A. M. Sukhov

Focal plane detector of the Dubna gas-filled recoil separator

The focal plane detector of the Dubna gass-filled recoil separator has been developed to study fusion reaction products from heavy-ion-induced nuclear reactions in the region of heavy nuclei with Z > 104. It consists of a 12 strip position-sensitive silicon detector to measure the energy/position of implanted nuclides and decay products and two low-pressure proportional chambers to generate a TOF signal. Additional information on ΔE from the STOP chamber is stored to provide an opportunity to discriminate alpha decays to the ground state of daughter product from the ones accompanied by conversion electron emission. The detector was successfully applied for the nuclear reactions which led to the discovery of new nuclides with Z = 108, 110 (A = 267, 273). An appropriate data acquisition system is described.

Synthesis of superheavy nuclei in 48Ca+244Pu interactions

This article reports the results of experiments aimed at producing hypothetical long-lived superheavy elements located near the spherical-shell closures with Z≥114 and N≥72. For the synthesis of superheavy nuclei, we used a combination of neutron-rich reaction partners, with a 244Pu target and a 48Ca projectile. The sensitivity of the present experiment exceeded by more than two orders of magnitude previous attempts at synthesizing superheavy nuclides in reactions of 48Ca projectiles with actinide targets. We observed new decay sequences of genetically linked alpha decays terminated by spontaneous fission. The high measured alpha-particle energies, together with the long decay times and spontaneous fission terminating the chains, offer evidence for the decay of nuclei with high atomic numbers. The decay properties of the synthesized nuclei are consistent with the consecutive alpha decays originating from the parent nuclides 288,289114, produced in the 3n-and 4n-evaporation channels with cross sections of about a picobarn. The present observations can be considered experimental evidence for the existence of the “island of stability” of superheavy elements and are discussed in terms of modern theoretical approaches.

Observation of enhanced nuclear stability near the 162 neutron shell

In bombardments of {sup 248}Cm with {sup 22}Ne the authors discovered two new isotopes, {sup 265}106 and {sup 266}106, by establishing genetic links between {alpha} decays of the 106 nuclides and SF or {alpha} decays of the daughter (grand-daughter) nuclides. For {sup 266}106 they measured E{sub {alpha}}=8.62{+-}0.06 MeV followed by the SF decay of {sup 262}104 for which they measured a half-life value of 1.2{sup +1.0}{sub {minus}0.5} s. For {sup 265}106 they measured E{sub {alpha}}=8.82{+-}0.06 MeV. They estimated {alpha} half-lives of 10-30 s for {sup 266}106 and 2-30 s for {sup 265}106 with SF branches of {approximately}50% or less. The decay properties of {sup 266}106 indicate a large enhancement in the SF stability of this N=160 nuclide and confirm the existence of the predicted neutron-deformed shell N=162.

Real-time mode detection of heavy ion-induced nuclear reaction products

Abstract Design of spectrometers of two nuclear research facilities, the Dubna Gas-filled Recoil Separator and KHIPTI is reviewed. The sources of backgrounds are discussed and techniques used to suppress these backgrounds in one-event detection experiments aimed at the synthesis of heavy elements are presented. The first system was used in 1998 in experiments on Z =114 superheavy element. We consider the possibility of detection of rare time and position correlated recoil-alpha and alpha–alpha sequences in real-time mode as basic techniques to suppress beam and target-like associated backgrounds. Fields of application of such a technique are discussed from the viewpoint of synthesis of heavy elements and by studying their chemical properties.

Parameter monitoring and control system of Dubna Gas-Filled Recoil Separator

A modified integrated parameter-monitoring and control system for experiments on the synthesis of superheavy nuclei at the U-400 cyclotron of the Flerov Laboratory of Nuclear Reactions is considered. Experiments on the irradiation of heavy actinide targets were performed at the Dubna Gas-Filled Recoil Separator (DGFRS) [1]. This system consists of three components, namely, the detection module parameter control subsystem, the beam parameter monitoring subsystem, and the separator technical parameter control subsystem. The system is designed in CAMAC standard, the software is written under Windows XP using RAD Borland C++ Builder 6.0 and Kk009Pci interface (LNR) [2, 3].

Method of active correlations: Present status

During the recent years, at the FLNR (JINR) a successful cycle of experiments has been accomplished on the synthesis of the superheavy elements with Z=112–118 with 48Ca beam. From the viewpoint of the detection of rare decays and background suppression, this success was achieved due to the application of a new radical technique - the method of active correlations. The method employs search in a real-time mode for a pointer to a probable correlation like recoil-alpha for switching the beam off. In the case of detection in the same detector strip an additional alpha-decay event, of “beam OFF” time interval is prolonged automatically. Reasonable scenarios of developing the method are considered. PC based data acquisition system as well as the monitoring and control system of the Dubna Gas Filled Recoil Separator is considered in brief too.

Synthesis of Superheavy Nuclei in 48Ca-Induced Reactions

neutron-rich isotopes 242,244 Pu, 243 Am, 245,248 Cm and 249 Cf with 48 Ca projectiles. The decay properties of the synthesized nuclei are consistent with the consecutive α-decays originating in the decays of parent nuclides 286,287,288,289 114, 287,288 115, 290,291,293 116 and 294 118 produced in the 2n- to 5n-evaporation channels. The present observations can be considered to be experimental evidence of the existence of the “island of stability” of superheavy elements.

Production and decay of the heaviest odd-Z nuclei in the 249Bk + 48Ca reaction

The reaction of 249Bk with 48Ca has been investigated with an aim of synthesizing and studying the decay properties of isotopes of the new element 117. The experiments were performed at five projectile energies (in two runs, in 2009-2010 and 2012) and with a total beam dose of 48Ca ions of about 9x1019 The experiments yielded data on a-decay characteristics and excitation functions of the produced nuclei that establish these to be 293117 and 294117 – the products of the 4n- and 3n-evaporation channels, respectively. In total, we have observed 20 decay chains of Z=117 nuclides. The cross sections were measured to be 1.1 pb for the 3n and 2.4 pb for the 4n-reaction channel. The new 289115 events, populated by α decay of 117, demonstrate the same decay properties as those observed for 115 produced in the 243Am(48Ca,2n) reaction thus providing cross-bombardment evidence. In addition, a single decay of 294118 was observed from the reaction with 249Cf – a result of the in-growth of 249Cf in the 249Bk target. The observed decay chain of 294118 is in good agreement with decay properties obtained in 2002-2005 in the experiments with the reaction 249Cf(48Ca,3n)294118. The energies and half-lives of the odd-Z isotopes observed in the 117 decay chains together with the results obtained for lower-Z superheavy nuclei demonstrate enhancement of nuclear stability with increasing neutron number towards the predicted new magic number N=184.

New data from the 243Am + 48Ca reaction give cross-bombardment verification of elements 113, 115 and 117

The reaction 243Am + 48Ca has been reinvestigated to provide new evidence for the discovery of elements 113, 115. Twenty eight new 288115 decay chains were detected in this reaction to increase from three to 31 the number of 288115 atoms observed. In addition, four new decay chains were observed for the first time and assigned to the decay of 289115. These new 289115 events have the same properties for their decay chains as those observed for 289115 populated in the alpha decay of 293117 produced in the 249Bk + 48Ca reaction to provide cross-bombardment evidence. These new high statistics data sets and the cross-bombardment agreement provide definitive evidence for the discoveries of the new elements with Z = 113, 115, 117.

Synthesis of the New Element with Z=117

The synthesis of the new chemical element with atomic number Z=117 is presented. The isotopes 293117 and 294117were produced in fusion reactions between 48Ca and 249Bk. The 249Bk was produced in the High Flux Isotope Reactor and chemically separated at Oak Ridge. Decay chains involving eleven new nuclei were identified by means of the Dubna Gas Filled Recoil Separator. The measured decay properties show a strong rise of stability for super-heavy nuclei toward N=184.