UZ9901138 EXPERIMENTS ON SYNTHESIS OF SUPERHEAVY ELEMENTS USING 48Ca BEAM AT FLNR JINR

Yu.Ts.Oganessian, A.V.Yeremin

Flerov Laboratory of Nuclear Reactions, JINR, Dubna, Russia

According to macro-microscopic nuclear theory the stability of heavy , under- going mainly a-decay and spontaneous fission, is determined by the structural properties of nuclear matter. Their binding energy and lifetime increase most prominently in the vicinity of closed shells, corresponding to the spherical nuclear shape in the ground state. Following the well-known shells with Z = 82 and N = 126 in the stable spherical nucleus 208Pb, the next closed shell is expected at N=184. Calculations by R.Smolanczuk et al. [1,2] who successfully reproduced the decay properties of the known most neutron-rich heavy nuclei, and P.Moller et al. [3] have shown that the stabilising effect of the spherical shell starts already at N > 168 for a broad region of isotopes with Z = 104 - 120. Such neutron-rich nuclei can be synthesised, as we have shown earlier (see for e.g. [4]), in fusion reactions using the heaviest isotopes of U, Pu and Cm as targets and a 48Ca ion beam. As a result of the significant mass excess of the doubly magic 48Ca nucleus, the excitation energy of the compound nucleus at the Coulomb barrier only amounts to about 30 MeV. The deexcitation of this nucleus should proceed by the emission of 3 and y-rays [5]. This circumstance should increase the survival probability of the evaporation residues (ER) as compared with the case of "hot" fusion reactions (Ex > 50 MeV), which were used for the synthesis of heavy isotopes of elements with atomic numbers Z=IO6, 108 and 110 [6]. On the other hand, the high asymmetry of the interacting nuclei in the entrance channel (AiJAr = 0.2; Zp- Zj= 1880) should decrease possible dynamical limitations on the fusion of massive nuclei as compared with more symmetrical cold fusion reactions [7]. To produce heavy isotopes of element 112 in the reaction 48Ca + 238U, two experiments at beam energies 231 MeV (E* = 31 MeV) and 238 MeV (E* = 39 MeV) were performed. In the first experiment, in a 25 - day irradiation of a 238U target with a beam dose of 3.5 x 1018 at the focal plane of the recoil separator VASSILISSA [8] two spontaneous fission events were detected. The most probable explanation of the data obtained in this experiment is that spontaneous fission is caused just by the decay of an even - odd isotope (N = 171) of element 112 produced in the reaction 238U(48Ca,3n)283112 with a cross section of about 5 pb [9]. The half-life of the new is 180 sec. In the second experiment the beam energy was 238 MeV (E* = 39 MeV). The total beam dose in that case was 2.2 x 1018. No events were detected. The only so far known isotope of element 112, 277112, was synthesised in the "cold" fusion reaction [10]. This isotope is an a. emitter with a half-life Tm = 0.24 ms. The obtained experimental data show that with an increase in the neutron number by 6 units, the half-life of element 112 increases by more than of 5 orders of magnitude. This is proof of existence of long-lived superheavy elements predicted theoretically in the region of the neutron shell N =

54 184. The production cross-section for element 112 is higher (5.0 pb) when using the "hot" fusion reaction and a 48Ca beam than the value obtained by using the "cold" fusion reaction and a70Zn beam (1.0 pb) [10]. The reaction 48Ca + 242Pu was investigated with the aim to synthesise a new isotope of the heaviest element Z = 114 [11]. The experiment was performed at beam energy resulting in excitation energy of the compound nucleus of 33.5 MeV. For the collected beam dose of 7.5xl018, we observed two decay chains both consisting of an implanted heavy nucleus, a subsequent a-decay and a spontaneous fission (SF). For the first sequence the measured a

particle energy and the corresponding time interval were: Ea =10.29 ± 0.02 MeV and At = 1.32s; for the spontaneous fission At = 559.6 s. For the second sequence a signal was

registered from an a particle, which had escaped the detector, AEa = 2.31 MeV, At = 14.45 s; for the spontaneous fission At = 228.6 s. The decay chains originated from the a-decay of the new isotope Z = 114 with 287 and was terminated by the SF of the previously investigated isotope 112. The new nuclide 114 was produced in the fusion reaction in the 3n-evaporation channel with a cross section of about 2.5 pb. The observed radioactive properties of the synthesised nuclei 287114 together with the data obtained earlier for the isotope 114 and the products of its a decay, viz the isotopes ' 112, may be considered as a direct proof of the existence of the "" of superheavy elements predicted by theory more than 30 years ago. This work has been performed with a support of the Russian Foundation for Basic Research under grant N 99-02-17447 and the INT AS grant N 96-662.

1. Smolanczuk, J, Skalski, and A. Sobiczewski, In Proceedings of the International Workshop XXIV on Gross Properties of Nuclei and Nuclear Excitations "Extremes of Nuclear Structure", Hirschegg, 1996 GSI, Darmstadt, 1996), p. 35. 2. Smolanczuk, Phys. Rev. C56, 812 (1997). 3. Moller, J.R. Nix, and K.-L. Kratz, Atomic Data and Nuclear Data Tables 66, 131 (1997). 4. Yu.Ts. Oganessian, In Proceedings of the International Conference on Nuclear Physics at the Turn of the Millennium "Structure of Vacuum and Elementary Matter", Wilderness, 1996 World Scientific, Singapore, 1997), p. 11. 5. Pustylnik, In Proceedings of the VI International School-Seminar "Heavy Ion Physics", Dubna, 1997 (World Scientific, Singapore, 1998), p. 431; 6. Yu.A. Lazarev et. al., Phys. Rev. C54, 620 (1996). 7. Blocki et.al., Nucl. Phys. A459, 145 (1986). 8. Yeremin et. al., Nucl. Instr. Meth. A350, 608 (1994). 9. Yu.Ts. Oganessian et. al., Eur. Phys. J. A5, 63 (1999). 10. Hofmann et. al., Z. Phys. A354 (1996) 229. 11. Yu.Ts. Oganessian et. al., Preprint JINR E7-99-111, Dubna 1999. To be published in Nature (1999).

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