中子俘获瞬发γ射线数据评价(英文)

简要地介绍了中子俘获瞬发γ射线数据及其衰变纲图的评价方法技术、主要程序及其功能、数据评价流程、强度平衡检验，以及对A=1-44的稳定核素和A>190的部分稳定核素的热中子俘获瞬发γ射线数据及其衰变纲图评价的具体应用.The method of prompt γ-ray data evaluation for neutron capture and how to calculate the prompt γ-ray intensities of neutron capture have been briefly presented. The prompt γ-ray data of thermal-neutron capture of some stable nuclei for A=1-44 and A>190 have been evaluated. The ENSDF format has been adopted. The checks of intensity balance and format have been made. The examples are given to illustrate its application.     

Method for measuring prompt γ-rays generated by D-T neutrons bombarding a depleted uranium spherical shell

The prompt γ-ray spectrum from depleted uranium(DU) spherical shells induced by 14 Me V D-T neutrons is measured. Monte Carlo(MC) simulation gives the largest prompt γ flux with the optimal thickness of the DU spherical shells 3–5 cm and the optimal frequency of neutron pulse 1 MHz. The method of time of flight and pulse shape coincidence with energy(DC-TOF) is proposed, and the subtraction of the background γ-rays discussed in detail. The electron recoil spectrum and time spectrum of the prompt γ-rays are obtained based on a 2 ×2 BC501 A liquid scintillator detector. The energy spectrum and time spectrum of prompt γ-rays are obtained based on an iterative unfolding method that can remove the influence of γ-rays response matrix and pulsed neutron shape.The measured time spectrum and the calculated results are roughly consistent with each other. Experimental promptγ-ray spectrum in the 0.4–3 Me V energy region agrees well with MC simulation based on the ENDF/BVI.5 library,and the discrepancies for the integral quantities of γ-rays of energy 0.4–1 Me V and 1–3 Me V are 9.2% and 1.1%,respectively.     

一个测量中子核数据的新技术——瞬发γ射线法

以测量2 3 9Pu(n ,2n)反应截面为例概述了直接测量的中子法和间接测量的瞬发γ射线法的原理及其优缺点 .阐明了瞬发γ射线法的物理内含 ,阐述了如何从测量的σm(k)约束程序中的反应机制及其参数和得到最小漏失的σm(0 ) ,并分析了σn ,2n的不确定度的来源及其贡献 .瞬发γ射线法适合不易采用直接方法的中子核数据的测量 ,具有一定的普适性 . The principles, merit and demerit of neutron and prompt γ-ray method are described through the cross section measurement for 239 Pu(n,2n) reaction. The population ways of low levels of the product nucleus are divided into a direct population from the (n,2n) reaction, a part from the upper level deexitation and a part from the all high level deexitation. The missed cross sections σ m(k) are defined. The minimum missed cross section σ m(0) that can not be measured experimentlly is obtained...     

Maxwellian Averaged Neutron Capture Cross Sections in 56Fe and 57Fe

Measurements of the keV-neutron capture cross sections and radiative γ-ray spectrum of 56Fe and 57Fe are performed based on a 7 Li（p,n）7 Be reaction neutron source. The incident neutron spectrum on a capture sample is measured by means of a time-of-flight （TOF） method with a 6Li-glass detector. The radiative capture 7-rays emitted from an iron CS Fe or 57 Fe） or standard gold （197Au） sample are detected by a large anti-Compton NaI（TI） spectrometer covered with a heavy shield. The capture yields of samples are obtained by applying a pulse-height weighting technique to the corresponding capture γ-ray pulse-height spectrum. The Maxwellian averaged neutron capture cross sections of 56Fe and 5T Fe are derived according to the present capture cross section results.     

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 nth＋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.     

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.     

Monte Carlo simulation of prompt γ-ray spectra from depleted uranium under D-T neutron irradiation and electron recoil spectra in a liquid scintillator detector

To overcome the problem of inefficient computing time and unreliable results in MCNP5 calculation, a two-step method is adopted to calculate the energy deposition of prompt γ-rays in detectors for depleted uranium spherical shells under D-T neutron irradiation. In the first step, the γ-ray spectrum for energy below 7 MeV is calculated by MCNP5 code; secondly, the electron recoil spectrum in a BC501 A liquid scintillator detector is simulated based on EGSnrc Monte Carlo Code with the γ-ray spectrum from the first step as input. The comparison of calculated results with experimental ones shows that the simulations agree well with experiment in the energy region 0.4–3 MeV for the prompt γ-ray spectrum and below 4 MeVee for the electron recoil spectrum. The reliability of the two-step method in this work is validated.     

A Monte Carlo simulation and setup optimization of output ef f iciency to PGNAA thermal neutron using ~(252)Cf neutrons

We present the design and optimization of a prompt γ-ray neutron activation analysis(PGNAA) thermal neutron output setup based on Monte Carlo simulations using MCNP5 computer code. In these simulations, the moderator materials, reflective materials, and structure of the PGNAA252 Cf neutrons of thermal neutron output setup are optimized. The simulation results reveal that the thin layer paraffin and the thick layer of heavy water moderating effect work best for the252 Cf neutron spectrum. Our new design shows a significantly improved performance of the thermal neutron flux and flux rate, that are increased by 3.02 times and 3.27 times, respectively,compared with the conventional neutron source design.     

Evaluation of the 1077 keV γ-ray emission probability from 6SGa decay

6SGa decays to the excited states of aSzn through the electron capture decay mode. New recommended values for the emission probability of 1077 keV γ-ray given by the ENSDF and DDEP databases all use data from absolute measurements. In 2011, JIANG Li-Yang deduced a new value for 1077 keV γ-ray emission probability by measuring the 69Ga（n,2n） 6SGa reaction cross section. The new value is about 20% lower than values obtained from previous absolute measurements and evaluations. In this paper, the discrepancies among the measurements and evaluations are analyzed carefully and the new values are re-recommended. Our recommended value for the emission probability of 1077 keV γ-ray is （2.72±0.16）%.     

Multi-modal calculations of prompt fission neutrons from 238U（n,f） at low induced energy

Properties of prompt fission neutrons from ²³⁸U(n, f) are calculated for incident neutron energies below 6 MeV using the multi-modal model, including the prompt fission neutron spectrum, the average prompt fission neutron multiplicity, and the prompt fission neutron multiplicity as a function of the fission fragment mass υ(A) (usually named “sawtooth” data) The three most dominant fission modes are taken into account. The model parameters are determined on the basis of experimental fission fragment data. The predicted results are in good agreement with the experimental data.     

Halo Effect on Direct Neutron Capture Process

We calculate the capture cross sections of the ^10Be(n,γ) ^11Be reaction by means of the asymptotic normalization coefficient method and demonstrate the halo effects on the capture cross sections for the direct radiative neutron capture where a p-, s- or d-wave neutron is captured into an s-orbit or p-orbit in ^11Be by emitting an E1 γ-ray, respectively. The result shows that the enormous enhancement of the capture cross section is just due to the large overlap of the incident neutron wave with the extended tail of the ha/o, which is clearly illustrated by the reduced transition amplitude function.     

Evaluation of the 1077 keV γ-ray emission probability from ~(68)Ga decay

68Ga decays to the excited states of68Zn through the electron capture decay mode. New recommended values for the emission probability of 1077 keV γ-ray given by the ENSDF and DDEP databases all use data from absolute measurements. In 2011, JIANG Li-Yang deduced a new value for 1077 keV γ-ray emission probability by measuring the69Ga(n,2n)68Ga reaction cross section. The new value is about 20% lower than values obtained from previous absolute measurements and evaluations. In this paper, the discrepancies among the measurements and evaluations are analyzed carefully and the new values are re-recommended. Our recommended value for the emission probability of 1077 keV γ-ray is(2.72±0.16)%.     

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.     

Prompt neutron multiplicity distribution for ~（235）U（n,f） at incident energies up to 20 MeV

For the n+²³⁵U 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 ²³⁵U 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.     

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

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 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.     

Nuclear uncertainties in the s-process simulation

The heavy elements in the Universe are formed during the s- and r-processes mainly in AGB stars and supernovae, respectively. Simulation of s- and r-nucleosynthesis critically depends on the neutron capture and weak decay rates for all the nuclei on the reaction chain. The present work analyzes systematically the neutron capture rates (cross sections) for the s-process nuclei, including ～3000 rates on ～200 nuclei. The network calculations for the constant temperature s-process have been performed using the different data sets selected as the nuclear inputs to investigate the uncertainties in the predicted s-abundances. We show that the available cross sections of neutron capture on many s-process nuclei still carry large uncertainties, which lead to low accuracy in the determination of s-process isotope abundances. We analyze the neutron capture cross section data for the same unique isobar nucleus accorded by year from previous work. Such an analysis indicates that the s-process has been studied for more than fifty years and there exist two research stages around 1976 and 2002, respectively. The needs and opportunities for future experiments and theoretical tools are highlighted to remove the existing shortcomings in the neutron capture rates.     

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,（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  and compared with the experimental and evaluated data.     

Systematical law of (n, γ) reaction cross sections of even-even nuclei

We have derived a formula for the neutron radiative capture cross section in the framework of a statistical model approach to nuclear reactions. Based on this formula, new systematics are established between the (n, γ) reaction cross section and the energy level density of a compound nucleus or a relative neutron excess of an even-even target nucleus for neutron incident energy above the resonance region to MeV. Good agreement with experimental data suggests that this new systematical law is helpful to analyze 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.     

Astrophysical S-factor of the 12C（α,γ） 16O reaction at solar energies

The astrophysical S-factor of the 4He＋12C radiative capture is calculated in the potential model at the energy range 0.1-2.0 MeV. Radiative capture 12C（α,γ） 16O is extremely relevant for the fate of massive stars and determines if the remnant of a supernova explosion becomes a black hole or a neutron star. Because this reaction occurs at low energies, the experimental measurements are very difficult and perhaps impossible. In this paper, radiative capture of the 12C（α,γ） 16O reaction at very low energies is taken as a case study. In comparison with other theoretical methods and available experimental data, good agreement is achieved for the astrophysical S-factor of this process.