Prof. Yuri
Oganessian
Flerov Laboratory of Nuclear Reactions
Joint Institute for Nuclear Research
141980 Dubna, Moscow region, Russian Federation
SYNTHESIS AND DECAY PROPERTIES OF SUPERHEAVY ELEMENTS
Abstract
A fundamental outcome of modern nuclear microscopic theory is the prediction of
the “islands of stability” in the region of hypothetical superheavy elements. A
significant enhancement in nuclear stability when approaching the closed
spherical shells with Z = 114 (possibly 120 and 122) and N = 184, which follow
the doubly magic 208Pb nucleus is expected for the nuclides with large neutron
excess. Because of this, for the synthesis of the Z = 112-116 and 118 elements
we chose the reactions 238U, 242,244Pu, 243Am, 245,248Cm and 249Cf + 48Ca, which
are characterized by evaporation residues with a maximal number of neutrons.
The formation and radioactive decay of the nuclei with Z = 112-116 and 118 were
registered with the use of the Gas-filled recoil separator, installed at the
beam of the heavy ion accelerator. The mechanism of formation of superheavy
nuclei was studied separately. From the yield of the nuclei measured at
different ion-beam energies it follows that they are formed in the process
involving the emission of two to five neutrons depending on the excitation
energy of the compound nucleus. The maximal cross section substantially depends
on the neutron number in the compound nucleus and its position relative to the
closed neutron shell N = 184.
The new nuclides undergo mainly sequential a decays, which are terminated by
spontaneous fission (SF). The total time of the decays ranges from 0.5 ms to ~1
d, depending on the proton and neutron numbers in the synthesized nuclei. The
experimental method used is demonstrated by the example of the synthesis of
elements 113 and 115 in the reaction 243Am + 48Ca. The evaporation of three
neutrons and the emission of g-rays by the compound nuclei of element 115,
produced in the fusion reaction, lead to the formation in the ground state of
the odd-odd nuclide with 115 protons and 173 neutrons. This nuclide is the
parent of a “radioactive family” consisting of the Z = 115®113®111®109®107®105
nuclei, formed as a result of 5 consecutive emissions of a particles, and
terminated by spontaneous fission of the Db isotope (Z = 105). Due to the long
lifetime (T1/2~1d), the atoms of element 105 was separated by a classical
off-line radiochemical method.
In the series of experiments in the 238U, 242,244Pu, 243Am, 245,248Cm and 249Cf
+ 48Ca reactions 29 new nuclides with Z=104-118 and N=163-177 were synthesized
for the first time. The comparison of the half-lives of the known nuclei with Z
= 110-112 with those of the newly observed neutron-rich isotopes of the same
elements shows that the increase of their mass by adding 6-8 neutrons brings
forth an increase in nuclear stability by a factor of 104-105.
The decay properties of the isotopes of the heaviest elements are now being
compared with the predictions of microscopic nuclear models. This comparison
gives evidence of the decisive influence of the nuclear structure of superheavy
elements on their stability with respect to different modes of radioactive decay.
A more detailed analysis shows that experiments do not solely reproduce the
theoretically expected decay scenarios, but also are consistent (within ~5%
accuracy) with the decay energies of all the synthesized 23 a-radioactive nuclei
with Z = 106-118. From this point of view, the obtained results can be
considered as first experimental evidence of the existence of “islands of
stability” in the region of the heaviest elements.
Another problem is to obtain longer-lived superheavy nuclides.
As the artificial synthesis of nuclei is limited, the possibilities to search
for the most stable nuclei with Z = 106 -110 and N ~ 180 in nature or in cosmic
rays is being considered. Among the possible candidates for the first experiment,
an isotope of element 108 (Hs) was chosen. The search for the long-lived Hs
isotope in its chemical homologue – a sample of metallic Os (500 g) – will be
done in the underground laboratory in Modane (France) by means of registering
the spontaneous fission of the nucleus that is looked for or of the products of
it’s a- or b-decay. The observation of a single spontaneous fission event (measured
as a neutron flash, accompanying the fission process) during a one-year
measuring period will correspond to a concentration amounting to 5×10-15 g/g of
element 108 in the Os-sample, assuming that its half-life is equal to 109 y or
5×10-19 g/g at T1/2≤105 y. In spite of the high sensitivity of the experiment,
the chances to find surviving superheavy nuclei are small. However, the absence
of any effect will give an upper limit for the half-life of the long-lived
nuclide.
The experiments were carried out at the Flerov Laboratory of Nuclear Reactions (JINR,
Dubna) in collaboration with the Analytical & Nuclear Chemistry Division of the
Lawrence Livermore National Laboratory (USA).