Mikhail G. Itkis

Fission modes in the reaction 208Pb(18O,f)

Results of fission fragment mass-energy distributions of the compound 226Th nucleus formed in the subbarrier fusion reaction 18O+H208Pb at an energy of 18O ions Elab=78 MeV are reported. The reaction has been studied twice using two different accelerators, and both sets of experimental data agree quite well. Performed analysis of the experimental data with the use of a new multicomponent method has shown that alongside the well-known modes, i.e., the symmetric (S) and two asymmetric modes standard-one and standard-two, a high-energy mode standard-three has manifested itself. The last named mode appears due to the influence of the close-to-sphere neutron shell with N≈50 in the light fission fragment group. Theoretical calculations of the prescission shapes of the fissioning nuclei 224,226Th confirm this conclusion.

Fission dynamics for capture reactions in 58,64Ni + 208Pb systems: New results in terms of thermal energy and neutron multiplicity correlated distributions

Abstract The neutron multidetector DeMoN has been used to investigate the symmetric splitting dynamics in the reactions 58,64 Ni + 208 Pb with excitation energies ranging from 65 to 186 MeV for the composite system. An analysis based on the new backtracing technique has been applied on the neutron data to determine the two-dimensional correlations between the parent composite system initial thermal energy (ECNth) and the total neutron multiplicity (νtot), and between pre- and post-scission neutron multiplicities (νpre and νpost, respectively). The νpre distribution shape indicates the possible coexistence of fast-fission and fusion-fission for the system 58 Ni + 208 Pb (Ebeam = 8.86 A MeV). The analysis of the neutron multiplicities in the framework of the combined dynamical statistical model (CDSM) gives a reduced friction coefficient β = 23 ± 1225 × 1021 s−1, above the one-body dissipation limit. The corresponding fission time is τf = 40 ± 2046 × 10−21s.

Fission of heavy and superheavy nuclei at low excitation energies

The talk presents the results of an investigation of the main characteristics (mass and energy distributions of fission fragments and multiplicity of neutrons) of the fission of the nuclei of 220 Ra, 226 Th, 256 No, 270 Sg, 286 112 produced in reactions with ions of 18 O, 22 Ne and 48 Ca at energies close to and essentially below the Coulomb barrier. The data obtained show that the form of the mass and energy distributions of the fission fragments of 226 Th and 270 Sg is accounted for by the multimodal nature of the fission. In addition, for 226 Th, a new phenomenon was established there is a significant difference between the numbers of prescission neutrons for symmetric and asymmetric fission modes. It was found that, for the low-energy fission of the nucleus of 286 112, the mass distribution of the fragments is of a clear-cut asymmetric form, contrary to what is observed for the spotaneous fission of the nuclei with Z = 100–104 and for the induced fission of 270 Sg.

Probing fission time scales with neutrons and GDR gamma rays

The time scales for nuclear fission have been explored using both pre-and postfission neutrons and GDR gamma rays. Four systems were investigated: 133-MeV 16O + 176Yb and 208Pb and 104-MeV 4He + 188Os and 209Bi. Fission fragments were measured in coincidence with PPACs. The neutrons were detected using eight detectors from the DEMON array, while gamma rays were measured using the US BaF2 array. The pre-and postfission gamma rays were determined using moving source fits parallel and perpendicular to the fission fragment emission directions. The time scales for fission for the neutrons were determined using the neutron clock technique. The gamma-ray data were fitted using a statistical model calculation based on the code CASCADE. The results of the fits from both data types were used to extract nuclear friction coefficients, γ, and fission time scales. The γ values ranged from 7 to 20, while the fission times were (31–105)×10−21 s.

The influence of the entrance channel on the formation and decay of the compound nucleus 250No

The mass-energy and angular distributions of binary fissionlike fragments produced in the reactions 44Ca + 206Pb and 64Ni + 186W, leading to the same compound nucleus 250No, have been measured at excitation energies of 30 and 40 MeV. The presence of the quasifission component was observed for both systems. But in the case of the 64Ni ion, the quasifission process dominates, while in the case of the 44Ca ion, the main process is the fusion-fission of compound nucleus 250No. From the angular distributions of reaction fragments, the time scales were found for quasifission and fusion-fission for both reactions.

The Peculiarities of the Production and Decay of Superheavy Nuclei

The interest in the study of the fission process of superheavy nuclei mainly deals with the opportunity to obtain information about the cross‐section of the compound nucleus (CN) formation at excitation energies E*≈15–30 MeV. It allows one to estimate the survival probability of the superheavy composite system after evaporation of 1–3 neutrons, i.e. in “cold” or “warm” fusion reactions. However, in order to solve this problem deeper understanding of the coalescence processes between colliding nuclei, the competition between fusion‐fission and quasi‐fission processes is needed. The characteristics of both processes, their manifestation in the experimental observables and the relative contribution to the capture cross‐section in dependence on the excitation energies, reaction entrance channel etc were investigated for a wide range of target‐projectile combinations. Results of the experiments devoted to the study of the fusion‐fission and quasi‐fission processes in the reactions of the formation of the super...

Fission dynamics in the proton induced fission of actinide nuclei at intermediate energies

A multi‐parameter correlation study of the reactions 232Th(p,f), 238U(p,f) and 242Pu(p,f) at Ep=13, 20, 40 and 55 MeV has been conducted. The fission fragment mass, total kinetic energy distributions, double differential neutron spectra and γ‐ray spectra have been measured. The three humped shape of mass distributions has been observed up to higher proton energy. Pre‐equilibrium, pre‐scission and post‐scission neutron and γ‐ray multiplicities measured in coincidence with primary fission fragments provided access to fission dynamics. Shell structure effects were observed in fission fragment mass distributions even at high excitation energy. Manifestation of the nuclear shell Z = 28 near fragment mass Afr = 78 has been detected.

Fusion-fission of heavy and superheavy nuclei

The process of fusion-fission of heavy and superheavy nuclei (SHE) with Z=82–122 formed in the reactions with 48Ca and 58Fe ions at energies near and below the Coulomb barrier has been studied. The experiments were carried out at the U-400 accelerator of the Flerov Laboratory of Nuclear Reactions (JINR) and at the XTU Tandem accelerator of the National Laboratory of Legnaro (LNL) using the time-of-flight spectrometer of fission fragments CORSET and the neutron multidetector DEMON. As a result of the experiments, mass and energy distributions (MED) of fission fragments; fission, quasifission, and evaporation residue cross sections; and multiplicities of neutrons and γ-quanta and their dependences on the mechanism of formation and decay of compound systems have been studied.

The use of SSNTD for the investigation of cluster radioactivity and spontaneous fission

Abstract The results of the investigation of some properties of polyethyleneterephtalate, phosphate glass and mica for the study of cluster decay and spontaneous fission of heavy nuclei are given. The investigation results of 230 U cluster decay and 226 Ra spontaneous fission investigation are presented. The probability of 230 U cluster decay in relation to α-decay is less than 7.5·10 14 . The partial half-life of 226 Ra spontaneous fission is more than 6.6·10 17 yrs. The fusion-fission cross sections for the ( 208 Pb + 16 O → 224 Th ) reaction, as the inverse reaction of cluster decay, in the subbarrier region of ion energy was measured. For the energies of 78, 75, 73 and 68 MeV 16 O (lab.system) the cross-sections are 7.8; 3.7·10 −2 , 8.5·10 −4 and 6·10 −6 mb, respectively.

Bimodal fission of Hs

Mass and energy distributions of fission fragments obtained in the reactions 22Ne + 249Cf, 26Mg + 248Cm, and 22Ne + 238U have been measured. A special attention will be paid on the properties of mass-energy distribution of fission fragments obtained in the reaction 26Mg + 248Cm at an excitation energy of 35 MeV. At this energy shell effects should become more effective in fission, the TKE distribution of symmetric fragments obtained in the reaction 26Mg + 248Cm differs strongly from a Gaussian shape. Besides a low-energy component, a high-energy component, not foreseen in the LDM, arises. This is attributed to the fact that both fission fragments are close to the spherical neutron shell N = 82. It means that for the compound nucleus 274Hs*, formed in the reaction 26Mg + 248Cm, the phenomenon of bimodal fission was observed for the first time. For the compound nucleus 260No* formed in the reaction 22Ne + 238U at the initial excitation energy of 41 MeV the bimodal fission as well as superasymmetric fission were observed.