Isomeric yield ratios of fission products in the system of 24 MeV proton-induced fission of238U

Isomeric yield ratios of 30 fission products in 24 MeV proton-induced fission of²³⁸U were measured by the use of the ion-guide isotope separator on-line. The obtained isomeric yield ratios were converted to the angular momenta of primary fission fragments based on the statistical model. The deduced angular momenta were examined from various aspects. It is found that in general the angular momentum continuously increases with the fragment mass number including the region of symmetric mass division. However, there are some exceptions. For Sn isotopes the deduced angular momenta are quite small due to the spherical shape of the nuclear shell configuration. It is also concluded from the consideration of the charge distribution that the angular momentum of fission product scatters considerably within the narrow range of mass division. The dependence of the angular momentum on the available energy of fragments at scission point indicates that the individual fragment possesses a characteristic deformation at scission and/or the deduced angular momentum is seriously affected by the particle excitation after scission.     

Generation of angular momentum of fission fragments in a cluster model

Based on the dinuclear system concept, the role of bending vibrations in creation of the angular momentum of primary fission fragments is investigated. For ²⁵²Cf spontaneous fission, the angular momenta of the fragments are calculated as a function of the neutron multiplicity and compared with available experimental data. Different cluster compositions of the ²⁵²Cf fission modes at the scission point are considered.     

The effect of angular momentum coupling on the angular distribution of fission fragments

We examine what information can be obtained from fission angular distributions through precise measurements and an analysis in terms of the simple statistical model of Ericson. We report on the systematics of the decoupling angle and present the role of the angular momentum coupling between the entrance and the exit channel. The results indicate that the directional coupling of the angular momentum in the entrance and the exit channel is always strong for heavy-ion induced fission, and the angular momentum coupling plays a decisive role on the angular distribution of fission fragments. The average channel spins of fission fragments 〈I_f〉 are deduced through the decoupling angles determined experimentally. They agree well with γ-multiplicity measurements.     

More Detailed Study of Deformation Effects in Prescission Particle Emission by a Diffusion Model

Deformation effects on particle emission in a fission process of 251Es nucleus as functions of excitationenergy, angular momentum, and viscosity coefficient have been investigated in detail within the framework of Smolu-chowski equation. Our calculations show that high excitation energy, low angular momentum, and large viscosity willenhance the influence of deformation on multiplicity of prescission particles, and that the roles of these three parameterswill become weak with decreasing deformation.     

Application of a two-step dynamical model to calculating properties of fusion-fission reactions

The two-step model of fusion-fission reactions that was proposed previously by the present authors is extended in such a way as to describe the multiplicity of light particles emitted from nuclearfission fragments. Calculations are performed for the reaction induced by ⁴⁸Ca + ²⁴⁴Pu collisions. The mass distribution of fragments, their mass-energy distribution, and the total multiplicity of neutrons and gamma rays are obtained for E _lab = 230, 238, 249, and 255 MeV. It is shown that the model reproduces qualitatively relevant experimental data. In order to attain quantitative agreement, it is necessary to take into account the angular momentum carried away by particles from the nucleus undergoing fission and various types of gamma rays emitted by the nucleus and its fission fragments.     

More Detailed Study of Deformation Effects in Prescission Particle Emission by a Diffusion Model

Deformation effects on particle emission in a fission process of ^251Es nucleus as functions of excitation energy, angular momentum, and viscosity coefficient have been investigated in detail within the framework of Smolu-chowski equation. Our calculations show that high excitation energy, low angular momentum, and large viscosity will enhance the influence of deformation on multiplicity of prescission particles, and that the roles of these three parameters will become weak with decreasing deformation.     

Investigation of the reaction 208Pb(18O, f): Fragment spins and phenomenological analysis of the angular anisotropy of fission fragments

The average multiplicity of gamma rays emitted by fragments originating from the fission of ²²⁶Th nuclei formed via a complete fusion of ¹⁸O and ²⁰⁸Pb nuclei at laboratory energies of ¹⁸O projectile ions in the range E _lab = 78–198.5 MeV is measured and analyzed. The total spins of fission fragments are found and used in an empirical analysis of the energy dependence of the anisotropy of these fragments under the assumption that their angular distributions are formed in the vicinity of the scission point. The average temperature of compound nuclei at the scission point and their average angular momenta in the entrance channel are found for this analysis. Also, the moments of inertia are calculated for this purpose for the chain of fissile thorium nuclei at the scission point. All of these parameters are determined at the scission point by means of three-dimensional dynamical calculations based on Langevin equations. A strong alignment of fragment spins is assumed in analyzing the anisotropy in question. In that case, the energy dependence of the anisotropy of fission fragments is faithfully reproduced at energies in excess of the Coulomb barrier (E _c.m. ? E _B ≥ 30 MeV). It is assumed that, as the excitation energy and the angular momentum of a fissile nucleus are increased, the region where the angular distributions of fragments are formed is gradually shifted from the region of nuclear deformations in the vicinity of the saddle point to the region of nuclear deformations in the vicinity of the scission point, the total angular momentum of the nucleus undergoing fission being split into the orbital component, which is responsible for the anisotropy of fragments, and the spin component. This conclusion can be qualitatively explained on the basis of linear-response theory.     

Bombarding energy dependence of angular momentum transfer in deep inelastic collisions

Measurements of first and second moments of γ-ray multiplicity distributions from deep inelastic collisions of ⁸⁶Kr + ¹⁵⁴Sm are reported. A global systematics of the angular momentum distributions from deep inelastic reactions with projectile masses ? 40 is presented. The average angular momentum is found to depend linearly on the incident channel average angular momentum, while no simple systematics for the second moment appears obvious. In order to illuminate the question whether the angular momentum transfer process reaches statistical equilibrium in deep inelastic collisions, numerical calculations have been performed on two models: a two-sphere classical model including the collective modes of twisting, bending, wriggling and tilting, and a statistical equilibrium Fermi-gas model. The two-sphere classical model is not able to account for the observed second moments, and neither does the Fermi-gas model give an explanation of the deep inelastic multiplicity data.     

Investigation of the reaction <Superscript>208</Superscript>Pb(<Superscript>18</Superscript>O, <Emphasis Type="Italic">f</Emphasis>): Folding angular distributions of fission fragments and gamma-ray multiplicity

Correlations between folding angular distributions of fission fragments and the gamma-ray multiplicity are studied for ¹⁸O + ²⁰⁸Pb interactions at energies of the beam of ¹⁸O ions in the range E _lab = 78–198.5 MeV. The probabilities are determined for complete-and incomplete-fusion processes inevitably followed by the fission of nuclei formed in these processes. It is found that the probability of incomplete fusion followed by fission increases with increasing energy of bombarding ions. It is shown that, for the incomplete-fusion process, folding angular distributions of fission fragments have a two-component structure. The width of folding angular distributions (FWHM) for complete fusion grows linearly with increasing energy of ¹⁸O ions. The multiplicity of gamma rays from fission fragments as a function of the linear-momentum transfer behaves differently for different energies of projectile ions. This circumstance is explained here by the distinction between the average angular momenta of participant nuclei in the fusion and fission channels, which is due to the difference in the probabilities of fission in the cases where different numbers of nucleons are captured by the target nucleus.     

Mass distribution of heavy ion fission within a dynamical treatment

Dynamical calculations for the fluctuations of the collective variables involved in fission are obtained by a proper decomposition of the full density. The final mass distributions of the fission fragments appear to be independent on the angular momentum and on the friction strength.     

Angular momentum effect in prescission particle multiplicities for a light system by diffusion model

The effect of angular momentum on the competition between fission and particle emission during light system fission process was studied via fission diffusion model. The prescission particle multiplicities were found to increase with decreasing angular momentum. The experimental prescission proton and α particle multiplicities can be fitted for 10.6 MeV/nucleon ⁸⁴Kr(²⁷Al,binary fission) reaction with this model. Entrance channel effect found in [1] is proved to be angular momentum effect.     

Target mass dependence of the average linear momentum transfer

The average transferred linear momentum in nuclear reactions induced by ³He, ⁴He, ¹⁴N, and ²⁰Ne projectiles on Co, Cu and Ag targets was determined by thick-target-thick-catcher recoil range techniques for bombarding energies between 10 AMeV and 90 AMeV. A comparison with results obtained from fission fragment angular correlations indicates a strong target mass dependence of the linear momentum transfer.     

Angular momenta of fission fragments in the α-accompanied fission of 252Cf

For the first time, average angular momenta of the ternary fission fragments ^{100, 102}Zr, ¹⁰⁶Mo, ^{144, 146}Ba and ^{138, 140, 142}Xe from the α-accompanied fission of ²⁵²Cf were obtained from relative intensities of prompt γ-ray transitions with the use of the statistical model calculation. Average values of the angular momenta were compared with the corresponding values for the same fission fragments from the binary fission of ²⁵²Cf. Results indicate the presence of a decreasing trend in the average values of angular momenta induced in ternary fission fragments compared to the same binary fission fragments. On the average, the total angular momentum extracted for ternary fission fragments is ∼1.4 ℏ lower than in binary fission. Consequently, results indicate that the mechanism of the ternary α-particles emission may directly effect an induction of angular momenta of fission fragments, and possible scenarios of such mechanisms are discussed. Further, the dependence of the angular momenta of ¹⁰⁶Mo and ¹⁴⁰Xe on the number of emitted neutrons from correlated pairs of primary fragments was obtained also showing a decreasing dependence of average angular momenta with increasing number of emitted neutrons. Consequences are briefly discussed.     

Further experimental study of fast fission with the Cl + Au system

The onset of fast fission was investigated with the Cl + Au system. The mass distribution of the fragments and the fusion cross sections were determined at six different bombarding energies. It turns out that the width of the symmetric component of the mass distribution increases strongly with the critical angular momentum. This could be related with the disappearance of the fission barrier due to centrifugal effects. A semi-static model is developed and the results are compared with the experimental data. In this framework, the observed broadening of the mass distribution can be attributed to the presence of the fast fission phenomenon.     

Dynamic picture of the formation of spins of induced fission fragments

A way of calculating the average spins of induced fission fragments is developed, based on the dynamic model of their angular distributions. The range of relaxation times for the degree of freedom associated with the orientation of the axis of symmetry of a fissioning nucleus relative to its total angular momentum is determined by analyzing experimental data on the energy dependences of average spins and the anisotropy of the angular distributions of fission fragments for the ¹²C, ¹⁶O + ²³²Th reactions at E _сm = 55–150 MeV.     

超重复合核的存活概率

考虑角动量相关性的情况下计算了超重核259Db的裂变位垒Bf.计算了258Rf,259Db,266Hs和267Mt 4个超重核单中子蒸发的存活概率Wsur及其随角动量分波的变化关系.给出了中子蒸发宽度、裂变宽度和超重核蒸发一个中子的概率对激发能与角动量分波的依赖关系. Considering the dependence on angular momentum， the fission barriers of super heavy nuclei 259Db have been calculated. The survival probability for 258Rf， 259Db，266Hs，267Mt compound and its relation with angular momentum have been estimated. The neutron emission width， the fission width and the realization probability for one neutron emission and their relation with excitation energy and angular momentum are also presented.     

裂变中子角关联分布的数值模拟

为了实现对自发裂变中子源能量、多重性和中子角关联分布的数值模拟,开发了根据裂变过程,逐步给出裂变中子多重性、能量和角分布等参数的NESF数值模拟程序,并将该程序集成到现有中子输运软件中.重点介绍我们开发的用于模拟自发裂变中子源能量、多重性和中子角关联分布的程序的背景、物理依据、程序流程,以及基于该程序开展的中子角关联分布特征与核部件特征关联的数值模拟.     

Angular distributions,relative orbital angular momenta,and spins of fragments originating from the binary fission of polarized nuclei

The angular distributions of fragments originating from the binary fission of odd and odd-odd nuclei capable of undergoing spontaneous fission that are polarized by a strong magnetic field at ultralow temperatures and from the low-energy photofission of even-even nuclei that is induced by dipole and quadrupole photons are investigated. It is shown that the deviations of these angular distributions from those that are obtained on the basis of the A. Bohr formula make it possible to estimate the maximum relative orbital angular momentum of fission fragments, this estimate providing important information about the relative orientation of the fragment spins. The angular distributions of fragments originating from subthreshold fission are analyzed for the case of the ²³⁸U nucleus. A comparison of the resulting angular distributions with their experimental counterparts leads to the conclusion that the maximum relative orbital angular momentum of binary-fission fragments exceeds 20, the fragment spins having predominantly a parallel orientation. The possibility is considered for performing an experiment aimed at measuring the angular distributions of fragments of the spontaneous fission of polarized nuclei in order to determine both the spins of such nuclei and the maximum values of the relative orbital angular momenta of fission fragments.     

Prompt neutron multiplicity distribution for 235U(n,f) at incident energies up to 20 MeV

For the n+<'235>U fission reaction, the total excitation energy partition of the fission fragments, the average neutron kinetic energy <ε>(A) and the total average energies E<,γ>(A) removed by γ rays as a function of fission fragment mass are given at incident energies up to 20 MeV. The prompt neutron multiplicity as a function of the fragment mass, ν(A), for neutron-induced fission of <'235>U at different incident neutron energies is calculated. The calculated results are checked with the total average prompt neutron multiplicities ν and compared with the experimental and evaluated data. Some prompt neutron and γ emission mechanisms are discussed.