I. V. Shirokovsky

Experimental Studies of the Formation and Radioactive Decay of Isotopes with Z = 104—109

B y Y U . T s . O G A N E S S I A N , M. H U S S O N N O I S * , A . G . D E M I N , Y U . P . K H A R I T O N O V , H . B R U C H E R T S E I F E R * * , O . C O N S T A N T I N E S C U * * * , Y U . S . K O R O T K I N , S . P. T R E T Y A K O V A , V . K . U T Y O N K O V , I. V . S H I R O K O V S K Y , and J. ESTEVEZ, Laboratory of Nuclear Reactions, Joint Institute for Nuclear Research, Dubna, USSR

On the stability of the nuclei of element 108 withA=263–265

In bombarding209Bi and207, 208Pb targets by55Mn and58Fe ions the yields of “cold fusion” reactions have been determined using a sensitive technique for detectingT1/2≧1 ms spontaneous fission and theα-decay of heavy actinide elements. It has been shown that theA = 263–265 isotopes of element 108, including the even-even isotope264108, undergo mainlyα-decay. The obtained results, together with the known data on the properties of the isotopes of elements 104 and 106, provide evidence for the enhanced stability of theZ=108 nuclei against spontaneous fission.

On the properties of the element 106 isotopes produced in the reactions Pb+54Cr

In54Cr ion bombardments of targets prepared from enriched lead isotopes the spontaneous fission probability for the isotopes of element 106 formed in the reactions Pb(54Cr, 1,2n)106 was investigated. The spontaneous fission half-life of the doubly-even isotope260106 has been shown to beTSF≳5 ms. The half-lives of neighbouring oddmass isotopes have also been estimated:TSF(259106)≳0.1 s andTSF(261106)≳0.4 s. The results of the experiments are discussed in comparison with the latest theoretical calculations.

Development of a new SARA/IGISOL technique for the study of short-lived products from heavy-ion-induced fusion—evaporation reactions

Abstract A new ion guide chamber has been designed for heavy-ion (HI) induced fusion—evaporation reactions. Its principle is based on the beam projectiles-evaporation residues separation which is achieved by taking advantage of their very different angular distributions after passing a thick target. In this work it is proved that the IGISOL technique can be applied with an overall efficiency > 10 −3 for the study of mass-separated exotic nuclei produced in (HI, x n) reactions.

Observation of Delayed Nuclear Fission in the Region of 180Hg

The formation of nuclides undergoing delayed fission with half-lives in the range of T1/2 1 s has been observed in fusion-evaporation reactions induced by 230 MeV 40Ca and 40Ar projectiles on isotopically enriched targets of 144Sm, 147Sm and 150Sm. The fission activity with T1/2 = 0.70+0.12-0.09 s produced in the 144Sm + 40Ca reaction has been found to be the most striking one. The origin of this activity is explained as being due to the β-delayed fission occurring in the chain . The discovery of β-delayed fission in the Hg region opens up valuable prospects for extending low-energy (E* ≤ Qβ+(EC)) fission studies to neutron-deficient nuclei of many elements belonging to the translanthanide part of the Mendeleev periodic table.

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.

Spontaneous fission of light californium isotopes produced in 206,207,208Pb + 34,36S reactions; new nuclide 238Cf

Abstract In bombardments of 206,207,208Pb with 34S and 206Pb with 36S, we identified a new spontaneously fissioning isotope 238Cf with T sf ≈ T 1 2 = 21 ± 2 ms and obtained evidence of the production of a new isotope 237Cf with T 1 2 = 2.1 ± 0.3 s . The spontaneous-fission (SF) decay mode was established for 240Cf; its SF branch was estimated to be bsf ∼ 2 × 10−2. We measured also bsf ⩽ 1.4 × 10−4 for 242Cf and estimated bsf ∼ 10−1 for 237Cf. The production cross sections of 238Cf in the 206,207,208 Pb + 34 S reactions were measured to be in the range of 0.3 to 1.1 nb. Finally, we probed the influence of the neutron excess in the N = 20 projectile 36S on cross sections of fusion-evaporation reactions occurring on lead targets.