Analysis of prompt fission neutron spectrum and multiplicity for 237Np（n,f） in the frame of multi-modal Los Alamos model

The improved version of Los Alamos model with the multi-modal fission approach is used to analyse the prompt fission neutron spectrum and multiplicity for the neutron-induced fission of 237Np. The spectra of neutrons emitted from fragments for the three most dominant fission modes (standard Ⅰ, standard Ⅱ and superlong) are calculated separately and the total spectrum is synthesized. The multi-modal parameters contained in the spectrum model are determined on the basis of experimental data of fission fragment mass distributions. The calculated total prompt fission neutron spectrum and multiplicity are better agreement with the experimental data than those obtained from the conventional treatment of the Los Alamos model.     

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.     

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.     

The study of prompt and delayed muon induced fission

Mass yield and total kinetic energy release (TKE) distributions of fragments from prompt and delayed muon induced fission, separately, have been measured for the isotopes²³⁵U,²³⁸U,²³⁷Np and²⁴²Pu. The distributions from prompt muon induced fission are compared with the corresponding distributions from hadronic reactions and from spontaneous fission (s.f.). The distributions from the delayed muonic fission processes are compared to the distributions for neutron and proton-induced fission. No mass distributions measured in the prompt muonic fission process show any signature, which can be attributed to the presence of the muon. Differences observed between the TKE distributions of prompt muon induced and hadron induced fission can be explained by the screening effect of the negative charge of the muon bound in the orbit of one of the fission fragments. The observed yield of symmetric muon induced fission was found to be defined merely by the value of the excitation energy.     

Alpha-induced reaction cross-section for Sm,U, Np targets: influence of hexadecapole deformation and deformed surface diffuseness

Alpha-induced reactions on~(154)Sm,~(233,235,236,238)U, and ~(237)Np deformed nuclei are studied theoretically.The effects of hexadecapole deformation, deformed surface diffuseness parameter, and orientation on barrier height and position, fusion cross-section at any angle, and fusion cross-section have been investigated. Both hexadecapole deformation and deformed surface diffuseness can affect barrier characteristics and enhance fusion cross-section. Good agreement between experimental data and theoretical calculations with quadrupole and hexadecapole deformation and deformed surface diffuseness were observed for the ~4He+~(154)Sm,~(235)U,~(237)Np reactions.     

Multimodal analysis of prompt neutron spectra for 237Np(n,f)

An attempt was made to analyzed the systematic variation of the prompt neutron spectra of ²³⁷Np(n,f) on the basis of the multimodal analysis of mass and kinetic energy distributions. The spectra of neutrons emitted from fragments of each mode (standard I, II, III, superlong) were calculated independently and the total spectra were synthesized. It was found that the partial spectrum for standard I mode is the softest, getting harder in ascending order for standard II, standard III and superlong modes. The calculated total spectra well represented the experimental data and were found to be more sensitive to the variation of the incident energy than the conventional treatment of the Madland-Nix model.     

Measurement of delayed-neutron yield from <Superscript>237</Superscript>Np fission induced by thermal neutrons

The delayed-neutron yield from thermal-neutron-induced fission of the ²³⁷Np nucleus was measured using a sample periodically exposed to a pulsed neutron beam with subsequent detection of neutrons during the time intervals between pulses. The experiment was realized on an Isomer-M setup mounted in the IBR-2 pulsed reactor channel equipped with a mirror neutron guide. The setup and the experimental procedure are described, the background sources are thoroughly analyzed, and the experimental data are presented. The total delayed-neutron yield from ²³⁷Np fission induced by thermal neutrons is ν _d = 0.0110 ± 0.0009. This study was performed at the Frank Laboratory of Neutron Physics (JINR, Dubna).     

Calculation of the wide-angle neutron spectra from the ~9Be(d,xn) reaction in a thick beryllium target

The multi-layer computing model is developed to calculate wide-angle neutron spectra, in the range from0° to 180° with a 5° step, produced by bombarding a thick beryllium target with deuterons. The double-differential cross-sections(DDCSs) for the ~9 Be(d, xn) reaction are calculated using the TALYS-1.8 code. They are in agreement with the experimental data, and are much better than the PHITS-JQMD/GEM results at 15°, 30°, 45° and 60° neutron emission angles for deuteron energy of 10.0 MeV. In the TALYS-1.8 code, neutron contributions from direct reactions(break-up, stripping and knock-out reactions) are controlled by adjustable parameters, which describe the basic characteristics of typical direct reactions and control the relative intensity and the position of the ridgy hillock at the tail of DDCSs. It is found that the typical calculated wide-angle neutron spectra for different neutron emission angles and neutron angular distributions agree quite well with the experimental data for 13.5 MeV deuterons. The multi-layer computing model can reproduce the experimental data reasonably well by optimizing the adjustable parameters in the TALYS-1.8 code. Given the good agreement with the experimental data, the multi-layer computing model could provide better predictions of wide-angle neutron energy spectra, neutron angular distributions and neutron yields for the ~9 Be(d, xn) reaction neutron source.     

Angular Distributions and Anisotropy of Fragments from the Neutron-Induced Fission of 237Np in the Energy Range of 1–200 MeV: Measurement Data and Model Calculations

JETP Letters - The experimental data on the angular distributions of fragments from the fission of 237Np nuclei induced by 1–200 MeV neutrons have been presented. Such measurements for...     

基于Geant4 的4π多单元载Gd 液闪探测系统的初步设计

采用基于蒙特卡罗方法的Geant4 软件设计了由40 个载Gd 液闪探测器组成的多单元4π 立体角探测系统。该探测系统与置于其几何中心的裂变室构成了一个研究平台,该平台主要用于可裂变核(n, 2n)反应截面的测量工作。首先对该系统的物理模型进行了初步检验,然后计算并分析了该探测系统的一些基本性能。通过计算发现,系统空腔半径的变化对中子探测效率的影响十分有限;而探测效率随探测器的厚度增加先是迅速增大,然后逐渐趋于平缓;计算还表明,Gd 同位素俘获中子后发射的级联γ 射线的能量主要沉积在局部几个相邻的探测器中。基于以上计算,初步确定了系统的厚度以及空腔半径的大小,并对下一步将要开展的工作做了初步规划。A gadolinium-loaded neutron detection system with 40 liquid scintillators is designed and simulated by using Geant4 Package based on Monte Carlo method. This system with a fission chamber placed in its center is mainly used for the cross section measurement of (n, 2n) reaction of fissile nuclides. We firstly test the physics model, and then calculate and analyze some basic performance of this system. According to the calculation, the detecting efficiency is not sensitive to the inner radius of the detecting system. The efficiency increase with the system thickness at limited thickness value . The calculated results also indicate that the energy of the γ rays emitted from Gd isotope after capturing a thermal neutron is mainly deposited in a few adjacent detectors. Based on above calculations, the thickness and inradius of this detecting system have been preliminarily determined.     

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.     

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.     

GEANT4 Simulation of the High-Energy Neutron Beam of the TOF Spectrometer GNEIS

Using the Geant4 software package, a numerical simulation of a neutron source of the time-offlight spectrometer GNEIS created on the basis of the SC-1000 synchrocyclotron with 1 GeV proton energy at the NRC Kurchatov Institute—PNPI (Gatchina) has been carried out. The influence of the structural features of the neutron source of the spectrometer on the spatial and energy distributions of neutrons has been studied. The intensity and spectral characteristics of the neutron flux in the range of 1–1000 MeV have been determined on the basis of the obtained information and detailed allowance for all elements of the neutron beam guide system. It is found that the best agreement between the experiment and calculation performed by means of Geant4 is observed when using the QGSP_INCLXX_HP model. In the neutron energy range of 1–200 MeV, the difference between the experimental and calculated shapes of the spectra is less than 25%.     

11B快中子非弹性散射的理论计算

本文采用直接作用模型和扭曲波玻恩近似（ＤＷＢＡ）理论,计算了１１Ｂ的２．１４ＭｅＶ激发态在入射中子能量为７:５４～２０．０ＭｅＶ的非弹性散射的积分截面和角分布,计算结果与评价的实验值进行了比较,符合较好．对实验上所缺乏的数据作出了理论预期值． BY using the direct reaction mode and the DWBA theory, We calculate the integrated sections and angular distributions of inelastic neutron scattering of 11B(n, n )11B 2. 14MeV excited state in energy range from 7. 54MeV to 20. 0MeV. The calculated results are comparaed with the experimental values, and a good agreement has been obtained. The theory values are given for absent experimental data.     

Multimode approach to 241Am and 237Np fission induced by 660-MeV protons

The results obtained by measuring cross sections for the formation of fragments originating from ²⁴¹Am and ²³⁷Np fission induced by 660-MeV protons are presented. The charge and mass distributions of fragments are analyzed within the multimode-fission model, symmetric and asymmetric fission channels being separated. The contributions of various fission components are estimated, and the fission cross sections for the ²⁴¹Am and ²³⁷Np nuclei are calculated along with the fissilities of these nuclei.     

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.     

厚靶D（d,n）^3He反应加速器中子源的产额、能谱和角分布

给出一种计算氘钛厚靶D(d,n)3He反应加速器中子源的产额、能谱和角分布的方法,并发展了一个计算机模拟程序,程序能够计算氘束流能量小于1.0 MeV的中子源的产额、能谱和角分布.计算时使用推荐的D(d,n)3He反应截面数据和来自SRIM-2003程序的氘在氘钛靶中的阻止本领数据.给出一些典型计算结果,包括中子积分产额、中子能谱和角分布.     

10B中子非弹性散射的理论计算

本文介绍了根据直接作用模型,采用DWBA方法,对~(10)B快中子非弹性散射的一组实验数据进行分析,获得了与实验数据符合较好的理论结果及一组参数,由所得参数,计算了入射中子能量为7.54～20MeV的~(10)B(n,n′)~(10)B~0.717MeV激发态非弹性散射的积分截面和角分布,补充了实验所缺的数据。 In this paper, we analysed the experimental data of reaction cross sections induced byfast neutron on ~(10)B by using the direct nuclear reaction theory and the DWBA method. We obtainedthe results in agreement with experimental data and reported a set of parameters. Furthermore, byusing the set of parameters we have calculated systematically the inelastic integrated cross sectionsand angular distribution of ~(10)B(n, n′) ~(10)B 0.717 MeV excited state for incident neutron energiesfrom 7.54 MeV to 20.0 MeV. ...     

Study of mass spectra of light-actinide fission isotopes Kr and Xe in the framework of a new statistical approach

Results on light-actinide fission fragments Kr and Xe yields calculated within the framework of a new statistical approach are presented. The proposed statistical method is derived from the equilibrium condition of the ensemble of all possible fission fragments. Using ²³⁷Np isotope as an example, we investigate the influence of excitation energy (nuclear temperature) and nuclear particle emission on the formation of mass fission channels. The results obtained are compared with the experimental data on Kr and Xe isotope yields at the fission of Th, U, Np, and Pu.