MEASUREMENTS OF CROSS SECTIONS AND DECAY PROPERTIES OF THE ISOTOPES OF ELEMENTS 112, 114, AND 116 PRODUCED IN THE FUSION REACTIONS 233,238U, 242,244Pu, AND 248Cm+48Са Yu.Ts. Oganessian, V.K. Utyonkov, Yu.V. Lobanov, F.Sh. Abdullin, A.N. Polyakov, I.V. Shirokovsky, Yu.S. Tsyganov, G.G. Gulbekian, S.L. Bogomolov, B.N. Gikal, A.N. Mezentsev, S. Iliev, V.G. Subbotin, A.M. Sukhov, A.A. Voinov, G.V. Buklanov, K. Subotic, V.I. Zagrebaev, M.G. Itkis, J.B. Patin1, K.J. Moody1, J.F. Wild1, M.A. Stoyer1, N.J. Stoyer1, D.A. Shaughnessy1, J.M. Kenneally1, P.A. Wilk1, and R.W. Lougheed1 1 University of California, Lawrence Livermore National Laboratory, Livermore, California 94551, USA According to nuclear theory, the limits of the existence of heavy nuclei, as well as their decay properties, are completely determined by nuclear shell effects. For the heaviest elements in the vicinity of the hypothetical closed spherical shells Z=114 and N=184, the increase of nuclear binding energy results in a considerable increase of stability with respect to various decay modes. We may also speculate that the high fission barriers of the superheavy nuclei in their ground states may persist at low excitation energies, resulting in an increase in their production cross section, or to be more precise, their survival probability in the process of de-excitation of the compound nucleus. In the present work, we investigated the survival probabilities of the compound nuclei by measuring excitation functions for producing evaporation residues with Z=112, 114, and 116 in reactions with different targets: 233,238U, 242,244Pu, and 245,248Cm . The 32-cm2 targets consisted of the isotopes 233 U, 238U, 242Pu, 244Pu, 245Cm, and 248Cm deposited to thicknesses of 0.44, 0.35, 0.40, 0.38, 0.35, and 0.35 mg cm2, respectively. The 48Ca-ion beam intensity was 1.2 pA. The evaporation residues (ER) recoiling from the target were separated in flight by the Dubna Gas-Filled Recoil Separator with a transmission efficiency of about 35-40%. To detect the decay sequences in low background conditions, the beam was switched off after a recoil signal was detected with parameters of implantation energy expected for the ERs, followed by an -like signal within a preset energy interval in the same strip and position, and a time interval up to 12 s. The duration of the pause in beam was determined from the observed pattern of out-of-beam decays and varied from 1 to 12 minutes. During these experiments , we measured the excitation functions for the reaction 244 Pu(48Ca,xn). We chose bombarding energies for the 48Ca ions of 243, 250 and 257 MeV in the middle of the target (E*=38.9-43.0, 44.949.0, and 50.4-54.7 MeV , respectively). At the 243-MeV 48Ca beam energy, we observed two decay chains each consisting of two consecutive decays terminated by SF. Identical decay chains were previously discovered in the same reaction at a lower energy of 236 MeV . One more event of this type was detected at beam energy of 250 MeV. In addition to these decay chains, two new isotopes of element 114 and their descendant nuclei were identified for the first time in this experiment with E=9.94 MeV, T1/2=0.8 s and E=10.02 MeV, T1/2=0.5 s (see Fig. 1 and Table I). Based on the results of these experiments, it was reasonable to assign the previously observed ER---SF chain  to the decay of the even-odd nuclide 289114 produced via 3nevaporation with a maximum cross section of about 1.7 pb. The two other nuclei should then be assigned to the decay of the neighboring even-even and even-odd isotopes of element 114, 288114 and 287114, produced via the 4nand 5n-evaporation channels with cross sections of 5.3 pb and 1 pb, respectively . One can note that a more solid mass identification is possible if the mass number of the target nucleus can be varied. Thus, if the suggested mass assignment is correct, the isotope 287114 should be observed in the 3nevaporation channel of the reaction 242Pu+48Ca and another even-even isotope, 286114, which has a higher -particle energy and lower lifetime, should be produced via the 4nevaporation channel. Fig. 1 Excitation functions for the 2n-5n evaporation channels from the fusion-evaporation reactions 233,238U, 242,244Pu, 248Cm+48Ca. The Bass barrier  is shown by an open arrow in each panel. Lines show the results of calculations . Error bars correspond to statistical uncertainties. Indeed, in the reaction 242Pu+48Ca, we observed the two isotopes 287114 and 286 114 . The decay properties of nuclei originating from 287114 produced in the 242 Pu(48Ca,3n) reaction coincide well with those for nuclei observed in the 244 Pu(48Ca,5n)287114 reaction. The even-even isotope 286114 undergoes SF and decay with T1/2=0.16 s, while 282112 decays by SF with TSF=0.5 ms. Together with cross section measurements, the other approach to the mass and atomicnumber identification of unknown nuclei is a series of cross bombardments. In our case this means the production of the same isotopes of element 114 as the daughter nuclei following the decay of heavier mother nuclei with Z=116 or the observation of the descendant nuclei of element 114 isotopes in the primary reaction leading to nuclei with Z=112. We used this method for the first time in an experiment aimed at the synthesis of nuclei with Z=116 in the reaction 248Cm+48Ca . Three similar decay sequences were observed. All of the decays following the first particles agree well with the decay chains of 289114, previously observed in the 244Pu+48Ca reaction . Thus, it was reasonable to assign the observed decays to the nuclide 293116. In the recent bombardment of a 248Cm target, two more decays of 293116 and six decays of 292116 were observed at higher 48Ca energy . In the reaction 245Cm+48Ca, we investigated the radioactive properties of other isotopes of element 116 . At a beam energy of 243 MeV in the middle of the target (E*=30.9-35.0 MeV), we detected two decay chains of 291116 and three decays of 290116. The decay energies and half-lives agree well between the daughter nuclei in the decay chains of 291116 observed in the 245 Cm(48Ca,2n) reaction, and the chains observed in the reactions 244Pu+48Ca at the maximum beam energy E*=53 MeV and 242 Pu+48Ca at E*=32.5-40.2 MeV, which were assigned to the decay of 287114, the 5nand 3n-evaporation products, respectively . Accordingly, the decay properties of the descendant nuclei of even-even 260116, the product of 3n evaporation, agree with those of 286 114 and 282112 observed in the reactions 242 Pu+48Ca at E*=40.2 and 45.1 MeV . In the 245Cm+48Ca reaction, the 2n- and 3nevaporation channels were observed with cross sections of about 0.9 pb and 1.3 pb, respectively. Moreover, the decay properties of descendant nuclei in the decay chain of 294 118 synthesized in the reaction TABLE I. Decay properties of even-Z nuclei. 118 No. Decay mode, Half-life c) a) observed branch b) . 294 1/1 1.8 75 1.3 ms 116 293 4/4 292 4/5 18 16 6 ms 40 ms 10.660.07 10.800.07 11.06 291 2/2 .6 6.3 11 2.5 ms 20 ms 10.740.07 10.890.07 10.91 290 2/2 15 26 6 ms 10 ms 10.850.08 11.000.08 11.08 289 9/9 2.6 10..27 s 2s 9.820.05 9.960.05 10.04 288 16/16 0.80 00..27 16 s 0.9 s 9.940.06 10.080.06 10.32 287 15/15 0.51 00..18 10 s 0.5 s 10.020.06 10.160.06 10.56 286 11/5 0.2 s 10.200.06 10.350.06 10.86 285 10/10 :0.4,SF:0.6 0.16 00..07 03 s 29 13 7 s 50 s 9.150.05 9.280.05 9.49 284 17 SF 97 31 19 ms 9.80 9.76 283 18/18 :1,SF:0.1 4.0 10..37 s 9.670.06 10.16 282 6 SF 0.50 00..33 14 ms 10.82 10.68 281 10 SF 11.1 52..07 s 9.00 9.30 279 21/2 108 275 2/2 106 271 2/1 104 267 1 Z 114 112 110 A 61 57 20 ms Expected half-life 0.4 ms E (MeV) Q (MeV) Qth (MeV) d) 11.650.06 11.810.06 12.11 80 ms 10.540.06 10.690.06 11.09 3s 9.540.06 :0.1,SF:0.9 0.18 00..05 03 s 0.15 00..27 06 s 0.2 s 9.700.06 9.840.06 10.24 0.8 s 9.300.07 9.440.07 9.41 :0.5,SF:0.5 2.4 14..03 min SF 2.3 198.7. h 0.8 min 8.530.08 8.650.08 8.71 8.22 7.4 Number of events used for calculations of half-lives / -particle energies, respectively. Branching ratio is not shown if only one decay mode was observed. c Error bars correspond to 68%-confidence level if more than one event was observed, for only one registered event, the error bars correspond to 95%. d Predicted decay energies . a 249 Cf(48Ca,3n)  are in agreement with 290116 and 286114 produced via the 245Cm+48Ca and 242 Pu+48Ca reactions. In the 238U+48Ca experiments, we detected 7 decay sequences of 283112 that were observed at beam energies EL=230-234 MeV, and one decay of 282112 observed at EL=240 MeV. Decays of the daughter nuclei in the chains observed following the reaction 242 Pu+48Ca coincide in all of the measured parameters (E, T, TSF, and ESF) with the decay chains observed in the 238U+48Ca reaction . We have also studied the reaction 233 U+48Ca at EL=240 MeV; despite an accumulated beam dose of about 81018 ions, we did not observe any decay chains that could b be attributed to the decay of isotopes of element 112. We calculated an upper cross section limit of 24n0.6 pb for the reaction 233 U(48Ca,2-4n)277-279112 at EL=240 MeV . The -particle spectra observed in these experiments from the decay of nuclei with Z=112, 114 and 116 are characterized by welldefined transition energies. They follow the relationship between the probability and energy of decay (Viola-Seaborg formula) that was determined from previously known even-even nuclei (Table I). This means that the observed transitions are as unhindered for the isotopes with odd mass numbers as they are for isotopes with even mass numbers. For the isotopes of lighter elements, the difference between measured and calculated T values increases resulting in hindrance factors of about 10 for 279Ds. A noticeable transition from spherical to deformed shapes occurs at Z=110, in agreement with results observed for odd-Z nuclei . Thus, the decay properties of the isotopes of element 114 are generally determined by the spherical shells Z=114 and N=184. According to calculations , the nucleus 287114 is almost spherical (2=0.088). In a succession of sequential decays, the descendant nuclei move away from the N=184 shell and approach the deformed shell at N=162. The terminating nucleus, 267Rf (N=163), is deformed (20.23) . The experimentally measured values of Q practically coincide with theoretical predictions for the deformed nuclei in the vicinity of neutron shells N=152 and N=162 and becomes somewhat less than the calculated values by 0.5 MeV for the more neutron-rich nuclides with N169. In the decay chain 291116…267Rf, we observed a similar variation in -decay energies as those reported for decay chains starting with 287115 or 288115 . The slope of Q vs. neutron number remains practically the same for elements 112-116 but increases significantly for nuclides with Z=111 and 110 (see Table I). Such an effect might be caused by the transition from spherical nuclear shapes to deformed shapes during successive decays, in agreement with calculations . If the theoretical predictions of the existence of closed nuclear shells in the domain of superheavy elements are correct, they should be characterized not only by high stability to various decay modes (longer halflives) but also by a relatively high probability of production. Indeed, the increase of the height of fission barriers due to the influence of the shell closure at N=184 is expected only for the neutron-rich nuclei with N>170 . For these nuclei, an increase of neutron number in the compound nucleus results in an increase of the production cross section (see Fig. 1). The low cross section for the formation of the isotope 278112 in the reaction 233 U+48Ca with NCN=169 has the same explanation. 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