Search

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.

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 $N=184$.

Experimental measurements of the basic physical properties of the melt NaF-LiF-BeF2 are presented as validation of the concept of a molten-salt reactor for burning actinides from spent fuel from light-water reactors. Compositions which are characterized by the minimal molar fraction LiF 15–17% and BeF2 25–27% and meet the special requirements for a fuel salt for the concept under study are found. The melts of the fluorides of three metals have an acceptable melting temperature (

The results of tests of the compatibility of nickel-based alloys and metal-fluoride melts are generalized for molten-salt reactors.

PC-based detection system for heavy element research at the Dubna Gas-filled Recoil Separator is presented. It allows to operate with the experimental data obtained from the detection system of GFRS which consists of 12-strip silicon position sensitive detector and two multiwire low-pressure proportional gaseous chambers to generate time-of-flight signal of incoming charge particles. Special attention is payed to the detection of genetically linked recoil-alpha sequences in a real-time mode. The results of the experiment aimed at the synthesis of new Z=113–115 elements with the mentioned technique are presented in brief.

The reaction of 249 Bk with 48 Ca have been reinvestigated to provide new evidence for the discovery of element 117 on a larger number of events. The experiments were performed at five projectile energies and with a total beam dose of 48 Ca of about 4.6×10 19 . Two isotopes 293,294 117 were synthesized in the 249 Bk+ 48 Ca reaction, providing excitation functions and α-decay spectra of the produced isotopes that establishes these nuclei to be the products of the 4n- and 3n-evaporation channels, respectively. Decay properties of 293,294 117 and of all the daughter products agree with the data of the experiment in which these nuclei were synthesized for the first time in 2010. The new 289 115 events, populated by α decay of 293 117, demonstrate the same decay properties as those observed for 289 115 produced in the 243 Am( 48 Ca,2n) reaction thus providing cross- bombardment evidence. In addition, a single decay of 294 118 was observed from the reaction with 249 Cf - a result of the in-growth of 249 Cf in the 249 Bk target.

Experimental work on transferring compact weapons plutonium by pyrochemical methods into its compounds was generalized. Powdered plutonium dioxide was obtained by interaction with moist air at 480–500°C. PuH3 was synthesized by interaction of the metal with hydrogen at 280°C. PuF3 was formed in an interaction of dioxides with HF and H2. The reaction of plutonium hydride with HF also resulted in plutonium trifluoride. PuF4 was synthesized by interaction of PuO2 with HF. In summary, compact plutonium can be transfered by water-free methods into its powdered compounds, which can be used in investigations for developing new types of fuel elements for power reactors. 6 figures, 2 tables, 16 references.

In bombardments of 248Cm with 22Ne we discovered two new isotopes, 265106 and 266106, by establishing genetic links between α decays of the 106 nuclides and spontaneous fission (SF) or α decays of the daughter (or granddaughter) nuclides. For 266106 we measured Eα = 8.63 ± 0.05 MeV followed by the SF decay of 262104 for which we measured a half-life value of 1.2+1.00.5 s. For 265106 we measured Eα = 8.71–8.91 MeV. We estimated α half-lives of 10–30 s for 266106 and 2–30 s for 265106 with SF branches of approximately 50% or less. The decay properties of 266106 indicate a large enhancement in the SF stability of this N = 160 nuclide and confirm the existence of the predicted deformed shells N = 162 and Z = 108.

Thirty‐four new nuclides with Z = 104–116, 118 and N = 161–177 have been synthesized in the complete‐fusion reactions of 238U, 237Np, 242,244Pu, 243Am, 245,248Cm, and 249Cf targets with 48Ca beams. The masses of evaporation residues were identified through measurements of the excitation functions of the xn‐evaporation channels and from cross bombardments. The decay properties of the new nuclei agree with those of previously known heavy nuclei and with predictions from different theoretical models. A discussion of self‐consistent interpretations of all observed decay chains originating from the parent isotopes 282,283112, 282113, 286–289114, 287,288115, 290–293116, and 294118 is presented. Decay energies and lifetimes of the neutron‐rich superheavy nuclei as well as their production cross sections indicate a considerable increase in the stability of nuclei with the approach to the theoretically predicted nuclear shells with N = 184 and Z = 114.

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.

The present paper reports the results of an attempt aimed at the synthesis of element 118 in the reaction {sup 249}Cf({sup 48}Ca,3n){sup 294}118. The experiment was performed employing the Dubna Gas-filled Recoil Separator and the U400 heavy-ion cyclotron at FLNR, JINR, Dubna. In the course of a 2300-hour irradiation of an enriched {sup 249}Cf target (0.23 mg/cm{sup 2}) with a beam of 245-MeV {sup 48}Ca ions, we accumulated a total beam dose of 2.5 x 10{sup 19} ions. We detected two events that may be attributed to the formation and decay of nuclei with Z=118. For one event, we observed a decay chain of two correlated {alpha}-decays with corresponding energies and correlation times of E{sub {alpha}1} = 11.65 {+-} 0.06 MeV, t{sub {alpha}1} = 2.55 ms and E{sub {alpha}2} = 10.71 {+-} 0.17 MeV, t{sub {alpha}2} = 42.1 ms and, finally, a spontaneous fission with the sum of the kinetic energies of the fission fragments E{sub tot} = 207 MeV (TKE {approx} 230 MeV) and t{sub SF} = 0.52 s. In the second event chain, the recoil nucleus decayed into two fission fragments with E{sub tot} = 223 MeV (TKE {approx} 245 MeV) 3.16 ms later, without intervening {alpha} decays.more » The probabilities that these events were caused by the chance correlations of unrelated signals are negligible. Both events were observed at an excitation energy of the compound nucleus {sup 297}118 of E* = 30.0 {+-} 2.4 MeV, close to the expected maximum of the 3n-evaporation channel. The relationship between the decay energy Q{sub {alpha}} and decay period T{sub {alpha}} shows that sequential {alpha}-transitions in the first event correspond to the decay chain with Z = 118{_}116{_}114. Decay characteristics of the newly observed nuclides are compared with radioactive decay properties of the even-even isotopes with Z = 116, 114 and 112 previously produced in the reactions {sup 244}Pu, {sup 248}Cm + {sup 48}Ca and calculations made in various nuclear models.« less

We measured the average electronic charge $〈q〉$ of heavy ions with atomic numbers $Z=89$ through 116 traversing dilute hydrogen gas with velocities ranging from 1 to 2.6 times the Bohr velocity. We observe a strong linear dependence of the average charge on the velocity of the ions, and only a weak dependence on their atomic number Z. A more detailed examination of the experimental results shows that the $〈q〉$ values reflect the influence of the $5f$ electron shell on the properties of these elements.

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.

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.