A.G. Demin

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.

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

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.

New mercury isotopes

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

Abstract Products of the reactions 170 Yb+ 20 Ne, 147 Sm+ 40 Ar and 169 Tm+ 22 Ne were collected by the helium sweeping technique, and their α-radiation analysed with a semiconductor spectrometer. The following measurements were carried out: energies of α-particles E α , half-lives T 1 2 of the observed emitters and the excitation functions of the reactions leading to their production. The following observed new α-groups have been identified as new isotopes of mercury E α ( MeV ) T 1 2 ( s ) 179 Hg 6.076±0.012 3.5±0.4 180 Hg 5.964±0.012 5.9±0.8 182 Hgα 0 5.838±0.012 9.6±0.2 182? Hgα 1 ≈ 5.74 ≈ 11 183? Hg ≈ 5.59 ≈ 26 Data were also obtained on the α-decay of 185 Hg, 179 Au, 181 Au, 183 Au, 185 Au, 176 Pt, 177 Pt, 178 Pt and 180 Pt, which agree with previous results.

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.

Experiments on the synthesis of neutron-deficient isotopes of kurchatovium in reactions with accelerated50Ti ions

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.

An energy shift between the fusion barrier and interaction barrier in heavy nuclear systems

ConclusionsA number of conclusions can be drawn from the aggregate of experimental results.The method of synthesis of transfermium elements in the irradiation of lead isotopes with ions with mass AI≥40 atomic units, investigated earlier [12] for the example of the reaction Pb(40Ar, xn)Fm, is also extremely effective using50Ti ions. The reactions Pb(40Ar, 2n)Fm and Pb(50Ti, 2n)Ku have approximately equal cross sections, which might have been expected on the basis of theoretical estimates. This permits us to hope that this method can be used successfully for the synthesis of heavier elements in the reactions induced by54Cr,55Mn, and58Fe ions.In all probability, the substantial changes in the systematics of the half-lives for even-even isotopes of kurchatovium are associated with the structure of the barriers to fission of these nuclei. From this standpoint, it seems important to investigate the properties of more neutron-deficient isotopes of kurchatovium, using, in particular, the reactions with204Pb, and to attempt to advance into the region of elements with Z≥ 106. On the other hand, a detailed theoretical analysis must be made of the data obtained on the basis of the modern theory of nuclear fission, which, in our opinion, will aid in a more reliable prediction of the properties of heavy and ultraheavy elements.