In this study, we investigated the potential and limitations of using joint X-ray and time-of-flight (TOF) neutron imaging for mapping the 3-dimensional organic carbon distribution in soil. This approach is viable because neutron and X-ray beams have complementary attenuation properties. Soil minerals consist to a large part of silicon and aluminium, and elements that are relatively translucent to neutrons but attenuate X-rays. In contrast, attenuation of neutrons is strong for hydrogen, which is abundant in soil organic matter (SOM), while hydrogen barely attenuates X-rays. In theory, TOF neutron imaging does further more allow the imaging of Bragg edges, which correspond to d-spacings in minerals. This could help to distinguish between SOM and clay minerals, the mineral group in soil that is most strongly associated with hydrogen atoms. We collected TOF neutron image data at the IMAT beamline at the ISIS facility and synchrotron X-ray image data at the I12 beamline at the Diamond Light source, both located within the Rutherford Appleton Laboratory, Harwell, UK. The white beam (the full energy spectrum) neutron image clearly showed variations in neutron attenuation within soil aggregates at approximately constant X-ray attenuations. This indicates a constant bulk density with varying organic matter and/or clay content. Unfortunately, the combination of TOF neutron and X-ray imaging was not suited to allow for a distinction between SOM and clay minerals at the voxel scale. While such a distinction is possible in theory, it is prevented by technical limitations. One of the main reasons is that the neutron frequencies available at modern neutron sources are too large to capture the main d-spacings of clay minerals. As a result, inference to voxel scale SOM concentrations is presently not feasible. Future improved neutron sources and advanced detector designs will eventually overcome the technical problems encountered here. On the positive side, combined X-ray and TOF neutron imaging demonstrated abilities to identify quartz grains and to distinguish between plastics and plant seeds.

中文翻译：

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结合中子和 X 射线成像量化土壤中局部碳含量的潜力

在这项研究中，我们研究了使用联合 X 射线和飞行时间 (TOF) 中子成像绘制土壤中 3 维有机碳分布的潜力和局限性。这种方法是可行的，因为中子和 X 射线束具有互补的衰减特性。土壤矿物的大部分成分是硅和铝，以及对中子相对透明但会减弱 X 射线的元素。相比之下，土壤有机质 (SOM) 中富含氢的中子衰减强烈，而氢几乎不衰减 X 射线。理论上，TOF 中子成像确实更允许对布拉格边缘进行成像，这对应于矿物中的 d 间距。这有助于区分 SOM 和粘土矿物，这是土壤中与氢原子最密切相关的矿物组。我们在 ISIS 设施的 IMAT 光束线上收集了 TOF 中子图像数据，在钻石光源的 I12 光束线上收集了同步加速器 X 射线图像数据，两者都位于英国哈威尔的卢瑟福阿普尔顿实验室内。白色光束（全能谱）中子图像清楚地显示了土壤聚集体中中子衰减的变化，X 射线衰减大致恒定。这表明具有变化的有机物和/或粘土含量的堆积密度恒定。不幸的是，TOF 中子和 X 射线成像的组合不适合在体素尺度上区分 SOM 和粘土矿物。虽然这种区分在理论上是可能的，但由于技术限制而无法实现。主要原因之一是现代中子源可用的中子频率太大而无法捕捉粘土矿物的主要 d 间距。因此，目前无法推断体素尺度 SOM 浓度。未来改进的中子源和先进的探测器设计最终将克服这里遇到的技术问题。从积极的一面来看，结合 X 射线和 TOF 中子成像显示了识别石英颗粒和区分塑料和植物种子的能力。