Dependence of Nucleon and Pion-Induced Fission Cross Sections on the Level Density Parameter in the Cascade-Exciton Model

A new method is used to compute the fission cross sections in which a change of the ratio of the level density parameter in fission to neutron emission channels is taken into account with the change of the incident energy of the projectile. It is shown that fission cross sections induced by nucleons and pions depend on the ratio of the level density parameter in the fission and evaporation modes, i,e. af /an, respectively. We are unable to describe well the cross sections for fission without using this new method. The computed values exhibit reasonable agreement with the experimental data found in the literature across a wide range of beam energies.     

Medium energy proton induced fission in Tb,La and Ag

Coincidence studies with silicon surface barrier detectors have been used to determine fragment kinetic energies, angular correlations and fission cross sections in the fission of Ag,¹³⁹La,¹⁵⁹Tb and U nuclei induced by 600 MeV protons. Symmetric mass distributions are deduced for Ag and Tb, whereas La shows an indication of a stable asymmetric mass distribution. We find no indication of the Businaro-Gallone point. Fission-spallation competition calculations are used to deduce values of macroscopic fission barrier heights and nuclear level density parameter values at deformations corresponding to the saddle point shapes. We find macroscopic fission barriers lower than those predicted by macroscopic theories. The total kinetic energies at symmetric mass divisions follow closely the Viola prediction.     

Cross sections for interactions of 8.8 GeVα particles with Ag,Au, Bi and Th

Makrofol was used as 4π track detector to determine reaction cross sections of Ag, Au, Bi and Th induced by 8.8 GeVα particles. The variation of cross sections as functions of theZ ²/A parameter of the target has been investigated. Data concerning geometry of events, momentum transfer and fission probabilities were discussed in order to discern fission products from those originating from more violent processes. Comparison with proton data has been done and differences were discussed.     

Inelastic interactions of fast nucleons with nuclei and fission of nuclei

The cross section for the fission of actinide nuclei that is induced by fast neutrons is considered as a fraction of the cross section for inelastic nucleon interaction with nuclei. In turn, inelastic nucleon interaction with a nucleus is treated as scattering on intranuclear nucleons. It is shown that this interaction model describes satisfactorily the cross section for the inelastic interaction of 60- to 2200-MeV nucleons for a broad set of nuclei and that the energy dependence of the cross section for the fission of actinide nuclei that is induced by 400- to 1000–MeV protons replicates the energy dependence of the cross section for inelastic interactions with respective nuclei. From the model used, it follows that the cross sections for proton-nucleus interactions exceed cross sections for respective neutron-nucleus interactions in the energy range extending up to 550 MeV; at higher energies neutron cross sections are larger than proton cross sections.     

裂变位垒参数的推荐和裂变位垒参数库

通过分析和计算锕系核裂变截面和全套中子反应截面获取了一组锕系核的裂变位垒参数.Fm)在此基础上,广泛收集实验裂变位垒参数,通过比对和评价,推荐了本组的129个核素(109Cd—255的实验裂变位垒参数.此外,对收集的不同裂变位垒参数作了分析与评价,推荐了7组实验和1组模型计算的裂变位垒参数,并组建了中国评价核参数库裂变位垒参数子库(CENPL FBPL). The fission barrier parameters for actinide nuclei were obtained by analysis and calculations of fission cross-sections and complete neutron reaction cross sections. A lot of experimental fission barrier parameters from different authors were collected. A set of fission barrier parameters for 129 nuclei ranging from~(109)Cd~( ) to~(255)Fm were recommended after performing comparisons, evaluations and systematics extrapolations for these extracted fission barrier parameters in model calculations of fissi...     

Investigation of fast neutron interaction with235U

The angular distribution of neutrons emitted by elastic, inelastic and fission processes on²³⁵U were measured at the incident neutron energies of 1.5, 1.9, 2.3, 4.0, 4.5, 5.0 and 5.5 MeV using nanosecond time-of-flight technique. The differential elastic scattering cross sections and their angular distributions at all the seven energies are presented. The total elastic scattering cross sections, angle and energy integrated cross sections for the inelastically scattered neutrons in energy bands of 200 keV, fission cross sections and the angular distributions of fission neutrons were extracted at 1.5, 1.9 and 2.3 MeV incident neutron energies. The energy distributions of the prompt fission neutrons and of the inelastically scattered neutrons are given at the incoming neutron energies of 1.5, 1.9 and 2.3 MeV; and the average fission neutron energies and the inelastic neutron evaporation temperatures were also evaluated at these energies.     

Sub-Coulomb transfer and fission in collisions of129Xe,132Xe and136Xe with238U

Integral cross sections for fission and for one- and two-neutron transfer reactions in the system¹³²Xe+²³⁸U were measured radiochemically in the energy range 0.7≦E/E _Coul≦1. The excitation functions for fission and transfer are found to be essentially parallel below 0.85×E _Coul. Even at the lowest energies the transfer cross sections exceed the fission cross section by more than one order of magnitude. With the other projectiles¹²⁹Xe and¹³⁶Xe different transfer cross sections illustrating their sensitivity for the ground stateQ-values,Q _gg , are observed while the fission cross sections are the same as in the¹³²Xe +²³⁸U reaction. The fission data are interpreted in terms of a continuous transition between Coulomb fission and several transfer-induced fission processes.     

Study of Survival Probability of Super Heavy Nuclei

The survival probability of super heavy nuclei produced in cold fusion reactions is studied by using the standard Fermi gas level density formula and analyzed with fission and neutron evaporation characteristics predicted in different theoretical models. The level density formula used in this letter suppresses the ratio of neutron emission width to fission width, Гn/Гf. The dependence of Гn/Гf on the saddle point level density parameter and excitation energy is also investigated.     

Electrofission of238U and232Th

Absolute electrofission cross sections for²³⁸U and²³²Th in the energy regionE _e =7 ?65 MeV and fission fragment angular distributions forE _e =7–30 MeV have been measured. The angular distributions show strong anisotropies for low energies. The relative dipole and quadrupole contributions as a function of excitation energy are discussed in terms of the low lying fission transition states above the fission barriers. The cross sections show significant deviations from the results of some earlier measurements, in particular in the energy region above the giant dipole resonance. From the difficulties of absolute electrofission cross section measurements and the ambiguities in their interpretation it is concluded that by this time the quantitative analysis of electrofission cross sections with respect to the contributions of the giant quadrupole resonances to the fission decay channel should be regarded as rather tentative.     

MENDF—一个计算200 MeV以下裂变核数据的程序(英文)

基于球型光学模型、 预平衡发射和Hauser-Feshbach统计等理论, 编制了MENDF（Medium Energy Nuclear Data for Fission）程序, 该程序适用于裂变核在入射粒子能量低于200 MeV的中低能区的全套核数据计算。 对于中子和质子在200 MeV以下诱发的核反应, 其全截面、 反应截面、 弹性散射微分截面、 裂变截面和裂变中子谱、 5种发射粒子的单举截面和相应的能谱等理论计算值与相应的实验值基本符合。 MENDF在我国已被广泛用于核数据计算及建立中能核数据库。Based on the spherical optical model, pre equilibrium and Hauser Feshbach statistical theory, the code MENDF (Medium Energy Nuclear Data for Fission) is written to calculate a complete set of nuclear data for fission nuclei in the medium low energy region (≤200 MeV). For neutron and proton induced reactions below 200 MeV, the total cross sections, reaction cross sections, elastic scattering differential cross sections, fission cross section, energy spectra of fission neutron and five kinds of emitting particles, etc. are calculated by MENDF. The calculated data generally agree with their corresponding experimental data. MENDF is widely used for nuclear data calculation and to establish ENDF 6 formatted files for the medium low energy region in China.     

Experimental Cross Sections of Fission Fragments of Thorium-232 Irradiated with Medium-Energy Protons

This paper is focused on fission of Th-232 nuclei induced by protons with energies ranging from 20 to 140 MeV. This energy range is the most informative for studying the competition between asymmetric and symmetric fission modes. Experimental cross sections of production of radionuclides in thorium targets have been determined a year after irradiation. The corresponding theoretical values are calculated using the cascade–evaporation–fission model. The theoretical and experimental cross sections (literature data included) are compared.     

Time features of delayed neutrons and partial emissive-fission cross sections for the neutron-induced fission of 232Th nuclei in the energy range 3.2–17.9 MeV

The energy dependence of the relative abundances of delayed neutrons and the energy dependence of the half-lives of their precursors in the neutron-induced fission of ²³²Th nuclei in the energy range 3.2–17.9 MeV were measured for the first time. A systematics of the time features of delayed neutrons is developed. This systematics makes it possible to estimate the half-life of delayed-neutron precursors as a function of the nucleonic composition of fissile nuclei by using a single parameter set for all nuclides. The energy dependence of the partial cross sections for emissive fission in the reaction ²³²Th(n, f) was analyzed on the basis of data obtained for the relative abundances of delayed neutrons and the aforementioned half-lives and on the basis of the created systematics of the time features of delayed neutrons. It was shown experimentally for the first time that the decrease in the cross section after the reaction threshold in the fission of ²³²Th nuclei (it has a pronounced first-chance plateau) is not an exclusion among the already studied uranium, plutonium, and curium isotopes and complies with theoretical predictions obtained for the respective nuclei with allowance for shell, superfluid, and collective effects in the nuclear-level density and with allowance for preequilibrium neutron emission     

Manifestation of pairing effects in the cross section for 238Pu fission induced by neutrons

The cross section for ²³⁸Pu fission induced by neutrons with energies between 1 keV and about 5 MeV is described within the statistical model. It is shown that the stepwise structure observed above the fission threshold (at incident-neutron energies of Es>1 MeV) is due to the step in the level density of the fissile ²³⁹Pu nucleus at deformations corresponding to the inner fission barrier. In turn, the step in the level density of the odd nucleus ²³⁹Pu is associated with the excitation of internal single-and three-quasiparticle states. The level density is described with allowance for collective, pairing, and shell effects.     

Electromagnetic fission of238U at 600 and 1000 MeV per nucleon

Electromagnetic fission of²³⁸U projectiles at E/A =600 and 1000 MeV was studied with the ALADIN spectrometer at the heavy-ion synchrotron SIS. Seven different targets (Be, C, Al, Cu, In, Au and U) were used. By considering only those fission events where the two charges added up to 92, most of the nuclear interactions were excluded. The nuclear contributions to the measured fission cross sections were determined by extrapolating from beryllium to the heavier targets with the concept of factorization. The obtained cross sections for electromagnetic fission are well reproduced by extended Weizsäcker-Williams calculations which include E1 and E2 excitations. The asymmetry of the fission fragments' charge distribution gives evidence for the excitation of the double giant-dipole resonance in uranium.Communicated by V. Metag     

Thermal neutron fission cross sections in the range 302≧A≧223

A least squares method is used to fit a smooth curve to known thermal neutron fission cross section data for nuclei of evenN. By making use of fission barriers resulting from a modified liquid drop formalism coupled with the Strutinsky Normalization Procedure, the result is generalized and applied to the prediction within this framework of some 59 cross sections in the actinide and adjacent sub-actinide and trans-actinide regions, as well as to the prediction of an additional 37 cross sections in the superheavy region. In the first group is included a number of nuclides for which presently available cross section data are necessarily cited as upper limits. In the second group, interest is focussed on the predicted island of stability surrounding a neutron shell closure at 184 and a proton shell closure at 114. The results, while they are regarded as plausible estimates and not precise determinations, indicate that many cross sections in this region can be expected to undergo a pronounced reduction in magnitude relative to the limited number of estimates previously given in the literature.     

高激发核相继裂变的限制

高温核衰变是当前中能重离子核物理中研究最热烈的课题.理论上对高温核的衰变进行了各种预言,相继裂变就是其中之一.本文从有关核裂变的时间尺度、裂前中子发射、裂变截面等随激发能的变化和多重碎裂的实验结果以及复杂碎片发射时间的实验结果分析了对高温核相继裂变的限制,提出了从复杂粒子蒸发向多重碎裂过渡的可能性. Sequential fission is one of the several kinds of theoratical production for hot nucleus de-cay.By analysing relevant experimental results such as fission time scale,prescission neutron emission,fission cross sections and their variation with excitation energy,the results of multifragmentation andthe time scale of complex fragment emission,a limit to the sequential fission of the hot nucleus is sug-gested in this paper.A transition from intermediate mass fragment evaporation to..     

Separation of Coulomb fission from nuclear fission at medium energies

The cross sections for fission induced by one neutron transfer and by the electromagnetic field in the reaction 24.3 MeV/u ²³⁸U + ¹⁹⁷Au are calculated and compared to the experimental data. It turns up that the two calculated cross sections differ by five orders of magnitude at a distance of closest approach of 25 fm. It is shown that in the experiment in which one is able to select the events corresponding to a large distance of closest approach, a separation of Coulomb fission from nuclear fission events can be efficiently obtained.     

P-odd asymmetries for the binary fission of oriented target nuclei, induced by cold polarized neutrons

P-odd asymmetries in binary fission of oriented target nuclei induced by cold polarized neutrons are described for the first time using methods of the quantum theory of fission. A spin matrix of the compound nucleus density is constructed with allowance for the interference of the fission amplitudes of various pairs of neutron resonances excited in the given nucleus during the capture of a cold polarized neutron by the oriented target nucleus. In the differential cross sections for this reaction we obtain not only P-odd T-even correlations with the angular dependence different from that of the earlier investigated correlations in the fission of unoriented target nuclei by polarized neutrons but also P-odd T-odd correlations that do not occur in the fission of unoriented target nuclei by polarized neutrons and oriented target nuclei by unpolarized neutrons.