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 predicted decay properties of nuclei, the 2-to-4 neutron evaporation products from the compound nucleus ²⁹²114 [8,9] should have -decay chains terminated by spontaneous fission.

We observed only two fission events during this ²⁴⁴Pu+⁴⁸Ca bombardment. Both SF events were observed as two coincident signals (two fission fragments) with energies E_F1=156+65 MeV and E_F2=171+42 MeV. The items in each sum mean energies deposited in the focal-plane and side detectors, respectively. We searched the data backwards in time from these events for preceding particles with E>8 MeV [7,8] and/or EVRs, in the same positions. The latter were defined as the nearest events characterized by the measured energies, TOF signals and estimated resulting mass values, that were consistent with those expected for a complete-fusion EVR, as determined in the calibration reaction. The full decay chains including these two fission events are shown in Fig. 1, with the suggested assignment of decays to specific nuclei.

Both decay chains are consistent with one another, in all the measured characteristics. The first -particles have similar energies E=9.87 MeV and E=9.80 MeV, and were detected in the focal-plane detector 0.77 s and 4.58 s after the implantation of the recoil nuclei in strips 2 and 8, respectively. The second -particles in corresponding chains, having the energies E=9.21 MeV and E=9.13 MeV, were observed at the same locations after 10.34 s and 18.01 s. Finally, 14.26 s and 7.44 s later, the SF events were observed. All events in the two decay chains appeared within time intervals of 25.4 s and 30.0 s and position intervals of 0.5 mm and 0.4 mm (Fig. 1), respectively.

To estimate the probability of random origin of the candidate decay chains we used two approaches. By applying a Monte Carlo technique [2,10] we determined the probability per fission of finding such correlated events at any position of the detector array. Following the procedure described in Ref. [11] we calculated probabilities of these decay sequences being caused by the chance correlations of unrelated events at the positions in which the candidate events occurred. The probability that both decay chains consist of random events is less then 10^-12.

By applying a Monte Carlo technique [2,10] we determined to estimate the probability of the candidate decay chains being due to random correlations we determined the probability per fission of finding such correlated events to be P_err=3x10^-7. By applying the Geiger-Nuttall relationship, we imposed a lifetime window for each -event. That reduced P_err to 3x10^-8. The probability that both decay chains consist of uncorrelated events to beis approximately 3x10^-15.

Following the procedure described in Ref. [11], the calculated probabilities P_err of these decay sequences being caused by the chance correlations of randomunrelated events in strips 2 and 8 at the positions in which the candidate events occurred, awere calculated to be 7x10^-7 and 8x10^-7, respectively, in agreement with the results of the Monte Carlo analysis.

We observed two three-member decay sequences. If we assume that they actually consisted of four decays (i.e., the spontaneous fission was due to element 108), the probability of missing one -event in both decay chains would be less than 3%.

The ⁴⁸Ca beam energy in the middle of the target corresponded to the excitation energy of 36-37-MeV of the compound nuclei, in both cases, suitable expected for evaporation of 4 neutrons from the ²⁹²114 compound nucleus. We should also note that Tthe observed decay chains, including two alpha-decays and terminated by SF, match the predicted decay scenario predicted for the even-even nuclide ²⁸⁸114dependence of Q on T for heavy even-Z nuclei with Z³110 [8,9]. For all the chain members, the basic rule of -decay, which defines the relation between Q andT_1/2 vs. TE values of the detected sequential decays correspond to the decays of the even-even nuclides with Z=114 and 112, is fulfilled. To illustrate this, Fig. 1 presents the expected half-lives, corresponding to the measured -particle energies for the genetically related nuclides with the specified atomic numbers. For the calculation of half-lives, the formula by Viola and Seaborg has been used, with parameters fitted to the T values of 58 even-even nuclei with Z>82 and N>126, for which both T and Q were measured [8,9]. On the other hand, substituting experimental T_1/2 and E values in this formula results in atomic numbers of 114.4 and 110.2 for the mother and daughter nuclides, respectively. The total deposited energies of the two fission events are 230-240 MeV, correcting for pulse-height defect. Despite the relatively wide distributions of the total kinetic energies released in spontaneous fission, these deposited energies also indicate the spontaneous fission of rather heavy granddaughter nucleus with Z 106 [12].

From the above considerationsThus, we can conclude thatthe most reasonable explanation of  the detectedregistered decay chains is by the consecutive -decays startingoriginate from the parent even-even nuclide ²⁸⁸114, produced in the ²⁴⁴Pu+⁴⁸Ca reaction via 4n-evaporation channel. On the basis of two detected events, the cross section of this reaction is about 1 pb.

The new decay sequences evidently originate from a different parent nucleus than a single chain that was observed previously in the reaction ²⁴⁴Pu+⁴⁸Ca and attributed to the decay of ²⁸⁹114 [2]. The new alpha-decaying nuclides are characterized by higher decay energies than the corresponding members of the chain observed in [2], while SF terminates the decay sequence at an earlier stage. Thus, comparison of the decay properties supports assignment of a previously observed longer sequence to the decay of a heavier odd nuclide ²⁸⁹114 [2].

Both series of experiments with the ²⁴⁴Pu+⁴⁸Ca reaction were performed under essentially the same experimental conditions with a resulting integral beam dose of 1.5x10¹⁹ ions. From the experimental dependencies of the cross sections of symmetric fission induced by ⁴⁸Ca ions on the ²³⁸U and ²⁴⁴Pu targets and recent calculations [13], Eevaporation of 3 or 4 neutrons from the compound nucleus ²⁹²114 could be expected, with close probabilities, aint the investigated excitation energy range of of 31.5-39 MeVV. In this respect, Recent semiempirical calculations [13] predict the cross section maxima for emission of 3 and 4 neutrons from the compound nucleus ²⁹²114 at the excitation energy of 30 MeV and 38 MeV, respectively. tThe observation of the two neighbouring isotopes of element 114 in the present difference in results of two experiments is thus understandable, keeping in mind the low number of events and the narrow widths expected for the excitation functions. Further, we plan to continue our experiment using ~5 MeV lower projectile energy to search for additional decays of ²⁸⁹114.

The radioactive properties of the new observed nuclides are in qualitative agreement with predictions of the macroscopic-microscopic nuclear theory [8,9]. The experimental data exactly reproduced the predicted for ²⁸⁸114 decay scenario, i.e., two consecutive -decays terminated by spontaneous fission. Moreover, the radioactive properties of new nuclides agree with those that could be expected from the decay properties of previously synthesized neighbouring odd isotopes ^287,289114 [2,3] and ²⁸⁵112 [2].

The properties of the observed even-even nuclei can be directly compared with calculations [8,9]. Alpha-decay energies of known isotopes of even-Z elements with Z 100 together with theoretical Q values [8,9] for even-even isotopes with Z=106-114 are shown in Fig. 2.

Comparison of the measured decay properties of the new even-even superheavy nuclei ²⁸⁸114 (E=9.84±0.05 MeV, T_1/2=1.9 s), ²⁸⁴112 (E=9.17±0.05 MeV, T_1/2=9.8 s), and ²⁸⁰110 (T_1/2=7.5 s) with theoretical calculations [8,9] indicates that nuclei in the vicinity of spherical shell closures with Z=114 and N=184 could be even more stable than is predicted by theory. It can be seen in Fig. 2 that alpha-decay energies of the heaviest new even-even nuclides with Z=112 and 114 are 0.4-0.5 MeV less than predicted values. The heaviest even-odd nuclides follow this trend, as well. Such a decrease in Q values leads to an increase of partial -decay lifetimes by an order of magnitude. Calculations are far less definite, regarding spontaneous fission; however, we note that the observed spontaneous fission half-life of ²⁸⁰110 exceeds the predicted value [8] by more than two orders of magnitude.

We would like to express our gratitude to I.V. Kalagin and the personnel of the U400 cyclotron and the associates of the ion-source group for obtaining an intense ⁴⁸Ca beam. We are grateful to the JINR Directorate, in particular to Profs. Ts. Vylov and, V.G. Kadyshevsky and A.N. Sisakian for the help and support we got during all stages of performing the experiment. We express our thanks to Dr. V.B. Zlokazov for mathematical analysis of data, Drs. V.Ya. Lebedev and S.N. Dmitriev for developing methods for preparation of the metal Ca samples for the ECR-ion source, and also to V.I. Krashonkin, V.I. Tomin, A.M. Zubareva, and A.N. Shamanin, for their help in preparing and carrying out the experiment.

The Livermore authors thank their Russian hosts for their hospitality during the experiment.

This work has been performed with the support of INTAS under grant No. 96-662. Much of support was provided through a special investment of the Russian Ministry of Atomic Energy. The ²⁴⁴Pu target material was provided by the U.S. DOE through ORNL. Much of the support for the LLNL authors was provided through the U.S. DOE under Contract No. W-7405-Eng-48. These studies were performed in the framework of the Russian Federation/U.S. Joint Coordinating Committee for Research on Fundamental Properties of Matter.

 

 

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