Shell effects in 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 multi-detector DEMON. As a result of the experiments, mass and energy distributions (MED) of fission fragments, fission, quasi-fission and evaporation residues cross sections, multiplicities of neutrons and γ quanta and their dependence on the mechanism of formation and decay of compound systems have been studied.     

Analysis of fusion-fission process with neutron evaporation in superheavy mass region

The fusion-fission process for the synthesis of superheavy elements is discussed on the basis of fluctuation-dissipation dynamics. Recently, experiments at Dubna on fission of superheavy nuclei were carried out, and the mass and total kinetic energy distributions of fission fragments were measured. The fusion-fission cross section was derived from the experiments. We analyze the data using a three-dimensional Langevin calculation. We present a clear understanding of the competition between the fusion and the quasifission. We emphasize that a one-or two-dimensional model of Langevin calculation is not sufficient to estimate the fusion cross section in the superheavy mass region. Also, experiments on the emission of neutrons in correlation with fission fragments were conducted. It is useful to investigate the fusion-fission dynamics. We take into account the neutron emission with a Langevin calculation and compare it with experimental data. Finally, we discuss the evaporation residue cross section for superheavy elements.     

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.     

Studies in nuclear macrophysics through fission and fission-like reactions

Recent developments in the study of fission and fission-like reactions are briefly reviewed. After a brief introduction of some of the important features of the fission process, binary fission and fission-like processes in heavy ion-induced reactions are discussed. It is shown that studies of the fission fragment angular distributions which provide a way to determine relative contributions of compound nucleus fission and non-equilibrium fission-like events in heavy ion-induced fission have proved to be quite valuable in investigating the very shortK-equilibration times of the order of 10⁻²⁰ s involved in the nuclear dynamics of the dinuclear complex on its way to compound nucleus formation following nucleus-nucleus collision.     

Reaction mechanism studies using the CN/ER spin distribution

To study basic properties of the fusion reaction dynamics for heavy compound systems the partial-wave distribution can be employed as an alternative to the classically used fusion/fission excitation functions. A variety of reactions leading to compound nuclei (CN) in the Pb region can be used to investigate features like the fusion-fission competition, the role of deformation in the fusion of heavy systems and a possible effect of the Z = 82 shell on the enhancement of evaporation residue (ER) production.Received: 5 November 2002, Revised: 26 March 2003, Published online: 9 March 2004PACS: 24.75. + i General properties of fission - 25.70.Gh Compound nucleus - 25.70.Jj Fusion and fusion-fission reactions     

Reaction dynamics at the barrier for heavy compound systems

To investigate basic properties of the fusion reaction dynamics for heavy compound systems the partial wave distribution σ _? extracted from measured γ multiplicities can be employed as an alternative to the classically used fusion/fission excitation functions. A variety of reactions leading to compound nuclei (CN) in the Pb region can be used to investigate features like the fusion-fission competition, the role of deformation in the fusion of heavy systems and a possible effect of the Z=82 shell on the enhancement of evaporation residue (ER) production. The measured spin distribution (SD) can provide information on the single partial wave cross sections which is hidden in the integral fusion cross section. We started a series of experiments to measure those properties for the reactions at the Laboratori Nazionali di Legnaro, Italy. In order to extract the CN spin distribution, γ multiplicities are measured by using the γ-detector array GASP and its inner ball in the multiplicity filter mode.     

Reaction dynamics at the barrier for heavy compound systems

To investigate basic properties of the fusion reaction dynamics for heavy compound systems, the partial wave distribution σ _l extracted from measured γ multiplicities can be employed as an alternative to the classically used fusion/fission excitation functions. A variety of reactions leading to compound nuclei in the Pb region can be used to investigate features like the fusion-fission competition, the role of deformation in the fusion of heavy systems, and a possible effect of the Z=82 shell on the enhancement of evaporation residue production. The measured spin distribution can provide information on the single partial wave cross sections, which is hidden in the integral fusion cross section. Moreover, it can reveal signatures in the high-spin region, which could be an indication of a stabilization due to an increase in the potential hole by shell correction energies E_shell in the vicinity of a closed shell. The systematic investigation and understanding of the fusion-fission reaction dynamics, together with the understanding of the structure of transfermium nuclei, which are stabilized only via shell effects, are essential for a successful program aiming at the synthesis of new elements at GSI, Dubna, or elsewhere. We started a series of experiments to measure those properties for the reactions at the Laboratori Nazionali di Legnaro, Italy. In order to extract the compound nucleus spin distribution, γ multiplicities are measured using the γ-detector array GASP and its inner ball in the multiplicity filter mode.     

Fission barriers of super-heavy nuclei produced in cold-fusion reactions

Excitation functions of super-heavy evaporation residues formed in cold-fusion reactions were analyzed with the aim of getting information on the fission barrier height of these nuclei. The method uses the location of the maximum of 1n and 2n excitation functions. The results obtained on nuclei from Z = 104 to 112 are compared to three theoretical predictions.PACS: 25.70.Jj Fusion and fusion-fission reactions - 24.75. + i General properties of fission     

Phenomenological nuclear-reaction description in deuterium-saturated palladium and synthesized structure in dense deuterium gas under γ-quanta irradiation

The observed phenomena of changes of chemical compositions in previous reports [1, 2] allowed us to develop a phenomenological nuclear fusion-fission model with taking into consideration the elastic and inelastic scattering of photoprotons and photoneutrons, heating of surrounding deuterium nuclei, following d-d fusion reactions and fission of middle-mass nuclei by “hot” protons, deuterons and various-energy neutrons. Such chain processes could produce the necessary number of neutrons, “hot” deuterons for explanation of observed experimental results [1, 2]. The developed approach can be a basis for creation of deuterated nuclear fission reactors (DNFR) with high-density deuterium gas and the so-called deuterated metals. Also, this approach can be used for the study of nuclear reactions in high-density deuterium or tritium gas and deuterated metals.     

Fission process of superheavy nuclei

The process of fusion-fission of superheavy nuclei with Z=102?122 formed in the reactions with ²²Ne, ²⁶Mg, ⁴⁸Ca, ⁵⁸Fe and ⁸⁶Kr 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) using a time-of-flight spectrometer of fission fragments CORSET and a neutron multi-detector DEMON. As a result of the experiments, mass and energy distributions of fission fragments, fission and quasi-fission cross sections, multiplicities of neutrons and gamma rays and their dependence on the mechanism of formation and decay of compound superheavy systems have been studied.     

原子核的大质量转移反应和丰中子超重核的产生(英文)

迄今为止,人们合成的超重核都是缺中子的,无论是熔合-裂变反应还是碎化过程都无法使产物达到周期表的“东北区域”。而重核之间(如U核之间) 的近垒大质量转移反应则可能是目前生成丰中子超重核和达到未知丰中子重核区域的唯一途径。在改进的量子分子动力学(ImQMD) 模型结合统计模型框架内,研究了U+U等反应体系的大质量转移反应,计算了反应产生的初生态碎片的质量和电荷分布,并成功再现了产物的终态质量和电荷分布。通过比较3 个反应136Xe+248Cm,48Ca+248Cm和238U+248Cm 产生的106号元素的截面大小,揭示了U+U等重核大质量转移反应对产生丰中子超重核是非常有利的。For elements with Z > 100 only neutron deficient isotopes have been synthesized so far. The “northeast area” of the nuclear map can be reached neither in fusion-fission reactions nor in fragmentation processes.The large mass transfer reactions in near barrier collisions of heavy (U-like) ions seem to be the only reaction mechanism allowing us to produce neutron rich heavy nuclei including those located at the superheavy(SH) island of stability and unexplored area of heavy neutron-rich nuclides. This study is extremely important for nuclear astrophysical investigations and, in particular, for the understanding of the r process. In this paper within the Improved Quantum Molecular Dynamics (ImQMD) model combining with the statistical-evaporation model, the large mass transfer reactions, like 238U+238U have been studied. The charge and mass distributions of transiently formed primary fragments are investigated within the ImQMD model and de-excitation processes of those primary fragments are described by the statistical decay model. The mass distribution of the final products in 238U+238U collisions is obtained and compared with the recent experimental data. Through compared the formation cross sections of transfermium element 106 by three reactions of 136Xe+248Cm, 48Ca+248Cm and 238U+248Cm, it is explored that the large mass transfer reactions, like U+U are very benefit for the production of SH nuclei.     

Study of fission dynamics with the three-dimensional Langevin equations

The dynamics of fission has been studied by solving one- and three-dimensional Langevin equations with dissipation generated through the chaos weighted wall and window friction formula. The average prescission neutron multiplicities, fission probabilities and the mean fission times have been calculated in a broad range of the excitation energy for compound nuclei ²¹⁰Po and ²²⁴Th formed in the fusion-fission reactions ⁴He + ²⁰⁶Pb , ¹⁶O + ²⁰⁸Pb and results compared with the experimental data. The analysis of the results shows that the average prescission neutron multiplicities, fission probabilities and the mean fission times calculated by one- and three-dimensional Langevin equations are different from each other, and also the results obtained based on three-dimensional Langevin equations are in better agreement with the experimental data.     

Mechanisms for emission of4He in the reactions of 334 MeV40Ar with238U

Emission of⁴He in the reaction 334 MeV⁴⁰Ar+²³⁸U has been studied by triple coincidence measurements that allow the separate identification of fusion fission and sequential fission. For the⁴He evaporative spectra from fusion fission the composite system is shown to be the predominant contributor; whereas, for sequential fission the dominant emission is from the fragments. This result demonstrates a correlation between evaporative emission probability and lifetime expectancy of the composite system. To account for the observed⁴He spectra two other mechanisms are necessary in addition to nuclear evaporation. At forward angles, the⁴He spectra from both fusion fission and sequential fission exhibit higher intensities and larger energies than those expected from purely evaporative processes. This forward-peaked component must be related to a very rapid or pre-thermalization stage of the reaction. At backward angles yet another component is observed for fusion fission. As it is sensitive to the fragment masses but does not carry the kinematic shift characteristic of their full acceleration, this component must originate near to the time of scission. The average⁴He energy for this component is approximately 17 MeV (c.m.), and its intensity is correlated with a plane perpendicular to the fission fragment separation axis. These signatures are similar to those for long range alpha particle emission in low energy fission. Alpha particles evaporated from the composite nuclei in fusion-fission reactions are shown to be preferentially associated with fission events which result in the more symmetric masses. This result is consistent with the notion that mass asymmetric fission is a faster process than symmetric fission. Such a correlation between mass asymmetry and lifetime is an essential part of the “fast fission” or “quasifission” idea, which has attracted much current attention.     

Recherche de fission provenant de l'lnteraction 84Kr+209Bi: La fusion entre ions krypton et noyaux lourds est-elle possible?

Fragments emitted in binary fission from complete fusion nuclei have been investigated for krypton induced reactions on heavy nuclei. Cross sections are between 25 and 5% of the total reaction cross section. It is deduced that complete fusion between krypton projectiles and heavy nuclei is a very improbable process. Most of the reaction products seem to result from a very inelastic interaction which looks like very asymmetric fission.     

Giant resonances in exotic nuclear systems: What we expect and what we know

The characteristic features of γ-rays resulting from the decay of the Giant Dipole Resonance in very deformed, hot systems and in systems with a large isospin are discussed. For both cases there are recent theoretical predictions but very few if any data. With respect to hot, deformed systems it is argued that the discrepancy which exists between the measured and calculated asymmetry of the GDR γ-rays emitted in the fusion-fission process of hot actinide nuclei, might well be due to γ-emission during the saddle-to-scission stage of the fission process, that is from very deformed hot nuclear systems. The prospect of studying experimentally the behavior of Giant Resonances in systems with large isospin (=large neutron excess) is also discussed. Experiments on the GDR are feasible and will give information on the deformation of the emitting system.     

Description of the two-humped mass distribution of fission fragments of mercury isotopes on the basis of the multidimensional stochastic model

The dynamical model proposed earlier for describing fusion-fission reactions is applied to describing the two-humped mass distribution of fission fragments of mercury isotopes. In this model, the calculation of the time evolution of collective coordinates of the system is broken down into two stages. The first stage is that within which the projectile approaches the target nucleus, while the second is that of the evolution of the system formed after the touching of the projectile and target nuclei. The dynamical evolution of the system within both stages of the calculation is described on the basis of Langevin equations. The shell structure of colliding nuclei is taken into account at either stage of the calculation. Mass distributions are calculated for fragments originating from the fission of the mercury isotopes ^{190, 184}Hg formed in the fusion-fission reactions ⁴⁸Ca + ¹⁴²Nd → ¹⁹⁰Hg and ⁴⁰Ar + ¹⁴⁴Sm → ¹⁸⁴Hg. The process in which the isotope ¹⁸⁰Hg undergoes fission from the ground state is also calculated. The results obtained in this way are compared with the results of previous theoretical calculations and with available experimental data.     

Production and study of heavy neutron rich nuclei formed in multi-nucleon transfer reactions

A new setup is proposed to produce and investigate heavy neutron-rich nuclei located along the neutron closed shell N = 126. This “blank spot” of the nuclear map can be reached neither in fusion–fission reactions nor in fragmentation processes widely used nowadays for the production of exotic nuclei. The present limits of the upper part of the nuclear map are very close to stability while the unexplored area of heavy neutron-rich nuclides along the neutron closed shell N = 126 is extremely important for nuclear astrophysics investigations and, in particular, for the understanding of the r-process of astrophysical nucleosynthesis. A new way was recently proposed for the production of these nuclei via low-energy multi-nucleon transfer reactions. The estimated yields of neutron-rich nuclei are found to be rather high in such reactions and several tens of new nuclides can be produced, for example, in the near-barrier collision of 136Xe with 208Pb. This setup could definitely open a new opportunity in the studies at heavy-ion facilities and will have significant impact on future experiments.     

Neutron-induced reaction rates for the <Emphasis Type="Italic">r</Emphasis>-process

Neutron-induced reaction rates for the formation of heavy and superheavy nuclei in the astrophysical r-process are presented. Neutron-induced reactions, including fission and neutron capture, at astrophysical temperatures are treated within the framework of the statistical model and calculated for targets with the atomic number 84 < Z < 118, using different mass and fission barrier predictions. The dependence of heavy element yields from nuclear explosions on the target nuclei, nuclear reactions, and nuclear models is discussed.