Dubna Gas-filled Recoil Separator Research Articles

Simulations of recoil trajectories in Dubna Gas-Filled Recoil Separator 2 by GEANT4 toolkit

Dubna Gas-Filled Recoil Separator (DGFRS-2) is a new facility at the Super Heavy Element Factory which allows us to study the properties of superheavy elements (SHEs) formed in complete fusion reactions in the femtobarn cross-section range. High dispersion and complex magnetic configuration make it difficult to tune the separator and find the optimal current values in the magnetic elements. This work presents a simulation model of the DGFRS-2 based on a GEANT4 toolkit. The main methods of trajectory simulations of heavy ions in gaseous media are discussed and several new processes are implemented in GEANT4. Calculation results agreed well with the data produced in test experiments which allows us to use this model for transmission determination and tuning of the separator in experiments aimed at SHE investigations.

DGFRS-2—A gas-filled recoil separator for the Dubna Super Heavy Element Factory

The main goal of development of the new Dubna Gas-Filled Recoil Separator (DGFRS-2) is to sufficiently improve the efficiency of studies on heavy and superheavy nuclei at the Super Heavy Element Factory of the Flerov Laboratory of Nuclear Reactions (FLNR) at the Joint Institute for Nuclear Research. The use of beams with the intensity up to 6×1013 s−1 (10 pμA) delivered by the DC280 cyclotron requires an effective setup providing high suppression of background reaction products. The described gas-filled separator is optimized for synthesis and study of heavy isotopes produced in complete fusion reactions of massive nuclei. Basic characteristics of DGFRS-2, as well as the results of the first test experiments, are presented. In comparison to the DGFRS-1, the transmission efficiency was doubled, and the background was reduced by a factor 200.

Synthesizing and Studying Superheavy Nuclei 294Ts and 294Og

Experimental works performed on the Dubna gas-filled magnetic recoil separator (GFS) to study the reactions of formation of 294Ts and 294Og nuclides and the properties of their radioactive decay are described. Future possibilities of synthesizing new elements with Z = 119 and Z = 120 and performing experiments to study the properties of heavier isotopes of known superheavy nuclei Lv (Z = 116), Ts (Z = 117), and Og (Z = 118) are discussed.

A Flexible Real-Time Algorithm for the RedStorm#2 Software Package

A new real-time algorithm for the C++ RedStorm#2 software package to radically suppress the background at the total fusion reactions on the new DC-280 cyclotron (Flerov Laboratory of Nuclear Reactions, Joint Institute for Nuclear Research) is reviewed. This algorithm assumes some flexibility with the execution of the data set program in contrast to earlier algorithms applied at the Dubna gas-filled recoil separator. Namely, the experimenter sets the allowable value of the relative real-time loss of target irradiation, but not a fixed time interval between the implantation of a nuclide into a silicon detector and its α-decay. The block diagram of the algorithm execution is presented and possible branches of the algorithm are considered. The edge effects and their role in collecting a nonequilibrium charge from the p–n junction of the DSSSD detector (double-sided silicon strip detector) of the focal plane of recoil separator are also considered.

Selection of rotating entrance window and target parameters for a gas-filled separator operating at high intensive heavy ion beams

The durability of the targets and entrance window foils of the Dubna gas-filled recoil separator working at high intensive heavy ion beams was considered. The temperature, as a factor determining their durability, was calculated as a function of time in the conditions of their pulsed heating by heavy ion beam, followed by radiative cooling by radiation emitted from their surfaces. The temperatures were calculated for 48Ca heavy ion beam with intensity expected for the DC-280 accelerator. With the temperature calculated as a function of time, optimal parameters of the entrance window and target operation were chosen.

Fusion reaction Ca48+Bk249 leading to formation of the element Ts ( Z=117 )

The heaviest currently known nuclei, which have up to 118 protons, have been produced in 48Ca induced reactions with actinide targets. Among them, the element tennessine (Ts), which has 117 protons, has been synthesized by fusing 48Ca with the radioactive target 249Bk, which has a half-life of 327 d. The experiment was performed at the gas-filled recoil separator TASCA. Two long and two short α decay chains were observed. The long chains were attributed to the decay of 294Ts. The possible origin of the short-decay chains is discussed in comparison with the known experimental data. They are found to fit with the decay chain patterns attributed to 293Ts. The present experimental results confirm the previous findings at the Dubna Gas-Filled Recoil Separator on the decay chains originating from the nuclei assigned to Ts.

Development of the Active Correlation Method: Theoretical-Methodological Aspect

The Dubna Gas-Filled Recoil Separator (DGFRS) of the Flerov Laboratory of Nuclear Reactions (FLNR), JINR, is the most advanced facility for the synthesis and study of new superheavy nuclei. In the recent years, new elements with Z = 114 to 118 (Fl, Mc, Lv, Ts, Og) have been successfully synthesized. The DGFRS detection system and a unique method of active correlations for background suppression have played a significant role in these discoveries. Theoretical-methodological aspects of further development of the active correlation method are considered, especially in view of the upcoming commissioning of the new FLNR high-intensity DC-280 cyclotron for acceleration of heavy ions and the new gas-filled recoil separator. A numerical model of the edge effects between the neighboring strips on the p–n junction side of the DSSSD detector is presented. The corresponding empirical examples are given. A more flexible real-time algorithm is considered as a possible substitute for the current version with the rigidly set parameters. Since stability of the calibration parameters is strongly required for applying the method, the radiation stability factor is also briefly considered.

Search for the heaviest atomic nuclei among the products from reactions of mixed-Cf with a Ca48 beam

The search for new decay chains of oganesson isotopes is presented. The experiment utilized the Dubna Gas Filled Recoil Separator and a highly segmented recoil-decay detection system. The signals from all detectors were analyzed in parallel by digital and analog data acquisition systems. For the first time, a target of mixed californium (51% Cf249, 13% Cf250, and 36% Cf251) recovered from decayed Cf252 sources was produced and irradiated with an intense Ca48 beam. The observation of a new decay chain of Og294 is reported. The prospects for reaching new isotopes Og295,296 are discussed.

Study of the 249-251Cf + 48Ca reactions: recent results and outlook

Experiment aiming at the synthesis of heavy isotopes of Z=118 (Og) using beam of 48Ca and a target of 249-251Cf was undertaken in October 2015 – April 2016 employing the Dubna Gas-Filled Recoil Separator (FLNR JINR). The target of mixed isotopes of 249-251Cf (50.7% of 249Cf, 12.9% of 250Cf, and 36.4% of 251Cf) was irradiated by 48Ca ions at two beam energies of 252 and 258 MeV with the corresponding accumulated beam doses of 1.6×1019 and 1.1×1019. A single event observed at lower beam energy was assigned to the isotope 294Og, the product of the reaction 249Cf(48Ca, 3n); its decay pattern and the observed radioactive properties of the nuclides in the decay chain reproduce in full those observed for 294Og in our earlier experiments of 2002-2005 and 2012. At higher beam energy we observed no decay chains that could be attributed to the isotopes of Og. The possibility of renewal of this experiment in the future is discussed.

On the volatility of nihonium (Nh, Z = 113)

Gas-phase chromatography studies of nihonium (Nh, $Z=113$ ) were carried out at the one-atom-at-a-time level. For the production of nihonium, the heavy-ion-induced nuclear fusion reaction of 48Ca with 243Am was used. This leads to isotopes 284,285Nh, as the direct descendants of the $\alpha$ -decaying precursors 288,289Mc. Combining the Dubna Gas-Filled Recoil Separator with gas-phase chromatographic separation, the experiment was sensitive to elemental nihonium and its adsorption behavior on Teflon, theoretically predicted by modern relativistic density functional theory. The non-observation of any decays of Nh after the chemical separation indicates a larger than expected retention of elemental Nh on a Teflon surface.

Synthesis of superheavy elements at the Dubna gas-filled recoil separator

A survey of experiments at the Dubna gas-filled recoil separator (Laboratory of Nuclear Reactions, JINR, Dubna) aimed at the detection and study of the “island of stability” of superheavy nuclei produced in complete fusion reactions of 48Са ions and 238U–249Cf target nuclei is given. The problems of synthesis of superheavy nuclei, methods for their identification, and investigation of their decay properties, including the results of recent experiments at other separators (SHIP, BGS, TASCA) and chemical setups, are discussed. The studied properties of the new nuclei, the isotopes of elements 112–118, as well as the properties of their decay products, indicate substantial growth of stability of the heaviest nuclei with increasing number of neutrons in the nucleus as the magic number of neutrons N = 184 is approached.

New trends in the development of “active correlations” technique

With reaching extremely high intensities of heavy-ion beams, new requirements for the detection system of the Dubna Gas-Filled Recoil Separator (DGFRS) will definitely be set. One of the challenges is how to apply the “active correlations” method [1–6] to suppress beam associated background products without significant losses in the whole long-term experiment efficiency value. Different scenarios and equations for the development of a method according to this requirement are under consideration in the present paper.

Study of neutron-deficient isotopes of Fl in the 239Pu, 240Pu + 48Ca reactions

The results of the experiments aimed at the synthesis of Fl isotopes in the 239Pu + 48Ca and 240Pu + 48Ca reactions are presented. The experiment was performed using the Dubna gas-filled recoil separator at the U400 cyclotron. In the 239Pu+48Ca experiment one decay of spontaneously fissioning 284Fl was detected at 245-MeV beam energy. In the 240Pu+48Ca experiment three decay chains of 285Fl were detected at 245 MeV and four decays were assigned to 284Fl at the higher 48Ca beam energy of 250 MeV. The α-decay energy of 285Fl was measured for the first time and decay properties of its descendants 281Cn, 277Ds, 273Hs, 269Sg, and 265Rf were determined more precisely. The cross section of the 239Pu(48Ca,3n)284Fl reaction was observed to be about 20 times lower than those predicted by theoretical models and 50 times less than the value measured in the 244Pu+48Ca reaction. The cross sections of the 240Pu(48Ca,4-3n)284,285Fl at both 48Ca energies are similar and exceed that observed in the reaction with lighter isotope 239Pu by a factor of 10. The decay properties of the synthesized nuclei and their production cross sections indicate rapid decrease of stability of superheavy nuclei with departing from the neutron number N=184 predicted to be the next magic number.

ORNL actinide materials and a new detection system for superheavy nuclei

The actinide resources and production capabilities at Oak Ridge National Laboratory (ORNL) are reviewed, including potential electromagnetic separation of rare radioactive materials. The first experiments at the Dubna Gas Filled Recoil Separator (DGFRS) with a new digital detection system developed at ORNL and University of Tennessee Knoxville (UTK) are presented. These studies used 240 Pu material provided by ORNL and mixed-Cf targets made at ORNL. The proposal to use an enriched 251 Cf target and a large dose of 58 Fe beam to reach the N = 184 shell closure and to observe new elements with Z = 124, 122 and 120 is discussed.

Automation of experiments at Dubna Gas-Filled Recoil Separator

Approaches to solving the problems of automation of basic processes in long-term experiments in heavy ion beams of the Dubna Gas-Filled Recoil Separator (DGFRS) facility are considered. Approaches in the field of spectrometry, both of rare α decays of superheavy nuclei and those for constructing monitoring systems to provide accident-free experiment running with highly radioactive targets and recording basic parameters of experiment, are described. The specific features of Double Side Silicon Strip Detectors (DSSSDs) are considered, special attention is paid to the role of boundary effects of neighboring p–n transitions in the “active correlations” method. An example of an off–beam experiment attempting to observe Zeno effect is briefly considered. Basic examples for nuclear reactions of complete fusion at 48Ca ion beams of U-400 cyclotron (LNR, JINR) are given. A scenario of development of the “active correlations” method for the case of very high intensity beams of heavy ions at promising accelerators of LNR, JINR, is presented.

Development of “active correlation” technique

With reaching to extremely high intensities of heavy-ion beams new requirements for the detection system of the Dubna Gas-Filled Recoil Separator (DGFRS) will definitely be set. One of the challenges is how to apply the active correlations method to suppress beam associated background products without significant losses in the whole long-term experiment efficiency value. Different scenarios and equations to develop the method according this requirement are under consideration in the present paper. The execution time to estimate the dead time parameter associated with the optimal choice of the life-time parameter is presented.

Stopping powers and ranges for the heaviest atoms

Slowing down and stopping of the heaviest atoms, products of the fusion–evaporation nuclear reactions, during their passage through the Dubna gas-filled recoil separator has been studied using TRIM simulations. The study is important for experiments on the synthesis of super-heavy elements (SHEs) with atomic numbers around ZP=114 produced with accelerated heavy ion (HI) beams and extracted with a separator for their detection. The average Mylar stopping power (SP) values obtained with the simulations for HIs with 82⩽ZP⩽92 reveal almost the same magnitudes, allowing extrapolation to the region of ZP>92. Similar extrapolation of the ranges in an He+Ar gas mixture leads to rather small values for the heaviest atoms (ZP⩾102) as compared to the range for U. The extrapolated values have large uncertainties and should be verified with different approaches. Available SP data obtained for HIs with 18⩽ZP⩽92 at energies E<20MeV/u have been analysed within various semi-empirical approaches. The analysis has shown that existing parameterizations give Mylar SP values for ZP⩾82 that are very different from each other at energies of interest (around 0.1MeV/u). We propose to use a general approach based on the HI effective charge parameterization obtained with available SP data for HIs and the hydrogen SP and effective charge corresponding to the same velocity and stopping medium as those for HIs. In this manner, the SPs of the gases H2, He, C4H10, and Ar as well as those of the solids Mylar, C, Al, and Ti have been obtained for any atoms with ZP⩾18 (including the heaviest ones) at their reduced velocities 0.03⩽Vred⩽5.0. The SP values derived in such a way seem to be more reliable compared to the existing semi-empirical calculations and can be used in the conditioning of experiments on the synthesis of SHEs.

Experiments on the synthesis of superheavy nucleiFl284andFl285in thePu239,240+Ca48reactions

Irradiations of $^{239}\mathrm{Pu}$ and $^{240}\mathrm{Pu}$ targets with $^{48}\mathrm{Ca}$ beams aimed at the synthesis of $Z=114$ flerovium isotopes were performed at the Dubna Gas Filled Recoil Separator. A new spontaneously fissioning (SF) isotope $^{284}\mathrm{Fl}$ was produced for the first time in the $^{240}\mathrm{Pu}+^{48}\mathrm{Ca}$ (250 MeV) and $^{239}\mathrm{Pu}+^{48}\mathrm{Ca}$ (245 MeV) reactions. The cross section of the $^{239}\mathrm{Pu}(^{48}\mathrm{Ca},3n)^{284}\mathrm{Fl}$ reaction channel was about 20 times lower than predicted by theoretical models and about 50 times lower than the maximum fusion-evaporation cross section for the $3n$ and $4n$ channels measured in the $^{244}\mathrm{Pu}+^{48}\mathrm{Ca}$ reaction. In the $^{240}\mathrm{Pu}+^{48}\mathrm{Ca}$ experiment, performed at 245 MeV in order to maximize the $3n$-evaporation channel, three decay chains of $^{285}\mathrm{Fl}$ were detected. The $\ensuremath{\alpha}$-decay energy of $^{285}\mathrm{Fl}$ was measured for the first time and decay properties of its descendants $^{281}\mathrm{Cn}, ^{277}\mathrm{Ds}, ^{273}\mathrm{Hs}, ^{269}\mathrm{Sg}$, and $^{265}\mathrm{Rf}$ were determined with higher accuracy. The assignment of SF events observed during the irradiation of the $^{240}\mathrm{Pu}$ target with a 250 MeV $^{48}\mathrm{Ca}$ beam to $^{284}\mathrm{Fl}$ decay is presented and discussed. The cross sections at both $^{48}\mathrm{Ca}$ energies are similar and exceed that observed in the reaction with the lighter isotope $^{239}\mathrm{Pu}$ by a factor of 10. The decay properties of the synthesized nuclei and their production cross sections indicate a rapid decrease of stability of superheavy nuclei as the neutron number decreases from the predicted magic neutron number $N=184$.

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.

Experimental studies of the249Bk + 48Ca reaction including decay properties and excitation function for isotopes of element 117, and discovery of the new isotope277Mt

Studies of superheavy nuclei produced in the ${}^{249}$Bk + ${}^{48}$Ca reaction were performed using the Dubna Gas Filled Recoil Separator. The cross section for the production of ${}^{293}$117 and ${}^{294}$117 isotopes was measured at five excitation energies of the ${}^{297}$117 compound nucleus ranging from 30 to 48 MeV and yielding maximum values of 1.1${}_{\ensuremath{-}0.6}^{+1.2}$ pb for the $3n$ and 2.4${}_{\ensuremath{-}1.4}^{+3.3}$ pb for the $4n$ reaction channels. Alpha emission from ${}^{281}$Rg competing with spontaneous fission ($\ensuremath{\alpha}$/SF decay probability 1:9) was observed for the first time leading to the identification of the new isotope ${}^{277}$Mt (${T}_{\mathrm{SF}}\ensuremath{\approx}5$ ms). The measured decay properties are in good agreement with those expected based on the properties of neighboring even-$Z$ and odd-$Z$ nuclei. The $\ensuremath{\alpha}$ energies and half-lives of odd-$Z$ isotopes observed in the ${}^{293}$117 and ${}^{294}$117 decay chains together with results obtained for lower-$Z$ superheavy nuclei demonstrate enhanced stability with increasing neutron number toward the predicted new magic number $N=184$.