中子成像技术在元素分析中的应用

中子成像作为一种快速、直观的无损检测技术,在核工业、航空航天、新能源、地质、考古、先进制造等多个领域得到广泛应用。中子成像利用中子不带电、穿透能力强、对轻元素敏感、可区分同位素和近邻元素等特性,非常适合开展含氢元素、近邻元素和同位素等材料的无损检测。本文概述了中子成像技术的基本原理及特点,并结合中国先进研究堆(CARR)中子成像装置上的应用案例, 重点介绍了国内外中子成像技术在储氢材料、燃料电池、岩石、核燃料元件、古代文物等领域的典型应用。随着中子成像技术的不断发展和广泛应用,有望为我国更多领域研究提供更强有力的技术支撑。     

反应堆瞬发γ活化成像技术研究进展

瞬发γ活化成像技术(Prompt Gamma Activation Imaging,PGAI)是基于瞬发γ中子活化分析技术(Prompt Gamma Neutron Activation Analysis,PGAA)的一种新型元素成像技术,具有PGAA非破坏性、高灵敏度的多元素分析特点,同时可研究大体积样品内元素含量的三维分布情况。与移动中子源(如同位素中子源、加速器中子源)相比,反应堆中子源具有高通量的冷/热中子,因此,基于反应堆中子源的PGAI装置具有更加广阔的应用前景。鉴于PGAI技术的重要性,主要介绍了PGAI技术原理和测量方法、列举了国内外具有代表性的PGAI装置及特点,并指出当前PGAI技术研究现状与应用进展。其中,与中子层析成像(Neutron tomography,NT)结合的PGAI是目前研究的重点。随着国内高通量中子反应堆的持续稳定运行以及PGAI装置的建立和技术发展,相信未来PGAI技术能在我国更多领域得到广泛和深入应用。     

元素成像技术在核法证中的应用研究进展

核法证学是打击非法贩卖核材料和放射性物质的重要工具,可用于对截获的材料进行分析溯源。过去的研究主要集中于元素和同位素特征的总体分析表征。近年来,随着微分析技术的发展,微米尺度和纳米尺度元素和同位素空间分布特征、微观形态学特征研究正成为核法证学研究的热点。本文基于近年来国内外核法证学的研究成果,详细介绍了元素成像技术在核法证学材料分析领域元素和同位素成像、微观形态学方面的应用研究进展。     

中子散射技术在化学研究中的应用

中子散射在化学、软物质、基础物理、地球科学、工程等领域应用广泛,在不同领域的科学探索过程中发挥了关键作用.中子具有不带电荷、穿透能力强、对轻元素敏感、能够进行无损检测等特点.一方面,弹性中子散射在氢元素富集的化学材料体系、高分子体系、生物大分子等体系的结构研究中独具优势;另一方面,非弹性中子散射测量的能量、散射矢量范围与这类材料体系的时间、空间尺度吻合,非常适合化学过程的动力学研究.本文将通过结合中国绵阳研究堆(CMRR)中子散射平台上的应用实例,重点介绍国内外利用小角中子散射、中子反射、中子衍射、非弹/准弹性中子散射等代表性技术在化学反应机理研究、溶液中的化学过程、高分子化学、生物化学、材料化学、核化学和放射化学等领域的典型应用及前景.     

中国先进研究堆瞬发γ元素成像技术研究

无损分析样品内部的元素分布,对于材料、考古、地质科学等领域的研究具有重要意义。本文基于瞬发γ活化分析方法,结合中子照相技术,分析了模拟样品的结构和对称性,首次利用中国先进研究堆(CARR)水平孔道聚焦的微束中子束流,开展了样品瞬发γ扫描分析和材料内部元素分布研究。通过蒙特卡罗软件(MCNP)对样品元素分布实验模型进行分析,模拟结果获得了样品不同区域的元素分布,基本实现了元素空间分辨。MCNP计算结果表明在瞬发γ扫描分析中,优化准直几何参数和提高中子通量能进一步提高元素空间分辨和元素测量准确度。     

单分子定位超分辨显微成像有机荧光探针的研究进展

在生物医学领域,对纳米尺寸级别的微小生物目标进行精确定位研究具有非常重要的意义,而光学显微成像技术为此提供了强有力的工具。 光学显微成像技术受到光学衍射极限的限制,难以分辨尺寸在衍射极限(<200 nm)以下的生物结构,无法直接获取微小生物结构信息,阻碍了生物医学的进一步发展。 近年来,随着纳米分辨显微成像技术的出现,新型荧光探针的开发、成像系统与设备的不断发展及成像算法不断完善地深入结合,促进了光学衍射极限以下尺寸微观目标的研究。 基于单分子定位的超分辨荧光显微成像(SMLM)包括光激活定位成像(PALM)与随机光学重构超分辨成像(STORM),将有机荧光探针与超分辨光学显微成像技术紧密结合在一起,荧光探针的光物理性质直接决定着超分辨成像结果的好坏。 因此,设计不同性能的荧光探针可以实现超精细结构的不同超分辨成像,为研究其生物学功能提供了有力的工具。 本文着重围绕基于SMLM的原理、有机荧光探针的设计要求、用于SMLM的荧光探针种类及其生物应用等方面进行总结综述,指出了单分子定位成像上存在的不足,并对其发展方向进行了展望,希望为对超分辨成像研究感兴趣或初涉该领域的研究者提供成像理论与探针设计方面的帮助。     

月球样品的中子活化分析技术研究

月球的起源是月球研究的核心问题。月球探测任务返回的数据和样品极大地提高了人类对地月系统的认知,同时也发现了更多未解之谜,亟待未来的探测任务和科学研究来解答。嫦娥五号月壤是我国首次地外天体采集返回的样品,也是继美国和苏联探月采样任务45年后人类再次获得的月球样品。鉴于月球样品的珍贵性和特殊性,利用先进技术开展其全元素含量的非破坏精准分析对于认识月球演化和月球资源就地开发利用具有重要的意义。依托大型核反应研究堆和加速器中子注量率优势,利用中子活化分析技术可实现月球样品中的全元素非破坏分析：1)仪器中子活化分析技术(INAA)可测量Na—U元素之间的60余种元素;2)瞬发γ中子活化分析技术(PGNAA)可补充测量INAA不适合测量的元素如H、B、C、N等;3)中子深度剖面分析技术(NDP)可测量样品近表面(微米级)聚变能源³He的浓度深度分布;4)缓发中子测量技术(DNC)可定量样品中痕量裂变核素如²³⁵U和²³⁹Pu,并结合INAA可测量²³⁸U/²³⁵U同位素比值;5)14 Me...     

基于可移动中子源的瞬发γ活化成像研究进展

瞬发γ活化成像技术基于瞬发γ射线中子活化分析,结合准直测量或者伴随粒子测量手段,以实现对样品内部中元素位置分布的测量。当前大多数相关研究都是基于大型中子源开展的,这限制了该技术的应用场景,可移动小型化中子源是该技术现场应用的必然要求。基于可移动中子源的瞬发伽马活化成像技术按照准直方式可分为基于单孔准直、编码孔准直及伴随粒子测量三类,本文对其技术原理、研究进展及仍存在的问题进行介绍与讨论。目前的技术在空间分辨率上还需要进一步提升,未来将进一步结合图像处理算法提高成像质量并降低测量时间。     

基于瞬发伽马谱学的元素重建算法

质子治疗中的瞬发伽马射线是质子和靶标之间核反应的产物,瞬发伽马射线的特征能量和强度可以用来确定靶中元素的种类和数量,在之前的实验中已经证明,无论反应截面多么复杂,一旦确定了被照射元素和入射质子的能量,元素浓度与伽马射线光子数之间就存在一定的线性关系。然而,这种线性关系很难应用于医学成像,而且氢的非线性行为迄今尚未研究。本文将这种线性关系推广到包括氢等非线性情况的混合元素材料,并提出了一种通用的数学形式,即基于瞬发伽马谱学的重建算法 (PGSRA)。PGSRA的基本假设是样品材料的PGS与元素的每摩尔伽马射线有某种关系。对于碳和氧,这种关系是线性的,而对于氢,这种关系是非线性的。由于2.23 MeV的伽马线来源于中子吸收辐射,我们仔细研究了氢非线性行为。利用蒙特卡罗模拟验证了碳、氧和氢的不同组合,如PMMA、戊二醇和乙醇二醇的线性和非线性关系。在这项工作中开发的PGSRA可能是PGS和医学成像之间的第一座桥梁。     

小角中子散射技术及其在大分子结构表征中的应用

小角中子散射(SANS)是一种表征从纳米到微米尺寸物质特征结构的有力工具,配合中子的强穿透性和同位素辨识等特性,在软物质大分子结构表征方面发挥着独特的作用.随着中国散裂中子源(CSNS)在2018年正式对外接受机时申请,国内SANS用户群逐年扩大.本文首先简要介绍小角中子散射技术的基本原理、谱仪结构和实验技巧,然后紧扣小角谱仪的特点和方法学方面的最新进展,介绍小角中子散射在高分子溶液、高分子共混物和复合材料、高分子结晶、凝胶、多孔材料、生物大分子等研究领域的结构表征方面的典型应用.小角中子散射和其他表征手段,如小角X射线散射(SAXS)相互紧密配合和补充,成为连接大分子内部多相多尺度的微观结构和宏观性的桥梁.     

High-temperature neutron diffraction study of the cation ordered perovskites TbBaMn2O5+x and TbBaMn2O5.5−y

The miscibility of TbBaMn₂O_5+x and TbBaMn₂O_5.5−y has been investigated at 100-600 °C using in situ powder neutron diffraction. No miscibility is observed, and the two phases remain oxygen stoichiometric (x,y=0) at 600 °C. Structure refinement results show that neither material undergoes a phase transition in this temperature range. TbBaMn₂O₅ is Mn²⁺/Mn³⁺ charge ordered and any charge melting transition is >600 °C. This symmetry-broken charge ordering is remarkably robust in comparison to that in other oxides.     

Synthesis and crystal structure of CaxCo4Sb12 skutterudites

New skutterudite compounds Ca_xCo₄Sb₁₂ (0<x?0.2) have been prepared by traditional metallurgical synthesis. The compounds have been characterized by X-ray powder diffraction (XRD), electron probe microanalysis (EPMA) and neutron powder diffraction. Rietveld refinement of the structures against neutron powder diffraction data (on Ca_0.1Co₄Sb₁₂, , a=9.0429 Å, χ²=1.55; wR_p=1.52) enabled the location of Ca in the voids of the skutterudite structure to be verified. The large displacement ellipsoid for Ca is consistent with “rattling” in the cage of the crystalline structure. XRD combined with EPMA analyses showed that the maximum occupancy of Ca atoms is about 0.2.     

Detection of dynamical transitions in hydrogenous materials using transmission measurements with very cold neutrons

We have tested the transmission of very cold neutrons as a method to measure dynamical transitions in hydrogenous materials. Transmitted intensities vs. temperature at 30 Å neutron wavelength were measured for four materials that undergo phase transformations associated with changes in dynamics: ammonium iodide, sodium borohydride, hexamethylbenzene, and dicesium dodecahydro-closo-dodecaborate. In some cases, neutron vibrational spectra above and below the transformation temperatures are compared to the transmission results. The measurements show changes in transmission at or near the transition for all these compounds, reflecting dynamical changes. The results demonstrate that the transmission method is sensitive to motional changes due to a wide range of structural transitions, from first-order to much more subtle order–disorder effects and for both small molecular species and larger molecules. The technique is valuable for rapid (ca. hours) scans of new materials to guide neutron inelastic scattering experiments or to complement the results of other techniques.     

Uses of neutron and X-ray beams to investigate magnetism

Experimental techniques using beams of neutrons and X-ray have more than proved their worth in studies of magnetism. Their similarities and differences will be discussed, and illustrated with reference to recent experimental studies of surface and bulk magnetism.     

Lattice effects in cubic La2Mo2O9: Effect of vacuum and correlation with transport properties

This study aims to investigate correlations between lattice effects and transport properties in cubic La₂Mo₂O₉. High temperature neutron diffraction data, recorded in air and under vacuum, are used to follow the evolution with temperature of selected structural parameters, i.e. bond lengths and angles. Results suggest a possible correlation with the experimentally observed decrease of the activation energy for oxygen migration at high temperature. The effect on the structural properties of the low oxygen partial pressure used during the measurements in vacuum is negligible and this represents a valuable information in view of possible applications of the material in solid state devices.     

Determination of the Bloch wall thickness in Fe-4 at %Si single crystals by neutron small angle scattering

Summary With aid of neutron small angle scattering from regular magnetic domain structure in the interior of bulky <110> Fe-4 at %Si single crystals the thickness of 90° Bloch walls and therefrom the exchange constant is determined.

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Zusammenfassung Mit Hilfe der Neutronenkleinwinkelstreuung an regelmäßiger Bereichstruktur im Inneren massiver <110> Fe-4 at %Si Einkristalle werden die Dicke von 90°-Blochwänden und daraus die Austauschkonstante bestimmt.

     

Frequency spectra of transport properties in lonic liquids: contribution of charge fluctuation modes

Abstract

The influence of charge fluctuation modes on the frequency spectra for shear viscosity, self-diffusion and thermal diffusion is discussed. Both the one-component plasma, and modifications which are anticipated in real ionic liquids to the charge fluctuation modes, are considered. The latter treatment utilizes a simple model into which is incorporated plasmon dispersion and plasmon damping, the damping being both at zero wave number k and at order k²

The main emphasis of the paper is on non-analytic behaviour with frequency, and the modifications which occur for damped charge fluctuation modes. Because of the dominant influence of mass fluctuation modes at very long time, which are not considered quantitatively here, the theory discussed should have relevance at intermediate times, such as are treated in present molecular dynamical calculations. Contact is made with such computer simulation of molten BeF₂ and a model of molten salts     

NeuDATool:an Open Source Neutron Data Analysis Tools,Supporting GPU Hardware Acceleration,and across-Computer Cluster Nodes Parallel

Empirical potential structure refinement is a neutron scattering data analysis algorithm and a software package. It was developed by the disordered materials group in the British spallation neutron source (ISIS) in 1980s, and aims to construct the most-probable atomic structures of disordered materials in the field of chemical physics. It has been extensively used during the past decades, and has generated reliable results. However, it implements a shared-memory architecture with open multi-processing (OpenMP). With the extensive construction of supercomputer clusters and the widespread use of graphics processing unit (GPU) acceleration technology, it is now possible to rebuild the EPSR with these techniques in the effort to improve its calculation speed. In this study, an open source framework NeuDATool is proposed. It is programmed in the object-oriented language C++, can be paralleled across nodes within a computer cluster, and supports GPU acceleration. The performance of NeuDATool has been tested with water and amorphous silica neutron scattering data. The test shows that the software can reconstruct the correct microstructure of the samples, and the calculation speed with GPU acceleration can increase by more than 400 times, compared with CPU serial algorithm at a simulation box that has about 100 thousand atoms. NeuDATool provides another choice to implement simulation in the (neutron) diffraction community, especially for experts who are familiar with C++ programming and want to define specific algorithms for their analysis.     

About different types of water in swollen polyelectrolyte multilayers

The review addresses swelling of polyelectrolyte multilayers in water. Different models for the determination of the water content are compared. It is clearly shown that voids under dry conditions present cavities for water which contribute to the water content of the multilayer in the swollen state. This so-called “void water” does not lead to any changes in thickness but in scattering length density during swelling. The “swelling water” leads to both changes in scattering length density and in thickness. Depending on the preparation conditions like the type polymers, polymer charge density, ionic strength and type of salt the ratio of “void water” differs between 1 and 15 vol.% while the amount of “swelling water” is of several ten's of vol.%.     

Magnetic structures of the M2TbF6 (M=Li, K, Rb) fluorides: A complex behavior resulting from frustration

Neutron powder diffraction has been performed on Li₂TbF₆, K₂TbF₆ and Rb₂TbF₆ fluoroterbates. Incommensurate long-range magnetic order is observed below T_N=2.02, 1.60 and 2.07 K. The square-modulating of the magnetic structures can be correlated with the geometric frustration induced by the pseudo-hexagonal packing of the [TbF₆]²⁻ chains in these hexafluorides. This frustration and the magnetic interactions are discussed on the basis of experimental data and topological considerations. The magnetic structures encountered in this series, and the particular thermal evolution of the Li₂TbF₆ magnetic structure may result from the competition between the magnetic interactions taking place in the chains and the magnetic interactions coupling the chains.