Chemistry Nexus

by WebElements: the periodic table on the web

Experiments conducted at the Flerov Laboratory of Nuclear Reactions (Joint Institute for Nuclear Research) at Dubna in Russia indicate that element 118 (ununoctium, Uuo) was produced. Not too much though, one atom in the spring of 2002 and two more in 2005.1, 2

The 2002 experiment involved firing a beam of 4820Ca at 24998Cf. The experiment took 4 months and involved a beam of 2.5 x 1019 calcium ions to produce the single event believed to be the synthesis of 294118Uuo.

24998Cf + 4820Ca → 294118Uuo + 31n

This ununoctium isotope loses three alpha particles in rapid succesion:

294118Uuo → 290116Uuh + 42He (1.29 milliseconds)

290116Uuh → 286114Uuq + 42He (14.4 milliseconds)

286114Uuq → 282112Uub + 42He (230 milliseconds)

The 282112Uub species then undergoes spontaneous fragmentation (denoted SF) to other species. It took a few years to carry out enough research to properly characterize the decompoition products.

In 2005 a similar experiment but with more sensitive detectors and a total beam dose of 1.6 x 1019 calcium ions resulted in the detection of two further events arising from the formation of 294118Uuo. This work is particularly significant given the scandal associated with the first report (now withdrawn) of element 118.

Abstract:1 The decay properties of 290116 and 291116, and the dependence of their production cross sections on the excitation energies of the compound nucleus, 293116, have been measured in the 24998Cf (48Ca, xn)293-x116 reaction. These isotopes of element 116 are the decay daughters of element 118 isotopes, which are produced via the 24998Cf + 48Ca reaction. We performed the element 118 experiment at two projectile energies, corresponding to 297118 compound nucleus excitation energies of E∗ = 29.2 ± 2.5 and 34.4 ± 2.3 MeV. During an irradiation with a total beam dose of 4.1 × 1019 48Ca projectiles, three similar decay chains consisting of two or three consecutive α decays and terminated by a spontaneous fission (SF) with high total kinetic energy of about 230 MeV were observed. The three decay chains originated from the even-even isotope 294118 (Eα = 11.65 ± 0.06 MeV, Tα = 0.89+1.07−0.31 ms) produced in the 3n-evaporation channel of the 24998Cf + 48Ca reaction with a maximum cross section of 0.5+1.6−0.3 pb.

Abstract:2 A fundamental outcome of modern nuclear theory is the prediction of the “island of stability” in the region of hypothetical superheavy elements. A significant enhancement in nuclear stability at approaching the closed shells with Z = 114 (possibly 120 and 122) and N = 184 is expected for the nuclei with large neutron excess. For this reason, for the synthesis of nuclei with Z = 112-116 and 118, we chose the reactions 238U, 242,244Pu, 243Am, 245,248Cm, and 249Cf + 48Ca, which are characterized by fusion products with a maximal neutron excess. The formation and decay properties of the heaviest nuclei were registered with the use of a gas-filled recoil separator installed at a 48Ca-beam of the heavy-ion cyclotron. The new nuclides mainly undergo sequential α-decay, which ends with spontaneous fission (SF). The total time of decay ranges from 0.5 ms to ~1 d, depending on the proton and neutron numbers in the synthesized nuclei. The atomic number of the new elements 115 and 113 was confirmed also by an independent radiochemical experiment based on the identification of the neutron-rich isotope 268Db (TSF ~ 30 h), the final product in the chain of α-decays of the odd-odd parent nucleus 288115. The comparison of the decay properties of 29 new nuclides with Z = 104-118 and N = 162-177 gives evidence of the decisive influence of the structure of superheavy elements on their stability with respect to different modes of radioactive decay. The investigations connected with the search for superheavy elements in Nature are also presented.

References

1. Oganessian, Yu. Ts., V. K. Utyonkov, Yu. V. Lobanov, F. Sh. Abdullin, A. N. Polyakov, R. N. Sagaidak, I. V. Shirokovsky, et al. 2006. Synthesis of the isotopes of elements 118 and 116 in the  Cf 249 and Cm 245 + Ca 48 fusion reactions . Phys. Rev. C. American Physical Society (APS), October 9. http://dx.doi.org/10.1103/PhysRevC.74.044602.
2. Oganessian, Yuri. 2006. Synthesis and decay properties of superheavy elements. Pure and Applied Chemistry. Walter de Gruyter GmbH, January 1. http://dx.doi.org/10.1351/pac200678050889.

December 12th, 2007

Posted In: Chemistry, Nuclear chemistry

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