Search
of the Decay of Heaviest Isotopes of the Element 112
Yu.Ts. Oganessian1*, A.V. Yeremin1*, G.G. Gulbekian1, B.N. Gikal1,
R.N. Sagaidak1, V.B. Kutner1, O.N. Malyshev1, A.G. Popeko1, A.P. Kabachenko1,
V.I. Chepigin1, J. Rochach1, V.A. Gorshkov1, A.Yu. Lavrentev1, S.L. Bogomolov1,
M. G. Itkis1, S. Hofmann2, G. Munzenberg2, M. Veselsky3, S. Saro4, K. Morita5,
N. Iwasa5
1 Flerov Laboratory of Nuclear Reactions, JINR, 141980 Dubna, Russia
2
Gesellshaft fur Schwerionenforschung, D-64220 Darmstadt, Germany
3
Institute of Physics, SK-84228 Dubravska 9, Bratislava, Slovakia
4 Department
of Physics, Comenius University, SK-84215, Bratislava, Slovakia
5 Institute of Physical and
Chemical Research (RIKEN), Wako-shi, Saitama 351-01, Japan
Abstract
To produce heavy isotopes of element 112 in the reaction 48Ca+238U
two experiments at different beam energies were performed.
In the first experiment the beam energy was 231 MeV which lead to the
production of the compound nucleus 286112 with an excitation energy
of Ex = 31 MeV.
In a 20-day irradiation of the 238U target with beam dose of
3.5*1018 on the focal plane of the
recoil separator VASSILISSA two spontaneous fission events were detected. No 
- particle emission preceding spontaneous
fission as well as any - correlation in
the energy range from 8 to 13 MeV in the time interval up to 10000 s. have been
observed.
The half life of the new spontaneous fissioning nuclide is about 100s. The
most probable explanation of the data obtained in this experiment is that the
observed spontaneous fission corresponds to the decay of the even-odd isotope
(N=171) of element 112 produced in the reaction 238U(48Ca,
3n)283112 with a cross section ~ 5pb.
In the second experiment the beam energy was 238 MeV which increase the
excitation energy of the compound nuclei up to Ex= 39 MeV. The total
beam dose in this case was 2.2*1018. No events due either to spontaneous fission or
sequential - decay in the energy range from 8 to 12 MeV and
time interval of 1000s. were detected. This data gives an upper limit of 3 pb
for the production cross section of the even-even isotope 282112 in
the reaction 238U(48Ca,4n)282112.
The work was performed in the Flerov Laboratory of Nuclear Reactions
JINR.
PACS: 21.10 Dr; 23.60.+e; 25.70.-z; 27.90.+b
*-Corresponding authors; Tel: ++7-09621-62261, Fax: ++7-09621-65083,
E-mail: spm111@yandex.ru
and spm111@yandex.ru
Introduction
According to the macro-microscopic theory
predictions, the stability of the super heavy nuclei has to increase sharply,
while approaching the spherical neutron shell N=184. The synthesis of the super
heavy spherical nuclides, even if they are distant from the shell N=184, will
take place only with a significant neutron excess in the colliding nuclei. Thus,
the isotopes of element 112, which have a spherical shape and which are
therefore relatively stable, could be produced in the reaction 48Ca+238U[1].
The compound nucleus 286112 formed
in this reaction appears to be weakly excited due to the significant mass
excess of the double magic 48Ca; the excitation energy at the
Coulomb barrier is only Ex = 33 MeV. At such excitation energy, the
shell effects are still presented in the heated nucleus. This may increase the
survival probability of the evaporation residues (ER). The high mass asymmetry
in the entrance channel (A1/A2 = 0.2, and Z1Z2=
1840) seems to decrease the dynamic limitations for the fusion of the
interacting nuclei, earlier observed for the more symmetric cold fusion
reactions [2].
Fig 1.
The
theoretical predictions for partial half-life of the isotopes with Z=112. Open
points and open squares connected by solid lines are T and Tsf taken from [3] respectively.
Blank point experimental data; circle - T for the isotope 377122 obtained in
the reaction 208Pb (70Ni, n) 277122; square -
Tsf for the isotope 288112 produced in the reaction 238U(48Cn,
3n)283112 present work
The expected decay
properties of the isotopes with Z=112 are quite specific. According to the calculations
provided by R. Smolanczuk [3], the even-even isotopes 282112
and 284112 have partial half lives for -decay respectively: T = 0.005 s. and 1.0 s. (Fig.1). Their spontaneous fission half-life (Ts.f.)
is slightly higher than T. At the same time the isotope 280110,
which is the a-decay daughter of the 284112 nucleus, will mainly
decay by spontaneous fission. As for the chain of sequential decays of the
other isotope 282112, all products with Z<106 will have Ts.f
<< T , too. For the isotope 283112
the predictions are less clear, because the odd number of neutrons may lead to significant
limitations for both -decay and
spontaneous fission. Here, the strong competition between the two modes of
decay is also highly expected.
It should be noted, that calculations performed
by P.Moller and R.Nix [4] , predict for the same nuclei the
half-life against -decay to be
hundreds and thousands times longer than T from [3]. However, it seems less significant, since it does
not change the main decay properties of the isotopes with Z=112, produced in
the reaction 48Ca + 238U. Either they will decay by
spontaneous fission, or by the chains of their -decay (one or several) will end by the
spontaneous fission anyway. On the other hand, the modern separation techniques
allow synthesizing and identifying the recoil nuclei, if their half-life is
more than one microsecond. It covers the range of all predictions and thus the
experimental results obtained using these techniques can be used as a direct
test of the predictive capabilities of the existing theoretical models.
Experiment
The production of the
intensive ion beam from the rare and extremely expensive 48Ca
isotope presents itself as a key problem in our issue on the synthesis of the
isotopes of element 112.
The neutral atoms of
calcium were injected into the plasma of ECR-4M ion source [5] by
the process of controlled heating the metallic sample of 48Ca(50mg)
enriched to 70%. The sample was made from the calcium oxide right before being
placed in the oven of the ECR source. The whole preparation procedure of
metallic Ca as well as the recuperation of the material from the ion source
chamber was controlled by measuring the yield of 
-rays from the isotope of 47Ca (4.5
d), produced in the 48Ca(,n)47Ca reaction.
The new injection system and the modified beam
optics of the cyclotron allowed to obtain the internal beam of 48Ca5+
ions with intensity up to 1 p
A at material consumption rate of about 0.3 mg/h. The beam extraction
from the cyclotron U-400 was provided by the stripping method. The variation of
the energy of the extracted beam in the range from 200 MeV to 280 MeV was performed
by smooth variation of the magnetic field and by precise positioning of the
stripping foil. The mean beam intensity of 48Ca on the target was
2.2*1012pps.
The evaporation residues were
separated in-flight from the beam particles and other reaction products with
the electrostatic recoil separator VASSILISSA[6]. Enriched isotope of 238U (99.999%) with
thickness of 0.3 mg/cm2 was used as a target material. It was
deposited uniformly with a thickness of 1.6 mg/cm2 on an aluminum
disk 125 mm in diameter. The disk was rotating with frequency 
=2000 rpm. The same
target design with enriched isotopes of Tb, Yb, and Pb was used in the several
test experiments.
For registration and
identification of the recoils a system of time-of-flight detectors and silicon
position sensitive strip detector array was installed in the focal
plane of the separator. After registration in the TOF detectors, the recoils
were implanted into the 16-strip silicon detector with an active area of 60x60
mm2. Each of the strips had longitudinal position sensitivity. Special
measurements of the position resolution along the each strip were performed. For
sequential - decays it was 0.6 mm, for correlated recoil
and -particle - 1.0 mm, and for correlated recoil
and spontaneous fission events it was 1.5mm. The energy resolution for -particles within the energy range from 6 to 9
MeV was 20 keV. The time accuracy for the recorded events was about 1sec. The strip detector was surrounded with
four other identical silicon detectors and the entire array had a shape of a
cube with dimensions 60x60x60mm3. The geometrical efficiency of the
silicon array was 85% of 4
. In the present series of experiments each of the four neighboring
strips of backward detectors were connected galvanicaly and formed 16 energy
sensitive segments.
Scattered low energy projectiles were the main
contribution to the background. An additional bending dipole magnet allowed
placing the detectors at the angle of an 8o with respect to the
primary beam direction. After all, the counting rate of the frontal detector
was only 25-30Hz. In front of the silicon detectors 3 m degrader foil
(mylar) was placed to reduce the number of very low energy projectiles reaching
focal plane of the separator.
The separation
efficiency of EVRs from the reaction 48Ca+238U was
obtained in the test experiments. The cross sections of xn-evaporation channels
were measured in the bombardment of targets of 159Tb, 174Yb
and 206,208Pb by the 48Ca projectile in wide energy
range. These measurements showed that about 25% of the recoils 286-x112,
produced on the U-target, would be implanted in the frontal detector. The
signals from the time-of-flight detectors were used both for measurements of
the velocity of the recoils and for distinguishing decays of the previously
implanted nuclei. The high efficiency of the TOF detectors allowed to obtain
very clean decay spectra and to widen significantly (up to several hours) the
time window for measuring decay chains. The latter was particularly important
for identification of the EVRs with a long half-life and the continuous time
structure of the beam.
Results
To estimate the yield
of EVRs in the reaction 238U (48Ca, xn) 286-x112
and to choose the optimal beam energy for 48Ca, a set of additional
experiments was performed. On the recoil separator VASSILISSA, the excitation
functions for the reaction 206,208Pb(48Ca,xn) 254,256-xNo
were measured. At the same time, on the time-of-flight fission fragment
spectrometer CORSET[7] data were obtained on the cross-sections and
mass distribution of fission fragments in the reactions 48Ca +
206,208Pb and 48Ca + 238U. For all the reactions
these compound system was formed at low excitation energies, close to the
Coulomb barrier.
These experiments will be described in full in
a separate article;[8] here we shall limit ourselves to the main
conclusions drawn from the data. As it follows from the analysis of the data
observed above, the highest cross-section for the reaction 48Ca + 238U is expected
for the 3n and 4n-evaporation channels. The excitation functions reached their
maximums at Ex = 31 and 39 MeV, which corresponds to the beam energy
in the middle of the target EL = 231 MeV, and 239 MeV, respectively.
The absolute value of the cross-section for xn-evaporation channels can be
estimated much less definitely. According to different calculations, the
cross-section at the maximum of the 3n-evaporation channel varies from one to a
few tens pb; although, it still remains by a factor of 3-5 higher than for the
4n evaporation channel.
On the basis the data presented above, two
long-term irradiation of a 238U target with 48Ca at beam
energies of 231 3 MeV and 238 3 MeV were performed. The doze of 3.5*1018 projectiles was collected in
the first experiment in March 1998. Two spontaneous fission events were
detected at that time. For both events the fission fragments were registered as
two coinciding signals with a high-energy deposition in both front and backward
detectors.
The total kinetic
energy (TKE) for the events was also measured (Fig.2). The detectors were
calibrated by fragments from spontaneous fission of the implanted recoils of 252
No, produced in the reaction 206Pb (48Ca, 2n). The
determined values of the TKE for the two registered events are 190 and 212 MeV
respectively. If the observed spontaneous fission relates to the decay of a
heavier nucleus, say with Z=110-112, the above mentioned values of TKE must be
increased roughly by 10 MeV.
The analysis of all events collected in the
experiment has been provided in order to find genetic decay links of the
implants. For that purpose, the data of the position and energy resolutions of
the correlated events, measured by the frontal detector for the various modes
of decay were used. Thus, for recoil-- as well as
recoil---SF for an -particle in the energy range from 8 to 13 MeV
and in the time interval up to 10000 s.
no correlation were found. But at the same time, there were
Fig. 2.
The
spectra of TKE for spontaneous fission of 252No. Points data obtained with VASSILISSA recoil
separator in the reaction 206Pb (48Ca, 2n)252No;
dashed linedata taken from [9].
Black
triangles correspond to the events from the reaction 48Ca+238U.
two correlated signals from the recoil and spontaneous fission found in
strips #12 and #15 within the position window 0.8 mm. In the first case (strip #12), the time
interval between the signals recoil and spontaneous fission was 182.4 s,
while in the second case (strip #15), the time interval was 52.0 s. As it
became evident from the data of the long-term detection, at the position window
0.8mm the signals, similar to EVRs, were
detected in strip #12 with a mean frequency of 0.001Hz, while in strip #15 the
frequency was 0.005 Hz.
In the second
experiment, the doze of 2.2*1018 projectiles was collected at the beam energy EL=238
MeV (Ex=39MeV). No spontaneous fission events were detected and no - correlations were observed in the whole -particle energy range from 8 to12 MeV in the
time interval up to 1000 s. The upper limit for EVRs cross-sections at this
excitation energy is 3.0 1.5 pb.
It is very important to note that the two
events observed in these experiments were collected practically with no
background conditions. The fact that no SF- isomers as well as any other known
SF-nuclei were observed in the two experiments with the total beam doze of 5.7*1018 makes evident an extremely high
selectivity of the device for evaporation residues detection. That is why we
also have to exclude a possibility to explain the effect in terms of a rare and
unknown decay mode of known nuclei formed in the reaction 48Ca + 238U.
As mentioned above no -particles, preceding
the spontaneous fission, were observed for either event despite the high
efficiency of the detector array. The most reasonable explanation is that we
are observing the spontaneous fission of the heaviest new even-odd isotope 283112
with the half-life T1/2=80.7+195.2-33.5 s.
Despite the fact that only spontaneous fission was observed in the experiment,
we can not exclude also the -decay with a branching ratio b 
50%. The cross-section for the new isotope is
5 2 pb (the statistical error is presented here;
the absolute cross-section accuracy value is within a factor of 2). The absence
of any effect at the excitation energy Ex=39 MeV is not surprising. The
cross-section of the 3n-evaporation channel decreases with the increase of the
excitation energy, while the cross-section of 4n-evaporation channel is several
times lower than it is for the 3n-channel, as it was already mentioned above.
Conclusion
The present article
describes a new attempt to synthesize super heavy spherical nuclei in the reaction
48Ca + 238U. Comparing to all previous experiments with 48
Ca ions involved, the present one has more than 100 times higher
sensitivity. Two spontaneous fission events were detected at beam energy below
the Coulomb barrier. As it follows from the data analysis, the events were most
probably triggered by the decay of the even-odd isotope 283112
formed in the 3n-evaporation channel. The half-life of the new isotope is about
100 sec. It could be that it also undergoes -decay with a branching ratio b 50%.
The half-life of the
new isotopes is more than 5 orders of magnitude longer than the already known
isotope 277112 (T1/2~ 0.15 ms), obtained in the cold fusion reaction
208Pb(70Zn, n)277112.[10] The
difference could indicate that the stability increases sharply when we enter
domain of the spherical shell corrections.
The present experiment
is the first one in our long-term research program with the 48Ca
beam, dedicated to the synthesis and properties of super heavy elements. We
have a strong intention to increase the intensity of the beam , which will
allow us to try to synthesize other isotopes with Z=110 and 114.
Acknowledgments
Authors would like to thank sincerely Prof. Ts. Vylov and V.G.
Kadyshevsky for their support at all stages of carrying out of this work. We
are thankful to Dr. B.I. Pustilnik for calculations of fusion cross sections
induced by 48Ca ions. We would like to thank as well Drs. R.
Smolanczuk and Y. Yano for their help and interesting discussions. We also
express gratitude to Drs. S.N. Dmitriev, A.B. Yakushev, and. V .Ya. Lebedev for
developing methods for preparation of the metal samples from the enriched 48Ca
isotope and its recuperation from the chamber source; to A.N. Shamanin and E.N. Voronkov for their help in maintenance
of the separator Vassilissa; and finally to the personnel of the cyclotron
U-400 under the direction of A.V. Tihomirov for obtaining an intensive and
stable 48Ca beam.
References
[1] Yu. Ts. Oganessian, in: Proc. Of Int. Conf. Nuclear Physics at the Turn of the Millenium: Structure of the Vacuum and Elementary Matter, Wilderness, South Africa, (World Scientific, Singapure), p.11 (1996).
[2] J.P. Blocki, et. al., Nucl. Phys. A 459, 145 (1986).
[3] R. Smolanczuk, Phys. Rev. C 56 812 (1997).
[4] P.
Moller, et. al., Atomic Data and Nuclear Data Tables 59 185 (1995) and P. Moller
et. al.,
Nuclear Properties for Astrophysics,
[5] V.B. Kutner, et. al., in: Proc. of 15th Int. Conf. On Cyclotrons and their Applications, 14-19 June, 1998, Caen, France (submitted).
[6] A.V. Yeremin, et. al., Nucl. Instr. And Meth. B 126 329 (1997).
[7] M.G. Itkis et. al., Il Nuovo Cim. (to be published).
[8] A.V. Yeremin, et. al., JINR Rapid Communuications, (1998) (in press).
[9] E.K. Hulet, Yad. Fiz. 57 1165 (1994).
[10] S. Hofmann, et. al., Z. Phys. A 354 229 (1996).