The synthesis of spherical superheavy nuclei in ⁴⁸Ca induced reactions.

Yu.Ts.Oganessian¹, A.V.Yeremin¹, A.G.Popeko¹, S.L.Bogomolov¹,G.V.Buklanov¹, M.L.Chelnokov¹, V.I.Chepigin¹, B.N. Gikal¹, V.A.Gorshkov¹, G.G.Gulbekian¹, M.G.Itkis¹, A.P.Kabachenko¹,A.Yu.
Lavrentev¹,O.N.Malyshev¹, J.Rohac¹,R.N.Sagaidak¹, S.Hofmann²,S.Saro³, G.Giardina⁴,K.Morita⁵.

¹ Flerov Laboratory of Nuclear Reactions, JINR, 141 980 Dubna, Russia.
² Gesellschaft fur Schwerionenforschung, D-64220 Darmstadt, Germany,
³ Department of Physics, Comenius University, SK--84215, Bratislava, Slovakia,
⁴ Dipartimento di Fisica dell'Universita di Messina, 98166 Messina, Italy,
⁵ Institute of Physical and Chemical Research (RIKEN), Wako-shi, Saitama, Japan.

According to macro-microscopic nuclear theory the stability of heavy nuclides, undergoing mainly -decay and spontaneous fission, is determined by the structural properties of nuclear matter. Their binding energies and lifetimes increase most prominently in the vicinity of closed shells, corresponding to a certain number of neutrons and protons in the nucleus. These “magic numbers” determine the most stable 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 neutron shell is expected at N=184, forming “the island of stability” of the super heavy nuclei for a broad region of isotopes with Z = 104 to 120.Unlike the known heaviest nuclei having the life times of about parts of milliseconds, nuclei close to the top of “island of stability” can live for years and perhaps thousands of years (see Fig.1). Note that entirely non-structural nuclear matter consising of about 300 neutrons and protons will undergo fission within the time of about 10^-20 seconds. The nuclear structure (shells) increases the life-times of such nuclei-giants by 25–30 orders of magnitude. The prediction of existence of “the island of stability” is one of milestones of modern nuclear theory and its experimental verification is very important.

It is very difficult or even impossible to synthesize the nuclei at the top of “the island of stability” due to a very large neutron abundance. But calculations by R.Smolanczuk et al. ^1,2 who successfully reproduced the decay properties of the known heavy nuclei, and P.Moller et al.³ have shown that the stabilizing effect of the spherical shell starts already at N>168. Such neutron-rich nuclei can be synthesized, as we have shown earlier⁴, 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 defect of the doubly magic ⁴⁸Ca nucleus(magic numbers N=28 and Z=20), the excitation energy of the compound nucleus at the Coulomb barrier only amounts to about 30 MeV. It corresponds to the ⁴⁸Ca bombarding projectile energy in the laboratory system E_lab= 230 – 235 MeV. The deexcitation of this nucleus should proceed by the emission of mainly 3 neutrons and -rays^{5,6. The calculations predict that the probability for the excited compound nucleus to reach the final state (evaporation residue) after the evaporation of 2 or 4 neutrons is one order of magnitude less at the bombarding ion energy near the Coulomb barrier. This circumstance should increase the survival probability of the evaporation residues (EVRs) as compared with the case of hot fusion reactions (E}_x^{~50 MeV), which were used for the synthesis of heavy isotopes of elements with atomic number Z=106, 108 and 110 7-9. On the other hand, the high asymmetry of the interacting nuclei in the entrance channel (A}_p^/A_T^{=0.2; Z}_p^Z_T^{=1880) should decrease possible dynamical limitations10 on the fusion of massive nuclei as compared with more symmetrical cold fusion reactions.}

^{In spite of these obvious advantages, previous attempts to synthesize new elements in 48Ca-induced reactions gave only the upper limits of the production cross sections of superheavy elements11-13. As is apparent now it could be explained by the low experiment sensitivity since the intensity of the 48Ca beam was not high enough.}

^{The first positive result was obtained in spring 1998 in the 48Ca+238U reaction with a total beam dose of 3.5·1018Ca ions. In this experiment two spontaneous fission events were observed, which were assigned to the decay of a new isotope of element 112 produced in the reaction 238U(48Ca, 3n)283112 with a cross section of} _3n^{=  picobarn (pb, 10-36 cm2)14. The cross section value of 1 pb corresponds to the observation of one wanted event within 4 days providing the beam intensity is 4·1012 particles per second, the number of target nuclei able to take part in the fusion reaction is 5·1017 and the experiment efficiency is 100 %. The half-life of the new nuclide, determined on the basis of the two events, was about 1.5 min. Thus it turned out to be by about 3·105 times longer than the -decay half-life of the known lighter isotope of element 112, synthesized in the reaction 208Pb(70Zn, 1n)277112 by S.Hofmann et al.15in 1996 (Fig. 1a).}

^{The next experiment, carried out at the end of 1998, was aimed at the synthesis of nuclei with Z=114 in the reaction 48 Ca+244Pu. In a 34-day irradiation with a beam dose of Ca ions of 5.3·10 18, a decay chain, consisting of three sequential -decays and spontaneous fission, all took about 34 min, was observed after the implantation of a heavy atom in the detector 16. This decay chain may be considered as a good candidate for originating from the -decay of the parent nucleus 289114 (T~20 s), produced in the 3n-evaporation channel with a cross section of about 1 pb. It is very important to have at least two isotopes of the 114 element for establishing a certain trend in the half-lives– whether it increases with an increase in the neutron number, thus indicating that we are reaching the shore of “the island of stability”.}

^{Here we report another experiment also aimed at the synthesis of nuclei with Z=114 in the reaction 48Ca+242Pu.}

  ^{If the identification performed in the first two experiments is correct, then it is not difficult to predict the properties of the new isotope of element 114. According to the calculations and the above mentioned data, the isotope 287114 (N=173) should undergo predominantly -decay to the daughter nucleus 283112, obtained earlier in the 48Ca+238U reaction. Thus one could expect a short decay chain, -SF, with a relatively short half-life with respect to -decay (of about several sec.) and a subsequent spontaneous fission with a considerably longer half-life (of about several min.)}

^Experiment

^{The expected cross section for the production of the new isotope 287114 at the maximum of the excitation function for the 3n-evaporation channel is very small (~1 pb)5,6. This requires high sensitivity of the experiment. From this point of view, the production of an intense ion beam of the rare and extremely expensive isotope 48Ca is the cornerstone of our attempts to synthesize superheavy elements.}

               ^{The low energy (E}_lab^{= 60 keV) 48Ca+5 beam was extracted from the ECR ion source17 and injected into the Dubna U400 heavy ion cyclotron, where it was accelerated with a following extraction from the cyclotron and transportation to the experimental target. The average intensity of the ion beam on the target was 4·1012 particles per second at the material consumption rate of about 0.3 mg/h. The beam energy was determined with a precision of ~1 MeV by measuring the energies of the scattered ions and using time-of-flight technique.}

               ^{The targets consisted of the enriched isotope 242Pu (97%) in the form of PuO}₂^{, deposited onto 0.75 mg/cm2 Ti foils 0.2 mg/cm2 in thickness. Each target had an area of 5 cm2 in the shape of an arc segment with an angular extension of 60° and an average radius of 55 mm. Six targets were mounted on a disk that was rotated at 2000 rpm perpendicularly to the beam direction. A "chopper" system switch out the beam to make a pause (0.6 ms) at the moment when the target disk frames passed the beam.}

               ^{The evaporation residues were separated in-flight from the beam particles and other reaction products by the electrostatic recoil separator VASSILISSA18 (Fig.2).}

^{For the registration of the EVRs and their radioactive decay a system of time-of-flight (TOF) detectors and a silicon position-sensitive strip-detector array was installed in the focal plane of the separator. After registration by the TOF detectors, consisting of two secondary-electron foil converters with microchannel plates, the recoil atoms were implanted into a 16-strip silicon detector, which had an active area of 60x60 mm2. Each strip had a longitudinal position sensitivity. A 3 m thick degrader foil (Mylar) was placed in front of the silicon detectors for reduction of the number of low energy projectiles reaching the focal plane of the separator. Special measurements of the position resolution along each strip were performed. For sequential - decays it was 0.6 mm (FWHM), for correlated EVR - - about 1 mm, and 1.5 mm for EVR – SF. The measurements were performed for recoil nuclei with energies of 4 15 MeV, which fully covered the expected energy interval for the signals originating from the implanted Z=114 nuclei.}

^{In order to increase the detection efficiency for -particles, the front detector was surrounded by four silicon detectors of the same size as the stop detector. The entire array had the shape of a cube with a 60 mm edge length. The efficiency of the silicon array in the detection of -particles with the full energy was 85% of 4. Thus, the probability that the -particle energy (E was determined directly by the front detector was 50% (energy resolution E =20 keV); it was 35% when E was the sum of two signals E =E}₁^+E ₂ ^{from the front and side detectors (energy resolution E =150 keV); and finally, it was 15% when the -particle could be emitted backwards leaving only an escape signal (E}₁^{) in the front detector if E}₁ ^{1 MeV. In recording the events the accuracy of the time registration was about 1s.}

^{The signals from the TOF detectors were used to measure the velocity of the recoils and to distinguish the radioactive decays of previously implanted nuclei. The high efficiency of the TOF detectors made it possible to obtain very clean decay spectra. The time window for measuring decay chains could be considerably widened (up to several hours). The latter was particularly important for the correlation of decays with long half-lives in the presence of a continuously running beam.}

^{At a beam intensity of ~4·1012 ions/sec. the total counting rate of all events at the focal plane detector was only 25-30 events/sec. The counting rate at a single strip in a position interval of 1.2 mm amounted: for the -like signals (in the absence of signals from the TOF detector) with an energy higher than 7.5 MeV - to less than 1 h-1; for the -like signals with an energy from 1 to 4 MeV (signals corresponding to escaped 's)- to about 3h-1; for recoil-like (EVR-like) signals (with a TOF signal) with an energy higher than 4 MeV - to about 4 h-1.}

^Results

^{The experiment was performed during March 3 - April 5, 1999. The beam energy in the middle of target was E}_lab^{=235±2 MeV. Over a period of 32 days a total of 7.5·1018 Ca projectiles was passed through the target.}

^{In the analysis of the experimental data we proceeded from the expected short decay chain of 287114 terminated by spontaneous fission of the daughter nucleus (or its grand-daughters). So, first of all, the decay events with a large energy deposit in the front detector (E>100 MeV) were selected. Four such events were observed in the reaction 48Ca+242Pu.}

^{Two events with energies E=144 MeV and 175 MeV were registered 59 s and 20 s after the implantation of the corresponding position-correlated recoil atoms respectively. We assigned those events to the SF -isomers, tentatively to the 24-s 241mfPu, produced in a transfer reaction, and being a neighbor of the 242Pu-target nucleus.}

^{For the other two events spontaneous fission was observed as two coincident signals (two fission fragments) with an energy E}_F1^{=130 MeV deposited in the front detector and E}_F2^{=65 MeV in the side detector with E}_tot^{=195 MeV for the first event and E}_F1^{=110 MeV, E}_F2^{=55 MeV, with E}_tot^{=165 MeV for the second event.}

^{We searched for the recorded data backwards in time from each of those two events for preceding -particles correlated in position. For the first case only one -particle was detected in the front detector 1.32 s after the implantation in the middle of the strip of a recoil nucleus with a measured energy E}_EVR^{=10 MeV. This value agrees with the energy expected for the element-114 recoils, and the TOF signal is consistent with that expected for a complete-fusion EVR, as determined in the calibration reactions. The energy of the -particle was E =10.29 MeV. The SF event was observed 559.6 s later.}

^{All the three signals (EVR, and SF) appeared within a position interval of 0.82 mm, which indicates that there is correlation between the observed decays. The probability for the observed correlation to be a random coincidence of signals imitating the decay chain (EVR, and SF) at a given position window amounts to ~ 2·10-4. It was calculated employing a method used in the case of low statistics [19].}

^{In the second case, spontaneous fission was observed 243 s after the registration of the implanted recoil nucleus with the energy E}_EVR^{=13.5 MeV. In the search for an -particle in the time interval EVR-SF only a single escape signal with E}_{1=2.31 MeV was found 14.45 s after the implantation of the recoil nucleus. All the three signals (EVR, and SF) appeared within a position interval of 1.0 mm, which again indicated correlation between the observed decays. The probability for the observed correlation to be a random coincidence of signals of the EVR, and SF type in this case is 3·10}^-3 _{. The entire position correlated decay chains are shown in Fig.3a. In both sequences the parent nucleus undergoes -decay. Note that the time intervals preceding the -particle emission differ by about the factor of 10. This is not surprising with the low statistics we have. As the total energy E for the second event is not determined, we assume that the -decay in both the cases proceeds from one and the same state. The decay properties of the parent nucleus are T =}