V. A. Gorshkov

Synthesis of nuclei of the superheavy element 114 in reactions induced by 48Ca

The stability of heavy nuclides, which tend to decay by α-emission and spontaneous fission, is determined by the structural properties of nuclear matter. Nuclear binding energies and lifetimes increase markedly in the vicinity of closed shells of neutrons or protons (nucleons), corresponding to ‘magic’ numbers of nucleons; these give rise to the most stable (spherical) nuclear shapes in the ground state. For example, with a proton number of Z = 82 and a neutron number of N = 126, the nucleus 208Pb is ‘doubly-magic’ and also exceptionally stable. The next closed neutron shell is expected at N = 184, leading to the prediction of an ‘island of stability’ of superheavy nuclei, for a broad range of isotopes with Z = 104 to 120 (refs 1, 2). The heaviest known nuclei have lifetimes of less than a millisecond, but nuclei near the top of the island of stability are predicted to exist for many years. (In contrast, nuclear matter consisting of about 300 nucleons with no shell structure would undergo fission within about 10−20 seconds.) Calculations indicate that nuclei with N > 168 should already benefit from the stabilizing influence of the closed shell at N = 184. Here we report the synthesis of an isotope containing 114 protons and 173 neutrons, through fusion of intense beams of 48Ca ions with 242Pu targets. The isotope decays by α-emission with a half-life of about five seconds, providing experimental confirmation of the island of stability.

Chemical characterization of element 112

The heaviest elements to have been chemically characterized are seaborgium (element 106), bohrium (element 107) and hassium (element 108). All three behave according to their respective positions in groups 6, 7 and 8 of the periodic table, which arranges elements according to their outermost electrons and hence their chemical properties. However, the chemical characterization results are not trivial: relativistic effects on the electronic structure of the heaviest elements can strongly influence chemical properties. The next heavy element targeted for chemical characterization is element 112; its closed-shell electronic structure with a filled outer s orbital suggests that it may be particularly susceptible to strong deviations from the chemical property trends expected within group 12. Indeed, first experiments concluded that element 112 does not behave like its lighter homologue mercury. However, the production and identification methods used cast doubt on the validity of this result. Here we report a more reliable chemical characterization of element 112, involving the production of two atoms of 283112 through the alpha decay of the short-lived 287114 (which itself forms in the nuclear fusion reaction of 48Ca with 242Pu) and the adsorption of the two atoms on a gold surface. By directly comparing the adsorption characteristics of 283112 to that of mercury and the noble gas radon, we find that element 112 is very volatile and, unlike radon, reveals a metallic interaction with the gold surface. These adsorption characteristics establish element 112 as a typical element of group 12, and its successful production unambiguously establishes the approach to the island of stability of superheavy elements through 48Ca-induced nuclear fusion reactions with actinides.

Indication for a volatile element 114

Abstract Recently, the chemical investigation of element 112 revealed a highly volatile, noble metallic behaviour, as expected for the last group 12 member of the periodic table. The observed volatility and chemical inertness were ascribed to the growing influence of relativistic effects on the chemical properties of the heaviest elements with increasing nuclear charge. Here, we report for the first time on gas phase chemical experiments aiming at a determination of element 114 properties. This element was investigated using its isotopes 287114 and 288114 produced in the nuclear fusion reactions of 48Ca with 242Pu and 244Pu, respectively. Identification of three atoms of element 114 in thermochromatography experiments and their deposition pattern on a gold surface indicates that this element is at least as volatile as simultaneously investigated elements Hg, At, and element 112. This behaviour is rather unexpected for a typical metal of group 14.

TWO-NEUTRON EXCHANGE OBSERVED IN THE 6HE + 4HE REACTION. SEARCH FOR THE DI-NEUTRON CONFIGURATION OF 6HE

Differential cross sections for the elastic scattering of 6 He exotic nuclei from a gaseous helium target has been measured in a wide angular range in the CM system at a 6 He beam energy of 151 MeV. The large cross-sections obtained at backward angles are discussed in terms of a two-neutron exchange process. The results of DWBA calculations show that this effect can account for the cross sections obtained between 1208 and 1608 assuming the spectroscopic factor to be about 1 for the di-neutron cluster as was predicted by theory for 6 He. q 1998 Published by Elsevier Science B.V. All rights reserved.

Cluster structure of 6He studied by means of 6He + p reaction at 25 MeV/n energy

Abstract Angular distributions for elastic scattering and 1n and 2n transfer reactions were measured for the 6 He + 1 H system at 25 MeV/nucleon. The experimental data were analyzed in the framework of the one-step finite range DWBA model. The results suggest a lower triton spectroscopic amplitude for the 6 He nucleus as compared to the shell model calculation.

The Vassilissa facility for electrostatic separation and study of complete fusion reaction products

Abstract The Vassilissa facility is described. The main component is an electrostatic separator of complete-fusion reaction products from the ion beam. The facility is capable of transmitting recoil nuclei emitted by the target in the beam direction within a solid angle of 10 −2 sr and has a ± 10% spread in energy and ionic charge. The time required to transport the nuclei to the detectors is 3 × 10 −6 s The separation of scattered ions is performed using a system of three electrostatic condensers. Separation factors obtained for Ar, P and Ne are equal to 10 10 , 10 11 and 10 12 , respectively.

High resolution line for secondary radioactive beams at the U400M cyclotron

Abstract For implementation of an experimental program for studying nuclear reactions with radioactive ion beams in the energy domain of 20 through 80 MeV A the high resolution beam line ACCULINNA was put into commissioning on a primary beam line of the JINR U-400M cyclotron. By means of nuclear fragmentation of the 14 N beam with the energy of 51 MeV A on the 170 mg/cm 2 carbon target radioactive beams of 6 He, 8 He and 8 B were obtained. Possibilities of further development of the set-up are discussed.

The 8He and 10He spectra studied in the (t, p) reaction

The low-lying spectra of 8He and 10He nuclei were studied in the 3H(6He, p)8He and 3H(8He, p)10He transfer reactions. The 0+ ground state (g.s.) of 8He and excited states, 2+ at 3.6–3.9 MeV and (1+) at 5.3–5.5 MeV, were populated with cross sections of 200, 100–250, and 90–125 μb/sr, respectively. Some evidence for a 8He state at about 7.5 MeV was obtained. We discuss a possible nature of the nearthreshold anomaly above 2.14 MeV in 8He and relate it to the population of a 1− continuum (soft dipole excitation) with a peak value at about 3 MeV. The lowest energy group of events in the 10He spectrum was observed at ∼ 3 MeV with a cross section of ∼ 140 μb/sr. We argue that this result is consistent with the previously reported observation of 10He providing the new 10He g.s. position at about 3 MeV

GABRIELA: A new detector array for γ-ray and conversion electron spectroscopy of transfermium elements

With the aid of the Geant4 Monte Carlo simulation package a new detection system has been designed for the focal plane of the recoil separator VASSILISSA situated at the Flerov Laboratory of Nuclear Reactions, JINR, Dubna. GABRIELA (Gamma Alpha Beta Recoil Investigations with the ELectromagnetic Analyser VASSILISSA) has been optimised to detect the arrival of reaction products and their subsequent radioactive decays involving the emission of α- and β-particles, fission fragments, γ- and X-rays and conversion electrons. The new detector system is described and the results of the first commissioning experiments are presented.

Evidences for resonance states in 5H

Resonance states of H-5 were investigated through the two-neutron transfer reaction t(t, P)(5) H. A triton beam at 57.5 MeV and a cryogenic liquid tritium target were used. The H-5 missing mass spectrum in triple coincidence, proton + triton + neutron, shows a resonance at 1.8 +/- 0.1 MeV above the t + 2n decay threshold. This energy is in good agreement with the result reported in Phys. Rev. Lett. 87 (2001) 092501. The resonance width, Gamma(intr) less than or equal to 0.5 MeV, is surprisingly small and difficult to reconcile with theory predictions