M. P. Ivanov

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

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.

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

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.

Complete and incomplete fusion of 6Li ions with Bi and Pt

Experimental excitation functions are presented for fusion–evaporation and transfer reactions in the interaction of 6Li with Bi and Pt targets. The data on the cross sections obtained for the produced evaporation residues are in agreement with calculations within the framework of the statistical model using the ALICE-MP code. Enhancement of the cross section for deuteron transfer reactions is observed at energies below the Coulomb barrier. The results are discussed from the point of view of how the nuclear cluster structure influences the probability of interaction at near-barrier energies.

Investigation of quaternary fission of 248Cm and 252Cf using two CsI(Tl) counters

The simultaneous emission of two light charged particles p, t, cx in the spontaneous fission of 248Cm and “‘Cf was investigated. Identification of the charged particles in triple and quaternary fission was performed by pulse shape analysis method using two CsI(T1) counters. The emission probabilities for two light charged particles per binary fission for these nuclei were determined. The energy spectra of alpha particles measured in quatemary fission were compared with triple alpha particle spectra. 2. The method Investigation of light charged particle (LCP) emission in spontaneous fission is a subject of numerous experimental and theoretical works [l]. The main reason for these studies is to obtain useful information on both the configuration and the dynamics of the fissioning nucleus in the final stage of the fission reaction. The probability if triple fission is not so high and contains about 2-3 counts per 10’ acts of binary fission. Ternary fission with the emission of long range alpha particles (LRA), (E&,) - 16 MeV, is well studied because the LRA emission probability is about 90% of the total LCP yields. Two identical detectors on the basis of CsI(T1) 020 X 0.5 mm2 (Harshaw Chemical Company) and a photomultiplier FEU-176 (Novosibirsk, Russia) with a photocathode diameter of 44 mm and the spectral range of 300-850 nm were used. They were positioned at a distance of 4 mm on opposite sides of the source, see Fig. 1. The total geometry New important information can be extracted from quatemary fission accompanied by simultaneous emission of two light charged particles. At present only two experimental works on this topic are known. Quatemary fission in the ?J + n,,, reaction and spontaneous fission of *‘*Cf were studied in ref. [2,3], respectively. It was found [3] that o-o coincidences were ~80% and a-t = 16% of the total yields and the summary probability of quatemary fission of *?f is not higher than 2 X IO-’ per binary fission. Using high intensity sources (~~10~ ff/s) and A&E semiconductor detectors for particle registration with a small geometrical efficiency the measurements were continued for several months. In this work we attempted to investigate quatemary fission of 248Cm and ?Zf with the intensity of -3 X 10’ ff/s using a high efficiency method

Total cross sections for 4,6He + 28Si reactions measured at 10–28 MeV/A

New results on the energy dependence of the total cross section (σR) for 6He scattering on 28Si in the incident energy range 10–28 MeV/A are obtained. The α-particle-production cross sections for the 28Si(6He, 4He)X channel are measured as well. The secondary beam of 6He with an intensity of up to 5×104 particle/s was generated by bombarding a thick beryllium target with ∼32-MeV/A7Li ions. In the energy region below 17 MeV/A, σR increases sharply. The experimental dependences of the total cross sections are compared with the results of σR calculations using the double-folding potential within the optical model. The energy dependence of σR for 6He differs from that for the neighboring nuclei, which can be associated with the structural features of the former nucleus. The energy spectra of α particles produced in the 6He interactions with silicon indicate two mechanisms of their production: transfer reaction and 6He breakup in the field of the 28Si nucleus.

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

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.

A setup for measuring total cross sections of nuclear reactions

An experimental technique and a setup for measuring the energy dependence of the total cross sections of nuclear reactions with stable and radioactive nuclear beams at kinetic energies approaching the Coulomb barriers are described. The modified transmission method, complemented with γ-ray detection in the 4π geometry and pulse-shape discrimination of particles by a semiconductor detector, is used.

Active collimators in experiments with exotic nuclear beams

The active collimator method for experiments with exotic nuclear beams at energies near the Coulomb barrier of nuclear reactions is described. The technique is based on the use of detectors composed of microchannel plates (MCPs) and thin strips of metal foils (Au, Ag, Al) with a thickness of 200–300 μg/cm2 or less, which are oriented along axes X and Y (axis Z is collinear to the beam axis). Exotic nuclei passing through the foils are detected by the MCPs by electron emission from these foils, which provides a means for obtaining spatial information on the particle trajectory and the time mark of an event. The proposed setup is characterized by the smallest (of the available techniques) material budget on the particle path (≤8.0 × 1017 atoms/cm2) in the zone of detected particles, the high efficiency (ɛ ∼ 90% for every MCP), and the time resolution sufficient for performing additional identification of radioactive nuclei using the time-of-flight method.

Investigation of the fragmentation of 20Ne and 40Ar ions at the COMBAS setup

Properties of the COMBAS fragment separator are compared with respective properties of similar setups. Results of experiments aimed at obtaining products of one-proton-stripping reactions induced by a beam of 40Ar ions with an energy of 35 MeV/A and two-neutron-stripping reactions induced by a beam of 20Ne ions with an energy of 52 MeV/A are presented. A high resolution of the fragment separator in obtaining secondary neutron-rich 39Cl and neutron-deficient 18Ne ion beams is demonstrated.

Studying the excitation function of the full cross section of a reaction using a modified transmission technique: Initial results

A method for direct and model-independent measurements of the full cross section for the reaction σR is described, along with the physical setting and the results from measuring σR(E) for the reaction 6He+197Au in the energy region near the Coulomb barrier. A comparison of cross section values measured by the direct method with a summed cross section of the reaction’s xn evaporation channels (x = 2–7) and a neutron’s disruption-pickup channels measured by means of activation analysis is presented, along with the familiar value in the literature for a full cross section obtained from processing with an optical model of experimental differential cross sections of the elastic scattering of 6He on 197Au.