Flerov Laboratory of Nuclear Reactions
Joint Institute for Nuclear Research
141980 Dubna, Moscow region, Russian Federation
SYNTHESIS AND DECAY PROPERTIES OF SUPERHEAVY ELEMENTS
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).