Observation
of the Decay of 292116
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, O.V. Ivanov, G.V. Buklanov,
K.Subotic, and M.G. Itkis
Joint Institute for Nuclear Research,
141980 Dubna, Russian Federation
K.J. Moody,
J.F. Wild, N.J. Stoyer, M.A. Stoyer, R.W. Lougheed and C.A.
Laue
University of California, Lawrence Livermore National Laboratory,
Livermore, California 94551, USA
Ye.A. Karelin
Research Institute of Atomic Reactors, 433510 Dimitrovgrad, Russian
Federation
A.N. Tatarinov
State Enterprise Electrohimpribor, 624200 Lesnoy, Russian Federation
We present the observation of
the first decay event of the new nuclide 292116 in the running experiment
on the synthesis of Z=116 nuclei in the reaction 248Cm + 48Ca.
The experiment is in progress at FLNR, JINR( Dubna).
The beam of 48Ca+5
ions was delivered by the FLNR, JINR, U400 cyclotron operated with ECR-4M ion
source. The average beam intensity at the target was 0.7 pmA at the consumption rate of the 48Ca material
of ~0.3 mg h-1.
The 32-cm2 rotating
target consisted of the enriched isotope 248Cm (96.3%) in the form
of CmO2 deposited onto 1.5-mm Ti foils to a thickness of
~0.32 mg cm-2.
The evaporation residues
(EVRs) recoiling from the target were separated in flight from the 48Ca
beam ions, scattered particles and transfer-reaction products by the Dubna
Gas-filled Recoil Separator [1]. The transmission efficiency of the
separator for Z=116 nuclei was
estimated to be about 35%.
In the separator’s focal plane
a detector array was situated, consisting of a time-of-flight system (TOF)
followed by a 4x12-cm2
semiconductor detector with 12 vertical position-sensitive strips, in which the
recoils were implanted. This detector was surrounded by 8 (4x4-cm2) side detectors, so that to
form a box-like detector array, open from the front side. The detection
efficiency of 
-decays of implanted nuclei
was 87% of 4
. The detection system was tested by registering the
recoil nuclei and - and SF-decays of the known
isotopes of No produced in the reactions
204,206-208Pb(48Ca, xn). With the measured
position resolutions more than 95% of genetically linked signals in the focal-plane
detector appear in position window 
y=1.4 mm.
Energy resolution for particles absorbed in
focal-plane detector was about 55 keV. For ’s escaping the
focal-plane detector and registered by side detectors, the energy resolution of
sum signals was ~190 keV.
Fission fragments of 252No
implants produced in the 206Pb+48Ca reaction were used
for fission-energy calibration. The measured fragment energies should be
corrected for the pulse-height defect of detectors. The energy of fragments registered
by the side detectors should be corrected also for energy loss in the detectors’
entrance windows, dead layers and pentane filling the detection system. The
mean sum energy loss of both fission fragments for 252No was about
20 MeV, for fission fragments escaping the focal-plane detector at small
angle this value could be higher.
We chose the bombarding energy
of 48Ca ions of 240 MeV in the middle of the target. With the
~1.5-MeV beam energy resolution, variation of the beam energy during irradiation
(±0.9 MeV) and energy
losses in the target (~2.8 MeV), we expected the resulting compound nuclei
296116 to have excitation energy within a range of 30.4 MeV to
35.8 MeV. Thus, the compound nuclei should deexcite most probably by the
evaporation of 3 or 4 neutrons and 
-rays which would result in production of the isotopes
of element 116 with neutron numbers N=176
and 177.
Alpha decays of the nuclides 292,293116
lead to the known isotopes of element 114, which were produced in our recent
experiments via the reaction244Pu(48Ca, 3-4n)288,289114 [2,3].
Their chains of sequential decays should be observed, following -particle emission from the parent nuclei with Z=116.
To improve background
conditions for detecting long-time decay sequences, a special measurement mode
was employed [1]. The beam was switched off after detecting recoil signal
with parameters of implantation energy and TOF, expected for Z=116 evaporation residues, followed by -like signal with energy of 10.25 MeV
E11.5MeV,
in the same strip, within a position window y=2 mm and time interval
of 1 s. Thus, all the expected sequential decays of the nuclides with Z=114
could be observed in the absence of beam-associated background. With the used beam intensity
the average counting rate of such “EVR-” events was
less than one per 2 h. The total counting rate of
-particles with E >8 MeV by the whole detector array during beam-off
pauses was less then 2 h-1.
On the 35-th day of
irradiation, with the accumulated beam dose of 6.6x1018 ions, the first event sequence
was observed, that can be assigned to the implantation and decay of the isotope
of element 116 with mass number 292, see Fig. 1a.
Implantation of a heavy recoil
in strip 4 of the focal-plane detector was followed, in 46.9 ms, by -particle with E=10.56 MeV. This sequence switched the ion
beam off, for one hour and further decays were detected under lower-background
conditions. Second -particle with E=9.81 MeV was observed
2.42 s later. Then, in 53.87 s the third -decay was registered by the side detector with the
energy of 8.63 MeV. The energy deposited by this -particle in the focal-plane detector was lower than
the detection threshold of 0.92 MeV. Thus it’s total energy is determined
with larger uncertainty, E=9.09±0.46 MeV; the probability
that the third -particle appeared in the
chain (t~1 min)
due to random count can be estimated as ~1%.
Finally, 6.93 s after the last -decay,
two coincident fission fragments with sum energy of 197 MeV were
registered by both the focal-plane and the side detectors. The low energy of
one fission fragment measured by side detector for this event means large
energy lost by this fragment in the dead layers.
Positions of the four events
(EVR, 1, 2, and SF) were measured to be within a window of about
0.5 mm, all events appeared within time interval of 63.26 s, which
points to a strong correlation between them. The probability of the random
origin of the observed event chain is negligible (<10-10).
All the decays following
the first 10.56-MeV -particle agree well with the
decay chains of 288114, previously observed in the 244Pu+48Ca
reaction, see Fig. 1b.
Thus, it is reasonable to
assign the observed decay the nuclide 292116, produced via
evaporation of 4 neutrons in the complete-fusion reaction 248Cm+48Ca.
All the decay chain members follow Geiger-Nuttall Q
vs. T
relationship for even-even
nuclei. Substituting the values (E
=9.83±0.05 MeV and T=1.8
s) measured
in the 244Pu+48Ca
reaction (mother nuclide) and 248Cm+48Ca (daughter
nuclide) into the formula by Viola and Seaborg with parameters fitted to the
all known even-even nuclides with Z>82 and N>126, results in the atomic
number Z=114.3
. Decay energy Q
=10.71 MeV of the new observed nuclide and
half-life estimated from one event T=33
ms agree with theoretical predictions of the stability island in
the domain of superheavy elements around Z=114 and N=184.
Experiments are in progress.
This work has been performed with the support of INTAS under grant No. 991-1344. Much of support was provided through a special investment of the Russian Ministry of Atomic Energy. The 248Cm target material was provided by the NIIAR, Dimitrovgrad. 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.
[1] Yu.Ts. Oganessian et al., in Proceedings of the Fourth International Conference on Dynamical Aspects of Nuclear Fission, Casta-Papiernichka, Slovak Republic, 1998 (World Scientific, Singapore), p.334.
[2] Yu.Ts. Oganessian et al., Phys. Rev. Lett. 83, 3154 (1999).
[3] Yu.Ts. Oganessian et al., Phys. Rev. C, (2000) (to be published).