PSI - Issue 64

M. Mizuta et al. / Procedia Structural Integrity 64 (2024) 214–219 Author name / Structural Integrity Procedia 00 (2019) 000 – 000

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3. Results and discussion In neutron imaging, the obtained images are a mixture of neutrons transmitted through the material and neutrons scattered within the material. Therefore, even in samples with uniform thickness and homogeneity along the neutron transmission direction, images with differences in shading between the surface and other areas are obtained. In this experiment, images are captured under conditions where the sample thickness and the distance from the sample to the detection surface varied along the cross-section shown in Figure 3. Such geometric variations affect the balance between neutrons passing through specific locations of the sample and neutrons scattered from the sample, making it difficult to separate the two. Therefore, the normalized axial brightness at two positions in the cross-section (near the cylinder axis and near the circumference) is compared to examine the influence of the circular sample shape. Figure 4 illustrates the normalized brightness before water absorption and after 4 hours of water absorption. Here, distance zero represents the water absorption surface, and the normalized brightness is over a width of 18 mm (400 pixels, with 1 pixel equivalent to 0.045mm) as average. Higher normalized brightness indicates more neutrons reaching the detection surface. The normalized brightness decreases since the presence of water reduces the neutrons reaching the detection surface due to increased hydrogen content in the sample. First, examining the normalized brightness distribution near the axis and circumference before water absorption reveals that both are highest at the water absorption surface and gradually decrease up to approximately 1000 pixels (45 mm). This indicates that the influence of scattered neutrons from the sample differs between continuous parts and the edge (water absorption surface). When focusing on water permeation from the edge, as in this study, changes over a large range where the influence of scattered neutrons is significant are captured. Thus, it becomes necessary to consider methods to remove or mitigate the influence of scattered neutrons. Next, comparing the shapes of the normalized brightness distributions near the axis and circumference before water absorption, both exhibited similar trends. However, normalized brightness exhibited variability above a distance of 1000 pixels near the circumference. This suggests that the neutron transmission distance differs between near the axis and near the circumference in circular samples, resulting in differences in hydrogen detection sensitivity. As a result, it is speculated that the areas near the periphery with shorter transmission distances reflect the presence of aggregates more effectively. Comparing normalized brightness distributions before and after water absorption, normalized brightness decreased after water absorption in both near-axis and near-circumference cases, with the decrease ranging up to approximately 1000 pixels from the water absorption surface in the near-axis case and up to 500 pixels in the near circumference case. This indicates a reduction in neutron transmission due to increased hydrogen content near the water absorption surface as a result of water penetration. Thus, for circular samples, neutron imaging is found to be capable of capturing water movement although the range of normalized brightness changes differs before and after water absorption. The difference between normalized images before and after water absorption (hereafter referred to as brightness difference) was analyzed to understand the water movement after 4 and 22 hours. This method was previously confirmed to allow quantitative determination of water content from normalized images in cubic (neutron

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Fig. 4. Comparison of normalized brightness before and after water absorption