Recent improvements in the methodology of neutron imaging

The focus of this article is on further improvements of methods in neutron imaging: the increased spatial resolution for microtomography and options for energy-selective neutron imaging. Before going into details, some common statements are given in respect to the state-of-the-art in neutron imaging. A relation to the X-ray methods is mentioned, where complementary results are obtained. The potential for the energy selection is of particular interest for future installations at the new pulsed sources, based on spallation (SNS, J-PARC, ISIS-TS2). First results from preliminary studies look very promising for future material and industrial research. Therefore, statements about the set-up of the best possible imaging systems are included in the article.      

准单能中子单粒子效应研究现状

对国际上用于单粒子效应(SEE)研究的准单能中子源进行了相关调研,对产生准单能中子源的⁷Li(p, n)⁷Be核反应、装置布局以及表征中子场性质的中子注量率、中子能谱、中子束流轮廓及其均匀性、热中子本底等参数的理论计算及实验测量进行了系统的介绍。进行准单能中子SEE实验要求中子源有较高的中子注量率水平、较大的束流轮廓、较好的束流均匀性以及较低的热中子本底,并且能测量出精确的中子能谱。对准单能中子SEE实验过程以及三种中子SEE截面的尾部修正方法进行了介绍。     

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。     

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。     

Electron volt neutron spectrometers

The advent of pulsed neutron sources has made available intense fluxes of epithermal neutrons (500 meV ≤E≤100 eV ). The possibility to open new investigations on condensed matter with eV neutron scattering techniques, is related to the development of methods, concepts and devices that drive, or are inspired by, emerging studies at this energy scale. Electron volt spectrometers have undergone continuous improvements since the construction of the first prototype instruments, but in the last decade major breakthroughs have been accomplished in terms of resolution and counting statistics, leading, for example, to the direct measurement of the proton 3-D Born–Oppenheimer potential in any material, or to quantitatively probe nuclear quantum effects in hydrogen bonded systems. This paper reports on the most effective methods and concepts for energy analysis and detection, as well as devices for the optimization of electron volt spectrometers for different applications. This is set in the context of the progress made up to date in instrument development. Starting from early stages of development of the technique, particular emphasis will be given to the Vesuvio eV spectrometer at the ISIS neutron source, the first spectrometer where extensive scientific, as well as research and development programmes have been carried out. The potential offered by this type of instrumentation, from single particle excitations to momentum distribution studies, is then put in perspective into the emerging fields of eV spectroscopy applied to cultural heritages and neutron irradiation effects in electronics.     

Russian measurements of neutron energy spectra on the Mir orbital station

Results of the experiments on neutron energy spectra measurements within broad energy range from 5 x 10(-7) to 2 x 10(2) MeV aboard the Mir orbital station and equivalent neutron dose estimation are presented. Four measurement techniques were used during the experiments. The shape of spectra and their absolute values are in good agreement. According to those experiments, an equivalent neutron dose depends upon effective shielding thickness and spacecraft mass. The neutron dose mentioned is comparable with that of ionizing radiation. Neutron flux levels measured aboard the Mir station have shown that a neutron spectrometer involving broad energy range will be used within the radiation monitoring systems in manned space flights.     

极化中子技术

极化中子技术是利用中子的自旋与样品及磁场的相互作用进行测量的一种技术,是中子散射技术中的重要组成部分。极化中子技术已经成功地应用于磁性、强关联、纳米、自旋电子、高分子和生物等材料中,为前沿材料学研究与工业应用提供了所亟需的探测与表征手段。另外,极化中子也被广泛应用于核物理和粒子物理相关领域的基础测量。文章对极化中子技术的发展进行了简要的介绍,概括了极化中子实验所涉及的理论知识,以及在实验中所使用的主要技术,并在此基础上展示了当前极化中子技术在科研中的应用及其所使用的实验装置。     

探测系统能量分辨在中子能谱数值模拟中的应用

针对中子能谱实验的数值模拟研究中涉及到的探测系统能量分辨问题,基于系统探测伽玛和中子的对应关系以及对伽玛射线的能量分辨率,介绍了一种简便实用的数值模拟方案,利用探测系统对中子、伽玛的光响应对应关系以及其对伽玛的能量分辨率,得到系统对中子的能量分辨率,再进一步把它应用到中子能谱的数值模拟中,并对数值计算结果与相关实验数据进行比较分析。结果表明,该方法可以很好地应用于中子能谱的数值模拟研究。     

High-energy neutron spectrometry

We review progress made in the last quarter of a century in the field of neutron spectrometry over a wide energy range from ??1 MeV to a few tens of GeV. We consider spectrometers and detectors constructed in various laboratories for neutron measurements in numerous fundamental and applied studies. We discuss the results of works devoted to the development of experimental methods and the elaboration of new detectors. We pursue some promising avenues of further investigations.     

D-T中子与平板型聚乙烯材料作用的次级中子角度谱实验研究

基于反冲质子法建立了一种测量D-T中子与平板型宏观样品作用的次级中子角度谱的实验方法.为保证探测器的能量线性并在较低的中子有效测量下阈(0.5 MeV)情况下获得好的中子-伽马射线甄别性能,采用高、低能段分别测量的方法.采用事件记录法,同时记录了次级中子和伴随伽马射线的脉冲形状甄别和脉冲幅度二维信息,利用基于ROOT数据分析平台编写的离线数据分析程序,完成了伴随伽马射线的挑选和扣除,以及高、低两能段反冲质子谱的拼接,并成功的将神经网络技术应用于中子能谱的解谱,获得了D-T中子与9和18 cm厚平板型聚乙烯材料作用的0.5-15 MeV的次级中子角度谱实验结果.实验模型的MC模拟由MCNP5完成,数据库采用ENDF-VI,实验结果和MC计算结果在实验不确定度范围内一致. 关键词： D-T中子 积分中子学 反冲质子法 次级中子能谱     

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.     

Coupled neutron transport for HZETRN

Exposure estimates inside space vehicles, surface habitats, and high altitude aircrafts exposed to space radiation are highly influenced by secondary neutron production. The deterministic transport code HZETRN has been identified as a reliable and efficient tool for such studies, but improvements to the underlying transport models and numerical methods are still necessary. In this paper, the forward–backward (FB) and directionally coupled forward–backward (DC) neutron transport models are derived, numerical methods for the FB model are reviewed, and a computationally efficient numerical solution is presented for the DC model. Both models are compared to the Monte Carlo codes HETC-HEDS, FLUKA, and MCNPX, and the DC model is shown to agree closely with the Monte Carlo results. Finally, it is found in the development of either model that the decoupling of low energy neutrons from the light ion transport procedure adversely affects low energy light ion fluence spectra and exposure quantities. A first order correction is presented to resolve the problem, and it is shown to be both accurate and efficient.     

SiC基中子探测器对热中子的响应

以SiC二极管和中子转换材料6LiF为基础,研制了SiC基中子探测器,并用241Am源与临界装置分别研究了SiC基中子探测器的粒子响应、热中子响应。结果表明：SiC基中子探测器能够满足241Am源粒子的计数测量,但由于SiC二极管灵敏区薄,故不能用于5.48 MeV的粒子能谱测量;SiC基中子探测器对热中子响应良好,不同功率下,脉冲幅度谱中可清晰看到由6Li(n,)3H的反应产物粒子、3H粒子形成的双峰;脉冲幅度甄别法可将射线及电子学噪声甄别掉;在直接测量与符合测量中,SiC基中子探测器的计数率均与临界装置功率成线性关系,且直接测量的线性度比符合测量的线性度好,最好可达0.999 97。研究表明：降低肖特基接触金属的厚度、增加其外延层厚度、提高其外延层品质,可将SiC二极管用于带电粒子能谱测量。     

Polarization of nondegenerate nonrelativistic neutron gas particles by a magnetic field

The occupation of energy subbands with the magnetic moments parallel and antiparallel to the direction of the magnetic field for a nondegenerate neutron system of a specified concentration is calculated in the framework of the statistical thermodynamics of equilibrium systems. The kinetic neutron energies for the two subbands as well as the total system energy with allowance made for Pauli paramagnetism are determined. Byelorussian State University, Brest State University. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 4, pp. 39–43, April, 2000.     

Thermodynamic functions of a nonrelativistic degenerate neutron gas in a magnetic field

The Fermi energy, partial concentrations of polarized neutrons, pressure, and volume energy density of a degenerate nonrelativistic neutron gas in a magnetic field are calculated using numerical methods taking into account the anomalous magnetic moment of a neutron. The results of calculations are a generalization of relations underlying the Oppenheimer-Volkov model of a neutron star to the case of an applied magnetic field. An ultrastrong (up to 10¹⁷ G) magnetic field changes the pressure and internal energy of the star and affects it static configuration and evolution. It is shown that a degenerate neutron gas in ultrastrong and weak magnetic fields is paramagnetic; the corresponding values of magnetic susceptibility differ by a factor on the order of unity. The possibility of experimentally verifying the results from analysis of pulsar-emitted radiation is discussed.     

2.45 MeV小型D-D中子管准直屏蔽结构设计

为获取小角度出射的单能中子源,采用蒙特卡罗软件对小型D-D中子管产生的能量为2.45 MeV的4立体角中子源进行了准直屏蔽结构设计。准直屏蔽结构分为准直器和捕获穴,准直器采用铁+含硼聚乙烯+铅的三层过滤结构,用于屏蔽照射野外杂散中子,捕获穴主要功能是增加反方向中子的弹性散射次数,从而降低出射束低能散射中子的比例。通过MCNP模拟得到了准直器各层材料的最佳厚度和出射孔尺寸以及捕获穴最佳结构。经验证,中子照射野处2.45 MeV的中子通量比照射野外大三个量级,中子照射野处低能中子通量比2.45 MeV的中子通量低一个量级,墙壁外总剂量率（中子+）在2.5 Gy/h以下。该研究对于小角度单能中子源的快速获取具有一定的实用价值,获取的中子源可用于中子剂量仪器有效性检验、中子监测仪性能测试等方面的研究。     

An automatic evaluation system for NTA film neutron dosimeters

At CERN, neutron personal monitoring for over 4000 collaborators is performed with Kodak NTA films, which have been shown to be the most suitable neutron dosimeter in the radiation environment around high-energy accelerators. To overcome the lengthy and strenuous manual scanning process with an optical microscope, an automatic analysis system has been developed. We report on the successful automatic scanning of NTA films irradiated with ²³⁸Pu---Be source neutrons, which results in densely ionised recoil tracks, as well as on the extension of the method to higher energy neutrons causing sparse and fragmentary tracks. The application of the method in routine personal monitoring is discussed.     

A new high P-T cell for neutron diffraction up to 7 GPa and 2000 K with measurement of temperature by neutron radiography

Abstract

This paper reports developments to enable neutron diffraction at simultaneous high temperatures and pressures using the Paris-Edinburgh cell. These include a new design of a cell assembly with internal heating. One of the novel features of our system is the use of neutron radiographic methods for measurement of temperature. Fully refinable neutron diffraction patterns obtained by time of flight technique with our apparatus are found to be of comparable quality to previous high-pressure studies at ambient temperatures. In this paper we describe the procedures for the generation and measurement of pressure and temperature and illustrate the quality of the data which can be obtained. The present system may be used on a routine basis for experiments up to 7 GPa and temperature approaching 2000 K. Current attempts are discussed for extending these measurements to a wider domain of pressures and temperatures.     

聚变中子能谱测量系统脉冲中子灵敏度的实验研究

为多种复杂环境下的稳态和脉冲DT聚变中子能谱测量建立了一种灵敏度优化反冲质子磁谱仪. 使用成像板和同位素α源测量了谱仪的反冲质子能量-位置投影关系. 利用稳态加速器中子源平台、通过单粒子计数方法结合三维带电粒子输运程序模拟,研究了谱仪脉冲中子灵敏度能量响应. 通过高探测效率参数设置使谱仪对DT中子的探测效率达到2×10^-5 cm²水平,从而在较弱中子源上获得了较高统计精度实验数据. 程序模拟结果与谱仪α粒子刻度和DT中子标定实验结果取得了良好的一致性,可由此发展精细解谱技术,以提高脉冲中子能谱测量的灵敏度和能量分辨. 关键词： 聚变中子能谱 磁反冲质子 脉冲中子灵敏度 粒子输运     

Study of the neutron flux generated at E-linac-driven neutron sources

We treat generic physical features of the production of neutrons from the high atomic number materials irradiated by the electron beam provided by linear electron accelerator. The bremsstrahlung of incident electrons inducing the photo-nuclear reactions is considered. The experimental data on photo-nuclear reactions are utilized to describe neutrons generating caused by the ??-radiation absorption. The generally received theoretical approaches are applied to explore the photo-neutrons energy distribution. The produced neutrons are primarily statistical, yet the direct neutrons share is noteworthy as well. The dependence of the neutron spectrum, mean neutron energy and total neutron yield on the energy and current of the electron beam and on the characteristics of irradiated samples is investigated. The analysis is plainly carried out in framework of the quantum electrodynamics and photo-nuclear physics, without having any recourse to the ??numerical Monte-Carlo simulations??. Our findings prove mainly to conform satisfactorily to the experimental measurements, so far as those are available for now.