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Introduction to neutron scattering 中子散射概论

Q2 CHEMISTRY, MULTIDISCIPLINARY

ChemTexts

Pub Date : 2023-10-25 DOI: 10.1007/s40828-023-00184-7

Walter Langel

Abstract Neutron scattering is a very high-performance method for studying the structure and dynamics of condensed matter with similar approaches in wide ranges of space and time, matching dimensions in space from single atoms to macromolecules and in time from atomic vibrations over crystal phonons to low-lying transitions in the microwave range, and to motions of large molecular units. Concerning the number and depth of physical concepts, neutron scattering may be compared to modern nuclear magnetic resonance. Neutrons have contributed essential results to the understanding of atomic and molecular processes and are, in this respect, complementary to other materials science probes. Among others, three properties of thermal neutrons make them especially appropriate for such work: the neutron mass is similar to atomic masses, and both neutron energies and the wavelengths of the neutron material wave match typical values for condensed matter. A further important feature of neutron scattering, making it especially valuable in biochemistry and polymer sciences, is that hydrogen and deuterium atoms very significantly and specifically contribute to the signal in both diffraction and spectroscopy. Additionally, neutrons are scattered at the nuclei and directly reflect the nuclear structure and motions. Results from neutron scattering are of great general interest. This paper aims to provide an introduction for chemists on a level understandable also to students and researchers who are not going to become part of the neutron community and will not be involved in the experiments, but shall be able to understand the basic concepts of the method and its relevance to modern chemistry. The paper focuses on basic theory, typical experiments, and some examples demonstrating the applications. As for many modern experimental techniques, the interpretation of the results of neutron scattering is based on theoretical models and requires a significant mathematical overhead. Most results are only meaningful when compared with computer simulations. For understanding this, in this paper, the theory of scattering is developed, starting with intuitive models and presenting typical concepts such as the scattering triangle, energy and momentum transfer, and the relation of inelastic and elastic scattering to space- and time-dependent information. The interaction of neutrons with matter, scattering cross sections, beam attenuation, and coherent versus incoherent scattering are explained in detail. Two further typical concepts that are not generally familiar to scientists outside the community are the use of wave and particle equivalence, and of handling results as a scattering function that depends simultaneously on momentum and energy transfers. The possibility of obtaining neutron beams for scattering experiments at a few research centers around high-performance sources is explained, and experimentally relevant features of research reactors and spallation sources are ment

中子散射是一种高性能的研究凝聚态物质结构和动力学的方法，在大的空间和时间范围内具有相似的方法，在空间上从单个原子到大分子，在时间上从晶体声子的原子振动到微波范围内的低洼跃迁，以及大分子单位的运动。就物理概念的数量和深度而言，中子散射可与现代核磁共振相比较。中子对原子和分子过程的理解做出了重要的贡献，在这方面，它与其他材料科学探测是互补的。其中，热中子的三个特性使它们特别适合这样的工作:中子质量与原子质量相似，中子能量和中子物质波的波长都符合凝聚态物质的典型值。中子散射的另一个重要特征是，氢和氘原子对衍射和光谱学中的信号都有非常显著的贡献，这使得它在生物化学和聚合物科学中特别有价值。此外，中子在原子核中散射，直接反映原子核的结构和运动。中子散射的结果具有广泛的意义。本文的目的是为化学家提供一个可以理解的介绍，也为那些不打算成为中子社区的一部分和不参与实验的学生和研究人员提供一个可以理解的水平，但应该能够理解该方法的基本概念及其与现代化学的相关性。本文着重介绍了该方法的基本原理、典型实验和应用实例。对于许多现代实验技术来说，中子散射结果的解释是基于理论模型的，需要大量的数学开销。大多数结果只有在与计算机模拟相比较时才有意义。为了理解这一点，本文发展了散射理论，从直观的模型出发，提出了散射三角形、能量和动量传递、非弹性和弹性散射与时空相关信息的关系等典型概念。详细解释了中子与物质的相互作用、散射截面、光束衰减以及相干与非相干散射。另外两个典型的概念通常不为学术界以外的科学家所熟悉，一是使用波和粒子等效，二是将结果作为同时依赖于动量和能量转移的散射函数来处理。说明了在少数几个研究中心围绕高性能源获得用于散射实验的中子束的可能性，并提到了研究堆和散裂源的实验相关特征。由于中子实验总是要处理小通量、延伸束和屏蔽，实验条件与实验室处理样品和仪器的方法相距甚远。为了使实验更容易理解，并使读者熟悉该方法，给出了实验细节。与此相关的是扩展了在各种不同环境中处理样本的可能性。在手稿的进一步部分，各种技术和典型的仪器提出，连同一些特点的应用，使活跃的理论发展到目前为止。这包括粉末衍射和液态水的结构，三轴光谱仪和晶格声子，后向散射光谱和旋转隧道，飞行时间光谱，同时探测低洼振动和扩散的能量和形状，滤波光谱仪和无选择规则的振动光谱，小角度中子散射和蛋白质展开，以及胶束，中子自旋回波光谱，聚合物动力学。

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