A. P. Kabachenko

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.

Large size foil-microchannel plate timing detectors

Abstract A large area foil-microchannel plate time-of-flight (TOF) system is presented. The parameters of the system are as follows: 55 cm 2 active area, 99.8% detection efficiency, 100% transparency and 700 ps time resolution. The TOF system, unique in size and conditions of exploitation, consists of two (or three) detectors used also as anticoincidence units for the analysing multistrip Si-detector array.

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.

Modernization of the detector system at the recoil separator Vassilissa

Abstract Since the past 10 years, the recoil separator VASSILISSA has been used for the investigations of evaporation residues produced in heavy-ion-induced complete fusion reactions. In the course of experimental work in the region of the elements with 92⩽Z⩽94 , 14 new isotopes have been identified by the parent–daughter correlations. For further experiments aimed at the synthesis of the superheavy element isotopes ( Z⩾110 ) with intensive 48 Ca extracted beams, improvements in the focal plane detector system have been made. As a result, energy and position resolutions of the detectors have been significantly improved, the detection efficiency for the α -particles, emitted from the implanted into the focal plane detector recoil nuclei, has been increased to 85%. The decay properties of the new isotope of element 112 and the isotope of the new element 114 with mass 287 have been measured in two-nuclei decays.

In-beam separation and mass determination of superheavy nuclei. Part I

Within the past 15 years, the recoil separator VASSILISSA has been used for the investigations of evaporation residues produced in complete fusion reactions induced by heavy ions. The study of decay properties and formation of cross-sections of the isotopes of elements 110, 112 and 114 was performed using high-intensity Ca beams and Th; U; Pu targets. For further experiments aimed at the synthesis of the superheavy element isotopes ðZX110Þ with the use of intense Ca extracted beams, improvements in the ion optical system of the separator and the focal plane detector system have been made. The results from the test reactions and new results for the isotope 112 are presented. r 2003 Elsevier B.V. All rights reserved. PACS: 23.60.+e; 25.85.Ca; 29.30.Cm; 29.40.Pe

The upgrade of the kinematic separator VASSILISSA - Experimental results and plans

Within the past twelve years, the recoil separator VASSILISSA has been used for the investigations of evaporation residues produced in heavy-ion-induced complete fusion reactions. In the course of the experimental work, fourteen new isotopes have been identified by the parent-daughter correlations in the region of elements with 92≤Z≤94. The study of the decay properties and formation cross sections of the isotopes of elements 110, 112, and 114 was performed using high-intensity 48Ca beams and 232Th, 238U, and 242Pu targets. At the beam energies corresponding to the calculated cross-section maxima of the 3n evaporation channels, the isotopes 277110, 283112, and 287114 were produced and identified. For further experiments aimed at the synthesis of the superheavy element isotopes (Z≥110) with the use of intensive 48Ca extracted beams, improvements in the ion optical system of the separator and the focal plane detector system have been made.

Decay widths of highly excited Ra compound nuclei

Abstract Excitation functions were obtained for the x n, p x n and αx n decay channels of 216,218,220 Ra compound nuclei produced in the reactions 22 Ne+ 194,196,198 Pt in the excitation energy range of 40–160 MeV. Due to the employment of three platinum isotope targets, the cross section ratios were measured with a high degree of accuracy for evaporation reaction channels resulting in the formation of fixed cold products after the termination of evaporation cascades of a different numbers of stages. Experimental values for the reduced neutron widths of highly excited compound nuclei were obtained from the measured cross section ratios. Evaporation reaction cross sections were well described in the framework of the statistical model taking into account shell effects according to Ignatyuk. Our present, and also earlier, investigations showed that calculations making use of one single set of the model parameters (the scaling factor to the liquid-drop fission barrier of Cohen, Plasil and Swiatecki, C = 0.6–0.7 and the ratio of the level density parameters, a f a v = 1.0 ) reproduces correctly the cross sections of the evaporation reactions in a wide range of compound nuclei extending from Bi to U. Moreover, the measured cross section ratios are exceptionally sensitive to the value of the parameter a f a v , i.e. are very suitable for a high accuracy estimation of this value. By fitting two sets of the experimental data — the absolute cross section values and cross section ratios — we were able to independently estimate the values of two principal parameters of the model, C and a f a y . Our calculations showed that a large part of the pre-fission neutrons is evaporated by hot pre-actinide compound nuclei before they reach the saddle point (50 to 100% of the total number of pre-fission neutrons are pre-saddle).

Cross sections of 102 element isotopes formation in the reactions of22Ne+236U and26Mg+232Th

The production cross sections of the isotopes252102,253102, and254102 were measured for the heavy ion fusion reactions of22Ne+236U and26Mg+232Th by using the kinematic separator VASSILISSA. The obtained excitation functions and the maximum production cross sections are compared with the ones for more asymmetric reactions leading to the same compound nucleus258102. The experimental cross sections and the results of statistical model calculations are compared and discussed.

The new isotopes223, 224U

In the heavy-ion complete fusion reaction208Pb+20Ne the new isotopes223,224U were produced. These nuclei were identified after in-flight separation with the kinematic separator VASSILISSA, followed by their implantation into a passivated ion implanted silicon detector and the observation of the genetic relationships of subsequent α-decays.223U was found to decay withEα=(8780±40) keV and T1/2=18−5+10μs. For224U the α-line atEα=(8470±15) keV and T1/2=0.7−0.2+0.5 ms was observed.

Large area high-efficiency time-of-flight system for detection of low energy heavy evaporation residues at the electrostatic separator VASSILISSA

Abstract A large-area thin timing detector for slow heavy nuclei was developed. The apparatus is based on the transport of secondary electrons by crossed electric and magnetic fields with subsequentamplification by micro-channel plate multipliers. To increase the registration efficiency secondary electrons are collected from the both sides of the emitter foil. An intrinsic time resolution of (380±30) ps and registration efficiency of (95±5)% were obtained for the single TOF detector using a 226 Ra α-source. A time resolution of (400±30) ps and registration efficiency close to 100% were measured for heavy ions for the single TOF detector. A time-of-flight system consisting of two such detectors is extensively used in the experiments with heavy nuclei at the kinematic separator VASSILISSA [1,2].