E. V. Prokhorova

The CORSET time-of-flight spectrometer for measuring binary products of nuclear reactions

The CORSET time-of-flight spectrometer has been developed at the Flerov Laboratory of Nuclear Reactions of the Joint Institute for Nuclear Research (Dubna, Russia) for investigating binary products of nuclear reactions. The spectrometer has been used to study the dynamics of fusion-fission and quasi-fission of superheavy elements. The design and the main characteristics of the spectrometer, as well as the algorithms for deducing the mass-energy distributions of fragments and the cross sections of nuclear reactions, are presented. The spectrometer contains two time-of-flight arms based on microchannel-plate detectors and three telescopes, each of which is composed of two microchannel-plate detectors and one semiconductor detector. A system of four semiconductor detectors is used to obtain the absolute value of a cross section. The time resolution of the time-of-flight arms is 150 ps, which allows the time-of-flight distances to be set at 10–20 cm, thus providing a mass resolution of 3 amu and an angular resolution of 0.3°. Owing to these characteristics, the spectrometer can be used as a trigger in multidetector setups for measuring light charged particles, neutrons, and γ rays in coincidence with reaction fragments.

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 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.

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...

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.

Fusion-fission of nuclei withZ > 100 at low excitation energies

SummaryThis paper presents new results obtained in the study of fission of superheavy nuclei256102,270106 and286112 formed in reactions with22Ne and48Ca ions at energies near or considerably lower than the Coulomb barrier. The experiments have been performed at the U-400 accelerator of the Flerov Laboratory of Nuclear Reactions (FLNR) with the use of the time-of-flight spectrometer of fission fragments CORSET.

Investigation of the fusion-fission reaction 12C+208Pb

For the first time the cross section and angular distributions of fission fragments have been measured for the compound nucleus 220 Ra produced in the reaction 12 C+ 208 Pb at three energy values in the range from 0.5 to 8.0 r MeV below the fusion barrier calculated in terms of the Bass model. In the investigated energy range there is a plateau characterized by the relation W (180°)/ W (90°)=1.2 in the angular anisotropy of fission fragments.

Fission of nuclei with Z=102-112 produced in reactions with 22Ne and 48Ca ions

The talk presents new results obtained in the study of fission of superheavy nuclei 256No, 270Sg and 286112 formed in reactions with 22Ne and 48Ca ions at energies near or considerably lower than the Coulomb barrier. The experiments have been performed at the U-400 accelerator of the Flerov Laboratory of Nuclear Reactions (FLNR) with the use of the time-of-flight spectrometer of fission fragments CORSET.

Angular Correlations Between Fragment Spin and Prompt Neutron Evaporation in Spontaneous Fission of 252Cf: CORA‐Demon Experiment

A novel method to search for the anisotropic emission of prompt neutrons in the center‐of‐mass system of fission fragments is presented. The anisotropy is conjectured to be due to the large spins of fission fragments are known to carry. Triple neutron‐ neutron‐fragment correlations in spontaneous fission of 252Cf were investigated in an exploratory experiment dubbed CORA‐DEMON experiment. Fission fragments were intercepted in a double ionization chamber while neutrons were spotted in 2 two‐dimensional cylindrical walls of Demon detectors with the target on the vertical cylinder axis. A new method of analysis of triple angular correlations between 2 neutrons and a fission fragment was applied. Preliminary results are reported.

Capture and Fusion‐Fission Processes in Heavy Ion Induced Reactions

Results of the experiments aimed at the study of fission and quasi‐fission processes in the reactions 12C+204Pb, 48Ca+144,154Sm, 168Er, 208Pb, 238U, 244Pu, 248Cm; 58Fe+208Pb, 244Pu, 248Cm, and 64Ni+186W, 242Pu are presented. The choice of the above‐mentioned reactions was inspired by the experiments on the production of the isotopes 283112, 289114 and 283116 at Dubna using the same reactions. The 58Fe and 64Ni projectiles were chosen since the corresponding projectile‐target combinations lead to the synthesis of even heavier elements. The experiments were carried out at the U‐400 accelerator of the Flerov Laboratory of Nuclear Reactions (JINR, Russia), the XTU Tandem accelerator of the National Laboratory of Legnaro (LNL, Italy) and the Accelerator of the Laboratory of University of Jyvaskyla (JYFL, Finland) using the time‐of‐flight spectrometer of fission fragments CORSET and the neutron multi‐detector DEMON. The role of shell effects and the influence of the entrance channel asymmetry and the deformatio...