中子因为其与物质特殊的作用规律，是除了X射线之外的又一种重要的中性检测射线，在科学研究、工业检测和医疗服务领域发挥着重要的作用。在开展中子应用的时候，需要中子源能够提供足够高的中子产额，以获得足够高的中子注量率。在常见的中子源中，同位素源、中子发生器中子产额低，使用寿命较短，反应堆和散裂中子源建造和使用成本过高，均无法为一般科学研究、工业检测和医疗单位提供适用的中子束流。相比之下，基于小型加速器的中子源可以在各项指标之间取得良好的平衡，本论文即针对基于电子加速器的光中子源物理与技术问题开展研究。论文研究工作主要包括光中子产生、中子慢化引出和光中子源实验及其应用三个部分，涵盖了光中子源研究的主要环节：光中子产生——讨论了当电子加速器的能量分别为低能、中能和高能时的中子源设计原则。当电子加速器的能量为低能和高能时，分别应该选择低（γ，n）阈值核素和高Z核素作为（γ，n）反应的中子转换靶；当电子加速器的能量为中能时，则建议使用低（γ，n）阈值核素和高Z核素构成的复合靶来将电子加速器的轫致辐射光子转换为中子，并研究了复合靶优化设计方法与算法，提出与模拟验证了面向中子束流引出效率的复合靶与中子束流引出结构耦合方法与算法。当光中子转换靶体积有限制时，复合靶结构可以有效提高中子产额。（入射电子能量18 MeV,转换靶半径不超过25 cm时，复合靶的中子产额比单一材料靶中子产额提高~10%）中子慢化引出——分别讨论面向热中子成像和布拉格边沿分析的中子慢化引出设计方法和优化计算结果。面向热中子成像，中子转换靶的尺寸、慢化体的厚度均对引出中子通量构成了显著影响；面向布拉格边沿分析，研究了“常温转换靶+去耦”与“低温慢化体”两种提高波长分辨率的方式：前者可以在36米的飞行距离下，对波长2~3埃的中子获得好于0.5%的波长分辨率；后者可以在30米的飞行距离下，对波长4埃的中子获得好于0.5%的波长分辨率，这为具有不同晶格常数材料的布拉格边沿分析提供了灵活的慢化体实现方式。光中子源及其应用实验——基于9 MeV/0.9 kW电子直线加速器设计实现了光中子源，并对其引出中子的能谱、通量和波长分辨率特性开展了比较完整的实验评价，并开展了中高Z核素共振分析和热中子成像实验。论文研究成果为更高产额强流光中子源的深入研究提供了有益基础。
Neutron acts as an important role in the areas of scientific research, industrial inspection and medical applications, as its counterpart of X-ray, for its unique property of interaction mechanism with matter. A qualified neutron source should be capable of delivering neutrons of high yield for providing the needed high flux. The traditional isotopic neutron sources and neutron generators are ruled out due to their quite low neutron yields and short life-spans. And the reactor or spallation neutron sources will also not be considered for their ultra-high constructing and operation costs. Therefore, the low energy accelerator driven neutron source might be the only proper choice that may strike the compromise among the factors related with a neutron source that could be practically deployed. In this dissertation, the physical and technical issues along with the photoneutron source were discussed in detail.There are three sub-topics addressed in this dissertation: the production of photoneutrons, the moderation of photoneutrons and the preliminary neutron measurement conducted with a 9 MeV/0.9 kW photoneutron source.Firstly, the production of photoneutrons is reaserched. When the energy of the electron linear accelerator (e-LINAC) works at different energies, the criteria for designing the photon-to-neutron convertor would differ. For the low energy e-LINAC, which works with the energy less than 16 MeV, the nuclides with low (γ,n) threshold should be used to convert photons to neutrons; and for the high energy e-LINAC delivering electrons with energy higher than 30 MeV, the nuclides with high atomic number should be used. For the medium e-LINAC delivering electrons with energy between 16 MeV and 30 MeV, a composite photon-to-neutron convertor, which involves both the low (γ,n) threshold nuclides and high atomic number nuclides, should be used to convert photons to neutron effectively. If the radius of the photon-to-neutron convertor is restricted to be less than 25 cm (when the energy of electrons is 18 MeV), the neutron yield of the composite photon-to-neutron convertor is improved by ~10% compared with that of a single material photon-to-neutron convertor.Secondly, the moderation of photoneutrons for the thermal neutron imaging and the Bragg-edge imaging is reaserched. For the thermal neutron imaging, the photon-to-convertor’s geometrical parameters, as well as that of the surrounding moderator were both studied in detail to find the optimal parameters to deliver the highest thermal neutron flux at the exit of the photon-to-neutron convertor. For the Bragg-edge imaging, in which the long wavelength neutrons of high energy resolution are desired, two different methods were investigated with the simulation. The first method was realized with the decoupled room temperature photon-to-neutron convertor, while the second method was realized with the aid of a 30 K solid methane convertor. For the 2~3 angstrom neutrons, the first method can be used to realize the ≤0.5% wavelength resolution with a 36-meter-flight-distance. For the 4 angstrom or even longer wavelength neutrons, the second method were expected to realize the ≤0.5% wavelength resolution with a 30-meter-flight-distance.The neutron beams delivered by the 9 MeV/900 W photoneutron source was experimentally evaluated for the neutrons’ spectrum, flux and wavelength resolution. And the preliminary thermal neutron imaging, as well as the resonant analysis of medium- and high-Z nuclides were also successfully conducted.The issues discussed in this dissertation would be beneficial to the future study of photoneutron sources with even high neutron yields.
