Journal of Nuclear and Radiochemical Sciences,Vol. 3, No. 1, pp. 217–218, 2002

217

Synthesis and Identification of Superheavy Elements in Reactions with 48 Ca Beams
Yu. Ts. Oganessian, A. V. Yeremin,∗ A. G. Popeko, O. N. Malyshev, and A. B. Yakushev
Flerov Laboratory of Nuclear Reactions, Joint Institute for Nuclear Research, 141980 Dubna, Moscow
region, Russia
Received: November 13, 2001; In Final Form: November 13, 2001
Experimental methods for the identification of newly synthesized heaviest isotopes of elements Z = 112 and 114
with the use of 48 Ca beams are discussed.
1. Introduction
For the identification of new nuclides undergoing α decay
a method for the investigation of consistent α decays, the socalled α-α correlation analysis, has been employed since mid60ties. This method is based on the fact that a decay chain starting from unknown isotopes should be ended in the known regionof isotopes with the known decay properties.
Using the neutron-rich isotope 48 Ca as a bombarding beam
for the synthesis of new heaviest nuclei we found ourselves in acompletely unknown region where all decay chains were started
and finished with isotopes having unknown decay properties.
According to the calculations1, 2 the decay chains which started
from the neutron-rich (N = 171–175) isotopes of elements 112–
114 after a few α decays should be terminated by spontaneous
fission in the region of elements 104–110. Taking into accountthe above mentioned problems we performed a number of experiments with the use of recoil separator VASSILISSA,3 each experiment being the basis for the following one. For example, weinvestigated the complete fusion reactions 48 Ca + 238 U → 286 112∗
and 48 Ca + 242 Pu → 290 114∗ in which the targets differ by an α
particle.4, 5 It means that the isotopes which can be produced ina subsequent experiment should decay into the isotopes whichwere obtained in the previous experiments. But for direct determination of Z of the detected nucleus it is necessary to havethe mass resolution at the level of a few mass units (≈1%) sothat one could distinguish between the xn and αxn deexcitation
channels. It is obvious that to have a possibility of mass determination, i.e. to distinguish between the xn and (x + 1)n deexcitation channels, the necessary mass resolution should be better
than 1 amu (0.3%).
2. Experimental Methods
2.1. Separator VASSILISSA. As a first step for improving
the analysis and identification of complete fusion reaction products, a new dipole magnet, having a deflection angle of 37 degrees, has been installed behind the separator VASSILISSA, replacing the old 8◦ magnet.5 In order to obtain sufficient position
separation between nuclides with different masses a new detector system, having a focal plane 32 strip detector 60 ×120 mm2in size, has been also developed.
Test experiments with 40 Ar and 48 Ca beams and Dy, Yb, andPb targets showed that the new magnet provided an additional
suppression for unwanted products by the factor of about 10 anda possibility to have the mass resolution for heavy nuclei with
masses of about 300 amu at the level of 2.5%.
Using a specific ion optical regime of the separator it waspossible to see at the focal plane of the separator the charge
distribution of scattered 40 Ar ions. These data were used forthe calibration of TOF-Energy spectra together with the relation
between the strip number (deflection angle) and Bρ (magneticrigidity) of ER’s.
∗Corresponding author. E-mail: eremin@sunvas.jinr.ru. FAX: +709621-65083.

The time resolution of the separator time-of-flight system
was about 0.7 ns, which corresponds to the value of about 1%(for slow evaporation residues). For each ER passing throughthe separator and detected at the focal plane, the time-of-flight(speed “v” of ER) and position of implantation/strip number(magnetic rigidity “Bρ” of ER) were measured.
It was possible to calculate the mass of the ER’s, detected
at the focal plane. For different charge states (Q = 17, 18, and19) the result was A = 198.1 ± 1, 197.8 ± 1, and 198.4 ± 1, respectively. Taking into account that the calculations were madefor the isotope 197 Po, formed in the complete fusion reaction
164Dy(40 Ar, 7n)197 Po, the obtained mass resolution could be estimated as about 2%.
In the case of the reaction 40 Ar + 164 Dy → 204 Po the cross section values reach hundreds of microbarns. For the reaction
40Ar + 208 Pb → 248 Fm these values are equal to tens of nanobarns.
Using the reaction 208 Pb(40 Ar, 2n)246 Fm we estimated that thenew dipole magnet provided an additional suppression factor ofabout 10 for scattered ions. Very clean α spectra were obtained.
From the comparison with literature data6 we could estimate thattransmission efficiency for 246 Fm ER’s was about 20%. The obtained mass resolution for 246 Fm ER’s, calculated according tothe measured TOF and strip number, ranged from 244.5 to 247.2
that corresponded to an accuracy of about 2.5%.
2.2. On-line Mass Spectrometer. The heaviest nuclei
newly synthesized with the use of 48 Ca beams are in the unknown region where all decay chains are started and finishedwith isotopes having unknown decay properties. In this case forunambiguous identification of such nuclei it is necessary to perform direct mass determination with a resolution of better than
1 amu at the level of 300 amu. It becomes possible with specially designed experimental setups, one of the variants is online mass spectrometers guided with ion sources, the so-called
ISOL systems. At the FLNR the project of such spectrometer
is now being developed. Neutron-rich isotopes of elements withZ = 112 and 114 which have rather long half-lives will be implanted in a hot catcher. After that they will diffuse from thecatcher into the volume of the ECR ion source. The atoms ionized in the ion source to the charge state +1 (with an efficiency
close to 100%) will be accelerated with 30–40 kV and massseparated. The mass resolution M/∆M of about 1000 will beobtained for the nuclei in the mass region of about 300 amu.
The name of this project is MASHA (Mass Analyzer of Super
Heavy Atoms).
2.3. Chemical Investigations of SHE. The relatively long
half-lives of the new isotopes with Z = 112 and 114, synthesizedin reactions with 48 Ca ions, open new prospects for the investigation of the chemical properties of superheavy elements. The
problem consists in finding out if these elements are homologuesof Hg and Pb and how strong is the influence of relativistic effects in electron shells in superheavy atoms. Of course the investigation of chemical properties could help in the determinationof the atomic number Z of the element but not the mass number,
however it will still be a big progress, providing the experiments

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

218 J.Nucl.Radiochem.Sci.,Vol. 3, No. 1, 2002
are successful.
The first attempt to identify chemically element 112 (E112)
was made in January 2000. A 2 mg/cm2 , 20 mm in diameter238
U(nat U) target, containing 100 µg of nat Nd was prepared ontoa 2 µm HAVAR foil, which also served as a vacuum window on asupporting Cu grate (its transparency was 62%). After a 10-day
irradiation with 48 Ca ions (262 MeV, 0.2–0.4 pµA), an integral
beam dose of 6.85 ×1017 was accumulated. The recoils werethermalized in pure helium at atmospheric pressure and transported through a 25 m long PTFE capillary to the detectors. The
transportation time was 25 s at a gas flow rate of 500 cm3 /min.
The detection apparatus consisted of 8 chambers connected inseries. The PIPS detectors in chambers 1–6 were coated withAu, and in chambers 7 and 8 with Pd. The chambers were positioned inside an assemblage of 84 3 He-filled detectors (in amoderator) to register prompt fission neutrons. If E112 behavedchemically like Hg and all the quantities measured for Hg werealso valid for E112, we could expect the detection of some three
SF events, taking into account the cross section value of about
5 pb. But no events with the characteristics described above
were registered. Thus the experiment did not provide an unambiguous answer as to the physical and chemical properties of element 112, namely whether the chemical properties of E112 aremore Rn-like, or the poor statistics resulted in a random value ofzero. However, the experiment undoubtedly showed a possibility of chemical identification of SF nuclei produced with picobarn cross sections. At the next stage of this work it is planned toincrease the beam dose at least twice and to upgrade the detector system: at the exit of the PIPS detector chambers the gas willpass through a special ionization chamber to measure α decays
and SF events of nuclei remaining in the gas.
3. Conclusion
The experiments performed with 48 Ca beams are the first stepin a long-term program aimed at the synthesis and study of decay properties of superheavy nuclei with neutron numbers close
to the predicted spherical shell. The relatively long half-lives of
the new isotopes with Z = 112 and 114 (even-odd and odd-odd

Oganessian
isotopes which could be obtained in reactions with 237 Np and243
Am targets could have even longer half-lives), synthesized in
the reactions with 48 Ca ions, dictate the necessity of upgrading
the existing experimental setups and developing new ones, thus
providing the possibility of direct mass measurements with anaccuracy of 1 amu, but on the other hand open new prospects
for the investigation of chemical properties of superheavy elements.
Acknowledgments. This work was performed partially under the financial support of the Russian Foundation for Basic
Research, contracts N 99-02-16447, 02-02-16116, and INTAS,
contract N 99-1344.
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