Yu. S. Tsyganov

The Dubna gas-filled recoil separator: status and developments

Within the Dubna-Livermore collaboration a series of experiments were performed. Experiments with the Dubna gas-filled recoil separator have resulted in the discovery of the new nuclides , , , , and . The ground state decay properties established for and revealed an enhancement in their stability against and spontaneous fission-decays by a factor of as compared with that of nuclides with lower Z and N values. A direct test of the theory was the observation of a decrease in stability for nuclides with Z,N beyond the predicted magic numbers. The -particle energy measured for with N = 163 provided evidence that a neutron shell closure indeed exists and is located at N = 162. Concerning the possibility to reach the Z = 114 region with gas-filled separator some improvements were made to suppress fast background particles.

PARAMETER OF EQUILIBRIUM CHARGE STATES DISTRIBUTION WIDTH FOR CALCULATION OF HEAVY RECOIL SPECTRA

Energy and time-of-flight spectra of heavy nuclei were calculated using a PC-based code with a q2 scaling of the energy loss in low pressure hydrogen, filling the working volume of the gas filled separator at a pressure of about 1 Torr. The χ2 distribution is used to describe the charge distribution of heavy nuclei passing through hydrogen. The measured recoil spectra used to extract the optimum parameter of equilibrium charge states distribution width were obtained in the heavy ion induced 197Au + 22Ne, natW + 34S and 206Pb + 48Ca reactions. A brief description of the detecting module of the Dubna Gas-Filled Recoil Separator (GFRS) is presented and precedes the calculation description.

Chemical procedure applied for the identification of Rf/Db produced in the 48Ca +243Am reaction

Summary A chemical separation procedure for Rf/Db is described which was applied to a long-lived decay product from the nuclear fusion reaction 48Ca+ 243Am. A 1.2 mg thick 243Am target was bombarded by 247 MeV 48Ca particles. The recoiling products were collected in a thick Cu catcher for about one day and then subjected to a chemical separation procedure that included an ion exchange from dilute HF solutions. Final samples were prepared on 30 μg/cm2 thick polyethylene (PE) foils and counted in 4π-geometry for α-particles and spontaneous fission (SF) coincidences. The detector arrays were surrounded by 3He detectors to also assay prompt neutrons. Decontamination factors from actinides of about 105 were achieved. Group 6 (W) to 14 (Pb) elements as models for their heavier homologues were shown to be separated from the Rf/Db fraction with more than 90%. In eight final samples, representing a total beam dose of 3.4 × 1018 particles, 15 SF events were detected. The decay pattern points to a single component with a half-life of ≈32h, which shows a chemical behavior similar to the lighter homologues of group 4 and 5 elements.

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.

Targets of uranium, plutonium, and curium for heavy-element research

The heavy-element research program of the Dubna gas-filled recoil separator requires the use of rather exotic, strongly radioactive targets which can withstand long-term, high-intensity heavy-ion bombardments. A number of targets with thicknesses of 0.1–0.8 mg/cm2 deposited on various backings by different techniques such as electrospraying, mechanical painting with organic solutions, as well as molecular plating or electrodeposition from organic solutions were tested. The best results were obtained for electroplated targets deposited on 1.5 μm Ti backings. Isotopically enriched targets of 235,236,238U, 242,244Pu, and 248Cm mounted on rotating disks were irradiated by ions ranging from neon to argon with intensities up to 2 × 1013pps delivered by the U400 cyclotron. During two months of irradiation the total beam dose of the 34S ions applied to the target of 244Pu reached 2.5 × 1019. Collaborative Dubna-Livermore experiments were performed in 1993–1995 by employing the Dubna gas-filled recoil separator and resulted in the discovery of the new nuclides 262104, 265106, 266106, 267108, and 273110. The experiments aimed at the synthesis of element 114 are under preparation. Target fabrication methods and experimental results for nuclear physics studies at Coulomb energies are 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.

An approximate formula for the angular dependence of the residual defect in a silicon surface barrier detector

Abstract A new formula for the angular dependence of the amplitude residual defect based on the surface recombination model for charge losses is presented. Experimental results for Xe and Ni ions are compared with calculated ones. An attempt to apply this formula in a computer code to generate the energy distribution of spontaneous fission events of nuclei implanted into the detector was made.

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.

Synthesis of new superheavy elements using the Dubna Gas-Filled Separator: The complex of technologies

All the new isotopes of superheavy elements in the reactions with 48Ca ions and actinide targets, synthesized at the Flerov Laboratory of Nuclear Reactions (FLNR), have been obtained at the Dubna Gas-Filled Recoil Separator (DGFRS). Success was achieved with the use of several techniques, algorithms and approaches with record performance in their area. The most sophisticated of them are rare event detection technology, methods of measuring the effective equilibrium charges of heavy ions and nuclei in hydrogen at low pressure, modified approaches to evaluation of probabilistic characteristics of detected multichain events, beam-free test methods of detection equipment, and some other ones. The author analyzes the approaches that have led ultimately to a real breakthrough and an advance in experiments on the synthesis of superheavy elements. The development of detection systems also attracts the authors’ attention. Contributions from some facilities—such as SHIP, TASCA, and GARIS (RIKEN)—to synthesis experiments are partially noted. Under consideration are methods of automation of similar experiments, including the development of control and shielding systems for operation with highly active actinide targets. Some techniques for detecting α decay of heavy nuclei that came before the current experiments with electromagnetic separators are reviewed briefly. The methods, algorithms and hardware presented cover a more than twenty-year period of commissioning and application in long-term experiments at FLNR DGFRS.