Production of heavy neutron-rich nuclei with radioactive beams in multinucleon transfer reactions

The production mechanism of heavy neutronrich nuclei is investigated by using the multinucleon transfer reactions of 136,148Xe+208pb and 238U+208pb in the framework of a dinuclear system model.The evaporation residual cross sections of target-like fragments are studied with the reaction system 148Xe+208pb at near barrier energies.The results show that the final isotopic production cross sections in the neutron-deficient side are very sensitive to incident energy while it is not sensitive in the neutron-rich side.Comparing the isotopic production cross sections for the reactions of 20spb bombarded with stable and radioactive projectiles,we find that neutron-rich radioactive beams can significantly increase the production cross sections of heavy neutron-rich nuclei.     

Neutron Emission,Mass Distribution and Energetics in 14.5 MeV-Neutron Induced Fission of Uranium-238

A study is performed on 14.5 MeV-neutron induced fission of ²³⁸U by means of three-parameter experiment in which the energies of both fragments and the time-of-flight of one fragment are measured. A mosaic-arrayed surface barrier detector of large sensitive area is used at the remote end of a flight tube. The pre- and post-neutron-emission fragment mass distributions are obtained, together with the average total kinetic energy of fragment as a function of its mass. The average number of neutrons emitted from an individual fragment and the average total number of emitted neutrons are also derived as a function of fragment mass. The results agree well with those calculated by the method developed in our laboratory for medium-excitation fission. The average number of emitted neutrons and the mass distribution of fission fragment are derived for the respective reactions of first-, second- and third-chance fission.     

The Plastic Box -- A 4? Detector for Intermediate Energy Heavy Ion Physics

A 4? array of plastic scintillator has been constructed and used in conjunction with high resolution solid state detectors. The array is employed principally to discriminate multinucleon transfer reactions from three-body (or more) final states. It should be useful for a broad class of experiments in which a global charged particle anticoincidence is required.     

Estimation of Neutron Yield from Individual Fragment in Medium-Excitation Fission

A method of calculation is described to estimate the average number of neutrons emitted per fragment in medium-excitation fission from published experimental data on neutron emission in thermal-neutron induced fission, average total kinetic energy as a function of fragment mass and mass yield in low- and medium-excitation fission reactions. Use is made of a relation of fragment excitation energy with internal excitation and deformation energies, and the difference in kinetic energy between the fission reactions at two-excitation energies. A tentative calculation is made for the fission of ²³⁸U induced by 12 MeV protons. The results are in good agreement with experimental data.

The method developed in the present work may make it possible to predict the average number of neutrons emitted from individual fragment in medium-excitation fission which has not yet been measured experimentally.     

Symmetry energy extraction from primary fragments in intermediate heavy-ion collisions

An improved method is proposed for the extraction of the symmetry energy coefficient relative to the temperature, asym/T, for the heavy-ion reactions near the Fermi energy region, based on the modified Fisher Model(MFM). This method is applied to the primary fragments of the Anti-symmetrized Molecular Dynamics(AMD)simulations for the reactions of40 Ca + 40 Ca at 35 Me V/nucleon. The density and the temperature at the fragment formation stage are extracted using a self-consistent method.     

10.7AGeV197Au诱发乳胶核反应α射弹碎片分布

对10.7AGeV¹⁹⁷Au诱发乳胶核反应α射弹碎片产生的分截面进行研究。讨论了¹⁹⁷Au核碎裂产生α多重碎片的产生率，并与其它能区的相应实验结果进行了比较。实验结果表明，α射弹碎片多重数分布服从KNO标度无关性。     

Exotic break-up modes in heavy ion reactions up to Fermi energies

We discussed recent studies, within the framework of transport theories, on heavy ion reactions between charge asymmetric systems, from low up to Fermi energies. We concentrated on the analysis of ternary breakup events of dynamical origin occurring in semi-central reactions, where the formation of excited systems in various conditions of shape, excitation energy and spin is observed. At beam energies around 20 A Me V, we showed how this fragmentation mode emerges from the combined action of surface(neck) instabilities and angular momentum effects, leading to the observation of three aligned massive fragments in the exit channel. At Fermi energies, a transition towards a prompt emission of small fragments from the neck region with larger relative velocity with respect to projectile and target remnants is observed. We also focus on isospin sensitive observables, aiming at extracting information on the density dependence of the isovector part of the nuclear effective interaction and of the nuclear symmetry energy.     

基于协变密度泛函理论的258Fm诱发裂变动力学研究

采用基于协变密度泛函理论的含时生成坐标方法研究了258Fm低能诱发裂变动力学性质,重点探讨了裂变位能曲面、裂变碎片总动能分布和碎片质量分布等。研究表明,258Fm位能曲面中存在显著的对称裂变谷,因而其低能裂变碎片总动能分布与质量分布均呈单峰结构,且随剪裂线判据Qn（脖子处粒子数）从4减至1,碎片总动能分布变窄,碎片质量分布的峰值从988%增至1028%。此外,随初态激发能从83 MeV增至173 MeV,碎片质量分布峰值从988%降至855%。     

Systematic study of anomalous fragment anisotropies in subbarrier complete fusion—fission reactions

The complete fusion-fission is separated from the transter-induced-fission with the fragment folding angle technique.The cross sections and fragment angular distributions for the complete fusion-fission reactions of 11B 238U(237Np),237NP,16O 232Th(238U) and 19F 232Th at near-and sub-barrier energies have been measured.The present fusion and fission standard models can reproduces both the excitation functions and the fragment anisotropies for the systems of 11B 238U(237Np)and 12C 237Np;but fail to explain both the experimental data for the other 3 systems simultaneously,The evidence of the entrance-channel dependence of fission-fragment anisotropies is revealsed by comparison of the 11B 237NP and ^16O 232Th data.Based on the observations a new version model of preequilibrium fission is put forward to explain the anomaly.     

The excitation energy of fragments in nuclear fission

The multiplication factor for chain reactions in a nuclear reactor depends essentially on the number of secondary neutrons emitted during fission, and this is determined completely by the excitation energy of the fragments. Therefore, calculation of the excitation energy of the fragments as a function of the atomic number Z and the atomic weight A of the fissioning nucleus is of considerable interest in atomic energy.In the work it was shown that the excitation energy of the fragments may be found by solving a system of equations for the deformation parameters of the fragments and the distance between them. The basic excitation energies of the fragments were found for some nuclei. An investigation was made of the dependence of the excitation energy on Z and A of the fissioning nucleus.In conclusion, the author would like to thank V. M. Galitskii and V. M. Strutinskii for an interesting discussion and I. G. Krutikova for carrying out the numerical calculations.     

Multi-parametric Measurement of Prompt Neutrons and Fission Fragments for 233U(n th,f)

The multiplicity and the energy of prompt neutrons from the fragments for ²³³U(n _th, f)were measured as functions of fragment mass and total kinetic energy. Average neutron energy against the fragment mass showed a nearly symmetric distribution about the half mass division with two valleys at 98 and 145 U. This shape formed a contrast with a saw-tooth distribution of the average neutron multiplicity. It indicates that the shell-effects, which are pronounced for the fragments having the proton number or neutron number close to the magic-number of 50 or 82, affected the neutron emission process. The slope of the neutron multiplicity with total kinetic energy depended on the fragment mass and showed the minimum at about 130 U. The obtained neutron data were applied to determine the total excitation energy of the system, and the resulting value in the typical asymmetric fission lied between 22 and 25 MeV. The excitation energy agreed with that determined by subtracting the total kinetic energy from the Q-value within 1MeV, thus satisfied the energy conservation. In the symmetric fission, where the mass yield was drastically suppressed, the total excitation energy is significantly large and reaches to about 40MeV: suggesting that fragment pairs are preferentially formed in a compact configuration at the scission point.     

Angle and energy distributions of fission neutrons

Using the statistical model of the nucleus the angle distribution of fission neutrons is determined taking into account the anisotropy in the angle distribution of the fission fragments. The latter quantity is approximated by a simple expression of the form 1 + kcos² α. It is assumed that the neutrons are emitted isotropically in the coordinate system in which the fragment is at rest. Using U²³⁸, a calculation is carried out to determine the values of P — the ratio of the intensity of fission neutrons emitted in the direction of the incident neutrons to that of neutrons emitted perpendicularly to the incident beam — at various energy values of the primary and secondary neutrons (in the region from 1 to 10 Mev). The mean value of P in fission in U²³⁸ by neutrons characteristic of a fission spectrum is found to be approximately 1.13. The energy spectra for fission neutrons at various primary-neutron energies are also obtained. The method of calculation can also be employed in investigations of the anisotropy of neutrons produced in fission of other nuclei.     

基于傅里叶级数展开的裂变位能曲面与碎片质量分布计算

裂变位能曲面是裂变核结构性质和裂变动力学研究的基础。本文使用傅里叶级数展开方法描述裂变过程中原子核的形状,采用基于Lublin Strasbourg Drop(LSD)宏观模型和Yukawa Folded微观模型计算了236U多维裂变位能曲面,研究了位能曲面随不同集体自由度(原子核拉长形变、左右碎片质量不对称度以及颈部宽度)的变化情况以及温度对位能曲面的影响。在位能曲面基础上采用基于Born Oppenheimer近似的三维集体模型描述原子核裂变过程,计算了236U裂变碎片质量分布,计算结果与实验数据符合较好,特别是质量分布的峰位,同时分析了核温度、零点能和颈部断裂概率半宽度对裂变碎片质量分布计算结果的影响。     

Distribution of specific ionization along a track as a function of the initial energy of U235 fission fragments

By means of a telescope consisting of 11 pulse ionization chambers and a two-dimensional pulse height analyzer, measurements were made of the distribution of specific ionization energy loss along a track as a function of the initial energy of fission fragments. Studies were made of U²³⁵ fission fragments produced by thermal neutrons in the initial energy ranges 78–115.5 MeV (light fragments) and 34–88 MeV (heavy fragments). The data obtained made it possible to discover the dependence of specific ionization on fission fragment velocity. Those relationsMps indicated some differences in the nature of ionization energy losses for light and heavy fragments.Translated from Atomnaya Énergiya, Vol. 19, No. 3, pp. 244–250, September, 1965     

Investigation of the statistical distribution of spontaneous U238 fission acts according to the energies of the two fragments

Using an apparatus consisting of a double pulse ionization chamber, two amplifying channels, a coincidence circuit and a two-beam pulsed oscillograph, we measured the kinetic energy of each of the two fragments formed during nuclear fission. 780 cases of spontaneous U²³⁸ fission and about 4500 cases of U²³⁵ fission by slow neutrons were recorded. Using these data we plotted the statistical distributions of acts of spontaneous fission and fission with slow neutrons according to the energies of the two fragments. These distributions gave the following curves for the two fission methods: a) the energy spectrum of all fragments and that of light and heavy fragments separately; b) the distribution of acts according to the total kinetic energy of the two nuclear fragments; c) the distribution of fission fragments according to mass; d) the relation of the average total kinetic energy of the two fragments to the ratio of their masses; and e) the relation of the average kinetic energy of light and heavy fragments to their total kinetic energy. The results of the two methods of fission are quite similar. It was found that the total kinetic energy of the two fragments in spontaneous U²³⁸ fission was, on the average, 4 Mev less than the energy in U²³⁵ fission by slow neutrons.     

Neutrons from the fission of excited nuclei

The time required for fragments to attain a given velocity was calculated and compared with the calculated lifetime of an excited nucleus with respect to neutron emission. On the basis of this, it is proposed that a certain fraction of neutrons (which increases with excitation) may be emitted before the time of complete acceleration of a fragment in the fission of nuclei excited to energies of 20 MeV and higher. Consideration of this situation makes it possible to eliminate some of the contradictions present in interpretations of experimental data. It is also pointed out that, because of this, the recentiy employed method for determining the dependence of neutron number on fragment masses by comparing the energies and velocities of the fragments may prove to be inapplicable in cases of high excitations.Translated from Atomnaya Énergiya, Vol. 19, No. 2, pp. 113–116, August, 1965     

Dissociation of fast HeH ions at glancing angle scattering from a crystal surface

Scattered H⁺ fragments resulting from glancing angle incidence of MeV HeH⁺ ions on the clean (001) surface of a SnTe crystal are distributed on an ellipse in the E-φ plane, where the angle φ is measured in the plane parallel to the surface and E is the energy of the fragment. The kinetic energies released upon dissociation derived from the diameters of the ellipses are smaller than those calculated from the explosion of fully ionized fragments.

Based on a simplified model, we have simulated the dissociative scattering of HeH⁺ ions at the surface, where the dissociation takes place via excitation of HeH⁺ ions and the charges of fragments change along the trajectory. It is concluded from the simulation that the cross section for dissociation in HeH⁺-electron collisions is of the order of 10⁻¹⁶ cm².     

Calcul du profil d'endommagement par une source isotrope de fragments de fission: Application aux gaines d'acier inoxydable du combustible UO2

The damage distribution produced by an isotropie fission fragments source in contact with a metallic target has been investigated. More precisely, the density of energy transferred by such a source to the target lattice has been computed as a function of the depth in the target. The basis of these calculations is Lindhard's theory which gives for each fission fragment emitted by the source the energy lost in atomic collisions. The results can be applied to the fission fragments damage produced by the fuel of a nuclear reactor in the clad which contains it.     

A New Insight into Energy Distribution of Electrons in Fuel-Rod Gap in VVER-1000 Nuclear Reactor

In order to calculate the electron energy distribution in the fuel rod gap of a VVER-1000 nuclear reactor,the Fokker-Planck equation(FPE)governing the non-equilibrium behavior of electrons passing through the fuel-rod gap as an absorber has been solved in this paper.Besides,the Monte Carlo Geant4 code was employed to simulate the electron migration in the fuel-rod gap and the energy distribution of electrons was found.As for the results,the accuracy of the FPE was compared to the Geant4 code outcomes and a satisfactory agreement was found.Also,different percentage of the volatile and noble gas fission fragments produced in fission reactions in fuel rod,i.e.Krypton,Xenon,Iodine,Bromine,Rubidium and Cesium were employed so as to investigate their effects on the electrons' energy distribution.The present results show that most of the electrons in the fuel rod's gap were within the thermal energy limitation and the tail of the electron energy distribution was far from a Maxwellian distribution.The interesting outcome was that the electron energy distribution is slightly increased due to the accumulation of fission fragments in the gap.It should be noted that solving the FPE for the energy straggling electrons that are penetrating into the fuel-rod gap in the VVER-1000 nuclear reactor has been carried out for the first time using an analytical approach.     

Projected-length distributions of fission-fragment tracks from U and Th thin film sources in muscovite

Thin films of natural uranium and thorium deposited on muscovite were used as sources of neutron-induced fission fragments. Fragment energy loss in thin-source geometry is negligible. In this way, the observed fragment range results from the interaction of the fragment with the detector material. This characteristic enables the investigation of asymmetric fission and etching, through measurements of projected track length distributions in muscovite micas coupled with thin films. The means and standard deviations of the etchable length distributions of the heavy and light fission-fragment tracks were estimated by fitting a theoretical equation to the experimental data.The light fission-fragment accounts for ∼54% and the heavy fission-fragment for ∼46% of the etchable length of a full fission track. This average partition is the same for tracks from thermal-neutron-induced fission of ²³⁵U and fast-neutron-induced fission of ²³²Th. The mean etchable length of uranium fission tracks is ∼2.5% longer than that of thorium fission tracks. This difference is at the resolution limit of these measurements but correlates with the difference in the mean combined initial kinetic energies of the fission fragments. The mean etchable length of uranium fission tracks in muscovite is ∼5% shorter than their calculated latent track length, supporting earlier estimates of a length deficit of this magnitude. The length deficit and the standard deviation of the etchable length distribution of the light fission-fragment tracks are twice the equivalent values for the heavy fission-fragment tracks. This is interpreted in terms of a v_t-profile (track etch rate) that depends on the mass of the track-forming particle.