Yu. V. Lobanov

Synthesis of several isotopes of fermium and determination of their radioactive properties

The article describes the results of experiments in which U238 and U235 were irradiated with accelerated ions of O16, using an internal beam of the OIYaI 310-cm cyclotron. In the procedure used, the products of the nuclear reactions were collected by means of a stream of gas, and the α-decay was then recorded with (Si+Au) detectors. The excitation functions of the (O16, xn)reactions were studied. Four isotopes of fermium, with mass numbers of 250, 249, 248, and 246, were investigated. Data on the energies and half-life periods of these isotopes were obtained.

On the synthesis of element 105

Abstract In bombardments of a 243 Am target with 22 Ne ions a spontaneous fission activity with a half-life of {ce:inline-formula}1.8±0.6s{/ce:inline-formula} has been observed. The yield corresponds to a formation cross section of {ce:inline-formula}(5.0±1.5) × 10 −34 cm 2 {/ce:inline-formula}, for the spontaneous fission decay. Integral angular distributions of products from the reaction 243 Am + 22 Ne and the formation cross section for the new isotope as a function of the bombarding ion energy have been measured. From these data and the results of test experiments it is concluded that the atomic number of the new spontaneous fission activity with {ce:inline-formula}T 1/2≈ 2s{/ce:inline-formula}, is 105. The spontaneous fission decay of the isotopes 256,257 103 has also been investigated. The spontaneous fission half-lives of these nuclei are shown to be larger than 10 5 s.

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.

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.

Study of the stability of the ground states and K-isomeric states of 250Fm and 254102 against spontaneous fission

By employing the 249Cf(4He, 3n) and 208Pb(48Ca, 2n) reactions, experiments to study the stability against spontaneous fission of the nuclides 250Fm and 254102 as well as of the two-quasi-particle (2q-p) K isomers 250mFm (T1/2 = 1.8 ± 0.1 s) and 254m102 (T1/2 = 0.28 ± 0.04s) have been performed. The ground-state spontaneous fission of the two nuclides has been discovered and the corresponding branching ratios bsf and partial half-lives Tsf, respectively, have been determined to be: (6.9 ± 1.0) × 10−5, 0.83 ± 0.15 yr for 250Fm; (1.7 ± 0.5) × 10−3, (3.2 ± 0.9) × 104s for 254102. As a by-product of these studies, new data about cross sections of the 206,208Pb(48Ca, xn) reactions have been obtained. Experiments designed to search for the spontaneous fission decay of the 2q-p K-isomeric states in 250Fm and 254102 have not revealed the effect in question. The lower limits of the ratios of the partial spontaneous fission half-lives for the 2q-p K-isomeric states to those for the respective ground states, Tsf*/Tsf, have been established to be ≥ 10−1 for 250mFm/250Fm and ≥ 5 × 10−3 for 254m102/254102. This means that the stability of the 2q-p K-isomeric states in 250Fm and 254102 against spontaneous fission is practically not inferior to that of the ground states of these nuclei. In accord with the experimental findings, the theoretical estimates of Tsf*/Tsf made in the present paper show that, due to the influence of the specialization and blocking effects on the potential energy and the effective mass associated with fission, spontaneous fission from 2q-p K-isomeric states cannot be facilitated but, on the contrary, should be essentially hindered compared with ground-state spontaneous fission.

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.

Refinement of data on the volatility of octavalent plutonium in the form of tetraoxide PuO4

The previous determinations [1–5] of the enthalpy of adsorption of PuO4 on the quartz surface, which was calculated with an error of ±6 kJ mol−1, are insufficiently accurate. Gas thermochromatographic experiments aimed to refine the temperature of PuO4 deposition under definite temperature and gas-dynamic conditions were performed. The carrier gas was an He-O2 mixture (1: 1). From the data obtained, more correct value of −δHa0 was calculated: 43 ± 4 kJ mol−1. The standard enthalpy of sublimation δH298, subl0 of PuO4 was estimated at 35 ± 10 kJ mol−1 using the linear correlation between the enthalpies of adsorption and sublimation of transition metal oxides.

Formation of Unusual U, Pu, and Cf Oxide Species under Conditions of Gas Thermochromatography

Experiments were performed on the preparation of volatile oxidation products from trace amounts of plutonium and on thermochromatographic (TC) separation of these species, taking into account the previously obtained data on different behaviour of some micro- and macrocomponents in the gas phase. The volatility of products formed by thermal oxidation of traces of U and 249Cf with an He-O2 (1 : 1 vol.) mixture was studied with the aim to simulate the behavior of Pu(IV) and Pu(VI) oxides. Hollow quartz columns were used. Under the experimental conditions, volatile UO2, UO3, and 249CfO2 were formed; the dioxides were deposited at 450±25°C, and UO3, at 250±25°C. The enthalpies of adsorption -ΔHa0 of these compounds on quartz were calculated. Experimental evidence of existence in the gas phase of volatile uranic acid was obtained. The TC behavior of 238,239Pu traces was studied under conditions of thermal oxidation in an He-O2 flow at the oxygen concentration cO2 varied from 50 to 10-7%. In the cO2 range of 50-5 × 10-2%, deposition of volatile Pu species at 460±25 and 250±25°C was observed. The identical deposition temperatures and close values of -ΔHa0 of the products of U and Pu oxidation suggest the transfer of Pu in the form of PuO2 and PuO3. In the cO2 range from 50 to 1%, an abnormally volatile Pu species was detected, which was deposited at -105±25°C. Its -ΔHa0, 41±6 kJ mol-1, was close to that of OsO4. The deposition zones of 185OsO4 and of the highly volatile Pu species were superimposed. As cO2 decreased, the yield of the highly volatile Pu species decreased; a similar trend is also characteristic of OsO4. This volatile Pu species was tentatively identified as Pu(VIII) oxide, PuO4.

Synthesis of260Ku in the irradiation of249Bk by15N ions

Conclusions1.In the249Bk(15N, 4n)260Ku reaction an isotope of kurchatovium (element 104) is obtained which decays by spontaneous fission with a half-life of ∼0.1 sec. This result is in complete agreement with conclusions of earlier works [1, 3, 5] in which260Ku was synthesized in other combinations of target and particle:242Pu+22Ne,246Cm+18O.2.The failure to observe260Ku with T1/2≈0.1 sec at Berkeley [10, 11] resulted from improper experimental arrangements and data processing.3.The detection of the spontaneous fission of nuclei of an unknown isotope when background is properly considered and suppressed permits the identification of the Z and A of the nuclei under study.