# Three-Dimensional High-Energy Electron Radiography Method for Static Mesoscale Samples Diagnostics
## 4 Conclusions and Discussions  
A new three-dimensional high-energy electron radiography method was proposed for mesoscale sciences diagnostics. The feasibility and validation of TDHEER are testified by start-to-end simulation studies. The TDHEER method is a combination of HEER and a three-dimensional reconstruction algorithm. With the TDHEER method, the diagnostics capability of HEER is enhanced greatly. The simulation results show that the internal structures of the sample can be attained from slice to slice along each axis. The inner flaw or structure of the sample can be exactly determined, and the spatial resolution can reach \(\upmu\)m, which is very useful for mesoscale sciences studies.  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Sample Number/Material** & **X (cm)** & **Y (cm)** & **Z (cm)** & **R (cm)** \\ \hline
1/aluminum & 0 & 0 & 0 & 0.01 \\
2/hollow & 0.005 & 0.006 & 0 & 0.002 \\
3/hollow & 0 & \(-\)0.004 & \(-\)0.005 & 0.003 \\
4/tungsten & \(-\)0.004 & 0.005 & 0 & 0.003 \\
5/tungsten & 0 & \(-\)0.006 & 0.005 & 0.002 \\ \hline \hline \end{tabular}
\end{table}
Table 4: The positions and material parameters of the complicated spherical sample with four small balls inside.  
Figure 6: Three-dimensional reconstruction slice images along the \(z\)-axis of the complicated spherical sample with four small balls inside. The slice numbers and the directions are shown in each picture.  
For further TDHEER development, some possible methods were investigated. First of all, TDHEER experimental studies will be undertaken in the near future, using the same process as shown in the simulation studies. The TDHEER experiment has been scheduled and will be performed at the Lanzhou HEER experimental platform [20]. Secondly, the imaging lens used in this paper has a magnification factor of 1, which restrains the HEER spatial resolution to several microns. For further studies, a high-magnification imaging lens should be used, such as 10 to 100, which will help and improve the spatial resolution to sub \(\upmu\)m [9] accordingly, improving the TDHEER resolution. Thirdly, here we used the FBP reconstruction algorithm with a total of 180 HEER images, in which data recording and images analysis took a long time. Therefore, we suggest that other three-dimensional reconstruction algorithms are used to achieve the three-dimensional reconstruction with fewer HEER images in less time, for example, with a 5 or 10 degree step. Fourth, in this paper, we considered the static mesoscale samples. We also considered TDHEER for the samples' dynamic process diagnostics, for which we cannot use rotating sample technology. However, we proposed three-orthogonal directions HEER with three radiography electron beam lines from one accelerator [2; 8]. Here, we propose the reconstruction of the sample's three-dimensional structures with even fewer HEER images and the development of a new reconstruction algorithm [21] in the future. Furthermore, the electron beam energy can be selected from tens of MeV to GeV to achieve high spatial resolution and image contrast depending on the sample characteristics. In summary, we propose and demonstrate a new three-dimensional radiography method with HEER, which is more powerful and suitable for mesoscale sciences diagnostic studies.  
Q.Z. and Y.M. have equal contributions, conceptualization, Q.Z. and Z.Z.; methodology, Y.M.; software, Y.M, Y.Z. and Z.R.; validation, Q.Z., Y.M., and J.X.; writing--original draft preparation, Q.Z. and Y.M.; writing--review and editing, S.C. and X.S.; project administration, Z.Z.; funding acquisition, Z.Z. This research was funded by the National Natural Science Foundation of China, grant number 11435015, the Ministry of Science and Technology of China 2016 YFE0104900 and the Chinese Academy of Sciences 28Y740010. The authors declare no conflict of interest.