S.P. Tretyakova

Experiments on the production of fermium neutron-deficient isotopes and new possibilities of synthesizing elements with Z > 100

Abstract Experimental and theoretical data on the production of the neutron-deficient isotopes 244 Fm and 246 Fm in the bombardment of lead and bismuth isotopes with 40 Ar and 37 C1 ions are presented. By using different lead isotopes, the cross sections for the ( 40 Ar, x n) reactions with x = 1, 2, 3 and 4 have been measured. It is shown that when a target of the “magic” 208 Pb nucleus or its neighbours is bombarded with ions of mass

Experiments on the synthesis of neutron-deficient kurchatovium isotopes in reactions induced by 50Ti Ions

= 40 , the compound nuclei formed appear to be weakly excited and to de-excite by emitting only 2 or 3 neutrons. New possibilities of synthesizing elements with atomic number Z > 100 are being discussed in the framework of the experimental data obtained.

Experiments to produce isotopes of superheavy elements with atomic numbers 114–116 in 48Ca ion reactions

Abstract By bombarding Pb isotopes with 50 Ti ions, two new isotopes of element 104 (kurchatovium) with mass numbers 255 and 256 have been synthesized. The isotope 256 Ku has been formed in the reaction 208 Pb ( 50 Ti, 2n) and has a spontaneous fission half-life of about 5 msec. The odd isotope 255 Ku has been formed with a maximum cross section in the reaction 207 Pb( 50 Ti, 2n) and has a half-life of several seconds. The experimental results obtained, together with the available data on the properties of kurchatovium, substantially change the understanding of the stability of heavy nuclei with respect to spontaneous fission. In this connection the influence of the structure of the fission barrier on the properties of heavy nuclei is discussed. On the basis of the experimental data on the synthesis of Fm and Ku neutron-deficient isotopes in reactions induced by ions heavier than argon, the possibilitie s of synthesizing elements with Z≧106 are investigated.

Synthesis of the new neutron-deficient isotopes 250102, 242Fm, and 254Ku

Experiments have been performed to synthesize isotopes of elements 114–116 in fusion reactions between 48Ca ions and 246, 248Cm, 243Am and 242Pu target nuclei. The detection of reaction products by recording spontaneous fission and α-decay events allowed us to set upper limits of 5 × 10−35 cm2 and ≈ 2 × 10 −34 cm2 on the production cross sections for superheavy elements with 2 h ≦ Tsf ≦ 1 y and 2 h ≦ Tα ≦ 10 d, respectively.

Excitation energy of spontaneously fissioning isomer 242mAm

Abstract The new neutron-deficient isotopes 250102, 242Fm and 254Ku have been synthesized by the reactions 233U(22Ne, 5n), 204Pb(40Ar, 2n) and 206Pb(50Ti, 2n), respectively. It is found that spontaneous fission is the main mode of decay. The spontaneous fission half-lives of these isotopes have been measured to be 0.25 msec, 0.8 msec and 0.5 msec, respectively. Some regularities in the spontaneous fission half-lives of transfermium isotopes are discussed.

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

Abstract The excitation function for the reaction 243 Am(n, 2n) 242 Am leading to the formation of a spontaneously fissioning isomer has been studied. The mono-energetic 8–14.4 MeV. neutrons were obtaained from the reaction D(d, n) 3 He using a gaseous deuterium target. The excitation function has a distinct threshold character. The neutron energy corresponding to the reaction threshold is found to be 9.2±0.3 MeV in the c.m. system. The difference between this value and the threshold of the reaction leading to the ground state, equal to 2.9±0.4 MeV, is considered as the excitation energy of the spontaneously fissioning state of 242 Am.

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

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 spontaneous fission of neutron-deficient isotopes of elements 103, 105 and 107

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.

A study of the spontaneously-fissioning isomer of 242Am through the 241Am(n,γ) reaction

Experiments are described on the determination of the spontaneous fission probability for the isotopes of elements 103, 105 and 107 produced by bombarding Tl, Pb and Bi targets with 50Ti, 51V, 54Cr, 55Mn and 58Fe ions. The upper limit of the spontaneous fission probability is established to be 2% for the isotopes 252, 253103. A new isotope of element 105 with mass number 257(T12 = 5 s) has been produced. In bombardment of 209Bi with 54Cr ions a new spontaneous fission activity with T12 ≈ 1–2 ms has been observed. The experimental data permit identification of this activity as the isotope of the new element 107 with mass number 261, which mainly undergoes α-decay (≈ 80%) resulting in the formation of the isotope 257105. The values of the partial α-decay and spontaneous fission half-lives of the isotopes 257105 and 261107 are compared with the systematics of the properties of transfermium nuclei.

Observation of Delayed Nuclear Fission in the Region of 180Hg

Abstract The excitation function for the 241 Am(n,γ) 242m Am reaction leading to the excitation of the 14 ms, spontaneously fissioning isomer has been studied in the range 0–6.5 MeV. The experimental results can be well explained taking into account the hypothesis of Strutinsky concerning the “shape isomerism”.