Journal of Nuclear and Radiochemical Sciences,Vol. 3, No. 1, pp. 125–127, 2002

125

Radiochemical Investigations at the FLNR
S. N. Dmitriev,∗ Yu. Ts. Oganessian, and M. G. Itkis
Flerov Laboratory of Nuclear Reactions, Joint Institute for Nuclear Research, Dubna, Moscow reg.,
141980, Russia
Received: November 13, 2001; In Final Form: April 3, 2002
The paper considers a programme of development of radiochemical investigations at the FLNR in the light of recent
results on the synthesis of isotopes of elements 108–116 in reactions between 48 Ca and 238 U, 242 Pu, 244 Pu, 248 Cm.
Three main lines of the investigation are discussed: chemical identification and study of chemical properties ofsuperheavy elements (SHE), on-line mass separation for the detection of SHE, and the setting of new experimentson the search of SHE in Nature.
1. Introduction
The results of experiments aimed at the synthesis of superheavy elements in the reactions between 48 Ca and 248 Cm(Ref. 1), 244 Pu (Ref. 2), 242 Pu (Ref. 3), and 238 U (Ref. 4) carriedout at the FLNR in 1999–2001 point to a substantial increase in
the stability of nuclides with Z = 108–116 and N = 170–175. The
observed decay chains of nuclei of superheavy elements consistof successive α decays and are terminated by spontaneous fission.
Relatively long half-lives of isotopes of elements 108–116
obtained in reactions with 48 Ca and chemical properties of SHE
predicted theoretically provide possibilities for new experiments
devoted to the chemical identification of SHE, study of theirchemical properties, on-line mass separation for the detectionof SHE, and search of SHE in Nature.
2. Chemical Identification of Elements 112 and 114
The experiments on chemical identification of elements 112
and 114 and study of their chemical properties are planned as
the first priority. The 238 U(48 Ca, 3n)283 112 reaction is chosen for
the first experiment. The 283 112 nuclide decays by spontaneous5
fission with the half-life TSF = 3+2.8−1.0 min. The cross section of
the above mentioned reaction is equal to about 5 pb.4
According to classical concepts, element 112 belongs togroup IIB of the Periodic Table and is a homologue of Zn, Cd,
and Hg. Theoretical works predict that element 112 has an enhanced volatility (Hg-like),6 and like Hg it must adsorb on metals due to the formation of quite strong intermetallic bonds.7
However, a number of works predict that element 112 is chemically inert like Rn.8
These theoretical concepts became the basis for the experiment, developed at the FLNR, on the chemical identification ofelement 112. A general scheme of the experiment is presented
in Figure 1.
The 238 U (∼2 mg/cm2 ) target is irradiated with a 48 Ca beam
264 MeV in energy and ∼6 ×1012 s−1 (1 pµA) in intensity. The
recoils are thermalized in helium at atmospheric pressure. Then
recoils are transported by helium through a Teflon capillary
to the detection system consisting of two subsequently linked
blocks: Au-coated PIPS-detectors and the ionization chamber.
Both blocks are surrounded with 3 He-counters registering neutrons from spontaneous fission. Thus the transport line (ca.
25 m) will provide separation of the volatile 112 element fromthe non-volatile reaction products, detection of SF events by
PIPS detectors will indicate that the 112 element is adsorbedon the Au surface (Hg-like), and detection of SF event by theionization chamber in the gas phase will indicate that the 112
element is inert gas (Rn-like). Optimistic estimates predict the∗Corresponding author. E-mail:dmitriev@flnr.jinr.ru.
FAX: (7)09621-65955.

registration of one nucleus per 2–3 days, thus the sensitivity of
the chemical experiment may be by >5 times higher than that ofphysical experiments with using kinematic separators. Technical details of the chemical experiment and the results of modelexperiments with short-lived Hg isotopes are described in Reference 9.
Experiments on chemical identification of element 114 willbe the next stage. We will use the 244 Pu(48 Ca, 3,4n) reaction.
The choice of effective methods of separation and detection will
depend on the experimental results of the experiment on thechemistry of element 112 and results of model experiments withshort-living Pb isotopes. One can expect that element 114 will
also have an enhanced volatility. Then the scheme of separation and identification will be similar to that used in the caseof element 112 and described above. The identification of element 114 is possible by its daughter isotopes of element 112:244
Pu(48 Ca, 4n)288 114 (α; 2.6 s) ⇒ 284 112 (α; 19 s) ⇒ 280 110 (SF;
7.5 s) and 244 Pu(48 Ca, 3n)289 114 (α; 30 s) ⇒ 285 112 (α; 15.4 min)
⇒ 281 110 (α; 1.6 min) ⇒ 277 108 (SF; 16.5 min). In the identification of decay chains, the background created by short-living
isotopes undergoing α decay, first of all the isotopes of Rn andPo, is of primary importance. One of possible schemes of the
low-background experiment is shown in Figure 2. The scheme
allows for carrying out the on-line experiment. The recoil nuclei
are thermalized, then are carried out by a gas flow and adsorbedon the first column at a set time (the second, third, and forthones are closed). Then — on the second column (the third andforth ones are closed and the first column is blown by He cleaning it out from Rn); then — on the third column (the forth one isclosed, He is blown through the second column, the first column
is heated up to a certain set temperature and desorbed nuclei of
elements 112 and 114 are transported by He into the first detection block). And finally — on the forth column (the first column
is cooled down to a temperature required for the adsorption ofelements 112 and 114, the second one is heated and the nuclei
are transported into the second detection block, the third one isblown by He). Then the cycle is repeated. It is possible to unite
the detection blocks into a single one. It is just a general scheme,
its realization requires detailed elaboration.
3. On-line Mass Separation for the Synthesis of SHE
Because of the short lifetimes of superheavy nuclides(100 µs–10 ms), kinematic separators have been traditionallyused for their synthesis. Such separators work as velocity (or energy) selectors providing in-flight separation of nuclides during
the time ∼ 1–10 µs. At present, six similar setups work all over
the world, namely, at the GSI (Darmstadt, Germany), RIKEN(Wako-Shi, Japan), GANIL (Caen, France), LBL (Berkley,USA), and JINR (Dubna, Russia, two separators). All of themseparate reaction products, but do not measure masses of separated atoms. The masses are determined indirectly by means ofc 2002 The Japan Society of Nuclear and Radiochemical Sciences
Published on Web 6/30/2002

126 J.Nucl.Radiochem.Sci.,Vol. 3, No. 1, 2002
48

Ca (~1 pmA)

Dmitriev

3

He-filled detectors

U (~2 mg×cm )

238

-2

Teflon capillaryHe

Au-coated PIPS

Ionization chamber

Figure 1. Scheme of the 112 element identification experiment.
Ca (~1 pmA)

48

Pu(~2 mg×cm + 0.1 mg

244

-2

144

Sm)

He (H2)
1,2,3,4 adsorption columns
He

He

He

He

Figure 3. Mass Analyzer of Super Heavy Atoms.
1

2

3

4

PIPS-detectors
PIPS-detectors
PIPS-detectors
PIPS-detectors

Figure 2. Scheme of “low background” experiment for chemical identification of 114 element.

investigating the properties of atoms implanted in a focal plane
detector.
Since the lifetimes of nuclides located within (or in the vicinity of) the “island of stability” are considerably longer, one
should revise the concept of separation itself. Instead of kinematic separators it is more reasonable to use classical separators,
which are less fast-acting and measure masses of atoms with ahigh accuracy. A design of such a separator is presented below.
Schematic view of the analyser is shown in Figure 3. We gaveit the name of MASHA (Mass Analyzer of Super Heavy Atoms).
Recoil nuclei emitted from the target are stopped in a hot
Ta catcher and heated up to the temperature T ∼ 2300 K. Implanted in the catcher atoms drift, due to thermodiffusion, towards its surface and enter the chamber of the ECR ion source( f = 2.45 GHz). In the ion source, the atoms are ionized with
Q = +1 and then are extracted from plasma, forming a beamwith the energy E = 40 keV. Having passed through magnetic
and electrostatic elements of the separator and mass analyzer,
the atoms reach the separator focal plane which is at a distanceof approximately 14 m from the ion source. In the separator, theECR exit slit (the object) is transformed into a dispersive image,the mass resolution is as high as M/∆M ∼ 1000–1200. A focal
plane detector (FPD) installed in the separator focal plane has
the position resolution ∆X ∼ 1 mm to maintain the above mentioned mass resolution. The detector has a semiconductor strip
structure.
Atoms of superheavy elements are to be separated from 4 He
atoms (the ion source gas), 48 Ca ions (primary beam ions), target fission fragments with A = 70–150, and target-like atoms,
the products of transfer nuclear reactions, with A = 200–250.
No background can be allowed in the registration zone so that
one could observe decays of the superheavy nuclides in positions along the focal plane, which are determined by the nuclide
masses.
The setup described above has the following advantages forthe separation of superheavy nuclei:( i ) Unlike kinematic separators, in which the maximum target thickness is limited by the necessity of preserving kinematiccharacteristics of a reaction and thus cannot be more than 0.2–
0.3 mg/cm2 , in a classical separator it can be increased up to
1.5–2.0 mg/cm2 . This circumstance allows synthesizing two (orprobably even three) isotopes simultaneously.
( ii ) All the elements with Z = 112–118, being chemical ana-

logues of Hg, Tl, Pb, Bi, Po, At, and Rn, have low boiling points
and high volatility. Their fast transport from the hot catcher tothe ion source chamber and high ionization efficiency in the ion
source plasma can be organized.
Taking into account the above mentioned, one can expect thatthe total efficiency of the new setup will be few times higher
than those of existing kinematic separators.
4. Search of SHE in Nature
Theoretical predictions concerning the existence of the “island of stability” made in the 1960s predetermined the theoretical research on predicting the chemical properties of SHE as
compared with their more light homologues as well experimental investigations connected with the search of SHE in Nature.
The most active stage of this work was in the 1970s and 1980s.
Special attention was paid to a group of nuclides with Z and N
in the vicinity of magic numbers Z = 114 and N = 184, for which
the lifetimes were assumed to be the longest ones and could becompared with the age of Earth. That is why the properties of
element 114 (Eka Pb) were predicted most fully. Keller et al.10
wrote that the chemistry of element 114 should be similar tothat of Pb+2 , with a more distinguished tendency to formationof complexes in solutions. Pitzer8 noted that element 114 would
be a volatile liquid or gas due to the effect of the closed shell
7p21/2 . Similarly to theoreticians, experimentalists initially concentrated their attention on the search of element 114 in Nature,and later included elements 110–117 into the orbit of search.
In the search for SHE, the highest sensitivity of 10−13 –−14
g/g was reached in the registration of events of neutron
10
emission multiplicity using detectors with 3 He counters11 andneutron detectors with liquid scintillators.12 A detailed description of the obtained results is presented in review works by
Seaborg,13 Flerov,14 Hermann,15 and Kratz.16 Note that no unambiguous data pointing to the existence of SHE in Nature wasobtained. In the 90s, the above mentioned work was practically
stopped due to lack of positive experimental results as well as
due to some new (for that time) theoretical predictions made byMöller et al.17 The prediction was that the nuclides 288 110 and
290
110 would be the most stable of all SHE, and their half-lives
would be just 200 days.
The results on the synthesis of isotopes of elements 112, 114,
and 116 obtained at the FLNR in combination with theoretical calculations based on the macroscopic-microscopic modelof the stability of even-even isotopes of elements 108–116(Smolańczuk18 ) allow one to revitalize the experiments aimed
at the search of SHE in Nature.
The setting of new experiments presupposes estimation ofprevious ones. Speaking about the search for SHE in Nature,
analysis of the known experiments (so far leaving apart the problem of formation mechanism in the process of nuclear synthesis)

Radiochemical Investigations at the FLNR

J.Nucl.Radiochem.Sci.,Vol. 3, No. 1, 2002

Age of the Earth15

Spherical
Shell

 - decay

Log T1/2 /second

10

108
110

5

- stable
nuclei

Deformed
Shell0

References

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products of copper-nickel concentrate sublimation. Then, using a cascade of gas centrifuges, a heavy fraction in the formof 192 OsO4 will be extracted. Although the content of the mostheavy isotope 192 Os in the natural mixture is 41%, one can expect that tetra-oxide of element 108 will be added to the first portions and thus the concentration co-efficient will be no less than
1000. Analysis of the obtained concentrate will be performed ina low background laboratory with the use of ionization chamber. The expected sensitivity of the experiment in assuming the
lifetime T1/2 = 109 years of the sought nuclide is about 10−16 g/g.

112

116

-10

127

190

Neutron number
Figure 4. Half-life of the 108–116 elements (theory and experiment).

comes to two principal questions: the choice of the registration
method and the choice of the object of search.
The data obtained in Reference 19 on the neutron multiplicities in the fission of superheavy nuclei (SHN) from a weakly
excited state make it possible to state that the neutron multiplicity (ν) in the fission of SHN from the ground state will be equal
to no less than 8. Thus a method of registration of SHE using scintillation and 3 He counters employed in previous works
seems to be justified, and the obtained results — quite correct.
It has been noted above that in the search for SHE the experimentalists were mainly focused on the investigation of samples
promising from the point of view of finding elements 110–117,
i.e. from Eka-Pt to Eka-At. According to modern concepts, themost stable nuclides will have atomic numbers Z ≤ 110. Figure 4 shows the calculation data18 on the lifetimes of even-evenisotopes of elements 108–116. It is predicted that the mostlong living one will be the nuclide 292 108 (log T1/2 SF (s) = 12.01;
log T1/2 α (s) = 13.73). Figure 4 also shows experimental values
for the half-lives of isotopes of elements 108–116 synthesized
to date. One can see that the lifetimes of nuclides 280 110, 284 112,
288
114, and 292 116 exceed the calculation values by 1–2 ordersof magnitude. One cannot exclude that this difference will be
growing with increasing N and that the lifetime of the nuclide292
108 will be comparable with the age of Earth.
However, note that the problem of the β stability of these nuclei has not been solved yet. According to Smolańczuk’s calculations the nuclei 276−286 108 should be stable ones. But he does
not exclude a possibility of existence of more β-stable nuclei inthis region.
The search of element 108 (Eka-Os) in Nature is regarded byus as an extremely interesting though problematic task. In thefirst series of experiments we will investigate Os samples. A
sample of metallic Os (about 1 kg) will be produced from the