LS301快中子探测器的效率刻度

主要介绍了利用252Cf标准裂变中子能谱刻度快脉冲LS301型液闪中子探测器的探测效率的实验方法和结果, 简要介绍了实验数据的处理过程, 得到了阈值分别为0.5, 0.7, 1.0和1.6 MeV, 以及中子能量在10 MeV以下的探测器效率, 并对测量结果进行了误差分析。同时为了检验实验结果的准确性, 将实验结果与理论计算结果进行比较, 两者在不确定度范围内符合得很好。Neutron detection efficiency of LS301 fast neutron detector was calibrated by measuring the neutron energy spectrum of 252Cf source, which has a standard fission neutron spectrum. A low mass, fast ionization chamber is used as the fission fragments detector in the time of flight(TOF) spectrometer and afforded the start signal of neutron flight. The stop signal was offered by the anode of LS301. A measured TOF spectrum was turned to the neutron energy spectrum which will be compared with the standard one. Consequently, the fast neutron detection efficiency of LS301 was obtained. Calibration procedures of experimental and data processing was given. Relative detection efficiencies were obtained separately for threshold settings of 0.5, 0.7, 1.0 and 1.6 MeV for neutron energies under 10 MeV. Experimental results accorded with theoretical efficiency curves which were calculated with the Monte Carlo code NEFF50.     

锂玻璃探测器中子探测效率的刻度

为了精确测量keV能区的中子俘获截面,中国原子能科学研究院正在建造一台4π 全吸收型γ 探测装置---GTAF,锂玻璃探测器将会作为中子束流监视器测量中子能谱。利用5SDH-2 加速器刻度了锂玻璃探测器在两个入射中子单能点(250 和565 keV) 的探测效率,并使用EANT4 和MCNP 程序模拟计算了锂玻璃探测器的相对探测效率。通过归一化实验数据和模拟结果,得到了锂玻璃探测器在10keV~1 MeV 能区的中子探测效率曲线。对于把锂玻璃探测器测量得到的飞行时间谱转化为中子束流能谱,是一项非常重要的工作,同时为探测器效率刻度提供了新方法。In order to accurately measure the neutron capture cross section in the energy range of keVMeV, 4 πgamma-ray total absorption facility (GTAF) is being constructed at China Institute of Atomic Energy (CIAE). The lithium glass detector will be used as a neutron beam monitor for GTAF. The detection efficiency of the lithium glass detector at two incident neutron energy points (250, 565 keV) was calibrated in 5SDH-2 accelerator, and the relative detection efficiency was simulated by GEANT4 and MCNP code. By the normalization of the experimental data and simulation result, the neutron detection efficiency curve of the lithium glass detector between 10 keV and 1 MeV was obtained. This work will be important to convert the Time-of-flight spectrum that be measured by Li-glass detector to the energy spectrum of neutron beam, and provide the new method for calibration of detection efficiency.     

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.     

D-D 中子发生器n-γ 密度测井的蒙特卡罗模拟研究

根据D-D 反应中子的能谱和角分布数据,建立了中子源模型;根据石灰岩地层标准刻度井群数据,建立了井模型。采用MCNP 程序模拟了井中中子和γ 射线的输运,得到了不同地层密度和不同源距处NaI 探测器中的混合γ 射线能谱和非弹γ 射线能谱。在混合γ 射线能谱2.5~5.0 MeV 能区开窗,研究了开窗区混合γ 射线相对计数随源距的变化关系,确定源距应选择在30~80 cm 范围,给出了密度与混合γ 射线计数之间的非线性关系。研究表明,可以利用D-D中子源的混合γ 射线能谱来实现n-γ 密度测井。A D-D neutron source model was developed according to the neutron spectrum and the neutron angular distribution of D-D reaction. A standard calibration well model was built. The transports of the D-D neutrons and γ-rays in the well were simulated using MCNP code. The mixed γ-spectra and inelastic γ-spectra in the NaI detector were obtained for both different distances from a neutron source and different densities. Mixed γ count in the energy range of 2.5 to 5.0 MeV as a function of distance shows that the NaI detector should be located at a distance of 3080 cm from the neutron source. The nonlinear relationships between the density and the mixed γ count were presented in this paper.It was demonstrated that the energy spectrum of mixed γ rays can realize the n-γ density logging.     

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.     

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

Calculations of prompt fission neutron spectra (PFNS) from the ²³⁵U(n, f) reaction were performed with a semi-empirical method for E_n=7.0 and 14.7 MeV 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 MeV whereas first-chance fission dominates the 7 MeV case.     

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.     

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.     

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.     

基于D-T紧凑型中子源的快中子照相准直屏蔽体设计及中子束特性模拟研究

中子照相是一种重要的无损检测技术,它能用于火工产品、毒品和核燃料元件等的检测。基于紧凑型D-T中子发生器,完成了一个用于快中子照相的准直屏蔽体系统（BSA）的物理设计。根据D-T中子源的能谱和角分布建立了中子源模型,采用MCNP4C蒙特卡罗程序,模拟了准直屏蔽体系统中中子和γ射线的输运,准直中子束相对于单位源中子的中子注量可以达到9.30×10-6 cm-2,准直中子束中主要是能量大于10 MeV的快中子;在设置的样品平面直径14 cm的照射视野范围,准直束中子注量的不均匀度为4.30%,准直束中中子注量与γ注量的比值为17.20,中子通量和中子注量比值J/Φ为0.992,说明准直中子束有好的平行性;准直屏蔽体外的泄露中子注量率与准直束中子注量率相比降低了2个量级。所设计的准直屏蔽体能满足快中子照相的要求。Neutron radiography is an important nondestructive testing technique. It can be used to detect the explosive devices, drug and the nuclear fuel element, etc. A beam-shaping-assembly (BSA) based on a compact D-T neutron generator is designed for fast neutron radiography in this paper. D-T neutron source model is constructed based on the neutron energy spectrum and angular distribution data. The transportation of neutron and γ-ray in the BSA is simulated using MCNP4C code. The neutron fluence of the collimated neutron beam with respect to the neutron source of the unit source is 9.30×10-6 cm-2. The collimated neutron beams is mainly fast neutrons with energies greater than 10 MeV. In the irradiation field range with a diameter of 14 cm, the neutron fluence uniformity of the collimated beam is 4.3%, the ratio of the neutron fluence to the gamma fluence in the collimated beam is 17.20, and the neutron flux and the neutron fluence ratio (J/Φ) is 0.992 which indicates that the collimated neutron beam has good parallelism. The leakage neutron fluence in outside of BSA is two orders of magnitude lower than that of the collimated neutron beam. The designed BSA can meet the need of fast neutron radiography.     

中国散裂中子源反角白光中子束流参数的初步测量

中国散裂中子源(CSNS)已于2018年5月建设完工,随后进行了试运行.其中的反角白光中子束线(Back-n)可用于中子核数据测量、中子物理研究和核技术应用等多方面的实验.本文报道对该中子束的品质参数测量实验过程以及最终实验结果.实验主要采用中子飞行时间法,利用~(235)U,~(238)U裂变室和~6Li-Si探测器测量了中子能谱和中子注量率,又利用闪烁体-互补金属氧化物半导体探测系统测量了中子束斑的剖面,得到了该束线的初步实验测量结果.其中白光中子的全能谱测量范围eV—100 MeV,给出了不确定度分析;给出了中子注量率两个实验厅位置的满功率值;给出了白光中子在直径60 mm情况下的全能区束斑.通过与模拟结果的比较探讨了以上结果的合理性,并提出了改进计划.这些实验结果为以后该束线的核数据测量和探测器标定实验奠定了基础.     

基于裂变γ标识技术的瞬发裂变中子谱测量新方法

瞬发裂变中子谱（prompt fission neutron spectrum,PFNS）是用于核实验诊断过程中十分重要的参数数据,传统的测量主锕系核素（U,Pu）PFNS的技术手段是采用裂变室,利用裂变碎片标识裂变中子,通过中子飞行时间技术获得裂变中子谱.目前出现了一种新的用于PFNS测量的技术,其原理是基于如下的物理事实：在一次裂变过程中,释放中子的同时伴随着释放7–8个γ射线光子,而非弹性散射效应产生的γ射线光子只有1–2个.据此,可以通过裂变γ射线的多重性将裂变中子和其他杂散中子甄选出来,达到测量PFNS的目的.本文建立了基于裂变γ标识技术的PFNS测量实验系统.利用该系统对²⁵²Cf中子源的PFNS进行了实验测量,测量结果与传统的裂变碎片标识法及ENDF/B-VⅡ数据库的标准谱进行了比较,对新方法的裂变标识率以及实验不确定度也一并进行了分析.     

贫化铀/聚乙烯球壳交替系统中铀-238中子俘获率的测量与分析

为校验次临界能源堆的概念设计,建立了贫化铀/聚乙烯球壳交替系统, 采用活化法测量²³⁸U的中子俘获率. 贫化铀片置于系统内与入射D离子束成90^o的方向上活化 ,用HPGe探测器测量²³⁸U俘获中子衰变产生的²³⁹Np 衰变产生的277.6 keV特征γ射线计数,实验修正了贫铀片对277.6 keV γ射线的自吸收, 得到了交替系统中²³⁸U (n, γ)反应率的径向分布,反应率的相对不确定度为3.5%-3.7%, 并计算得到系统上整个贫铀区中²³⁸U的总中子俘获率为2.24 ± 0.09. 用MCNP5程序在常用ENDF库下进行了模拟计算, ²³⁸U (n, γ)反应率分布计算与实验一般在5%以内符合, 总俘获率在1%以内符合.     

Neutron fluences and dose equivalents measured with passive detectors on LDEF

Neutron fluences were measured on LDEF in the low energy (< 1 MeV) and high energy (> 1 MeV) ranges. The low energy detectors used the ⁶Li(n,)T reaction with Gd foil absorbers to separate thermal (< 0.2 eV) and resonance (0.2 eV−1 MeV) neutron response. High energy detectors contained sets of fission foils (¹⁸¹Ta, ²⁰⁹Bi, ²³²Th, ²³⁸U) with different neutron energy thresholds. The measured neutron fluences together with predicted spectral shapes were used to estimate neutron dose equivalents. The detectors were located in the A0015 and P0006 experiments at the west and Earth sides of LDEF under shielding varying from 1 to 19 g/cm².

Dose equivalent rates varied from 0.8 to 3.3 μSv/d for the low energy neutrons and from 160 to 390 μSv/d for the high energy neutrons. This compares with TLD measured absorbed dose rates in the range of 1000–3000 μGy/d near these locations and demonstrates that high energy neutrons contribute a significant fraction of the total dose equivalent in LEO.

Comparisons between measurements and calculations were made for high energy neutrons based on fission fragment tracks generated by fission foils at different shielding depths. A simple 1-D slab geometry was used in the calculations. Agreement between measurements and calculations depended on both shielding depth and threshold energy of the fission foils. Differences increased as both shielding and thereshold energy increased. The modeled proton/neutron spectra appeared deficient at high energies. A 3-D model of the experiments is needed to help resolve the differences.     

铀氧化物中238U含量的定量研究

准确定量铀氧化物样品中238U的含量对于裂变产额测量等工作具有重要意义,其定量方法的研究对于涉及238U的绝对测量的诸多实验都具有参考价值。利用四川大学2.5 MeV质子静电加速器产生的准单能中子对八氧化三铀样品进行中子活化,用高纯锗探测器测量活化产物的β衰变子体239Np的277.6 keV特征γ射线,利用已知的活化截面,算出样品中238U的质量占比为79.1%,不确定度为6.2%。利用电感耦合等离子体质谱法对样品中238U的含量进行了测量,其质量占比为59.2%。此外,基于EA3000元素分析仪、卡尔费休电位滴定仪和TGA-DSC2同步热分析质谱联用仪,测知样品中不含N元素,含水量在2.2%以下。并利用PIXE方法排除了样品中含有Al元素以上杂质,利用EPBS方法测知样品中U/O原子数比为1:3.6,误差约2%,推知样品中238U质量占比为80%,验证了活化法测得的238U含量。最终测得样品中238U的质量占比为79.1%,不确定度6.2%。对照按U3O8分子式折算的84.5%,考虑到水分含量等,此结果合理。The accurate quantification of 238U in uranium oxides is of great importance for measurements of fission yield. The study for the method of 238U quantification has significance to lots of experiments involving absolute measurement of 238U. The reaction (n, γ) in the triuranium octoxide samples was induced by T(p,n)3He quasi-monoenergetic neutrons provided by the 2.5 MeV proton electrostatic accelerator in Sichuan University. The 277.6 keV characteristic γ ray of 239Np that comes from β-decay of the activation product 239U was detected by a HPGe detector. With the known (n, γ) cross section, the mass percentage of 238U in samples was determined to be 79.1%, with the uncertainty of 6.2%. ICP-MS was also used in the quantification of 238U in samples, its result was 59.2%. Meanwhile, the samples were analyzed by other several methods and the results showed that less than 2.2% H₂O, no nitrogen, and no other impurity elements above Al were contained in the samples. EPBS was used to measure the atomic ratio of U and O and it was found that the mass percentage of 238U in samples was about 80%, verifying the result 79.1% of INAA.     

Further Analysis of Neutron Double-Differential Cross Section of n ＋ ^16O at 14.1 MeV and 18 MeV

By using a new reaction model for light nuclei, the double-differential cross section of total outgoing neutron with LUNF code for n＋^16O reactions at En=14.1 MeV and 18 MeV have been calculated and analyzed. In this paper the opened reaction channels, which have contribution to emitting the neutrons, are listed in detail. To improve the fitting results the direct inelastic scattering mechanism is involved. The calculating results agree fairly well with the experimental data at E,~ = 14.1 MeV and the deviation from calculated results and experimental data in low energy region at En= 18 MeV has been analyzed. Since the possibility of 5He has been affirmed theoretically [J.S. Zhang, Sci. Chin. G 47 （2004） 137], so 5He emission from n＋ ^16O reaction is taken into account, which plays an important role at the region of the outgoing neutron energy εn〈3 MeV in total outgoing neutron energy-angular spectrum. The calculated results indicate that the pre-equilibrium mechanism dominates the whole reaction processes, and the recoil effect in light nuclear reactions is essentially important.     

Gd, U, Pu and Cm alpha particle energy analysis using tracks in solids

This paper presents advances in a procedure for alpha particle analysis using the nuclear tracks formed in solid-state materials. This method is based on the relationship between the energy deposited in the material by ionizing particles and the track developed after a well-established chemical process. The experimental study included alpha particles in the energy range from 3.2 to 5.8 MeV emitted by ¹⁴⁸Gd, ²³⁸U, ²³⁹Pu and ²⁴⁴Cm. The quantitative results provide a clear signature to identify each one of the radioisotopes based on the formed track parameters. The track analysis is performed with a digital image analysis system associated with a PC mathematical processor. The wide range energy response makes this method a promising analysis system.     

Reaction filters. Charged-particle multiplicity and linear momentum transfer

The relation between charged-particle multiplicity and linear momentum transfer to heavy reaction residues has been investigated with a 4π charged-particle detector for the reactions ³⁶Ar+²³⁸U at E/A=35 MeV and ¹⁴N+²³⁸U at E/A=50 MeV. The multiplicity of charged particles at backward angles (θ > 35°) incrreases linear momentum transfer while the multiplicity of charged particles in the forward direction is almost independent of the linear momentum transfer.     

Excitation function of (p, pn) reactions at high energy

A closed theoretical formula for the (p, pn) reaction cross section at high energies in light, medium and heavy nuclei has been derived, and a comparison of the calculated cross sections with the experimental datais made for ¹²C, ¹⁹F, ⁵⁴Fe, ⁵⁸Ni, ⁵⁹Co, ⁶⁴Zn, ⁶⁵Cu, ¹⁰⁰Mo, ¹²⁷I, ¹⁴²Ce, ¹⁸²Ta and ²³⁸U at incident energies above 50 MeV. Good agreement between experimental and calculated cross sections is obtained in the entire energy range considered. In deducing the formula a direct reaction with a pure knock-out mechanism has been assumed.     

反应堆235U裂变源辐射防护材料的优化设计

以热中子反应堆²³⁵U裂变源为辐射源,利用MCNP程序对其能谱进行模拟并研究其辐射防护,结果表明对²³⁵U裂变源所发射的能量高于3MeV的瞬发中子,重金属具有良好的屏蔽效果,而对于能量低于1MeV的中子,轻氢材料的防护效果更好;W/LiH,W/B₄C,TiH₂/W三种复合材料当质量比满足：W：LiH=19：1,W：B₄C=9：1,W：TiH₂=3：1时材料的屏蔽效果最佳;通过遗传算法结合MCNP模拟,得到W/TiH₂/B₄C,TiH₂/Cu/Gd,TiH₂/B₄C/Gd三种复合材料的最优组分配比,源每次裂变产生的粒子穿过这三种材料后在等效组织中造成的剂量当量率（10^-11Sv·h^-1）与材料厚度呈指数下降关系,三种材料分别可近似为1.071exp（-0.187 8x）,1.077exp（-0.166 2x）和1.608exp（-0.171 9x）,x为材料厚度（cm）.