Synthesis of Superheavy Spherical Nuclei in the 48Ca + 244Pu-Reaction

A fundamental outcome of macro-microscopic nuclear theory is the prediction of an “island of stability” of superheavy elements. Calculations performed over more than 35 years with different versions of the nuclear shell model predict substantial enhancement of the stability of heavy nuclei when approaching the closed spherical shells Z = 114 and N = 184. Isotopes of element 114 with neutron numbers from 172 to 176, close to the predicted spherical nuclei and, consequently, being relatively stable, can be produced in fusion reactions of the heavy isotopes ^242,244Pu with ⁴⁸Ca ions [1].

With the doubly-magic ⁴⁸Ca projectile, the resulting compound nuclei ^290,292114 are weakly excited; their excitation energy at the Coulomb barrier is about 33 MeV. Correspondingly, nuclear shell effects are still expected to persist in the excited nucleus, increasing the survival probability of the evaporation residues (EVRs), as compared to “hot fusion” reactions (E_x 50 MeV), which were used for the synthesis of heavy isotopes of elements with atomic numbers Z = 106, 108 and 110 [2]. Additionally, the high mass asymmetry in the entrance channel should decrease the dynamical limitations on nuclear fusion arising in the more symmetrical reactions.

In spite of these advantages, previous attempts to synthesize new elements in ⁴⁸Ca-induced reactions with actinide targets gave only upper cross-section limits for their production [3]. The first positive result was obtained in 1998. Two spontaneous fission (SF) events were observed in the ⁴⁸Ca + ²³⁸U reaction with an integrated beam dose of 3.5x10¹⁸ ions. These were assigned to the decay of a new isotope of element 112 produced in the reaction ²³⁸U(⁴⁸Ca,3n)²⁸³112 (N=171) with a cross section of 5 ± 2 pb [4].

In the ⁴⁸Ca+²⁴⁴Pu reaction the ²⁹²114 compound nuclei are expected to deexcite by emission of 2 to 4 neutrons. From calculations by Pustylnik, based on the experimental cross sections of “hot fusion” reactions, and diffusion model calculations by Wada [5] the maximum cross section for producing evaporation residues in the ⁴⁸Ca + ²⁴⁴Pu reaction is expected for the 3n-evaporation channel at an excitation energy of 35 MeV. This agrees also with the experimental data for the reaction ⁴⁸Ca + ²³⁸U [4]. The absolute cross section values of the xn-channels are estimated with larger uncertainty; the calculated cross section at the maximum of the 3n-channel varies from 1 to 10 pb.

Smolanczuk et al. [6,7], who successfully reproduce the decay properties of the most neutron-rich known heavy nuclei, have calculated that the even-even isotopes ²⁸⁸114 and ²⁹⁰114 will have partial -decay half-lives T= 0.14 s and 0.7 s, respectively. Their SF half-lives should be considerably longer: T_SF= 2x10³ s and 4x10⁵ s, respectively. For their daughter nuclei - isotopes of element 112 - the values of T and T_SFare more comparable, but -decay should still prevail. The -decay granddaughters - the isotopes of element 110 - are expected to decay primarily by spontaneous fission. For the odd isotopes, in particular for the ²⁸⁹114 nucleus, the predictions are less definite; the odd neutron can lead to significant hindrance of both -decay and spontaneous fission. Here one expects competition of the two decay modes in the daughter products with Z112 and somewhat longer chains of sequential -decays than in the case of the neighboring even-even isotopes.

It is noteworthy that T calculations by Mller et al. [8] for ^288-290114 give values exceeding those of [6,7] by orders of magnitude (e.g., T of 7x10⁴ s for ²⁸⁹114). This, however, does not change the expected decay pattern for these isotopes of element 114 and their daughters. We can expect a sequence of two or more decays terminated by a spontaneous fission as it recedes from the stability region around N=184.

We present here initial results of an experiment to synthesize superheavy nuclei with Z=114 in the vicinity of predicted spherical nuclear shells in the complete fusion reaction ⁴⁸Ca + ²⁴⁴Pu.

A ⁴⁸Ca⁺⁵ beam was extracted from the ECR ion source and injected into the Dubna U400 heavy ion cyclotron. The average intensity of the ion beam on the target was 4x10¹² pps at the material consumption rate of about 0.3 mg/h. The beam energy was verified with a precision of ~1 MeV, by measuring the energies of scattered ions, and by a time-of-flight technique.

The targets consisted of the enriched isotope ²⁴⁴Pu (98.6 %) in the form of PuO₂, deposited onto 1.5-mm Ti foils to a thickness of ~0.37 mg/cm². Each target had an area of 3.5 cm² in the shape of an arc segment with an angular extension of 40° and an average radius of 60 mm. Nine targets were mounted on a disk which was rotated at 2000 rpm across the beam direction.

We used a ⁴⁸Ca bombarding energy of 236 MeV, corresponding to the calculated maximum of the 3n-evaporation channel to form the isotope ²⁸⁹114. EVRs recoiling from the target were separated in flight from beam particles and various transfer-reaction products by the Dubna Gas-filled Recoil Separator [9], passed through a time-of-flight (TOF) system and were implanted in the focal-plane detectors. At a beam intensity of 4x10¹²pps, the overall counting rate of the detector system was 15 Hz. The collection efficiency of the separator was estimated from the results of test experiments in the bombardment of ^natYb and enriched ^204,206-208Pb targets with ⁴⁸Ca ions. We deduce that 40% of the recoiling Z=114 nuclei formed in the ²⁴⁴Pu-target would be collected on the focal plane.

The TOF detector was used to measure the time of flight of recoiling nuclei (detection efficiency of ~99.7%) and to distinguish the focal-plane detector signals of particles passing through the separator from those of the radioactive decay of the implanted nuclei. The focal-plane detector consisted of three 40x40 mm² silicon Canberra Semiconductor detectors, each with four 40-mm-high x 10-mm-wide strips having position sensitivity in the vertical direction. To increase the detection efficiency for ’s escaping the focal-plane detector, we arranged 8 detectors of the same type without position sensitivity in a box array surrounding the focal-plane detector. Employing the side detectors increased a-particle detection efficiency to ~87% of 4. Behind the front detector, a set of 3 similar “veto” detectors was situated in order to eliminate signals from low-ionizing light particles, which partially passed through the focal-plane detector without being detected in TOF system.

Alpha-energy calibrations were periodically performed using the peaks from nuclides produced in the test reactions mentioned above. The fission-energy calibration was obtained by detecting fission fragments from the spontaneous fission of ²⁵²No [10]. The energy resolution for detection of -particles in the focal-plane detector was 45 keV; for detection by the side detectors of ’s escaping from the focal plane, the energy resolution was 180 keV. We determined the position resolution of the signals of correlated decays of nuclei implanted in the detectors: For sequential - decays the position resolution (at 95% confidence level) was ±0.8 mm; for correlated EVR- signals, ±1.2 mm; and for correlated EVR-SF signals, ±1.1 mm.

The experiment was performed during November and December, 1998. Over a period of 40 days a total of 5.2 x 10¹⁸ projectiles were delivered to the target. In the analysis of the experimental data, we assumed that the “island of stability” of superheavy elements has a border at which nuclei are unstable against spontaneous fission. As long as any -decay chain leads to the edge of the “island of stability”, it should be terminated by SF.

In test experiments, we observed that 95% of SF events from the ²⁵²No implants produced in ²⁰⁶Pb+⁴⁸Ca reaction are characterized by total energy release exceeding 130 MeV (without corrections for the pulse-height defect). Four such events were observed in the ²⁴⁴Pu+⁴⁸Ca bombardment.

Two events, with measured energies E = 149 MeV and E = 153 MeV, were detected 1.13 ms and 1.07 ms after the implantation of a recoil nucleus at the same position. For the second SF event both fission fragments were registered by the focal-plane and side detectors. We tentatively assign these events to the spontaneous fission of the 0.9 ms ^244mfAm isomer, a product of transfer reactions with the ²⁴⁴Pu target. Another signal was registered at the edge of the sensitive layer of the outermost detector strip No.12 and since its analysis is not straightforward, we have deferred it to a later time.

The last SF event was observed as two coincident signals (two fission fragments) with energy deposited in the focal-plane detector E_F1= 120 MeV and in the side detector E_F2 = 52 MeV; E_tot = 172 MeV. With a total kinetic energy value of 194.3 MeV [10] for implanted ²⁵²No nuclei that gave similar focal-plane-to-side-detector energy splits, this corresponds to a total energy release of 190 MeV. We searched the data backwards in time from this event for preceding particles in the same position with E>8 MeV [6-8]. The preceding position-correlated events are shown in Fig.1a. An -particle was detected in the focal-plane detector 30.4 s after the implantation in the middle of the 8^th strip of a recoil nucleus with a measured energy E_EVR= 6.1 MeV. This last value fits the energy expected for element-114 recoils. The TOF signal is consistent also with that expected of a complete-fusion EVR, as determined in the calibration reactions. The energy of the first -particle was E= 9.71 MeV. A second -particle, having an energy E= 8.67 MeV, was observed at the same location after 15.4 min. A third a-particle, escaping the front detector leaving an energy E₁= 4.04 MeV and absorbed in the side detector with E₂= 4.79 MeV, was measured 1.6 min later. Finally, 16.5 min later, the SF event was observed.

All 5 signals (EVR, ₁, ₂, ₃, SF) appeared within a position interval of ±0.8 mm (Fig.1b), which strongly indicates that there was a correlation among the observed decays. Assuming that the decay sequence for a good event will terminate with SF, two methods to estimate the probability of the candidate event being due to random correlations has been used. The calculation was based on Monte Carlo technique over the whole detector array and entire experiment duration as well as a random correlated event rates in strip No. 8 in the position in wich the candidate event occured. The probability of the detected decay sequence being caused by chance correlation was determined to be less than 10^-4 with both approaches.

All events of the decay chain are correlated in time and position. These events arise as a result of the -decay of a parent nucleus (E= 9.71 MeV) and continue until spontaneous fission takes place. This matches the decay of a superheavy nucleus that is predicted by theory. The detected sequential decays have larger E vs. T_1/2 values than the known radioactive nuclides. For the whole decay chain the basic rule for -decay, defining the relation between Q and T, is fulfilled. This can be seen in Fig.1a where the expected half-lives, corresponding to the measured -particle energies for the isotopes with the specified atomic numbers of the radioactive family, are shown. For the calculation of half-lives, the formula of Viola and Seaborg with parameters from Refs. [6,7] has been used. The best candidate for this parent nucleus, expected also from the experimental conditions, is the even-odd isotope ²⁸⁹114, produced in the 3n-evaporation channel. This one event corresponds to a cross section of about 1 pb. The half-lives of the observed nuclei are also in agreement with Tvalues predicted in the calculations [6,7], assuming reasonable hindrance factors for -decay of nuclei with an odd neutron number.

The lifetimes of the new isotopes of element 112, with mass number 285, and element 110, with mass number 281, produced in the reaction ⁴⁸Ca + ²⁴⁴Pu, are more than 10⁶ larger than the known nuclei ²⁷⁷112 and ²⁷³110 [11,2] which have eight fewer neutrons. From this point of view, the observed radioactive properties of the synthesized nuclides, together with the data obtained earlier for ²⁸³112 [4] constitute the first experimental proof of the existence of enhanced stability in the region of superheavy elements.

During the performance of the experiments we lost two of our most active colleagues, Drs. V. B. Kutner and B. I. Pustylnik. To their closest relatives we address our deepest appreciation of the great contribution our friends gave to the completion of the present work. We are grateful to the JINR Directorate, in particular to Profs. Ts. Vylov and V. G. Kadyshevsky for the help and support we got during all stages of performing the experiment. We express our thanks to 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. We would like to express our gratitude to the personnel of the U400 cyclotron and the associates of the ion-source group for obtaining an intense ⁴⁸Ca beam.

The Livermore authors thank their Russian hosts for their hospitality during the experiment. The help of Dr. E. K. Hulet in the early phases of our collaboration is gratefully acknowledged.

This work has been performed with the support of the Russian Foundation for Basic Research under grant No. 96-02-17377 and of INTAS under grant No. 96-662. 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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Joint Institute for Nuclear Research, 141980 Dubna, Russian Federation