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1.
RK Choudhury 《Pramana》2001,57(2-3):585-600
Nuclear fission process involves large scale shape changes of the nucleus, while it evolves from a nearly spherical configuration
to two separated fission fragments. The dynamics of these shape changes in the nuclear many body system is governed by a strong
interplay of the collective and single particle degrees of freedom. With the availability of heavy ion accelerators, there
has been an impetus to study the nuclear dynamics through the investigations of nucleus-nucleus collisions involving fusion
and fission process. From the various investigations carried out in the past years, it is now well recognized that there is
large scale damping of collective modes in heavy ion induced fission reactions, which in other words implies that nuclear
motion is highly viscous. In recent years, there have been many experimental observations in heavy ion induced fission reactions
at medium bombarding energies, which suggest possible occurrence of various non-equilibrium modes of fission such as quasi-fission,
fast fission and pre-equilibrium fission, where some of the internal degrees of freedom of the nucleus is not fully equilibrated.
We have carried out extensive investigations on the fission fragment angular distributions at near barrier bombarding energies
using heavy fissile targets. The measured fragment anisotropies when compared with the standard saddle point model (SSPM)
calculations show that for projectile-target systems having zero or low ground state spins, the angular anisotropy exhibits
a peak-like behaviour at the sub barrier energies, which cannot be explained by the SSPM calculations. For projectiles or
targets with large ground state spins, the anomalous peaking gets washed out due to smearing of the K-distribution by the intrinsic entrance channel spins. Recently studies have been carried out on the spin distributions of
fission fragments through the gamma ray multiplicity measurements. The fission fragments acquire spin mainly from two sources:
(i) due to rigid rotation of the nascent fragments at scission and (ii) due to statistical excitation of the spin bearing
collective modes in the fissioning nucleus. One of the collective modes — the tilting mode depends on the K quantum number and is responsible for the emission angle dependence of fragment spin. In our studies, we have shown conclusively
that the collective statistical spin modes get strongly suppressed for high K values corresponding to large rotational frequencies along the fission axis. These results bring out the importance of the
dynamical effects in the heavy ion induced fusion-fission reactions. The present article will review the work carried out
on the above aspects in heavy ion fission reactions as well as on the fission time scales, and some of the recent studies
on the mass-energy correlations of fission fragments at near-barrier bombarding energies. 相似文献
2.
The paper is focused on reaction dynamics of superheavy-nucleus formation and decay at beam energies near the Coulomb barrier. The aim is to review the things we have learned from recent experiments on fusion-fission reactions leading to the formation of compound nuclei with Z≥102 and from their extensive theoretical analysis. Major attention is paid to the dynamics of formation of very heavy compound nuclei taking place in strong competition with the process of fast fission (quasifission). The choice of collective degrees of freedom playing a fundamental role and finding the multidimensional driving potential and the corresponding dynamic equation regulating the whole process are discussed. A possibility of deriving the fission barriers of superheavy nuclei directly from performed experiments is of particular interest here. In conclusion, the results of a detailed theoretical analysis of available experimental data on the “cold” and “hot” fusion-fission reactions are presented. Perspectives of future experiments are discussed along with additional theoretical studies in this field needed for deeper understanding of the fusion-fission processes of very heavy nuclear systems. 相似文献
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M. G. Itkis S. Beghini A. A. Bogatchev L. Corradi O. Dorvaux F. Hanappe A. Gadea G. Giardina I. M. Itkis M. Jandel J. Kliman G. N. Kniajeya N. A. Kondratiev I. V. Korzyukov E. M. Kozulin L. Krupa L. Latina T. Materna G. Montagnoli Yu. Ts. Oganessian I. V. Pokrovsky E. V. Prokhorova N. Rowley A. Ya. Rusanov F. Scarlassara A. M. Stefanini L. Stuttgé S. Szilner M. Trotta V. M. Voskressenski 《Acta Physica Hungarica A》2004,19(1-2):9-18
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. 相似文献
5.
Fragment angular distributions in fission is one of the oldest and well understood aspects of fission theory. However, recent
heavy ion-induced fission and fission-like reactions have added a new dimension to this problem. We review here our present
understanding of the fragment angular distribution theory in fission and fission-like reactions. 相似文献
6.
M. G. Itkis S. Beghini A. A. Bogatchev L. Corradi O. Dorvaux A. Gadea G. Giardina A. A. Goverdovski F. Hanappe I. M. Itkis M. Jandel J. Kliman G. N. Kniajeva N. A. Kondratiev I. V. Korzyukov E. M. Kozulin L. Krupa L. Latina T. Materna G. Montagnoli K. J. Moody Yu. Ts. Oganessian I. V. Pokrovsky V. A. Ponomarenko E. V. Prokhorova N. Rowley A. Ya. Rusanov F. Scarlassara A. M. Stefanini L. Stuttgé S. Szilner M. Trotta A. M. Vinodkumar V. M. Voskressensky 《Physics of Atomic Nuclei》2003,66(6):1118-1124
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. 相似文献
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《Physics Reports》1997,284(6):381-416
The study of heavy-ion induced fission fragment angular distributions continues to be a source of rich information as regards fission process in general and fission dynamics in particular. Considerable progress has been made towards understanding many features of the fission phenomenon. While some of the new sets of data measured in the last few years have confirmed the theoretical expectations, the others have provided surprises not quite anticipated. In the present review article the emphasis will be mainly on the recent experimental results of heavy-ion-induced fission fragment angular distributions at energies near the fusion barrier, their implications and new puzzles in this area which require not only suitable explanation but also additional measurements. 相似文献
9.
S S Kapoor 《Pramana》1989,32(4):405-417
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. 相似文献
10.
ZHANG HuanQiao LIN ChengJian JIA HuiMing ZHANG ChunLei ZHANG GaoLong YANG Feng LIU ZuHua AN GuangPeng WU ZhenDong XU XinXing & JIA Fei China Institute of Atomic Energy P. O. Box Beijing China Particle Astrophysics Center 《中国科学:物理学 力学 天文学(英文版)》2011,(Z1)
The angular distributions of fission fragments for the 32S+184W reaction near Coulomb barrier energies are measured. The ex perimental fission excitation function is obtained. The measured fission cross sections are decomposed into fusion-fission, quasi-fission and fast fission contributions by the dinuclear system (DNS) model. The hindrance to completing fusion both at small and large collision energies is explained. The fusion excitation functions of 32S+90,96Zr in an energy range from above to below the ... 相似文献
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The nucleon exchange process between two nuclei in close proximity and its application to an explanation of fragment mass
and charge distributions in fission and in heavy ion deep inelastic collisions are reviewed. An analysis of the measured correlations
between the energy loss from relative motion and the fragment mass and charge variances in the heavy ion deep inelastic collisions
is presented. The recent data on fragment mass and charge variances as a function of the fragment kinetic energy in thermal
neutron induced fission of235U, lends added support to the hypothesis that the nucleon transport process plays a similar role both in fission and in heavy
ion deep inelastic collisions. 相似文献
13.
Experimental data from the literature concerning nucleus-nucleus collisions at beam energies of few A MeV above the Coulomb barrier are investigated in this work with emphasis on projectile-like fragment distributions. In peripheral collisions at beam energies below 10 A MeV, deep-inelastic transfer is shown to be the dominant reaction mechanism. In addition, the mechanism may involve an extension of the nuclear profile in the window (neck) region, observed primarily in reactions with very heavy target nuclei. Isoscaling observed at these energies can be used as a tool to predict the production rates of exotic nuclei in reactions induced by exotic secondary beams. 相似文献
14.
使用Monte-Carlo模拟计算了垒下16O+232Th系统转移裂变和复合核裂变碎片角分布及角关联.转移过程、熔合过程和裂变过程分别用半经典模型、耦合道模型及鞍点过渡态统计模型进行模拟.考虑了各物理量分布产生的运动学效应及裂前中子发射和裂后碎片粒子蒸发对碎片角分布及角关联的影响.模拟结果和实验测量的分布相一致.使用折叠角技术借助Monte-Carlo模拟区分转移裂变和复合核裂变是可能的.考虑了转移裂变和裂前中子发射的影响,复合核裂变碎片角分布各向异性异常仍然存在. 相似文献
15.
Systematic studies of heavy-ion induced fission reactions at near-barrier energies carried out in the last decade have brought
out many interesting aspects of fission process in general. The recent experimental findings which show dependence of fission
fragment angular distributions on entrance channel, shape, size and spin of the interacting nuclei and shell closure of the
intermediate compound nucleus are summarised in the present paper. 相似文献
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G. N. Knyazheva M. G. Itkis S. V. Khlebnikov E. M. Kozulin V. G. Lyapin V. A. Rubchenya W. Trzaska 《Physics of Particles and Nuclei Letters》2008,5(1):21-28
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. 相似文献
18.
We have searched for Coulomb fission induced by 84Kr ions on a 238U target at energies ranging from the interaction barrier down to 37 MeV below (408–458 MeV lab). No event attributed to Coulomb fission was detected; it was deduced that the cross section for this reaction is lower than 0.3 mb/sr near the interaction barrier. This value was compared to theoretical predictions. However, fission events originating from transfer reactions at the interaction barrier have been detected. 相似文献
19.
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. 相似文献
20.
《Physics letters. [Part B]》1988,207(4):385-388
Fragment angular and mass distributions have been measured simultaneously for photon energies near the fission barrier of 236U. Different angular distributions have been observed for particular fragment mass regions. This establishes a clear relationship between the quantum numbers of the transition states and the fragment mass division. The results are discussed within the transition channel concept of the double humped barrier for different fission paths and the more recent model of multi-ecit channels. 相似文献