Calculation of prompt fission neutron spectrum for 233U（n,f）reaction by the semi-empirical method~

The prompt fission neutron spectra for the neutron-induced fission of ²³³U for low energy neutrons (below 6 MeV) are calculated using nuclear evaporation theory with a semi-empirical method, in which the partition of the total excitation energy between the fission fragments for the n_th+²³³U fission reactions is determined by the available experimental and evaluation data. The calculated prompt fission neutron spectra agree well with the experimental data. The proportions of high-energy neutrons of prompt fission neutron spectrum versus incident neutron energies are investigated with the theoretical spectra, and the results are consistent with the systematics. The semi-empirical method could be a useful tool for the prompt evaluation of fission neutron spectra.     

Dependence of mass fission asymmetry on the compound excitation energy

The influence of the excitation energy of compound fissioning nucleus on the potential deformation energy as a function of the fission and asymmetry coordinates has been investigated. The Fermi distribution of nucleons at a certain temperature is assumed and taken into account in the Strutinsky shell-correction method. It has been found that the structure of the potential energy surface becomes smoother with increasing excitation energy and shell effects disappear at a compound-nuclear temperature of 2.0 to 2.5 MeV.     

Comparison between the symmetric fission and fusion paths

The symmetric fission path leading to smooth extended shapes and the fusion or new fission path going through the two tangent sphere configuration are investigated within the liquid-drop model including the nuclear proximity energy. Analytical formulae are given for the various shape-dependent functions which govern the dynamics. The quadrupole moment, the perpendicular moment of inertia and the Coulomb energy are similar in the two paths. In contrast, the neck radius, the rupture point between the fragments, the parallel and effective moments of inertia, the surface energy and the critical angular momentum against fission are quite different in the two valleys. The introduction of the proximity energy strongly lowers the deformation energy in the fusion valley and for the light, medium and very heavy nuclei the barrier heights are nearly equal in the two paths. This flattening of the potential surface by the proximity forces allows to better understand the sudden transition between the one- and two-body configurations.     

Realistic Langevin Treatment of Fission-Fragment Energy Distribution

The average kinetic energy and its variance of fission fragments for the compound nucleus ²¹³At are calculated by four-dimensional Langevin equation. Two collective coordinates are taken into account: the distance p between the centers of the nascent fragments and the neck parameter h using the {c, h, a} parametrization. The model allows for a realistic coordinate dependence of all coeficien ts appearing in the Langevin equation as computed by the Werner-Wheeler method. The diffusion process for thc fissioning nuclear system from the ground state to the scission line on the realistic energy surface is investigated. The results are in good agreement with the experiments.     

Calculation of multidimensional potential energy surfaces for even-even transuranium nuclei: systematic investigation of the triaxiality effect on the fission barrier

Static fission barriers for 95 even-even transuranium nuclei with charge number Z = 94-118 have been systematically investigated by means of pairing self-consistent Woods-Saxon-Strutinsky calculations using the potential energy surface approach in multidimensional(β_2, γ, β_4) deformation space. Taking the heavier (252)~Cf nucleus(with the available fission barrier from experiment) as an example, the formation of the fission barrier and the influence of macroscopic, shell and pairing correction energies on it are analyzed. The results of the present calculated β_2 values and barrier heights are compared with previous calculations and available experiments. The role of triaxiality in the region of the first saddle is discussed. It is found that the second fission barrier is also considerably affected by the triaxial deformation degree of freedom in some nuclei(e.g., the Z =112-118 isotopes). Based on the potential energy curves, general trends of the evolution of the fission barrier heights and widths as a function of the nucleon numbers are investigated. In addition, the effects of Woods-Saxon potential parameter modifications(e.g.,the strength of the spin-orbit coupling and the nuclear surface diffuseness) on the fission barrier are briefly discussed.     

Multi-modal analysis for low energy neutron-induced fission of ~(235)U

Low energy neutron induced fission of 235U is studied in the framework of the multi-modal fission model. The fission fragment properties, such as the yields, the average total kinetic energy distribution and the average neutron separation energy, are investigated for incident neutron energies from thermal to 6.0 MeV. The multi-modal fission approach is also used to evaluate the prompt fission neutron multiplicity and spectra for the neutron-induced fission of 235U with an improved version of the Los Alamos ...     

Antineutrino energy spectrum unfolding based on the Daya Bay measurement and its applications

The prediction of reactor antineutrino spectra will play a crucial role as reactor experiments enter the precision era.The positron energy spectrum of 3.5 million antineutrino inverse beta decay reactions observed by the Day a Bay experiment,in combination with the fission rates of fissile isotopes in the reactor,is used to extract the positron energy spectra resulting from the fission of specific isotopes.This information can be used to produce a precise,data-based prediction of the antineutrino energy spectrum in other reactor antineutrino experiments with different fission fractions than Day a Bay.The positron energy spectra are unfolded to obtain the antineutrino energy spectra by removing the contribution from detector response with the Wiener-SVD unfolding method.Consistent results are obtained with other unfolding methods.A technique to construct a data-based prediction of the reactor antineutrino energy spectrum is proposed and investigated.Given the reactor fission fractions,the technique can predict the energy spectrum to a 2% precision.In addition,we illustrate how to perform a rigorous comparison between the unfolded antineutrino spectrum and a theoretical model prediction that avoids the input model bias of the unfolding method.     

Temperature dependence of nuclear surface properties

Thermal properties of nuclear surface are investigated in a semi-infinite medium. Explicit analytical expressions are given for the temperature dependence of surface thickness, surface energy and surface free energy. In this model the temperature effects depend critically on the nuclear incompressibility and on the shape of the effective mass at the surface. To illustrate the relevance of these effects we made an estimate of the temperature dependence of the fission barrier height.     

Application of pulse shape discrimination in Si detector for fission fragment angular distribution measurements

Pulse shape discrimination (PSD) with totally depleted transmission type Si surface barrier detector in reverse mount has been investigated to identify fission fragments in the presence of elastic background in heavy ion-induced fission reactions by both numerical simulation and experimental studies. The PSD method is compared with the other conventional methods adopted to identify fission fragments with solid-state detectors such as ΔE–E telescope and single thin ΔE detector and the data for the¹⁰B +²³²Th fission reaction are presented. Results demonstrate the usefulness of a single transmission-type surface barrier detector for the identification of fission fragments and projectiles like heavy ions     

Multi-modal analysis for low energy neutron-induced fission of ~(235)U

Low energy neutron induced fission of ~(235)U is studied in the framework of the multi-modal fission model. The fission fragment properties, such as the yields, the average total kinetic energy distribution and the average neutron separation energy, are investigated for incident neutron energies from thermal to 6.0 MeV. The multi-modal fission approach is also used to evaluate the prompt fission neutron multiplicity and spectra for the neutron-induced fission of ~(235)U with an improved version of the Los Alamos model for incident neutrons below the (n, nf) threshold. The three most dominant fission modes are taken into account. The model parameters are determined on the basis of experimental data. The calculated results are in good agreement with the experimental data.     

4He闪烁裂变中子探测器的中子灵敏度

对4He闪烁裂变中子探测器的中子灵敏度进行了理论和实验研究。采用蒙特卡罗方法模拟了不同能量中子和不同厚度裂变靶产生的裂变碎片在4He中的能量沉积,计算结果表明:中子在4He气中的能量沉积曲线和裂变碎片的能量沉积曲线能够互补,从而使探测器对中子的能量响应变得更平坦;探测器的中子灵敏度为10-15 Ccm2量级。并对探测器的中子灵敏度进行了实验标定,实验结果与理论计算结果较为一致。     

Twin position sensitive ionization chamber,utilizing backgammon and time projection method

The anomalous dependence of average prompt fission neutron (PFN) multiplicity on the fission fragment total kinetic energy of ²⁵²Cf(sf) was investigated using digital signal processing (DSP). A twin Frisch-grid ionization chamber (TGIC) was used for fission fragment (FF) spectroscopy and PFN was registered with help of fast neutron detector (FND) on liquid scintillator base. The DSP apparatus was build up around the 8-channel fast synchronously sampling waveform digitizers (WFD). It was found that the main limitation of the experimental method, developed in ref. [1] for investigation of the PFN properties in neutron induced fission reaction ²³⁵U(n,f), ²³⁹Pu(n,f) etc is related to the FF energy loses of in the target backing. A modification of the method was suggested to overcome such a limitation.     

基于GEM工艺的裂变时间投影室中裂变碎片的讨论

基于GEM工艺的裂变时间投影室兼具GEM工艺的高计数率、高空间分辨本领等优点,我们拟通过裂变时间投影室在单次测量中准确获取裂变产物的核电荷数、质量数以及能量等多种信息。本文主要讨论了基于GEM工艺的裂变时间投影室中工作气体的种类、气压以及裂变碎片在气室中的能量损失和有效电荷问题,发现对于轻重两种裂变碎片的能量损失而言,重裂变碎片的分布宽度更窄。并针对裂变碎片在气室中的能量损失,提出了一种通过裂变碎片的有效电荷获取核电荷数的方法。这对于裂变时间投影室的研制工作以及实现裂变产物的多参量测量具有重要意义。     

Method for measuring the efficiency of energy removal by fission fragments through the fissile-substance surface

Part of the fission energy released in some volume of fissile substance is removed through the surface of this volume by fission fragments and can efficiently be used to pump gas lasers. The possibility of deducing this energy fraction from the results obtained by directly measuring the yield of alpha particles originating from the alpha decay of fissile substance and escaping through the surface of the volume being considered is examined. The respective estimates were derived in a first approximation on the basis of an extremely simple computational model.     

Semi-empirical interpretation of nuclear fission based on deformed-shell effects

The main features of the fragment mass and charge distribution in the fission of nuclides from Po to Fm are accounted for using a correlation of minimum potential energy with fragment yield. The potential energy is calculated from a liquid drop surface with deformed-fragment shell and pairing corrections at a particular deformation for the nascent fission fragments.     

Recoil studies of fission products formed in the fission of U by protons of energies 25–85 MeV

The recoil properties of ten fission products with masses ranging from 72 to 136 formed in the fission of ²³⁸U with protons of energies 25–85 MeV have been determined radiochemically by the integral-range method. From the recoil properties of the products and the Monte Carlo cascade calculations the average kinetic energy, cascade deposition energy, and anisotropy parameter for each fission product has been calculated. The kinetic energy and the excitation energy of the primary fragments leading to the observed fission product, and the total kinetic energy and the total excitation energy of the primary fragment pair have also been calculated.

The results indicate that up to a bombarding energy of 40 MeV fission takes place predominantly by the compound nucleus mechanism, with an increasing contribution of the direct interaction mechanism as the bombarding energy increases. The kinetic energy deficit was found to decrease with increasing bombarding energy. The fission products formed from the symmetric mode of fission have a larger separation distance between the charge centres of their respective primary fragments than those for the asymmetric mode of fission.     

Micro‐Raman spectroscopy and SEM/EDX applied to improve the zircon fission track method used for dating geological formations

The zircon mineral is widely studied in geochronology. In the case of the fission track method (FTM), the age is determined by the density of fission tracks at the zircon surface, which can be observed with an optical microscope after an appropriate chemical treatment (etching). The etching must be isotropic at the zircon grain surface to be used in the FTM, which leads those zircon grains whose etching is anisotropic to be discarded. The only reason for this discarding is the nonuniform morphology of the surface grain seen by optical microscopy, that is, no further physicochemical analysis is performed. In this work, combining micro‐Raman and scanning electron microscopy (SEM) to study the etching anisotropy, it was shown that zircon grains that present at least one area at the surface where the density of fission track is uniform can be used in the FTM. The micro‐Raman showed characteristic spectra of the standard zircon sample either from the areas where there are tracks or from where there are not. The only difference found was in the Raman bandwidths, which were broader for the areas with higher density of fission tracks. This suggests simply a decrease in the relative percentage of the crystalline/amorphous phases at these areas. The SEM/energy dispersive spectrometry (EDX) showed that there were no significant differences in the principal chemical composition at the areas with and without fission tracks. Copyright © 2008 John Wiley & Sons, Ltd.     

Ternary fission of nuclei in the adiabatic approximation

On the basis of a generalization of integral formulas for nuclear-decay widths to the three-body case, the spontaneous and the low-energy induced ternary fission of nuclei are investigated by using the adiabatic approximation. The properties of energy distributions, of partial fission widths, and of the angular distributions of fission fragments are analyzed for the case of ternary fission. Conditions are found under which the angular distributions of two heavy fragments originating from ternary fission are similar to the analogous distributions of fragments originating from binary fission. The features of angular distributions are investigated, along with the parities and angular momenta of the third (light) ternary-fission fragment.     

多维Kramers公式的研究

本文研究了鞍点附近多维的位能曲面及鞍点所在位置;并用Werner Wheeler及无旋液体等两种方法计算了多维的质量系数与粘滞系数,然后用多维Kramers公式计算了裂变速率.发现裂变速率随着维数的增加而适当增大.不同的形变参量以及不同的计算质量和粘滞系数方法对计算核裂变速率影响不大.从结果看,采用三维计算裂变几率已足够准确.     

Study of Fission Barrier Heights of Uranium Isotopes by the Macroscopic-Microscopic Method

Potential energy surfaces of uranium nuclei in the range of mass numbers 229 through 244 are investigated in the framework of the macroscopic-microscopic model and the heights of static fission barriers are obtained in terms of a double-humped structure. The macroscopic part of the nuclear energy is calculated according to Lublin-Strasbourg-drop (LSD) model. Shell and pairing corrections as the microscopic part are calculated with a folded-Yukawa single-particle potential. The calculation is carried out in a five-dimensional parameter space of the generalized Lawrence shapes. In order to extract saddle points on the potential energy surface, a new algorithm which can effectively find an optimal fission path leading from the ground state to the scission point is developed. The comparison of our results with available experimental data and others' theoretical results confirms the reliability of our calculations.