MEASUREMENTS OF CROSS SECTIONS AND DECAY PROPERTIES OF THE
ISOTOPES OF ELEMENTS 112, 114, AND 116 PRODUCED IN THE FUSION
REACTIONS 233,238U, 242,244Pu, AND 248Cm+48Са
Yu.Ts. Oganessian, V.K. Utyonkov, Yu.V. Lobanov, F.Sh. Abdullin, A.N. Polyakov,
I.V. Shirokovsky, Yu.S. Tsyganov, G.G. Gulbekian, S.L. Bogomolov, B.N. Gikal, A.N. Mezentsev,
S. Iliev, V.G. Subbotin, A.M. Sukhov, A.A. Voinov, G.V. Buklanov, K. Subotic, V.I. Zagrebaev,
M.G. Itkis, J.B. Patin1, K.J. Moody1, J.F. Wild1, M.A. Stoyer1, N.J. Stoyer1, D.A. Shaughnessy1,
J.M. Kenneally1, P.A. Wilk1, and R.W. Lougheed1
1
University of California, Lawrence Livermore National Laboratory, Livermore, California 94551, USA
According to nuclear theory, the limits of
the existence of heavy nuclei, as well as their
decay properties, are completely determined
by nuclear shell effects. For the heaviest
elements in the vicinity of the hypothetical
closed spherical shells Z=114 and N=184, the
increase of nuclear binding energy results in a
considerable increase of stability with respect
to various decay modes. We may also
speculate that the high fission barriers of the
superheavy nuclei in their ground states may
persist at low excitation energies, resulting in
an increase in their production cross section, or
to be more precise, their survival probability in
the process of de-excitation of the compound
nucleus.
In the present work, we investigated the
survival probabilities of the compound nuclei
by measuring excitation functions for
producing evaporation residues with Z=112,
114, and 116 in reactions with different
targets: 233,238U, 242,244Pu, and 245,248Cm [1].
The 32-cm2 targets consisted of the isotopes
233
U, 238U, 242Pu, 244Pu, 245Cm, and 248Cm
deposited to thicknesses of 0.44, 0.35, 0.40,
0.38, 0.35, and 0.35 mg cm2, respectively.
The 48Ca-ion beam intensity was 1.2 pA. The
evaporation residues (ER) recoiling from the
target were separated in flight by the Dubna
Gas-Filled
Recoil
Separator
with
a
transmission efficiency of about 35-40%. To
detect the decay sequences in low background
conditions, the beam was switched off after a
recoil signal was detected with parameters of
implantation energy expected for the ERs,
followed by an -like signal within a preset
energy interval in the same strip and position,
and a time interval up to 12 s. The duration of
the pause in beam was determined from the
observed pattern of out-of-beam  decays and
varied from 1 to 12 minutes.
During these experiments [1], we measured
the excitation functions for the reaction
244
Pu(48Ca,xn). We chose bombarding energies
for the 48Ca ions of 243, 250 and 257 MeV in
the middle of the target (E*=38.9-43.0, 44.949.0, and 50.4-54.7 MeV [2], respectively). At
the 243-MeV 48Ca beam energy, we observed
two decay chains each consisting of two
consecutive  decays terminated by SF.
Identical decay chains were previously
discovered in the same reaction at a lower
energy of 236 MeV [3]. One more event of
this type was detected at beam energy of
250 MeV. In addition to these decay chains,
two new isotopes of element 114 and their
descendant nuclei were identified for the first
time in this experiment with E=9.94 MeV,
T1/2=0.8 s and E=10.02 MeV, T1/2=0.5 s (see
Fig. 1 and Table I).
Based on the results of these experiments, it
was reasonable to assign the previously
observed ER---SF chain [3] to the decay of
the even-odd nuclide 289114 produced via 3nevaporation with a maximum cross section of
about 1.7 pb. The two other nuclei should then
be assigned to the decay of the neighboring
even-even and even-odd isotopes of element
114, 288114 and 287114, produced via the 4nand 5n-evaporation channels with cross
sections of 5.3 pb and 1 pb, respectively [1].
One can note that a more solid mass
identification is possible if the mass number of
the target nucleus can be varied. Thus, if the
suggested mass assignment is correct, the
isotope 287114 should be observed in the 3nevaporation channel of the reaction 242Pu+48Ca
and another even-even isotope, 286114, which
has a higher -particle energy and lower lifetime, should be produced via the 4nevaporation channel.
Fig. 1 Excitation functions for the 2n-5n
evaporation channels from the fusion-evaporation
reactions 233,238U, 242,244Pu, 248Cm+48Ca. The Bass
barrier [4] is shown by an open arrow in each
panel. Lines show the results of calculations [5].
Error bars correspond to statistical uncertainties.
Indeed, in the reaction 242Pu+48Ca, we
observed the two isotopes 287114 and
286
114 [1]. The decay properties of nuclei
originating from 287114 produced in the
242
Pu(48Ca,3n) reaction coincide well with
those
for
nuclei
observed
in
the
244
Pu(48Ca,5n)287114 reaction. The even-even
isotope 286114 undergoes SF and  decay with
T1/2=0.16 s, while 282112 decays by SF with
TSF=0.5 ms.
Together with cross section measurements,
the other approach to the mass and atomicnumber identification of unknown nuclei is a
series of cross bombardments. In our case this
means the production of the same isotopes of
element 114 as the daughter nuclei following
the  decay of heavier mother nuclei with
Z=116 or the observation of the descendant
nuclei of element 114 isotopes in the primary
reaction leading to nuclei with Z=112.
We used this method for the first time in an
experiment aimed at the synthesis of nuclei
with Z=116 in the reaction 248Cm+48Ca [6].
Three similar decay sequences were observed.
All of the decays following the first  particles
agree well with the decay chains of 289114,
previously observed in the 244Pu+48Ca
reaction [3]. Thus, it was reasonable to assign
the observed decays to the nuclide 293116. In
the recent bombardment of a 248Cm target, two
more decays of 293116 and six decays of 292116
were observed at higher 48Ca energy [1].
In the reaction 245Cm+48Ca, we investigated
the radioactive properties of other isotopes of
element 116 [1]. At a beam energy of
243 MeV in the middle of the target
(E*=30.9-35.0 MeV), we detected two decay
chains of 291116 and three decays of 290116.
The decay energies and half-lives agree well
between the daughter nuclei in the decay
chains of 291116 observed in the
245
Cm(48Ca,2n) reaction, and the chains
observed in the reactions 244Pu+48Ca at the
maximum beam energy E*=53 MeV and
242
Pu+48Ca at E*=32.5-40.2 MeV, which
were assigned to the decay of 287114, the 5nand 3n-evaporation products, respectively [1].
Accordingly, the decay properties of the
descendant nuclei of even-even 260116, the
product of 3n evaporation, agree with those of
286
114 and 282112 observed in the reactions
242
Pu+48Ca at E*=40.2 and 45.1 MeV [1]. In
the 245Cm+48Ca reaction, the 2n- and 3nevaporation channels were observed with cross
sections of about 0.9 pb and 1.3 pb,
respectively. Moreover, the decay properties
of descendant nuclei in the decay chain of
294
118
synthesized
in
the
reaction
TABLE I. Decay properties of even-Z nuclei.
118
No.
Decay mode,
Half-life c)
a)
observed
branch b)
.
294
1/1

1.8 75
1.3 ms
116
293
4/4

292
4/5

18 16
6 ms
40 ms
10.660.07
10.800.07
11.06
291
2/2

.6
6.3 11
2.5 ms
20 ms
10.740.07
10.890.07
10.91
290
2/2

15 26
6 ms
10 ms
10.850.08
11.000.08
11.08
289
9/9

2.6 10..27 s
2s
9.820.05
9.960.05
10.04
288
16/16

0.80 00..27
16 s
0.9 s
9.940.06
10.080.06
10.32
287
15/15

0.51 00..18
10 s
0.5 s
10.020.06
10.160.06
10.56
286
11/5
0.2 s
10.200.06
10.350.06
10.86
285
10/10
:0.4,SF:0.6 0.16 00..07
03 s
29 13

7 s
50 s
9.150.05
9.280.05
9.49
284
17
SF
97 31
19 ms
9.80
9.76
283
18/18
:1,SF:0.1
4.0 10..37 s
9.670.06
10.16
282
6
SF
0.50 00..33
14 ms
10.82
10.68
281
10
SF
11.1 52..07 s
9.00
9.30
279
21/2
108
275
2/2
106
271
2/1
104
267
1
Z
114
112
110
A
61 57
20 ms
Expected
half-life
0.4 ms
E (MeV)
Q (MeV)
Qth (MeV) d)
11.650.06
11.810.06
12.11
80 ms
10.540.06
10.690.06
11.09
3s
9.540.06
:0.1,SF:0.9 0.18 00..05
03 s

0.15 00..27
06 s
0.2 s
9.700.06
9.840.06
10.24
0.8 s
9.300.07
9.440.07
9.41
:0.5,SF:0.5 2.4 14..03 min
SF
2.3 198.7. h
0.8 min
8.530.08
8.650.08
8.71
8.22
7.4
Number of events used for calculations of half-lives / -particle energies, respectively. Branching ratio is
not shown if only one decay mode was observed. c Error bars correspond to 68%-confidence level if more
than one event was observed, for only one registered event, the error bars correspond to 95%. d Predicted decay energies [8].
a
249
Cf(48Ca,3n) [7] are in agreement with 290116
and 286114 produced via the 245Cm+48Ca and
242
Pu+48Ca reactions.
In the 238U+48Ca experiments, we detected
7 decay sequences of 283112 that were
observed at beam energies EL=230-234 MeV,
and one decay of 282112 observed at
EL=240 MeV. Decays of the daughter nuclei in
the chains observed following the reaction
242
Pu+48Ca coincide in all of the measured
parameters (E, T, TSF, and ESF) with the
decay chains observed in the 238U+48Ca
reaction [1].
We have also studied the reaction
233
U+48Ca at EL=240 MeV; despite an
accumulated beam dose of about 81018 ions,
we did not observe any decay chains that could
b
be attributed to the decay of isotopes of
element 112. We calculated an upper cross
section limit of 24n0.6 pb for the reaction
233
U(48Ca,2-4n)277-279112 at EL=240 MeV [1].
The -particle spectra observed in these
experiments from the decay of nuclei with
Z=112, 114 and 116 are characterized by welldefined transition energies. They follow the
relationship between the probability and
energy of  decay (Viola-Seaborg formula)
that was determined from previously known
even-even nuclei (Table I). This means that the
observed transitions are as unhindered for the
isotopes with odd mass numbers as they are
for isotopes with even mass numbers. For the
isotopes of lighter elements, the difference
between measured and calculated T values
increases resulting in hindrance factors of
about 10 for 279Ds. A noticeable transition
from spherical to deformed shapes occurs at
Z=110, in agreement with results observed for
odd-Z nuclei [9]. Thus, the decay properties of
the isotopes of element 114 are generally
determined by the spherical shells Z=114 and
N=184. According to calculations [8], the
nucleus 287114 is almost spherical (2=0.088).
In a succession of sequential  decays, the
descendant nuclei move away from the N=184
shell and approach the deformed shell at
N=162. The terminating nucleus, 267Rf
(N=163), is deformed (20.23) [8].
The experimentally measured values of Q
practically
coincide
with
theoretical
predictions for the deformed nuclei in the
vicinity of neutron shells N=152 and N=162
and becomes somewhat less than the
calculated values by 0.5 MeV for the more
neutron-rich nuclides with N169. In the
decay chain 291116…267Rf, we observed a
similar variation in -decay energies as those
reported for decay chains starting with 287115
or 288115 [9]. The slope of Q vs. neutron
number remains practically the same for
elements 112-116 but increases significantly
for nuclides with Z=111 and 110 (see Table I).
Such an effect might be caused by the
transition from spherical nuclear shapes to
deformed shapes during successive  decays,
in agreement with calculations [8].
If the theoretical predictions of the
existence of closed nuclear shells in the
domain of superheavy elements are correct,
they should be characterized not only by high
stability to various decay modes (longer halflives) but also by a relatively high probability
of production. Indeed, the increase of the
height of fission barriers due to the influence
of the shell closure at N=184 is expected only
for the neutron-rich nuclei with N>170 [10].
For these nuclei, an increase of neutron
number in the compound nucleus results in an
increase of the production cross section (see
Fig. 1). The low cross section for the
formation of the isotope 278112 in the reaction
233
U+48Ca with NCN=169 has the same
explanation. We consider this to be a major
advantage of using the complete fusion
reactions
involving
the
neutron-rich
48
transuranic target nuclei and the Ca projectile
for the synthesis of superheavy elements.
This work has been performed with the
support of the Russian Ministry of Atomic
Energy and grant of RFBR No. 04-02-17186.
The 233U, 242Pu, and 245Cm target material
were provided by RFNC-VNIIEF, Sarov,
Russia. The 244Pu and 248Cm target material
were provided by the U.S. DOE through
ORNL. Much of the support for the LLNL
authors was provided through the U.S. DOE
under Contract No. W-7405-Eng-48. These
studies were performed in the framework of
the
Russian
Federation/U.S.
Joint
Coordinating Committee for Research on
Fundamental Properties of Matter.
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