experiment.
The increase of the ion-beam intensity and the creation of new, more
efficient set-ups will make it possible to perform fundamental investigations
in nuclear physics (determination of nuclear mass limits) and chemistry
(relativistic effect), affecting at the same time inter-disciplinary ï¬elds: the
models of nucleosynthesis, astrophysical aspects, studies of the structure of
superheavy atoms and molecules, and so on.
One of the new-generation set-ups, MASHA (Mass Analyzer of Super
Heavy Atoms), is designed to be the ï¬rst step in achieving this aim. This
new set-up, compared to the already existing kinematical separators, will
be a few times more efficient; it will have high selectivity and identiï¬cation
ability for the mass numbers of the separated atoms.
18
At the same time, it
is a sophisticated detector that can be used in different chemical techniques,
for studying the chemical behaviors of the superheavy elements. After the
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48
Ca-Induced Reactions
9
Fig. 5.
Nuclide chart of elements with
Z
≥
104.
16
determination of the physical and chemical properties of elements 112 and
114 in the
242
,
244
Pu+
48
Ca reactions (such experiments have been performed
in our laboratory in the last two years), further investigations will be carried
out in conjunction with the MASHA set-up.
Another problem is obtaining longer-lived superheavy nuclides. As the
artiï¬cial synthesis of nuclei is limited, the possibility of searching for the
most stable nuclei with
Z
= 106–110 and
N
∼
180 (the estimates are
T
1
/
2
∼
10
4
–10
6
y with high uncertainties) in nature is being considered.
Among the possible candidates for the ï¬rst experiment, an isotope of ele-
ment 108 (Hs) was chosen. The search for the long-lived Hs isotope in its
chemical homologue — a sample of metallic Os (500 g) — is being performed
in the underground laboratory in Modane (France). The observation of a
single spontaneous ï¬ssion event (measured as a neutron flash, accompany-
ing the ï¬ssion process) over a one-year measuring period will correspond
to a concentration amounting to 5
×
10
−
15
g/g of element 108 in the Os
sample, assuming that its half-life is equal to 10
9
y. This small value is
∼
10
−
16
times less than the concentration of uranium in the Earth’s crust.
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Yu. Oganessian
Fig. 6.
Half-lives of the isotopes with
Z
≥
108 synthesized in cold fusion (open symbols)
and
48
Ca-induced reactions (black symbols). The lines trace through calculated
α
-decay
half-lives obtained in the MM-model.
17
In spite of the high sensitivity of the experiment, the chances of ï¬nding
surviving superheavy nuclei are small. However, the absence of any effect
will give an upper limit for the half-life of the long-lived nuclide at the level
of
T
1
/
2
≤
5
×
10
7
y (shown in Fig. 6).
Acknowledgments
The experiments were carried out at the Flerov Laboratory of Nuclear
Reactions (JINR, Dubna) in collaboration with the Analytical & Nuclear
Chemistry Division of the Lawrence Livermore National Laboratory (USA).
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Ca-Induced Reactions
11
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