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.     

Calculation of prompt fission neutron spectra for 235U(n,f)

The prompt fission neutron spectra for the neutron-induced fission of ²³⁵U at E_n < 5 MeV are calculated using nuclear evaporation theory with a semi-empirical model, in which the nonconstant and constant temperatures related to the Fermi gas model are taken into account. The calculated prompt fission neutron spectra reproduce the experimental data well. For the n(thermal)+²³⁵U reaction, the average nuclear temperature of the fission fragment, and the probability distribution of the nuclear temperature, are discussed and compared with the Los Alamos model. The energy carried away by γ rays emitted from each fragment is also obtained and the results are in good agreement with the existing experimental data.     

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.     

Calculation of prompt fission neutron spectra for 235U(n,f)

The prompt fission neutron spectra for the neutron-induced fission of ²³⁵U at E_n < 5 MeV are calculated using nuclear evaporation theory with a semi-empirical model, in which the nonconstant and constant temperatures related to the Fermi gas model are taken into account. The calculated prompt fission neutron spectra reproduce the experimental data well. For the n(thermal)+²³⁵U reaction, the average nuclear temperature of the fission fragment, and the probability distribution of the nuclear temperature, are discussed and compared with the Los Alamos model. The energy carried away by γ rays emitted from each fragment is also obtained and the results are in good agreement with the existing experimental data.     

Research on the prompt neutron multiplicity of fission fragments for 235U(n,f)

According to some experimental and evaluated data, the total excitation energy partitioning way between both of the fission fragments was given with a semi-empirical method. With the calculated energy partitioning way, the prompt neutron multiplicity as a function of fragment mass, (-v)(A), for neutron-induced fission of 235U at En=0.0253 eV, 3 MeV, and 5 MeV was calculated. The results are checked with the total average prompt neutron multiplicities (-v) and compared with the experimental and evaluated data.     

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.     

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.     

Yields and energy spectra of light charged particles emitted in neutron induced fission of235U

The yields and energy spectra of light charged particles emitted in the fission of²³⁵U have been measured in the neutron energy range of 100 keV to 1 MeV. The yield of long range alpha particles is found to increase around 200 keV neutron energy compared to thermal fission. A low energy component observed in the energy spectrum was assigned to the tritons emitted in fission. The yield of this triton component is seen to have a marked increase around 500 keV. These results indicate that LCP yield is influenced by the transition state level characteristics.     

On the question of fine structure in νp and EK for 233U and 235U

The question of fine structure in the variation of the average number of prompt neutrons, $\text{ν}{}_{\mathrm{<mtext>p</mtext>}}$, emitted per fission with energy in the neutron fission of ²³³U and ²³⁵U has been examined. Consistent structure has been found in measurements of both $\text{ν}{}_{\mathrm{<mtext>p</mtext>}}\text{and}\text{E}{}_{\mathrm{<mtext>K</mtext>}}$ (the average total fission fragment kinetic energy) for ²³³U. Channel analysis of the neutron fission cross section of ²³³U allows the structure to be calculated quantitatively provided the collective energy at the second hump in the fission barrier is weakly coupled to the nuclear degrees of freedom at scission. A similar calculation for neutron fission of ²³⁵U supports the case for the absence of fine structure in $\text{ν}{}_{\mathrm{<mtext>p</mtext>}}\text{and}\text{E}{}_{\mathrm{<mtext>K</mtext>}}$ for this nucleus.     

The investigation of LRA fission of235U by non-shielded semiconductor telescope

The LRA particles energy spectrum from ternary thermal neutron induced fission of²³⁵U has been measured. A nonshielded semiconductor telescope technique has been used. The experimental spectrum has been fitted by the Gaussian distribution with average alpha energy of 15·65 ±0·09 MeV and full width at half maximum of 9·56±0.10 MeV.     

关於热能中子所致铀235分裂时发出的中子数目的讨论

An investigation on the average number v of prompt neutrons emitted per thermal neutron induced fission of U²³⁵ has been made with the Fermi gas statistical model. Weizsacker-Fermi semi-empirical mass equation has been used in calculating the neutron binding energies of the fission fragments. Using Stern's value for the mass of U²³⁵, the total excitation energy E_e of the fission fragments has been estimated to be of the order of 10 to 20 Mev for different hypotheses regarding the primary fission products. The results of calculation (given in the third table) show that only the hypothesis of equal radioactive chain lengths together with the assumption (A) that the excitation energy E_e is shared by the two fragments in proportion to their masses yields values of v exceeding 2. The latter assumption is not in accord with the experimental finding of Fraser that the light fragment group emits on the average 30% more neutrons than does the heavy. However, a shift of mass of U²³⁵ towards larger values or of kinetic energy of fission fragments towards lower values so that 5 Mev more excitation energy is available would make v considerably larger than 2 even with the assumption (B) that the excitation energy is shared by the two fragments in inverse proportion to their masses, thus making possible conformity with Fraser's discovery. Even then, in no case has the value of v thus calculated exceeded 3 (as shown in the fourth table), which may then be taken as a theoretical upper bound for the value of ν for thermal neutron induced fission of U²³⁵. The results of this investigation are thus seen to be in harmony with the recently announced experimental value 2.5 ±0.1 for ν.     

Prescission neutrons in the fission of 235U and 252Cf nuclei

Experimental data obtained previously for the energy-angular distribution of neutrons originating from the fission of ²⁵²Cf (spontaneous fission) and ²³⁵U (thermal-neutron-induced fission) nuclei are analyzed, the angle being measured with respect to the direction of fission-fragment motion. A regularity common to all independent experiments is revealed: at an angle of about 90°, there exists an excess of neutrons (30% for ²⁵²Cf and 60% for ²³⁵U) that does not admit explanation within the model of neutron emission from fully accelerated fragments. Two possible explanations of this experimental fact—neutron emission during the acceleration process and the existence of an additional source of neutrons (predominantly, prescission neutrons)—are considered. It is shown that the latter conjecture describes the observed features for both nuclei more adequately. The total yield of prescission neutrons and their energy and angular distributions are determined.     

Nuclear-charge polarization at scission in the 12 MeV proton-induced fission of <Superscript>232</Superscript>Th

The most probable charges of secondary fragments, produced after neutron evaporation from primary fragments, have been evaluated using fractional cumulative and mass yields in the 12MeV proton-induced fission of ²³²Th . The nuclear-charge polarization of primary fragments at scission has been obtained by correcting the most probable charge of secondary fragments for neutron evaporation. The fragment mass dependence of the nuclear-charge polarization at scission shows good agreement with that for thermal neutron-induced fission of ²³⁵U , indicating that the nuclear-charge polarization is nearly insensitive to mass and excitation energy of the fissioning nucleus for asymmetric fission in the actinide region.     

基于支持向量机的~(252)Cf中子裂变信号时频特征分析及识别

基于裂变中子(252Cf)对裂变链(235U系统)依存关系,在对252Cf中子裂变信号的测量原理及信号特点分析基础上,开展了基于支持向量机的中子裂变信号时频特征分析及识别研究工作。采用小波分解和去噪小波包分解方法,提取不同状态下随机核信号的时频能量特征,借助于统计学习理论的支持向量机(SVM)分类器原理进行训练和分类。研究结果表明:通过直接小波分解或去噪小波包分解,以获取核信号特征的方法是有效的;去噪小波包分解特征提取方式,较之直接小波分解特征提取方式更能反映中子裂变核系统的内部特征和规律;基于SVM核信号样本的分类,训练后的SVM分类器有着大于70%以上的正确率,且较好地克服了训练样本数较少的问题,验证了方法的可行性和有效性。     

Prompt fission neutron spectra of n+~(235)U above the(n,nf) fission threshold

Calculations of prompt fission neutron spectra(PFNS) from the235U(n, f) reaction were performed with a semi-empirical method for E n = 7.0 and 14.7 Me V neutron energies. The total PFNS were obtained as a superposition of(n,xnf) pre-fission neutron spectra and post-fission spectra of neutrons which were evaporated from fission fragments, and these two kinds of spectra were taken as an expression of the evaporation spectrum.The contributions of(n,xnf) fission neutron spectra on the calculated PFNS were discussed. The results show that emission of one or two neutrons in the(n,nf) or(n,2nf) reactions influences the PFNS shape, and the neutron spectra of the(n,xnf) fission-channel are soft compared with the neutron spectra of the(n,f) fission channel. In addition,analysis of the multiple-chance fission component showed that second-chance fission dominates the PFNS with an incident neutron energy of 14.7 Me V whereas first-chance fission dominates the 7 Me V case.     

Study of α-particles produced in thermal neutron induced reactions on 235U

The ²³⁵U(n_th, α) reaction and the α-particles emitted in the thermal neutron induced fission of ²³⁵U were measured using a very pure thermal neutron beam of the Grenoble high flux reactor. An upper limit of 0.66 mb was determined for the ²³⁵U(n_th, α) reaction cross section, which is lower than all the previous results. The energy distribution of the α-particles produced by the ²³⁵U(n_th, f) reaction was measured down to 7 MeV with a non-shielded surface-barrier detector telescope assembly. The measured distribution has a quasi-Gaussian shape; it reveals, however, a pronounced deviation from such a shape at lower α-energies. Several possible explanations for this deviation are discussed.     

中子诱发233,235,238U裂变机制的多通道理论研究

用改进的多通道和无规颈断裂模型,计算了能量E_n=0—6MeV的中子诱发²³³U、²³⁵U裂变和E_n=1.3—5.3MeV中子诱发²³⁸U裂变的碎片的质量分布、动能分布和瞬发中子数分布,理论计算在定量上与实验符合较好.     

Multiparameter studies of polar emission in 236U fission

The energy distributions and relative intensities of protons, deutrons, tritons and α-particles emitted along the fission axis during thermal neutron fission of ²³⁵U were measured simultaneously with both fission fragment energies. The mass distributions of fragments, the total kinetic energy (TKE), the dependence of the mean TKE on the fragment mass, as well as the mean kinetic energy dependence of polar particles on the fragment mass and energy were subsequently deduced from these data. Although some experimental results agree remarkably well with the hypothesis that polar particles are evaporated in flight from fission fragments, the general conclusion is that these particles are emitted according to some other mechanism.     

裂变产物产额数据的评价

以中子诱发235U裂变产生的135Xe 的累积产额数据为例, 利用了所有可利用的实验数据, 介绍了裂变产物产额数据的评价方法, 包括数据的收集、 修正、 评价、 误差调整和数据处理, 特别是协方差数据的评价、 半经验模型理论计算数据的应用和相关数据的处理, 给出了135Xe热能点产额的推荐值以及0～20 MeV产额-能量关系曲线。 Takeing 135Xe produced by neutron induced fission of 235U as an example, the evaluation method for fission yield was introduced using the all experimental data available. The evaluation procedure includes the experimental data collection, correction, evaluation, error assessment and data analysis. The study of the uncertainty covariance, application of semi empirical theoretical calculation and the yield energy dependence of 135Xe in the incident energy range 0~20 MeV were emphasized in this work.     

Fission decay properties of ultra neutron-rich uranium isotopes

The fission decay of highly neutron-rich uranium isotopes is investigated which shows interesting new features in the barrier properties and neutron emission characteristics in the fission process. ²³³U and ²³⁵U are the nuclei in the actinide region in the beta stability valley which are thermally fissile and have been mainly used in reactors for power generation. The possibility of occurrence of thermally fissile members in the chain of neutron-rich uranium isotopes is examined here. The neutron number N = 162 or 164 has been predicted to be magic in numerous theoretical studies carried out over the years. The series of uranium isotopes around it with N = 154–172 are identified to be thermally fissile on the basis of the fission barrier and neutron separation energy systematics; a manifestation of the close shell nature of N = 162 (or 164). We consider here the thermal neutron fission of a typical representative ²⁴⁹U nucleus in the highly neutron-rich region. Semiempirical study of fission barrier height and width shows that ²⁵⁰U nucleus is stable against spontaneous fission due to increase in barrier width arising out of excess neutrons. On the basis of the calculation of the probability of fragment mass yields and the microscopic study in relativistic mean field theory, this nucleus is shown to undergo exotic decay mode of thermal neutron fission (multi-fragmentation fission) whereby a number of prompt scission neutrons are expected to be simultaneously released along with the two heavy fission fragments. Such properties will have important implications in stellar evolution involving r-process nucleosynthesis.